AU3912100A - Photonic integrated circuit for optical CDMA - Google Patents

Description

WO 00/70804 PCT/US00/07685 PHOTONIC INTEGRATED CIRCUIT FOR OPTICAL CDMA PRIORITY This application claims priority from provisional patent application 60/100,223, filed 5 September 14, 1998, which provisional patent application is incorporated by reference in its entirety. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to optical communication systems and, more particularly, to 10 optical code-division multiple access communications systems implemented using one or more photonic integrated circuits. 2. Description of the Related Art Recent years have seen rapidly expanding demand for communications bandwidth, resulting in the increased use of technologies such as satellite communications, video 15 programming distribution networks such as cable television, and spread-spectrum telephony including, for example, code-division multiple access telephony. Such technologies have become common and well integrated into everyday communications. Growing demand for communications bandwidth has brought significant investments in new communications technologies and in new communications infrastructure. For 20 example, the cable television industry, telephone companies, Internet providers and various government entities have invested in long distance optical fiber networks and in equipment for fiber networks. The addition of this infrastructure has, in turn, spurred demand for bandwidth use, resulting in demand for yet additional investment in new technologies and infrastructure. 25 Installing optical fibers over long distances is expensive. Additionally, conventional optical fiber or other optical communication networks utilize only a small fraction of the available bandwidth of the communication system. There is consequently considerable interest in obtaining higher utilization of fiber networks or otherwise increasing the bandwidth of optical fiber systems. Techniques have been developed to increase the 30 bandwidth of optical fiber communication systems and to convey information from plural sources over a fiber system. Generally, these techniques seek to use more of the readily available optical bandwidth of optical fibers by supplementing the comparatively simple coding schemes conventionally used by such systems. In some improved bandwidth fiber systems, the optical fiber carries an optical channel on an optical carrier signal consisting of 35 a single, narrow wavelength band and multiple users access the fiber using time-division multiplexing (TDM) or time-division multiple access (TDMA). Time division techniques

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WO 00/70804 PCT/USOO/07685 transmit frames of data by assigning successive time slots in the frame to particular communication channels. Optical TDM requires short-pulsed diode lasers and provides only moderate improvements in bandwidth utilization. In addition, improving the transmission rates on a TDM network requires that all of the transceivers attached to the 5 network be upgraded to the higher transmission rates. No partial network upgrades are possible, which makes TDM systems less flexible than is desirable. On the other hand, TDM systems provide a predictable and even data flow, which is very desirable in multi user systems that experience "bursty" usage. Thus, TDM techniques will have continued importance in optical communications systems, but other techniques must be used to obtain 0 the desired communications bandwidth for the overall system. Consequently, it is desirable to provide increased bandwidth in an optical system that is compatible with TDM communication techniques. One strategy for improving the utilization of optical communication networks employs wavelength-division multiplexing (WDM) or wavelength-division multiple access 5 (WDMA) to increase system bandwidth and to support a more independent form of multiple user access than is permitted by TDM. WDM systems provide plural optical channels each using one of a set of non-overlapping wavelength bands to provide expanded bandwidth. Information is transmitted independently in each of the optical channels using a light beam within an assigned wavelength band, typically generated by narrow wavelength band 0 optical sources such as lasers or light emitting diodes. Each of the light sources is modulated with data and the resulting modulated optical outputs for all of the different wavelength bands are multiplexed, coupled into the optical fiber and transmitted over the fiber. The modulation of the narrow wavelength band light corresponding to each channel may encode a simple digital data stream or a further plurality of communication channels 5 defined by TDM. Little interference will occur between the channels defined within different wavelength bands. At the receiving end, each of the WDM channels terminates in a receiver assigned to the wavelength band used for transmitting data on that WDM channel. This might be accomplished in a system by separating the total received light signal into different wavelengths using a demultiplexer, such as a tunable filter, and 0 directing the separated narrow wavelength band light signals to receivers assigned to the wavelength of that particular channel. At least theoretically, the availability of appropriately tuned optical sources limit the number of users that can be supported by a WDM system. Wavelength stability, for example as a function of operating temperature, may also affect the operational characteristics of the WDM system. 5 As a more practical matter, the expense of WDM systems limits the application of this technology. One embodiment of a WDM fiber optic communication system is described -2- WO 00/70804 PCTIUSOO/07685 in U.S. Patent No. 5,579,143 as a video distribution network with 128 different channels. The 128 different channels are defined using 128 different lasers operating on 128 closely spaced but distinct wavelengths. These lasers have precisely selected wavelengths and also have the well-defined mode structure and gain characteristics demanded for 5 communications systems. Lasers appropriate to the WDM video distribution system are individually expensive so that the requirements for 128 lasers having the desired operational characteristics make the overall system extremely expensive. The expense of the system makes it undesirable for use in applications such as local area computer networks and otherwise limits the application of the technology. As is described below, o embodiments of the present invention can provide a video distribution network like that described in U.S. Patent No. 5,579,143, and embodiments of the invention can provide other types of medium and wide area network applications, making such systems both more flexible and more economical. Unlike the WDM many laser system of U.S. Patent No. 5,579,143, embodiments of the present invention may be sufficiently flexible and cost 5 effective to be used in at least some types of local area networks. Embodiments of the present invention, as described below, use spread spectrum communication techniques to obtain improved loading of the bandwidth of an optical fiber communication system in a more cost-effective manner than known WDM systems. Spread spectrum communication techniques are known to have significant advantages and 0 considerable practical utility, most notably in secure military applications and mobile telephony. There have consequently been suggestions that spread spectrum techniques, most notably code-division multiple access (CDMA), could be applied to optical communications technologies. Spread spectrum techniques are desirable in optical communications systems because the bandwidth of optical communications systems, such 5 as those based on optical fibers, is sufficiently large that multi-dimensional coding techniques can be used without affecting the data rate of any electrically generated signal that can presently be input to the optical communications system. Different channels of data can be defined in the frequency domain and independent data streams can be supplied over the different channels without limiting the data rate within any one of the channels. 0 From a simplistic point of view, the WDM system described above might be considered a limiting case of a spread spectrum system in that plural data channels are defined for different wavelengths. The different wavelength channels are defined in the optical frequency domain and time domain signals can be transmitted over each of the wavelength channels. From a CDMA perspective, the distinct wavelength channels of the WDM 5 communication system described above provide a trivial, single position code, where individual code vectors are orthogonal because there is no overlap between code vectors. -3- WO 00/70804 PCT/USOO/07685 There have been suggestions for optical CDMA systems that are generally similar to traditional forms of radio frequency CDMA, for example in Kavehrad, et al., "Optical Code Division-Multiplexed Systems Based on Spectral Encoding of Noncoherent Sources," J Lightwave Tech., Vol. 13, No. 3, pp. 534-545 (1995). As opposed to the WDM system 5 described above, the suggested optical CDMA system uses a broad-spectrum source and combines frequency (equivalently, wavelength) coding in addition to time-domain coding. A schematic illustration of the theoretical optical CDMA suggested in the Kavehrad article is presented in FIG. 1. The suggested optical CDMA system uses a broad spectrum, incoherent source 12 such as an edge-emitting LED, super luminescent diode or an erbium 0 doped fiber amplifier. In the illustrated CDMA system, the broadband source is modulated with a time-domain data stream 10 and the time domain modulated broad-spectrum light 14 is directed into a spatial light modulator 16 by a mirror 18 or other beam steering optics. Within the spatial light modulator 16, light beam 20 is incident on a grating 22, which spatially spreads the spectrum of the light to produce a beam of light 24 having its 5 various component wavelengths spread over a region of space. The spatially spread spectrum beam 24 is then incident on a spherical lens 26 which shapes and directs the beam onto a spatially patterned mask 28, which filters the incident light. Light spatially filtered by the mask 28 passes through a second spherical lens 30 onto a second diffractive grating 34, which recombines the light. Mask 28 is positioned midway between the pair of 0 confocal lenses 26, 30 and the diffraction gratings 22, 34 are positioned at the respective focal planes of the confocal lens pair 26, 30. The broad optical spectrum of the incoherent source is spatially expanded at the spatially patterned mask 28 and the mask spatially modulates the spread spectrum light. Because the spectrum of the light is spatially expanded, the spatial modulation effects a modulation in the wavelength of the light or, 5 equivalently, in the frequency of the light. The modulated light thus has a frequency pattern characteristic of the particular mask used to modulate the light. This frequency pattern can then be used to identify a particular user within an optical network or to identify a particular channel within a multi-channel transmission system. After passing through the mask 28, the spatially modulated light passes through the o lens 30 and the wavelength modulated light beam 32 is then spatially condensed by the second grating 34. The wavelength modulated and spatially condensed light beam 36 passes out of the spatial light modulator 16 and is directed by mirror 38 or other beam steering optics into a fiber network or transmission system 42. The portion of the CDMA system described to this point is the transmitter portion of the system and that portion of 5 the illustrated CDMA system down the optical path from the fiber network 42 constitutes the receiver for the illustrated system. The receiver is adapted to identify a particular -4- WO 00/70804 PCT/US0O/07685 transmitter within a network including many users. This is accomplished by providing a characteristic spatial mask 28 within the transmitter and detecting in the receiver the spatial encoding characteristics of the transmission mask from among the many transmitted signals within the optical network. As set forth in the Kavehrad article, it is 5 important for the mask 28 to be variable so that the transmitter can select from a variety of different possible receivers on the network. In other words, a particular user with the illustrated transmitter selects a particular receiver or user to receive the transmitted data stream by altering the spatial pattern of the mask 28, and hence the frequency coding of the transmitted beam 40, so that the transmitter mask 28 corresponds to a spatial coding 0 characteristic of the intended receiver. The receiver illustrated in FIG. 1 detects data transmitted from a particular transmitter by detecting the spatial (frequency or wavelength) modulation characteristic of the transmitter mask 28 and rejecting signals having different characteristic spatial modulation patterns. Light received from the optical fiber network 42 is coupled into two 5 different receiving channels by coupler 44. The first receiver channel includes a spatial light demodulator 46 that has a mask identical to the one used in spatial light modulator 16 and the second receiver channel includes a spatial light demodulator 48 of similar construction to the transmitter's spatial light modulator 16, but having a mask the "opposite" of the transmitter mask 28. Each of the spatial light demodulators 46, 48 0 performs a filtering function on the received optical signals and each passes the filtered light out to an associated photodetector 50, 52. Photodetectors 50, 52 detect the filtered light signals and provide output signals to a differential amplifier 54. The output of the differential amplifier is provided to a low pass filter 56 and the originally transmitted data 58 are retrieved. 5 FIG. 2 provides an illustration of the receiver circuitry in greater detail. In this illustration, spatial light demodulators 46 and 48 are generally similar to the spatial light modulator 16 shown in FIG. 1 and so individual components of the systems are not separately described. Received light 60 is input to the receiver and is split using coupler 62, with a portion of the light directed into spatial light demodulator 46 and another 0 portion of the light directed into the other spatial light demodulator 48 using mirror 64. Spatial light demodulator 46 filters the received light 60 using the same spatial (frequency, wavelength) modulation function as is used in the transmitter's spatial light modulator 16 and provides the filtered light to photodetector 50. Spatial light demodulator 48 filters the received light using a complementary spatial filtering function and provides the output to 5 the detector 52. Amplifier 54 subtracts the output signals from the two photodetectors. To effect the same filtering function as the transmitter's spatial light modulator 16, the spatial -5- WO 00/70804 PCT/US0O/07685 light demodulator 46 includes a mask 66 identical to the transmitter mask 28. Spatial light demodulator 48 includes a mask 68 that performs a filtering function complementary to masks 28 and 66 so that spatial light demodulator 48 performs a filtering function complementary to the filtering function of spatial light modulators 16, 46. In the Kavehrad 5 article, each of these masks 16, 66, 68 is a liquid crystal element so that the masks are fully programmable. The particular codes embodied in the masks must be appropriate to the proposed optical application. Although CDMA has been widely used in radio frequency (RF) domain communication systems, its application in frequency (wavelength) domain encoding in 0 optical systems has been limited. This is because the success of the RF CDMA system depends crucially on the use of well-designed bipolar code sequences (i.e., sequences of +1 and -1 values) having good correlation properties. Such codes include M-sequences, Gold sequences, Kassami sequences and orthogonal Walsh codes. These bipolar codes can be used in the RF domain because the electromagnetic signals contain phase information that 5 can be detected. RF CDMA techniques are not readily applicable to optical systems in which an incoherent light source and direct detection (i.e., square-law detection of the intensity using photodetectors) are employed, because such optical systems cannot detect phase information. Code sequences defining negative symbol values cannot be used in such optical systems. As a result, only unipolar codes, i.e., code sequences of 0 and 1 values, can 0 be used for CDMA in a direct-detection optical system. The Kavehrad article suggests the adaption of various bipolar codes for the masks within the system illustrated in FIGS. 1 & 2, including masks provided with a unipolar (only O's and l's) M-sequence or a unipolar form of a Hadamard code. For these sorts of bipolar codes, the Kavehrad article indicates that the bipolar code of length N must be 5 converted into a unipolar code sequence of length 2N and that a system including such codes could support a total of N-1 users. The Kavehrad article primarily sets forth a theoretical discussion of a CDMA system, with little discussion of the practical implementation of such a system. A more practical example of an optical CDMA system including a converted bipolar o code sequence has been proposed for transmission and detection of bipolar code sequences in a unipolar system. This system is described in a series of papers by L. Nguyen, B. Aazhang and J.F. Young, including "Optical CDMA with Spectral Encoding and Bipolar Codes," Proc. 29th Annual Conf. Information Sciences and Systems (Johns Hopkins University, March 22-24, 1995), and "All-Optical CDMA with Bipolar Codes," Elec. Lett., 5 16th March 1995, Vol. 3, No. 6, pp. 469-470. This work is also summarized in U.S. Patent No. 5,760,941 to Young, et al., and this work is collectively referenced herein as the Young -6- WO 00/70804 PCT/US0O/07685 patent. In Young patent's system, schematically illustrated in FIG. 3, the transmitter 80 employs a broad spectrum light source 82, the output of which is split by a beam splitter 84 into two beams 86 and 88 that are processed by two spatial light modulators 90 and 92. The first spatial light modulator 90 comprises a dispersion grating 94 to spectrally disperse 5 the light beam 86 and a lens 96 to collimate and direct the dispersed light onto a first spatial encoding mask 98 which selectively passes or blocks the spectral components of the light beam. Lens 100 collects the spectral components of the spatially modulated light beam and recombination grating 102 recombines the spread beam into encoded beam 104. The "pass" and "block" state of the encoding masks represent a sequence of O's and l's, i.e., 0 a binary, unipolar code. The code 106 for the first mask 98 has a code U®U*, where U is a unipolar code of length N, U* is its complement and "0" denotes the concatenation of the two codes. The second encoder 92 (details not shown) is similar in structure to the first encoder 90 except that its encoding mask has a code U*@U. Symbol source 108 outputs a sequence of pulses representing O's and l's into a first ON/OFF modulator 110 and through 5 an inverter 112 into a second ON/OFF modulator 114. The two modulators 110 and 114 modulate the two spatially modulated beams of light and the two beams are combined using a beam splitter 116 to combine the two encoded light beams 118 and 120. The modulated light beams are alternately coupled to the output port depending on whether the bit from the source is 0 or 1. D This system can then use a receiver with differential detection of two complementary channels, as illustrated in the receiver of FIG. 2. The receiving channels are equipped with masks bearing the codes U*@U and UoU*, respectively, and sequences of O's and l's are detected according to which channel receives a signal correlated to that channel's mask. The system proposed in the Young patent allows the use of the bipolar 5 codes developed for RF CDMA technologies to be used in optical CDMA systems. However, for a mask of length 2N, only N codes can be defined since the code U and its complement U* must be concatenated and represented on the mask. Therefore, it is an object of the invention to provide a frequency-domain CDMA encoding/decoding scheme and an optical communication system incorporating such a D scheme where the number of users is maximized without raising interference unduly. It is another object of the invention to provide a system providing a relatively simple system for encoding and decoding the light but efficiently using the available spectrum. The optical CDMA systems described above and those optical CDMA systems described in the applications listed below and incorporated by reference in the related 5 application section are all built up from discrete optical elements such as gratings, lenses and detectors. The use of discrete optical elements is in many instances desirable, since -7- WO 00/70804 PCTUSOO/07685 discrete optical elements readily provide precision and flexibility. On the other hand, discrete optical systems tend to be large and expensive. For some aspects of the present invention to be most widely used, it is desirable to provide a more compact optical system formed from integrated optical elements. An optical CDMA communication system 5 implemented as optical or photonic integrated circuits should have the advantage of smaller size and should be more rugged than discrete optical systems. Desirably, an optical CDMA system implemented with photonic integrated circuits could be sufficiently inexpensive and rugged as to be used in local network and home applications. There have been attempts to implement in part code division techniques in 0 communications systems implemented at least partially in optical integrated circuits. For example, U.S. Patent No. 4,989,199 to Rzeszewski, entitled, "Photonic Switch Architecture Utilizing Code and Wavelength Multiplexing," describes a communications system that incorporates both wavelength division multiplexing (WDM) and a phase-type of code division. The Rzeszewski system uses a plurality of sets of coherent sources, each set of 5 coherent sources having the same wavelength, and performs phase modulation on the sources to impart a phase encoding to distinguish the sources within one set. Each set of coherent sources is input into a corresponding one of the phase encoders. Different phase encoders modulate sets of sources having different wavelengths, so that the outputs from the different phase encoders can be combined in a wavelength division-multiplexing 0 scheme. Because this system uses phase encoding to distinguish some of its channels, any optical system through which these signals are transmitted must preserve phase and coherence. Consequently, when signals encoded by the Rzeszewski patent's technique are transmitted over optical fibers, special fibers must be used and even then the system will be unable to transmit signals over fibers for as long of distances as are desirable for a fiber 5 communication system. It is consequently desirable to provide an integrated optical communication system that does not rely on the use of coherent signals. SUMMARY OF THE PREFERRED EMBODIMENTS These and other objects are obtained with an optical CDMA system in which a spatial encoder is implemented at least partially within a photonic integrated circuit. A 0 broad-spectrum light source is modulated with data to be transmitted. The broad-spectrum light beam is spatially dispersed, for example using a diffraction grating, and passed through a spatial spectrum-coding mask embodied within the photonic integrated circuit. The spatial coding mask preferably embodies a unipolar code belonging to a set of unipolar codes that are preferably derived from a set of balanced bipolar orthogonal codes. The 5 dispersed frequencies of the encoded modulated light beam are then recombined to provide -8- WO 00/70804 PCTIUS0O/07685 a modulated, encoded spread spectrum optical signal for injection into an optical fiber or another optical communication system. Alternately, aspects of the invention may provide an optical CDMA system in which a receiver is implemented at least partially within a photonic integrated circuit. A broad spectrum light source modulated with a spatial encoding function U is received by the receiver. The received light is split into two components and provided to a pair of complementary decoders. Within each of the complementary encoders, the received portion of the light beam is spatially dispersed, for example using a diffraction grating, and passed through a spatial decoding mask embodied within the photonic integrated circuit. One of the decoders includes a spatial decoding mask that embodies the spatial encoding function U of the original transmitting mask and the other, complementary decoder includes a complementary function U*. The spatial encoding functions U, U* preferably embody a unipolar code belonging to a set of unipolar codes that are preferably derived from a set of balanced bipolar orthogonal codes. Within each of the complementary decoders the spatially spread light signals are recombined after passing through the decoding masks. The signals passing through the complementary decoding masks are then provided to different inputs of a differential detector and the data originally modulated within the light is recovered. An appropriate differential detector might include back to back diodes, for example. Other aspects of the invention relates to an optical communication system including a data source providing a data stream and encoder. The encoder provides an optical output modulated with the data stream and embodying a first code, where the first code is selected from a set of unipolar codes in which each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code, and the codes in the set are defined as sequences of N digits each digit having one of at least two values, each of the N digits of the codes corresponding to one of N potential spectral ranges that could be output from the encoder. The optical output from the encoder comprises M components corresponding to M different spectral ranges within the N potential spectral ranges, wherein each of the M components is characterized by an M-th component optical level corresponding to the value of a corresponding code digit. Consequently, the optical output of the encoder is broad spectrum light modulated with a data stream and with a spectrally defined code function. The encoder comprises a photonic integrated circuit. Still other aspects of the invention provide an optical communication system having a decoder coupled to receive a light signal from an optical communication system and to recover transmitted data. The decoder includes an optical power separator for splitting a received light signal into approximately equal power first and second light components. -9- WO 00/70804 PCTIUSOO/07685 First and second spectral filters are coupled to receive the first and second light components, with the first spectral filter embodying a first code and the second spectral filter embodying a complement of the first code. The first and second spectral filters output first and second filtered components of the received light. An optical detector is receives the first and second filtered components of the received light and provides an electrical signal output. The first code is selected from a set of unipolar codes in which each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code, the codes in the set defined as sequences of N digits each digit having one of at least two values, each of the N digits of the codes corresponding to one of N potential spectral ranges over which the set of codes is defined. The received light signal is broad spectrum light modulated with a data stream and with a spectrally defined code function having M components corresponding to M different spectral ranges within the N potential spectral ranges, and the decoder comprises a photonic integrated circuit. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a conventional optical fiber mediated CDMA communication system. FIG. 2 provides a more detailed view of one receiver configuration that might be used in the system of FIG. 1. FIG. 3 schematically illustrates an overall view of an optical CDMA system using photonic integrated circuits. FIG. 4 illustrates one configuration of a PIC configuration of an encoder. FIG. 5 illustrates one configuration of a PIC configuration of an decoder. FIG. 6 illustrates an alternate configuration of the system of FIG. 3. FIG. 7 illustrates another configuration of a PIC configuration of an encoder or decoder. FIG. 8 illustrates another configuration of a PIC configuration of an encoder or decoder. FIG. 9 illustrates another configuration of a PIC configuration of an encoder or decoder. FIG. 10 illustrates a particularly compact configuration of an array waveguide grating. FIG. 11 schematically illustrates an apparatus for generating an array of N broad spectrum optical sources having sufficient intensity to generate light beams for N channels of communication over a fiber using methods in accordance with the present invention. 5 FIG. 12 schematically illustrates a polarization insensitive beam separator that is preferred in accordance with preferred embodiments of the present invention. -10- WO 00/70804 PCT/USOO/07685 FIG. 13 illustrates in greater detail the optical detection circuitry schematically illustrated in FIG. 3. FIG. 14 illustrates a modification to the source generation mechanism of FIG. 14. FIG. 15 illustrates a set of data streams (a)-(c) that might be used to modulate a 5 source in accordance with aspects of the present invention. FIG. 16 illustrates a circuit that may be used to generate a pulse stream such as that illustrated in FIG. 15(b) or 15(c) from a data stream such as that illustrated in FIG. 15(a). CROSS-REFERENCED PATENT AND APPLICATIONS D The following patent and applications relate to the present application and are each incorporated by reference in their entirety into this application: 1. "High Capacity Spread Spectrum Optical Communications System," U.S. Patent No. 5,867,290, issued February 2, 1999. 2. "Optical CDMA System," application Serial No. 09/126,310, filed July 30, 1998. 5 3. "Optical CDMA System Using Sub-Band Coding," application Serial No. 09/126,217, filed July 30, 1998. 4. "Method and Apparatus for Reduced interference in Optical CDMA," application Serial No. 09/127,343, filed July 30, 1998. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Particularly preferred embodiments of the present invention provide an optical CDMA communications system applying spread spectrum techniques to communication over optical fibers or over other communication links to achieve better bandwidth utilization. Components of the optical fiber communications system are preferably implemented in photonic integrated circuits. Implementation of, for example, the encoders 5 or decoders of an optical CDMA system can make the system less expensive, more rugged and easier to align than implementations of optical CDMA systems using entirely discrete components. Certain particularly preferred embodiments of the invention provide integrated encoders and decoders, with optical components implemented as photonic integrated circuits and electrical elements implemented as integrated circuits. These implementations advantageously incorporate at least some photonic integrated circuit components on the same semiconductor substrate as is used for implementing electrical integrated circuits. Optical CDMA systems in accordance with the present invention preferably transmit signals using a spatial encoder with binary or analog encoding of a single channel 5 and receive signals using a two-channel spatial decoder. The spatial decoder includes two channels, with one of the decoder channels embodying the same transmit code as the -11- WO 00/70804 PCTUSOO/07685 encoder and the other of the decoder channels embodying the complement of the transmit code. Preferably, the encoder spatially modulates a broad-spectrum light source with a code that identifies the channel through which the data are transmitted. Most typically the broad spectrum light source is at least partially incoherent. Spatial modulation of the light 5 source is accomplished by spatially dispersing the input light, for example using an array waveguide grating or a diffraction grating. The spatially spread spectrum is then provided to a spectrum-coding mask. In some embodiments this is accomplished by providing the spread spectrum to an array of optical switches that can be selected to represent the spatial modulation function. If the switches need not be selectable, the encoding mask can be more 0 simply implemented, such as either providing an optical link or no optical link between points on either side of each of the discrete wavelength or frequency bins defined within the mask. Most preferably, the spectrum of the light signal is spread so that the spectrum of the light signal is divided into a plurality of bins of equal bandwidth or alternately into a 5 plurality of bins of equal intensity. The spatial coding mask used to modulate the light signal provides a corresponding number of bins that independently attenuate the signals presented to the bins. Each bin might, for example, be switched to pass or block the light signal input to the bin and so might perform a form of binary encoding. Because each successive bin in the mask is presented light within a sequential range of wavelengths, the D spatial modulation of the mask imparts on the light a frequency distribution which may be used as a code in the CDMA communication system. The described spatial coding mask preferably embodies a unipolar code belonging to a set of unipolar codes that are preferably derived from a set of balanced bipolar orthogonal codes. After the spatial modulation, the dispersed frequencies of the encoded light beam are recombined to provide a modulated, 5 encoded spread spectrum optical signal for injection into an optical fiber or another optical communication system. In addition to the spatial encoding modulation, the input light is preferably also modulated in the time domain with a data stream. In the preferred embodiments of the invention which incorporate a photonic integrated circuit ("PIC") encoder, the modulation of D the input light signal may be accomplished within the PIC. For example, if the encoder is formed from an appropriate composition of III-V semiconductors (e.g., an appropriate composition of InGaAsP, AlGaAs or other similar system), the modulation may be accomplished by a component of the photonic integrated circuit. Similarly, the PIC encoder may accomplish the modulation if the encoder is implemented in an electro-optic material 5 such as LiNbO3. For either of these embodiments, the switches used to define the spatial mask function can be constructed to effect the time domain modulation of the light signal. -12- WO 00/70804 PCT/USOO/07685 Recovery of the transmitted signal is through the use of a decoder that corresponds to the spectral modulation function embodied within the encoding mask or code. At any receiver, a beam separator, which is in some particularly preferred embodiments a polarization insensitive splitter, divides the beam into two equal intensity parts. Each of 5 the parts of the light signal is spatially spread using an array waveguide grating or a diffraction grating to spatially separate the spectrum of that part of the light signal. The spatially spread received signal is presented to one of two different masks, one of which is identical to the encoding mask for the channel to be recovered and the other of which is complementary to the encoding mask for the channel to be recovered. The light passing 0 through the two decoding masks is recombined, again using an array waveguide grating or a diffraction grating, and the two light signals are converted to an electrical signal by differential detection. For example, the two decoded light signals might be provided to different ones of two back-to-back diodes. The resulting electrical signal is preferably low pass filtered and then, in particularly advantageous embodiments, the electrical signal is 5 provided to a limiting element that removes the negative components of the electrical signal. In particularly preferred embodiments, the masks in the encoder and decoder include unipolar binary codes comprising O's and l's such as Walsh codes. The advantages, utility and derivation of those codes are discussed in the applications referenced and 0 incorporated by reference in the cross-referenced application section, above. Those sections of those applications are not repeated here in the interests of brevity, but those sections form a part of this disclosure. Other encoding schemes, including the use of discrete analog basis functions, are also described in those related applications. Within the illustrative embodiments described here, there is particular emphasis on the use of binary masks, that 5 is, masks for which bins are either on or off, or transmitting and not transmitting. In such embodiments, it is possible to provide an L position mask to define L-1 communication channels for a total of L-1 users. Those of ordinary skill will appreciate that the photonic integrated circuits described below might be used to generate analog transmission functions for each of the bins of the mask and so are well adapted to implement sets of 0 orthogonal basis functions. Systems embodying aspects of the present invention will now be described with particular emphasis on implementation of the optical CDMA system within one or more photonic integrated circuits. FIG. 3 shows schematically an embodiment of a CDMA communications system including both a PIC encoder and a PIC decoder. A broadband light source 90, such as a 5 super luminescent diode (SLD) or erbium-doped fiber source (EDFS), is coupled to a CDMA PIC encoder 92. Encoder 92 may provide functionality somewhat similar to the spatial -13- WO 00/70804 PCTIUSOO/07685 light modulator 16 shown in FIG. 1 that spatially encodes the modulated broad-spectrum light beam. More preferably, however, the encoder 92 includes both a spatial modulation function and a time domain data modulation function by which the encoder 92 time domain modulates the input light based upon data or other information from the data source 94 5 using, for example, keying or pulse code modulation. As is illustrated schematically in FIG. 4, the photonic integrated circuit encoder 92 includes a substrate 120 having an array waveguide grating or a diffraction grating 122 that spatially spreads the spectrum of the modulated light beam along an axis. Array waveguide gratings are known in the art and are described, for example, in U.S. Patent No. 5,002,350 to Dragone, which patent is o incorporated by reference in its entirety. Suitable embodiments of the waveguides may be formed in III-V or silicon semiconductor systems, with the light propagating through waveguides defined compositionally or with the light propagating through semiconductor channels formed above the semiconductor substrate by etching into the semiconductor substrate. The array waveguide grating might also be implemented in an electro-optic 5 material such as LiNbO 3 , defining waveguides either by doping, for example with titanium, or by etching channels. An alternate PIC embodiment of the spatial spreading element is a curved grating implemented within a semiconductor, electro-optic photonic circuit or another suitable waveguide material. An example of such a system, provided within a PIC of AlGaAs, is shown in U.S. Patent No. 5,355,237 to Lang, et al., which patent is o incorporated herein by reference in its entirety. The embodiment of FIG. 4 uses a curved grating to achieve significant levels of spatial separation of the components of the light in a smaller space than can be achieved using a conventional planar grating. Still another possible implementation uses a silica on silicon assembly, which is discussed below. Advantageously, preferred PIC implementations of the spatial spreading device incorporate 5 both the beam shaping elements (i.e., collimating and focusing) and the dispersive elements (gratings or array waveguides) within the PIC. The number of different bins into which the spatial spreading element disperses the beam will vary for different applications. For the present discussion a total of 128 bins will be assumed, although systems with only sixteen bins are both immediately available and immediately useful in many present applications. 0 The spatially spread light beam is then passed through the encoding mask and modulator array 124. The encoding mask and modulator 124 includes a number of independently switchable optical elements that define the attenuation provided by the different bins of the mask/modulator, with the total number of bins equal to the number of bins (e.g., 128) defined by the spatial spreading element 122. Both a mask definition 5 function and a time-domain modulation function are input to the array of switches that make up the mask/modulator 1.22. The signal 94 input to the mask/modulator selectively -14- WO 00/70804 PCT/USO0/07685 turns on or off the various switch elements of the mask/modulator and applies the time domain modulation function to at least those switch elements that are turned on or are transmitting. Typically the same time domain modulation function is applied to all of the switch elements regardless of their switching state. The mask/modulator provides a 5 spatially encoded, modulated beam of light that is collected and recombined back to a broad spectrum beam by a second array waveguide grating, a curved diffraction grating or another similar element 126. The spectrally recombined, spatially and time-domain modulated signal is then output for injection into a fiber 96, which may be a single mode optical fiber. An optical coupler 98 such as a star coupler, a Y coupler or the like might be 0 used to couple the encoded beam from the fiber onto a larger network for long distance transmission. Alternately, the network of FIG. 3 may simply have a star configuration, with users other than the two illustrated not shown. Referring still to FIG. 3, the modulated light signal is transmitted over an optical fiber or other optical communication link 100 and is then provided to a decoder, which has 5 two channels 104, 106 coupled to two decoding PICs 108 and 110. Light signals containing a potential plurality of spread spectrum signals are diverted from the fiber 100 using an optical coupler (not shown), and split into two portions with a separator 102. The separator is most preferably a polarization insensitive element like that illustrated in FIG. 12 and discussed below with reference to that figure. One portion of the received light is provided 0 over a fiber or other link 104 to a CDMA PIC decoder 108; the other portion of the received light is provided over another fiber or other link 106 to a second CDMA PIC decoder 110. Each of the CDMA PIC decoders 108, 110 may have a fixed mask or may have a selectable mask embodied in an array of switches. Such preferred embodiments of the invention are better suited to a network environment in which any one user may want to transmit and 5 receive data with any other user attached to the network. For such switchable mask decoders, it is preferred that each of the masks in the decoders 108, 110 receives a respective mask signal 112, 114 that selects the mask distribution. Generally, the preferred masks will embody encoding masks that are complements of each other. Consequently the mask signals 112, 114 may be bit-wise complements of each other. 0 FIG. 5 shows an embodiment of the PIC decoders 108, 110 provided on a semiconductor, silica/silicon, or electro-optic substrate 130 that might be used in the system of FIG. 3. Referring to FIG. 5, incoming light is spread spatially along an axis by an array waveguide grating or a curved diffraction grating 132 and is then passed through a detection or decoding mask 134. For one of the decoders 108 it is preferred that the 5 decoding mask 134 embody the same coding function U used to encode the desired channel of light for transmission and it is preferred that the other of the decoders 110 embodies the -15- WO 00/70804 PCT/US00/07685 complement U* of the transmission coding function. Light passed through the decoding masks 134 is then provided to a second array waveguide grating or another diffraction grating 136 that recombines the spatially spread light into a broad spectrum beam and this beam is the output of the encoder. Most preferably, the encoder and decoders of FIG. 3 5 include masks that are either fixed at or switchably configured to embody a converted binary Hadamard code. The outputs from the decoders 108, 110 are then provided to a differential detector 116, which may be a pair of back-to-back two detector diodes that naturally perform a subtracting operation. Other detection configurations might be used and certain ones of 0 those are discussed below. Two gain control circuits may also separately process the two signals output from the decoders, such as by using multichannel detection within the decoder itself. The differential electrical signal is then detected for data recovery. Data recovery for digital data streams may include, for example, integrating and square-law detecting the difference signal in the processor 118. 5 Embodiments of the present invention can be implemented in a variety of different communications environments, including within backbone communications links, wide area networks, video distribution networks, and others. Certain of these applications are described and illustrated in greater detail in the applications identified and incorporated by reference in the related applications section, above. Aspects of the present invention may 0 also find application in hybrid systems in which a discrete optical encoding system is used in a centralized transmission or retransmission facility and photonic integrated circuit (PIC) CDMA receivers are provided in a remote receiving location. For example, such a system may find particular application in a video distribution system. In such an application, a discrete optical CDMA system may be provided at a central distribution 5 point, such as a head-end of a cable television distribution system, and a PIC-CDMA receiver capable of selecting different codes may be provided at a local receive station to selectively tune the receiver to receive different channels of data. Ideally, the local receive station would be within a decoder box for use in the home. Alternately, aspects of the invention may be implemented in a local area or other computer network using only PIC 0 CDMA components. In a CDMA system, the basic requirement for a spectral encoding/decoding scheme is that the decoding apparatus at a receiving user be able to recover data signals from the corresponding transmitting user while reducing or eliminating interference from signals from all other users. For some systems, the receiving masks will be fixed so that a 5 particular receiver always receives the same channel of data. In this way, a centralized and switchable broadcasting station can direct a data flow to a selected receiver. For other -16- WO 00/70804 PCT/USOO/07685 systems, the receiving masks will be programmable so that different signal sources can be selected from many sources that are transmitted over a common transmission network or system. In a spread spectrum CDMA system using an incoherent light source, because an incoherent optical system can only transmit positive signals (light intensities), and no phase information is available, only unipolar codes may be used for encoding. A unipolar binary code may be represented by a sequence of binary digits, such as ui=110011110101011, where subscript i designates the i1h user pair (or channel). The number of digits in the sequence, N, is referred to as the length of the code. In practice, for the particularly preferred binary unipolar code mask, each of the code values corresponds to a fixed interval slot, either transparent or opaque, on the spatially patterned mask that in turn corresponds to a fixed frequency or wavelength interval in the spatially modulated broad spectrum beam of light. When a single mask is used for encoding and decoding, the codes are preferably chosen such that they are orthogonal, or: ut e u { M ifi=j - 0 ififw j where "-" denotes the bit-wise dot product of two codes, and M is a constant. When orthogonal codes are used, each transmitting user may transmit signals using a single encoding mask, and the corresponding receiving user may use a single decoding mask identical to the encoding mask to recover the signal from the corresponding transmitting user while rejecting interfering signals from all other users. This desirable outcome, however, occurs only when the codes are chosen as the binary basis vectors: u1 = 000......001 U2 = 000......010 UN 100-.--000 5 This set of codes is undesirable in that, since only one digit of the entire code is 1, only one frequency bin of the mask passes power through it while the great majority of the bins are blocked. Such a system can be viewed as an incoherent wave division multiple access (WDMA) system. Such codes are undesirable as only about 1/N of the source power is D transmitted and the rest of it is wasted. -17- WO 00/70804 PCTIUSOO/07685 In the encoding and decoding system shown in FIG. 3, in which a single mask is used for encoding and two masks are used for decoding, a set of unipolar codes may be used even though a code ui in the set is not orthogonal to the other codes uj in the set according to the definition of orthogonality set forth above. Rather, the code ui is selected to be orthogonal to the difference between any other code uj and its complement uj*, i.e. ui * (u -u*) = { .ij (2) 0 ift ej where M' is a constant. Derivation of appropriate code sets, such as Hadamard codes converted to a unipolar code set, are described in the applications listed above and incorporated by reference in the related application section, above. In addition to the various combinations of multiplexing schemes that are possible, various network algorithms may also be implemented. For example, the present invention may be applied to various fiber communication system architectures, such as a network environment in which a plurality of users si, S2, . . . sN are connected to an optical fiber medium 130 and each user sj may communicate with any other user si over the optical fiber. Each user or node sj is assigned a code u for receiving data from other users, and different users are preferably assigned different codes. When a user si transmits data to a user sj, the transmitting user si encodes the optical signal using the code assigned to the receiving user sj, and the receiving user decodes the signal using its assigned code. This may require that the transmitting user be able to dynamically vary the code it uses to transmit data depending upon the code of the intended recipient user. The codes for any one node may be assignable from one or more master nodes distributed throughout the network. Hence, when a node in a network comes on line, it requests a code or codes for encoding to select one of the possible spread spectrum channels over which to communicate. When that node leaves the network, the code that had been used by that particular node may be reassigned to a different node in the network. Various schemes may be used for making such requests such as CSMA/CD technique or token passing on a permanently assigned channel. Alternatively, token passing techniques may be used for gaining codes to secure one of the code division channels. While only CDMA techniques have been described above, those of ordinary skill in the field will readily understand that depending upon system parameters, the system may 5 be used in combination with wavelength (frequency) division multiplexing and time division multiplexing. For example, different coding schemes may be used for different portions of -18- WO 00/70804 PCT/USOO/07685 the optical spectrum so that wavelength division multiplexing may be used. In addition, the codes may be shared on a time sharing basis to provide for time division multiplexing. Also, optical spatial (frequency domain) CDMA can be combined with time-domain optical CDMA to increase the number of codes and the users in the network. In the time domain 5 spread spectrum embodiments, several users are provided with different time domain spread spectrum codes for encoding the data before the data is provided to the optical encoder. However, these users can share the same wavelength encoding schemes discussed above. Of course, at the decoder, once the received optical information is converted back into the electrical digital domain, the digital signal must be processed according to the time o domain spread spectrum code to recover the desired transmitted information. A variation of the embodiment of FIG. 3 is illustrated in FIG. 6. This illustration does not provide an external broad band light source. Rather, the PIC CDMA encoder includes an array of N lasers emitting light in N different wavelength ranges. The N wavelength outputs of the N lasers are combined using an array waveguide grating or 5 curved diffraction grating to provide the output from the encoder 140. The mask and modulation functions can be accomplished entirely by turning on the lasers in the spectral pattern assigned to that channel, corresponding to a unipolar code. All of the lasers are modulated with the same time-domain modulation function, which distinguishes this CDMA embodiment from a wavelength division modulation system. By selection of 0 appropriate spatial spreading elements in the decoders, the remaining components illustrated in FIG. 6 are the same as the embodiment of FIG. 3. FIG. 7 shows a variation on the encoder of FIG. 4, which might also be used as the encoder of FIG. 5. Light input to the FIG. 7 CDMA PIC is provided through a coupler or circulator 150 and to an array waveguide grating or diffraction grating 152. The grating 5 152 spreads the spectrum of the input light in the manner described above and provides the spread spectrum light to the mask/modulator 154, which functions in the manner described above to modulate the input signal with a code defined in the optical frequency domain. Preferably the code is a binary one defined by selecting between two attenuation levels, i.e., a high and a low attenuation level within each bin of the mask. The signals output from o the mask/modulator are provided over or waveguides individually coupled to the bins of the masks, as before. Instead of recombining the spread spectrum signal as in the previous embodiments, however, the signals are reflected back through the mask/modulator 154 by high reflectivity coatings on the ends of the fiber optics or waveguides. The light modulated by the mask/modulator 154 is then recombined by the array waveguide grating or 5 diffraction grating 152 used in dispersing the broad band light initially. This embodiment has the advantage of not requiring a second array waveguide grating or diffraction grating -19- WO 00/70804 PCTIUS0O/07685 within the PIC. Output of the spatially modulated signal is then provided through the coupler or circulator 150. A further variation of the FIG. 7 embodiment, which also might be substituted for the encoder and decoders of FIGS. 4 and 5, is shown in FIG. 8. The FIG. 8 encoder/decoder 5 provides a fixed mask embodiment of the decoder and encoder that might be used in the system of FIG. 3. No array of switches is provided. Rather, the outputs of the array waveguide grating or diffraction grating 152 are selectively applied with high reflection or anti-reflection coatings in a pattern dictated by the code assigned to that communication channel. By selectively applying high reflection and anti-reflection coatings, the bins 0 corresponding to one binary state, as dictated by the code selected for that channel, reflect the light output from the grating 152. The bins corresponding to the other binary state do not reflect light. The light reflected back through the array waveguide grating is recombined to provide a broad band light signal spatially modulated in a manner appropriate for use within the communications system of FIG. 3. Other aspects of the FIG. 5 8 embodiment are similar to those of the FIG. 7 embodiment. The FIG. 8 embodiment is advantageous for certain embodiments of the invention because the FIG. 8 embodiment provides a lower cost, fixed encoder and decoder that allows a communications system to be implemented in a range of applications where cost and simplicity are of concern. Still another embodiment of an encoder/decoder is shown in FIG. 9. The FIG. 9 0 embodiment is a silica on silicon embodiment of an encoder/decoder in accordance with the present invention, generally similar to that illustrated in FIGS. 4 and 5. The FIG. 9 CDMA PIC is formed with array waveguide gratings 160, 164 provided by silica on silicon construction methods. The mask 162 provided between the two array waveguide gratings in this embodiment also has a silica on silicon construction. In particular, the mask 5 consists of an array of thermal switches constructed in the known manner, to alter the attenuation through a portion of a silica waveguide. For this embodiment, it is preferred that the broad spectrum light source be modulated external to the illustrated CDMA PIC circuit, since the illustrated thermal switch array is typically too slow to modulate the optical signal at a desired speed. Because of this, the mask control signal provided by the 0 mask controller 166 generally only controls the on/off states of the mask elements and does not include a time domain modulation function. A partial integration of an array waveguide grating that may be used in accordance with the present invention is illustrated in FIG. 10. The illustrated array waveguide grating may be implemented in any of the semiconductor, electro-optic or passive (i.e., silica 5 on silicon) materials discussed here. The illustrated waveguide has the particular advantage of compactness in efficiently using the diffraction path of the waveguide. This -20- WO 00/70804 PCT/USOO/07685 compact arrangement may be integrated into PICs having two gratings and an array of switches to make up a mask. Alternately, the outputs of the waveguides might be selectively blocked to effect a particularly compact fixed mask encoder/decoder. As will be described in greater detail below, it is desirable in many implementations 5 of aspects of the present invention to provide many very similar optical sources. For example, it is desirable in some embodiments to provide at least 128 light sources that are essentially identical in their extent and intensity distribution. Presently envisioned systems generally require many sources. A particularly desirable and particularly economical implementation of multiple sources having desirable spectral similarities is to 0 provide a single originating light source that is coupled to a fiber, where the output of the source is split, for example into four components by a star splitter. Each of the split off components is then amplified to an appropriate level and then each of the split off and amplified components is provided to a separate startsplitter. A hierarchical structure of an original source that is split and amplified, with each successive source channel being split 5 and amplified, can be used to develop a great many sources having essentially identical spectral characteristics. A difficulty observed by the present inventors when implementing this source strategy is an undesired level of temporal correlation between the different sources. This level of correlation can give rise to undesirable levels of correlation between the different 0 communication channels associated with the different sources. Consequently, preferred embodiments decorrelate the different sources. This might be accomplished by inserting different optical delays along each of the output paths of the different source channels. Such optical delays could consist of optical delay lines. Causing each of the sources to pass through different lengths of fiber delay lines might provide appropriate delays. Delays 5 might alternately be generated using free space propagation through different optical paths. Fiber delays are preferred since they can be implemented using only minimal space in a manner immune to vibrations, so that the overall optical system can be provided in a sufficiently small space as to allow a wider range of implementations for systems embodying this aspect of the present invention. 0 FIG. 11 illustrates a preferred apparatus for generating a plurality of broad spectrum sources in a cost effective manner using a single erbium-doped fiber source and a hierarchy of erbium-doped fiber amplifiers to provide enough channels of sources, each with sufficient intensity for driving a channel of the optical communications system. As shown, a single erbium-doped fiber source 300 outputs light with an acceptably broad spectrum, 5 generally providing a bandwidth of about 28 nanometers in wavelength over which the intensity of the source varies by less than about 5 dB. The 28-nanometer bandwidth -21- WO 00/70804 PCT/USOO/07685 corresponds to a system bandwidth of about 3.5 THz. The output of the erbium-doped fiber source, also known as a super luminescent fiber source, is provided over a fiber to a splitter such as a star coupler 302 which splits the input source signal and provides the output over four fibers to an array of four fiber amplifiers 304. 5 As the output of the fiber source 300 is split into four different sources, the intensity drops in the expected manner. Each of the four split off sources is thus amplified by the four fiber amplifiers to provide four broad-spectrum light beams preferably each having an intensity approximately equal to the original source 300 intensity. For the illustrated 128 channel system, this process is repeated through several further hierarchical stages. Thus, 0 the outputs from the four fiber amplifiers 304 are provided over fibers to a corresponding set of four splitters 306, which may also be star couplers. The splitters 306 split the output from the fiber amplifiers into a plurality of outputs also of reduced intensity. The split off output from the splitters 306 are then provided to a further array of fiber amplifiers 308, which preferably amplify the intensity of the plural channels of broad-spectrum light to 5 provide a next set of source light beams 310 having an appropriate intensity. This process is repeated until a sufficient number of broad-spectrum sources having an appropriate intensity are generated, for example 128 independent sources for the illustrative 128 channel fiber communication system. This hierarchical arrangement is preferred as using a single originating source and a number of fiber amplifiers to obtain the desired set of 0 broad-spectrum light sources, which advantageously takes advantage of the lower price of fiber amplifiers as compared to the fiber source. After sufficient channels of source light have been generated, the channels of source light are provided to an array of spatial light modulators or encoders each of which has the configuration shown in FIGS. 3 and 4. The 128 different encoders use a 128-bin mask to 5 spatially encode the input light signal, with each of the 128 masks presenting a different one of a unipolar Hadamard code vector generated in the manner discussed above. Most preferably, each of the masks is a fixed mask, with the mask having a total of 128 equal sized bins, with the bins spanning the usable width of the linear mask. Thus, the 128 bins span a total of about 3.5 THz (28 nanometers) in bandwidth, with each adjacent bin 0 defining a subsequent frequency interval providing about 25 GHz of bandwidth. Each of the equal sized bins of the fixed mask, which might be the encoder/decoder of FIG. 8, is assigned according to the code vector to have one or the other of two binary values. Each of the 128 channels of the communication system is then defined by a distinct spatial encoding function and each of the channels is also modulated with a time-domain signal, for example 5 using a modulator. After the various channels are modulated both spatially (equivalently, infrequency) and temporally, the 128 channels are combined and injected into a fiber. -22- WO 00/70804 PCT/USOO/07685 Long distance transmission for this fiber communication system is managed in a manner similar to the manner other conventional fiber communication systems are managed. As is conventional, it is typical to use a single mode fiber. In addition, the signals on the fiber will undergo dispersion and losses. It is preferable that the signals on the fiber be amplified using a conventional fiber doped amplifier at regular intervals, for example, every forty to eighty kilometers. At the other end of the transmission fiber, the combined light signals are split, amplified, and provided to an array of 128 receivers, each corresponding to one of the fixed mask channels defined by the 128 transmitters coupled into the fiber. The primary purpose of the illustrated embodiment is to expand the usage or loading on the fiber, so the receivers also include fixed masks so that each receiver is dedicated to a single one of the 128 channels. The receivers, which may have the structure shown in FIGS. 3 and 8, are each dedicated to a particular channel defined by a particular transmitter by including within the receiver one mask identical to the transmitter mask and a second mask that is the bit-wise complement of the transmitter mask. For the illustrated embodiment, the use of fixed masks on both the transmitting and receiving ends of the communication system provides a reduced cost system that provides significantly improved bandwidth for a high volume fiber link. As discussed above, recovery of optical signals from the fiber communication link is ) accomplished using a receiver that separates the light beam received from the optical system into two components that should have substantially similar power levels. A particularly preferred aspect of the present invention is illustrated in FIG. 12, which shows a beam separator that is preferably used at the input to the decoder. A separator in accordance with the present invention is capable of separating the received light into two portions of sufficiently equal power levels to allow the preferred differential detection scheme of the optical CDMA receivers to effectively detect a desired user channel. An embodiment of a polarization insensitive beam separator might consist of a first polarization sensitive element that divides the received light beam into first and second channels of light with each channel having a different one of two orthogonal polarizations. For example, one channel of light might include the vertically polarized component of the received light and the other channel might include the horizontally polarized component of the received light beam. The polarization of one of the channels is then converted to the polarization of the other light beam. For linearly polarized light this might consist of rotating the polarization of the light. The two channels of light are then recombined and provided to a beam splitter. This beam splitter is typically a polarization sensitive element -23- WO 00/70804 PCT/USOO/07685 that accurately splits the combined beams into two beams of substantially equal power because the polarization of the combined beams is well defined and predictable. Referring to FIG. 12, a specific embodiment is described in which light is received from a single mode fiber 350. Since the fiber 350 is generally not polarization preserving 5 and the light within the fiber 350 is likely linearly polarized in an arbitrary direction, it is convenient to use a conventional linear polarizer as a beam splitter 352 or polarization analyzer. The polarization sensitive element 352 preferably separates the input light beam into two orthogonal polarization components and provides those two components to two different optical paths 354, 356. Generally different power levels will be present along each 0 path. The illustrated optical paths may propagate through free space or may proceed through polarization preserving fibers. In either case, the polarization of the light within each arm will be of a uniform polarization until the polarization is altered. One component of the light is provided along optical path 354 and maintains a vertical linear polarization 358 throughout the optical path 354. Along the other optical 5 path 356, the polarization is initially horizontal 360 and then the polarization is rotated by 90* by a rotation element 362 so that the polarization of the second optical path's light becomes linear vertical as indicated at 364 in FIG. 12. When the second optical path 356 propagates through free space, the rotation element may be a 1/2 waveplate or an appropriate Faraday rotator. When the second optical path 356 propagates through a 0 polarization-preserving fiber, the rotation element 362 is most preferably effected by a mechanical rotation of the fiber by 900. Most generally the rotation of the fiber will proceed continuously over a length of the fiber. Of course, it is possible to perform the rotation through other means, such as by inserting a rotation element at an end of the fiber of the second optical path. 5 Once the two beams on the two optical paths have had their polarizations properly oriented, the two beams are recombined and then split into a pair of substantially equal power beams to propagate along two additional beam paths. After the beams from paths 354 and 356 are combined, it is possible to use a typical polarization sensitive beam splitter 366 to divide the beams into two substantially equal power beams. The two desired output 0 beams are provided along optical paths 368 and 370 preferably through single mode optical fiber with linear polarizations in the illustrated embodiment. The split, received beams are then provided to the decoders 108, 110 of FIG. 3. Those of ordinary skill will appreciate that the polarization insensitive beam splitter as described above with respect to fibers might be accomplished in other waveguides, for example in combination with rotational 5 elements. -24- WO 00/70804 PCT/USOO/07685 In the illustrated optical CDMA system, it is very desirable to reduce the interference between different channels of users or of different multiplexed signals so that a greater number of channels can be provided over a single fiber. Various mechanisms have been identified to perform this task and are described in the present application and in the 5 other applications incorporated by reference herein. A fundamental way in which the present system reduces interference is by injecting light into the optical communication system only to indicate one binary state. The source is time-domain modulated so that the source produces an output intensity to indicate one logical binary state, for example, a logical 1. No light is provided to indicate a logical 0. This has the effect of reducing the 0 overall interference in the system. Of course, the particularly preferred coding scheme, including the receiving system including different channels with complementary filtering functions, provides a very significant and basic mechanism for reducing interference. It is possible to also achieve reductions in interference within the signal detection circuitry of the decoders or receivers. Signals within the two channels of a receiver are 5 preferably detected in a differential fashion, for example by coupling the light from each channel to different ones of a pair of photodiodes in a back-to-back configuration. The electrical output from the photodiodes will then be a difference measurement of the signal received in the two channels. In particularly preferred embodiments of the present invention, the electrical output signal is low pass filtered and then provided to an electrical ) square law circuit element such as a diode. This square law element or limiter preferably removes the negative going portions of the received electrical signal and might also be used to amplify the positive going portions of the received electrical signal. The negative going portions of the electrical signal are immediately identifiable as noise and so can be removed to improve the signal to noise ratio of the overall system. 5 The preferred electrical system, illustrated schematically in FIG. 13, also provides a mechanism for reducing interference. The subsystem illustrated in FIG. 13 provides further detail on the back-to-back diode arrangement indicated at 116 in FIG. 3. The two complementarily filtered optical signals are provided to the back-to-back diodes, which effect both a square law optical detection but also a differential amplification function. Other combinations of optical detectors, difference detection and electrical amplification are known and might well be substituted for these functions. In particularly preferred embodiments of the present invention, the electrical output signal from the diode pair and is then low pass filtered by filter 380. For those decoders implemented in semiconductor devices, the detection circuitry might also be provided within the same chip or with a single 5 PIC circuit. The low pass filtering is performed to remove high frequency noise signals. In the illustrated system which might receive one of plural channels video data from the -25- WO 00/70804 PCTIUSOO/07685 optical communication system at a data rate of approximately 622 MHz, the filtering might pass frequencies below about 630-650 MHz. The filtered electrical signal is then provided to an electrical square law circuit element 382 such as a diode. This square law element or limiter preferably removes the negative going portions of the received electrical signal and 5 might also be used to amplify the positive going portions of the received electrical signal. The negative going portions of the electrical signal are immediately identifiable as noise and so can be removed to improve the signal to noise ratio of the overall system. The electrical signal output from the limiter 382 is then analyzed to detect signals above a threshold value, which signals are recognized as transmitted ones. 10 Another method of reducing interference is to reduce the correlation between different ones of the noise signals. A difficulty observed by the present inventors when implementing the source strategy shown in FIG. 11 is an undesired level of temporal correlation between the different sources. This level of correlation can give rise to undesirable levels of correlation of noise sources or of correlation between the different 15 communication channels associated with the different sources. Consequently, preferred embodiments decorrelate the different sources. This might be accomplished by inserting different optical delays along each of the output paths of the different source channels. One simple mechanism for accomplishing this is illustrated in FIG. 14. A large number of distinct sources 400-403 are defined, for example using the technique illustrated in FIG. 11 20 and discussed above, so that the sources provide similar optical outputs with similar spectral bandwidths and spectral power distribution. While four sources are shown, the system will typically include 128 or more total sources corresponding to 128 or more users. The outputs of each of the sources 400-403 are passed through a delay to reduce the temporal correlation between the different sources. Such optical delays could consist of 25 optical delay lines or extended optical propagation paths. Causing each of the sources to pass through different lengths of fiber delay lines is the most preferred mechanism for providing appropriate delays. Delays might alternately be generated using free space propagation through different optical paths. Fiber delays are preferred since they can be implemented using only minimal space, so that the overall optical system can be provided 30 in a sufficiently small space as to allow a wider range of implementations for systems embodying this aspect of the present invention. Referring once again to FIG. 14, appropriate delays are effected by passing the output of each of the sources 400-403 through different lengths of single mode fibers 404-407. The different length fibers are selected to impose a delay of between about one and about two or more times the data rate 35 on successive sources. Considering a data rate of approximately 622 Mbt/sec, an appropriate delay can be fashioned by adding about one and a half feet of optical fiber -26- WO 00/70804 PCT/US0O/07685 (equivalent to -1.5 GHz) for each desired delay. Thus, for the first source 400, no additional length of fiber would be added as this represents the baseline. For the second source 401, 1.5 feet of additional fiber 405 would be included in the output path and for the third source 402, a three foot length of fiber 406 beyond the baseline length of fiber 404 is 5 provided. Similarly, the output from source 403 is coupled through a fiber 407 that is about 4.5 feet (-4.5 GHz) longer than fiber 404. Each of the users within a system, which may total 128 users or more or equivalently might total 128 channels of multiplexed data, is provided with a source originating from a central source and delayed by an amount different from all of the other sources. It will of course be appreciated that different 0 mechanisms for achieving optical delays are known and could be practiced to achieve similar results. Another method of reducing interference, and one that has been observed to be particularly effective, is the use of a data modulation scheme that limits the amount of time that the source is maintained in an on state. Time domain modulated data are provided to 5 the optical communication system by modulating the sources. Sources may be directly modulated or may be modulated by passing the source light through an element that can modulate the source. In preferred embodiments of the present invention, modulation is accomplished so that a light pulse of predetermined intensity is provided to the optical system when one binary value is to be transmitted and no light is provided to the optical 0 system when the other binary value is to be transmitted. A schematic example of the modulation of a source with a data stream is shown in FIG. 3. In a modulated data stream, there is typically a clock defining a data rate for modulated binary data and these binary data streams are typically characterized by a duty cycle. This is illustrated schematically in FIG. 15, where various data streams (a)-(c) are 5 shown on a background of clock cycle starts identified by the vertical dashed lines. Conventionally, each clock cycle defines a data period and the data can consume some or all of the clock cycle. If all of the clock cycle is consumed by the data, then the duty cycle is said to be 100%. If the data consumes only half of the clock cycle, then the duty cycle is said to be 50%. This is shown in FIG. 15(a) and consists of a signal that may be "ON" as 0 much as one half of the time. It is desirable to further reduce the amount of time that light is being injected into the system to further reduce the amount of interference that exists between the various users or channels within the system. As such, data modulation is accomplished in particularly preferred embodiments of the present invention using a data stream with a duty cycle of 25% like that shown in FIG. 15(b) or even shorter such as that 5 shown in FIG. 15(c) which has a duty cycle of 12.5%. -27- WO 00/70804 PCT/USOO/07685 In particularly preferred embodiments of the present invention, it is preferred that the duty cycle be reduced to a low but still detectable level, generally using a duty cycle of less than 50%. This has the effect of reducing the total optical signal that is present within the optical fiber system at any point in time. In other words, the use of shortened duty 5 cycles reduces the amount of light within the system, thereby reducing the noise signals and the amount of interference experienced by a desired signal. Circuitry exists for reducing duty cycles considerably. As a practical matter, however, the reduction in duty cycle must be limited in extent so that the optical signal remains detectable. The amplification of the input source or the amplification of the detection scheme must be 0 increased in proportion to the reduction in the data duty cycle. The noise floor associated with the amplifier then establishes the limit on how small the duty cycle might be reduced. The duty cycle cannot be reduced below the level at which amplifier noise comes to dominate the signal. The data source might be selected to provide data streams with the desired duty 5 cycle characteristics. On the other hand, it is typically preferable to provide greater flexibility so that any input data stream, for example the 50% duty cycle stream illustrated in FIG. 15(a), can be converted into a comparatively short duty cycle pulse. FIG. 16 schematically illustrates a device for converting a input data stream like that of FIG. 15(a) into a data stream like that shown in FIGS. 15(b) or 15(c). The circuit of FIG. 16 is placed 0 between the data source and the modulator. A data stream is input from the data source to the pulse modifier 420 shown in FIG. 16. The electrical signal travels along two paths so that a portion of the signal passes through a delay element 422. Delay element 422 creates a delay with respect to the other path's undelayed signal. The two signals are recombined by a recombiner 424 in a manner that produces a positive pulse only while the undelayed !5 and the delayed signals are both "1". The delay circuit 422 may be a programmable delay or it might consist of a series of inverters. The recombiner might, for example, be an exclusive OR gate. By use of the FIG. 16 circuit, pulses of any chosen duty cycle can be provided. While the present invention has been described with particular emphasis on certain 0 preferred embodiments of the present invention, the present invention is not limited to the particular embodiments described herein. Those of ordinary skill will appreciate that certain modifications and variations might be made to the particular embodiments of the present invention while remaining within the teachings of the present invention. For example, while the above embodiments have been presented in terms of communications 35 systems mediated over fiber, aspects of the present invention are immediately used in an -28- WO 00/70804 PCT/USOO/07685 over the air optical system. As such, the scope of the present invention is to be determined by the following claims. -29-

Claims (46)

1. An optical communication system, comprising: a light source; a data source providing a data stream; and an encoder receiving an optical output from the light source, the encoder including a 5 first spectral filtering assembly embodying a first code, the first code being a sequence of N digits each having one of at least two values, the assembly spectrally encoding the output of the light source with the first code by separating the output of the light source into N spectral components each corresponding to a digit of the first code, attenuating each spectral component according to the value of a corresponding code digit, and recombining 0 the spectral components to generate an encoded light signal, wherein the encoder is coupled to the data source to modulate the output of the light source so that the encoder generates a data modulated, encoded light signal.

2. The optical communication system of claim 1, wherein the first code is selected from 5 a set of unipolar codes in which each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code.

3. The optical communication system of claim 1, further including a decoder coupled to receive a light signal from an optical fiber and to recover transmitted data from the optical 0 fiber, the decoder comprising: a phase insensitive optical power separator for splitting a received light signal into approximately equal power first and second light components.

4. The optical communications system of claim 3, wherein the decoder further 5 comprises: second and third spectral filtering assemblies coupled to receive the first and second light components, the second spectral filtering assembly embodying the first code and the third spectral filtering assembly embodying a complement of the first code, the second and third spectral filtering assemblies outputting first and second filtered components of the O received light; and an optical detector provided to receive the first and second filtered components of the received light, the optical detector providing an electrical signal output.

5. The optical communications system of claim 1, wherein the encoder is a photonic 5 integrated circuit. -30- WO 00/70804 PCT/USOO/07685

6. The optical communications system of claim 1, wherein the first spectral filtering assembly comprises an array waveguide grating.

7. The optical communications system of claim 4, wherein the spectral filtering 5 assemblies comprise an array waveguide grating.

8. The optical communications system of claim 4, wherein the spectral filtering assemblies comprise a plurality of array waveguide gratings. 0

9. The optical communications system of claim 4, wherein the decoder is at least partially formed as a photonic integrated circuit.

10. The optical communications system of claim 9, wherein the encoder is at least partially formed as a photonic integrated circuit. 5

11. The optical communication system of claim 4, wherein the electrical signal output represents a differential measurement between the first and second filtered components of the received light. 0

12. The optical communication system of claim 4, wherein the electrical signal output is provided to a limiting circuit that removes electrical noise signals having a sign opposite of the recovered data.

13. The optical communication system of claim 4, wherein the electrical signal output is 5 provided to a limiting circuit comprising an electrical square law detector.

14. The optical communication system of claim 3, wherein the phase insensitive optical power separator includes: a first polarization sensitive element positioned to receive the light signal and to o separate the light signal into a first and a second light component, the first light component having a first polarization and the second light component having a second polarization as output from the first polarization sensitive element; a first beam path along which the first light component travels and a second beam path along which the second light component travels; -31- WO 00/70804 PCT/USOO/07685 a polarization modifier positioned along the second beam path, the polarization modifier changing the polarization of the second light component to be predominantly the first polarization; a beam splitter receiving the first and second light components and splitting the 5 first and second light components into third and fourth light components.

15. An optical communication system, comprising: a data source providing a data stream; and an encoder providing an optical output modulated with the data stream and 10 embodying a first code, the first code selected from a set of unipolar codes in which each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code, the codes in the set defined as sequences of N digits each digit having one of at least two values, each of the N digits of the codes corresponding to one of N potential spectral ranges that could be output from the encoder, 15 wherein the optical output from the encoder comprises M components corresponding to M different spectral ranges within the N potential spectral ranges, wherein each of the M components is characterized by an M-th component optical level corresponding to the value of a corresponding code digit, so that the optical output of the encoder is broad spectrum light modulated with a 20 data stream and with a spectrally defined code function, and wherein the encoder comprises a photonic integrated circuit.

16. The optical communication system of claim 15, wherein the first code is a discrete analog function and M is equal to N. 25

17. The optical communication system of claim 15, wherein the first code is binary and M is less than M.

18. The optical communication system of claim 15, wherein the first code is defined 30 within a set of N waveguides.

19. The optical communication system of claim 18, wherein M of the waveguides are terminated in a reflective coating. 35

20. The optical communication system of claim 15, wherein the encoder comprises an array of N sources. -32- WO 00/70804 PCTIUSOO/07685

21. The optical communication system of claim 15, wherein the encoder comprises a spectrally dispersive element which is a photonic integrated circuit. 5

22. The optical communication system of claim 21, wherein the spectrally dispersive element comprises an array of waveguides.

23. The optical communication system of claim 22, wherein the spectrally dispersive element is a curved grating. 0

24. The optical communication system of claim 15, wherein the first code is defined within a set of N waveguides by switches that alter optical properties of the waveguides.

25. The optical communication system of claim 24, wherein the switches generate heat 5 to alter optical properties of the waveguides.

26. The optical communication system of claim 24, wherein the data source comprises a broad band light source modulated with the data stream. o

27. The optical communication system of claim 15, further including a decoder coupled to receive a light signal from an optical fiber and to recover transmitted data from the optical fiber, the decoder comprising: a phase insensitive optical power separator for splitting a received light signal into approximately equal power first and second light components. 5

28. The optical communications system of claim 27, wherein the decoder further comprises: second and third spectral filtering assemblies coupled to receive the first and second light components, the second spectral filtering assembly embodying the first code and the 0 third spectral filtering assembly embodying a complement of the first code, the second and third spectral filtering assemblies outputting first and second filtered components of the received light; and an optical detector provided to receive the first and second filtered components of the received light, the optical detector providing an electrical signal output. 5 -33- WO 00/70804 PCTIUSOO/07685

29. The optical communication system of claim 28, wherein the electrical signal output represents a differential measurement between the first and second filtered components of the received light. 5

30. The optical communication system of claim 31, wherein the electrical signal output is provided to a limiting circuit that removes electrical noise signals having a sign opposite of the recovered data.

31. The optical communication system of claim 28, wherein the electrical signal output 0 is provided to a limiting circuit comprising an electrical square law detector.

32. The optical communication system of claim 27, wherein the phase insensitive optical power separator includes: a first polarization sensitive element positioned to receive the light signal and to 5 separate the light signal into a first and a second light component, the first light component having a first polarization and the second light component having a second polarization as output from the first polarization sensitive element; a first beam path along which the first light component travels and a second beam path along which the second light component travels; 0 a polarization modifier positioned along the second beam path, the polarization modifier changing the polarization of the second light component to be predominantly the first polarization; a beam splitter receiving the first and second light components and splitting the first and second light components into third and fourth light components. Z5

33. An optical communication system, comprising a decoder coupled to receive a light signal from an optical communication system and to recover transmitted data, the decoder comprising: an optical power separator for splitting a received light signal into approximately 0 equal power first and second light components; first and second spectral filters coupled to receive the first and second light components, the first spectral filter embodying a first code and the second spectral filter embodying a complement of the first code, the first and second spectral filters outputting first and second filtered components of the received light; and 5 an optical detector provided to receive the first and second filtered components of the received light, the optical detector providing an electrical signal output, -34- WO 00/70804 PCT/USOO/07685 wherein the first code is selected from a set of unipolar codes in which each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code, the codes in the set defined as sequences of N digits each digit having one of at least two values, each of the N digits of the codes corresponding to one 5 of N potential spectral ranges over which the set of codes is defined, wherein the received light signal is broad spectrum light modulated with a data stream and with a spectrally defined code function having M components corresponding to M different spectral ranges within the N potential spectral ranges, and wherein the decoder comprises a photonic integrated circuit. 0

34. The optical communication system of claim 33, wherein the first code is a discrete analog function and M is equal to N.

35. The optical communication system of claim 33, wherein the first code is binary and 5 M is less than M.

36. The optical communication system of claim 33, wherein the first code is defined within a set of N waveguides. 0

37. The optical communication system of claim 36, wherein M of the waveguides are terminated in a reflective coating.

38. The optical communication system of claim 33, wherein the first filter comprises a spectrally dispersive element which is a photonic integrated circuit. 5

39. The optical communication system of claim 38, wherein the spectrally dispersive element comprises an array of waveguides.

40. The optical communication system of claim 38, wherein the spectrally dispersive 0 element is a curved grating.

41. The optical communication system of claim 33, wherein the first code is defined within a set of N waveguides by switches that alter optical properties of the waveguides. 5

42. The optical communication system of claim 41, wherein the switches generate heat to alter optical properties of the waveguides. -35- WO 00/70804 PCT/USOO/07685

43. The optical communication system of claim 33, wherein the electrical signal output represents a differential measurement between the first and second filtered components of the received light. 5

44. The optical communication system of claim 43, wherein the electrical signal output is provided to a limiting circuit that removes electrical noise signals having a sign opposite of the recovered data. 10

45. The optical communication system of claim 33, wherein the electrical signal output is provided to a limiting circuit comprising an electrical square law detector.

46. The optical communication system of claim 33, wherein the optical power separator includes: 15 a first polarization sensitive element positioned to receive the light signal and to separate the light signal into a first and a second light component, the first light component having a first polarization and the second light component having a second polarization as output from the first polarization sensitive element; a first beam path along which the first light component travels and a second beam 20 path along which the second light component travels; a polarization modifier positioned along the second beam path, the polarization modifier changing the polarization of the second light component to be predominantly the first polarization; a beam splitter receiving the first and second light components and splitting the 25 first and second light components into third and fourth light components. -36-