JP2004282793A - Optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system capable of dealing with wavelength fluctuation on the transmitting side. <P>SOLUTION: The optical transmission system is equipped with a plurality of optical transmitters for outputting signal beams and an optical multiplexer for adding together the signal beams from the optical transmitters for sending to one or more optical transmission lines. Each optical transmitter includes a light source, a band pass filter for passing light from the light source, a beam splitting means for splitting the light passed through the band pass filter into at least two split beams, a photo-detector for converting one of split beams into an electric signal with a level corresponding to its intensity, and a control means for receiving the electric signal and controlling a wavelength of the light source to fix the intensity of the split beam, and the other split beam is sent as a signal beam. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical transmitter, an optical transmission system and an optical reception system for wavelength multiplexed optical transmission, and an optical filter array applicable to these systems.

  In recent years, with the advent of erbium-doped optical fiber amplifiers (EDFAs), transmission systems for directly amplifying light have been studied. Since the EDFA has a wide amplification band of several tens of nm in the 1.5 μm band, it is possible to collectively amplify the wavelength division multiplexed (WDM) signal light using a plurality of light sources. For this reason, for example, for the purpose of increasing the capacity of a trunk system, research and development on a transmission system to which EDFA and WDM are applied have been activated.

  In a transmission system in which a plurality of optical repeaters equipped with EDFAs are used, the overall amplification band (a band that can be used for transmission from the viewpoint of fluctuations in optical output and SN ratio) is the amplification band of the EDFA alone. However, it is considered that the overall amplification band can still be secured at about 15 nm. In this case, simply calculating, it is possible to transmit 16 signal lights arranged at equal intervals with a wavelength interval of 1 nm.

  As described above, when performing high-density WDM in which the wavelength interval is reduced to about 1 nm (in the WDM conventionally studied, a wavelength interval of about 10 nm is assumed, meaning higher density compared to this). On the transmitting side, it is necessary to manage the relative wavelength intervals of the transmission light sources and the absolute wavelength. The reason is as follows.

  (1) A laser diode (LD) used as a light source has a fluctuation in oscillation wavelength due to a temperature fluctuation. The fluctuation of the oscillation wavelength is, for example, 0.1 nm / ° C. Even if the temperature of the LD module is controlled, if the ambient temperature fluctuates greatly, the temperature of the LD also slightly fluctuates, causing a wavelength fluctuation. The reason that the temperature of the LD changes despite the temperature control is that the distance between the LD and the temperature sensor for temperature control is large, so even if the temperature of the temperature sensor becomes constant, This is because a temperature gradient is generated between the temperature sensors, and the temperature gradient changes with a change in the outside air temperature.

  (2) There is a wavelength variation due to the aging of the LD. Although the magnitude of the deterioration with time is unknown at present, it is considered that it is necessary to design a system on the assumption that there is at least a fluctuation of about 0.5 nm in order to ensure stable operation.

  (3) When direct modulation of the LD is performed, the bias current applied to the LD is controlled in order to keep the optical output and the modulation amplitude constant. If the bias current is changed during this control, the oscillation wavelength changes.

  If the oscillation wavelength fluctuates on the transmitting side, the wavelength of the signal light deviates from the receivable wavelength band of the wavelength selective receiver for selectively receiving the signal light of the desired wavelength on the receiving side, and the reception quality is degraded. Or may be unreceivable.

  If the wavelength selective optical filter built into the wavelength selective receiver has a tracking function to follow the wavelength variation of the signal light to receive the transmission wavelength, withstand a certain amount of wavelength variation on the transmitting side Can be. However, when the wavelength of the transmission light source fluctuates greatly and approaches the wavelength of the signal light of the adjacent channel in WDM, the signal power of the adjacent channel becomes a crosstalk component of the signal originally desired to be received, and the reception quality is degraded.

  By the way, when WDM is applied to a closed system connecting one point to one point, the absolute value of the wavelength of the transmission light source is not so important. Therefore, the wavelength may be appropriately determined so that the wavelength characteristics of the optical component and the EDFA satisfy the required values, and it is sufficient to mainly control the relative wavelength interval between the transmission light sources. However, when WDM is applied to a point-to-multipoint or multipoint-to-multipoint system in which various nodes are on a network, it is necessary to manage the absolute wavelength of each transmission light source. It is becoming.

  As described above, in a transmission system to which WDM is applied, in order to maintain stable operation on the receiving side, relative or absolute stabilization of the wavelength of signal light on the transmitting side is necessary. . Specifically, in order to avoid the influence of crosstalk on the receiving side, in the case of a wavelength interval of about 1/10 to 1/5 of a wavelength interval between adjacent channels in WDM, that is, in the case of a wavelength interval of 1 nm, 0.1 to 1.0 is used. It is necessary to stabilize the wavelength interval to about 1 to 0.2 nm. Note that this value depends on the performance of the wavelength selection filter built in the receiver. Even when the absolute wavelength needs to be managed, it is necessary to manage the wavelength with an accuracy of about 0.1 nm or less at least as in the above-described wavelength interval.

  By the way, another problem of the transmission system to which WDM is applied is to secure a stable operation when a channel is added or suspended according to a required transmission capacity or when a device breaks down. For example, when an LD is used as a transmission light source when adding a channel, when the light source is started (cold start), the bias current is increased to a predetermined current value, and the temperature of the LD is stabilized at the set temperature. During this time, a wavelength fluctuation of about several nm occurs. The same occurs when the light source is stopped.

  Further, when a failure occurs in a part of the system, particularly when a failure occurs in the LD itself or a bias current control circuit or a temperature control circuit of the LD, sudden wavelength fluctuation may occur. On the transmitting side, it is desired to ensure stable operation so that the influence of the wavelength fluctuation of the light source does not affect other light sources.

  Therefore, a main object of the present invention is to cope with wavelength fluctuation on the transmission side. Other objects of the present invention will become clear from the following description.

  According to one aspect of the present invention, a plurality of optical transmitters for outputting signal light, and an optical multiplexer for adding the signal light from each of the optical transmitters and transmitting the combined signal light to one or more optical transmission lines are provided. Each of the optical transmitters includes a light source, a band-pass filter that allows light from the light source to pass therethrough, an optical branching unit that splits the light that has passed through the band-pass filter into at least two split lights, A light receiver for converting one of the lights into an electric signal having a level corresponding to the intensity of the light, and a control means for receiving the electric signal and controlling the wavelength of the light source so that the intensity of the split light is constant. And an optical transmission system for wavelength division multiplex transmission in which another one of the split lights is transmitted as the signal light.

  As described above, according to the present invention, there is an effect that it becomes possible to cope with wavelength fluctuation on the transmission side. Also, an optical filter required for WDM can be obtained with high accuracy and stability, and by using this, a more practical WDM system can be constructed.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram of a WDM transmission system to which the present invention can be applied. The signal lights respectively output from the plurality of optical transmitters 2 (# 1, # 2,..., #N) to which different wavelengths are set are added by the optical multiplexer 4 and sent out to the optical transmission line 6. You. In the middle of the optical transmission line 6, a plurality of optical repeaters 8 including, for example, EDFAs are provided.

  The WDM signal light transmitted through multiple relays is first distributed by the optical distributor 10 to channels corresponding to the number of multiplexes on the receiving side. Each of the distributed lights is supplied to the wavelength selection optical filters 12 (# 1, # 2,..., #N). Then, the selected signal lights are received by the optical receivers 14 (# 1, # 2,..., #N).

  FIG. 2 is a block diagram showing a basic configuration of the optical transmitter of the present invention applicable to the system of FIG. The optical transmitter includes, for example, a transmission light source 16 including an LD and a driving circuit thereof, a band-pass filter 18 that allows light from the light source 16 to pass, and light that passes through the band-pass filter 18 into at least two branch lights. It comprises an optical coupler 20 and a light receiver 22 for converting one of the split lights into an electric signal having a level corresponding to the intensity.

  As the light receiver 22, for example, a photodiode is used. The other one of the split lights split by the optical coupler 20 is transmitted to an optical transmission line (not shown).

  The electric signal from the light receiver 22 is amplified by the amplifier circuit 24 and supplied to the control circuit 26. The control circuit 26 controls the wavelength of the transmission light source 16 so that the intensity of the split light in the optical coupler 20 becomes constant.

  The bandpass filter 18 has, for example, a narrow band characteristic in the relationship between the transmittance and the wavelength. In order to maintain this characteristic, the temperature control circuit 28 for keeping the temperature of the bandpass filter 18 constant is used. Is provided.

  As the band-pass filter 18, an interference filter in which dielectric films are stacked in multiple layers or a bulk-type or waveguide-type filter using a grating can be used. The aging of the interference filter is 0.1 nm or less, and it is possible to obtain sufficient stability for performing the above-described WDM with a wavelength interval of about 1 nm. If the required stability can be obtained without performing temperature control on the bandpass filter 18, the temperature control circuit 28 is unnecessary.

  In this embodiment, the bandpass filter 18 is used as a wavelength reference. Therefore, when WDM is performed using a plurality of optical transmitters, even if the wavelength of the transmission light source 16 fluctuates greatly, the signal light of another channel is caused by the fluctuation. The effect on is avoided. That is, the band-pass filter 18 functions as a wavelength reference and also functions as a protection filter.

  Therefore, when starting up the optical transmitter (cold start), it is not necessary to shut off the signal light using an optical switch or the like and synchronize control with another channel.

  FIG. 3 is a block diagram showing a first embodiment of the optical transmitter. In this embodiment, a laser diode 30 is used as a transmission light source. The light output from the laser diode 30 passes through the bandpass filter 18 and is split by the beam splitter 32. One of the split lights is sent out to an optical transmission line (not shown), and the other of the split lights is supplied to the light receiver 22.

  In this embodiment, the control means for receiving the electric signal from the light receiver 22 and controlling the wavelength of the light source so that the intensity of the branched light becomes constant includes an oscillator 34 for outputting a low-frequency signal and a laser diode using the low-frequency signal. It includes means for slightly modulating the frequency of the signal 30 and synchronous detection means for receiving the low-frequency signal from the oscillator 34 and the electric signal from the light receiver 22 and detecting the low-frequency component contained in the electric signal. Then, control is performed so that the wavelength of the laser diode 30 coincides with the wavelength giving the peak in the characteristics of the bandpass filter 18. Specifically, it is as follows.

  The low frequency signal from the oscillator 34 is supplied to an LD drive circuit 36 and a synchronous detection circuit 38. The LD drive circuit 36 frequency-modulates the laser diode 30 with a small modulation factor based on the low-frequency signal from the oscillator 34.

  To the synchronous detection circuit 38, in addition to the low frequency signal from the oscillator 34, the electric signal from the optical receiver 22 amplified by the amplifier 24 is supplied. The output signal of the synchronous detection circuit 38 is supplied to a PID controller 42 through a low-pass filter 40.

  In the following description, a signal supplied from the oscillator 34 to the synchronous detection circuit 38 is referred to as a “low-frequency signal”, and a signal supplied from the amplifier 24 to the synchronous detection circuit 38 is referred to as an “electric signal”. The synchronous detection circuit 38 detects a phase difference between the low-frequency signal and the electric signal. Therefore, this phase difference is reflected on the DC signal output from the low-pass filter 40. Then, based on the DC signal, the PID controller 42 controls the oscillation wavelength of the laser diode 30 based on the principle of proportional / integral / differential control. Control is performed so as to coincide with the wavelength giving the peak in the characteristic.

  The principle of synchronous detection will be described with reference to FIG. FIG. 4A shows the relationship between the transmittance and the wavelength in the bandpass filter 18. Reference numeral 44 indicates a characteristic curve of the band-pass filter 18.

  If the wavelength of the output light of the laser diode 30 is lower than the peak wavelength in the characteristic curve 44 and the frequency of the laser diode 30 is modulated based on the low frequency signal indicated by reference numeral 46A, the band-pass filter The output light 18 is intensity-modulated in phase with the low-frequency signal as indicated by reference numeral 48A.

  On the other hand, when the wavelength of the output light of the laser diode 30 is larger than the peak wavelength in the characteristic curve 44 and the frequency of the laser diode 30 is modulated by the low frequency signal indicated by reference numeral 46C, the bandpass filter 18 Will be intensity-modulated in phase opposite to that of the low-frequency signal as indicated by reference numeral 48C.

  Even if the laser diode 30 is frequency-modulated by the low-frequency signal when the wavelength of the output light of the laser diode 30 matches the peak wavelength in the characteristic curve 44, the low-frequency signal component is included in the output light of the bandpass filter 18. Does not occur. That is, in this case, the output light of the band-pass filter 18 is intensity-modulated by a frequency twice the frequency of the low-frequency signal, as indicated by reference numeral 48B.

  Therefore, the frequency detection (wavelength discrimination) of the output light of the laser diode 30 can be performed by extracting the frequency component of the low-frequency signal included in the branched light by the synchronous detection circuit 38.

  FIG. 4B shows the relationship between the error voltage (the level of the DC signal from the low-pass filter 40) and the separation wavelength from the peak wavelength. It can be seen that a characteristic (reference numeral 50) obtained by differentiating the characteristic curve 44 of the bandpass filter 18 is obtained according to the above-described principle. When the wavelength of the output light from the laser diode 30 is larger than the peak wavelength, the error voltage is positive. When the wavelength is smaller than the peak wavelength, the error voltage is negative. When the wavelength matches the peak wavelength, the error voltage is 0. Become. Therefore, by performing the BID control using the proportional region 52 of the differential characteristic, the wavelength of the output light of the laser diode 30 can be made to coincide with the peak wavelength of the characteristic curve of the bandpass filter 18.

  FIG. 5 shows a block diagram of a conventional optical transmitter to explain the superiority of the optical transmitter of the present invention. Throughout the drawings, substantially the same parts are denoted by the same reference numerals.

  In this conventional example, a molecular absorption cell 54 is provided as a wavelength reference. The molecular absorption cell 54 is constructed by enclosing a gas such as HCN or NH3 at a suitable pressure in a vacuum transparent container, and this cell is arranged on the optical path of the output light of the laser diode 30.

  Since the molecular absorption cell 54 slightly absorbs light having a wavelength corresponding to the natural frequency determined by the type of the sealed gas, a characteristic curve having a shape opposite to the characteristic curve of FIG. 4A can be obtained.

  Therefore, the wavelength of the output light from the laser diode 30 can be made to match the absorption wavelength in the molecular absorption cell 54 according to the principle of the synchronous detection described above.

  However, in this conventional example, when the wavelength of the output light of the laser diode 30 fluctuates greatly due to, for example, a cold start of the optical transmitter or a failure of the control circuit, the output light of the laser diode 30 is supplied to the optical transmission line. There is no means for blocking the transmission, so that when this optical transmitter is applied to a WDM transmission system, it is impossible to avoid the influence on other channels.

  On the other hand, in the optical transmitter of the present invention, a bandpass filter is used as a wavelength reference, and in particular, in the embodiment of FIG. 3, a narrowband bandpass filter is used. Does not pass through the bandpass filter, and therefore has little effect on signals on other channels when the optical transmitter is applied to a WDM transmission system.

  By the way, in an optical transmitter applied to a trunk system, external modulation is generally performed to suppress wavelength fluctuation at the time of intensity modulation to cope with high-speed transmission. As an external modulator, a Mach-Zehnder type optical modulator using lithium niobate (LiNbO3) as a substrate material has been put to practical use.

  In this type of optical modulator, control is performed such that a low-frequency signal is superimposed on a DC bias supplied to the optical modulator, the optical output is partially branched and received, and the operating point of the optical modulator is stabilized by synchronous detection ( ABC (Automatic Bias Control) may be performed.

  Also, in the optical transmitter, control (APC; Automatic Power Control) for stabilizing the optical output is performed. Note that the above-described wavelength stabilization control is called AFC (Automatic Frequency Control).

  Hereinafter, an embodiment of an optical transmitter of an external modulation system in which ABC, APC, and AFC are performed will be described with reference to FIG. FIG. 6 is a block diagram showing a second embodiment of the optical transmitter according to the present invention.

  While the transmission light source (laser diode) is directly modulated by its drive circuit in the previous embodiments, in this embodiment, the laser diode 30 outputs light of a constant intensity, and this output light is subjected to light modulation. The intensity is modulated by the device 56. The optical modulator 56 is a Mach-Zehnder type optical modulator using, for example, LiNbO3 as a waveguide substrate material. The optical modulator 56 is supplied with a modulation signal based on transmission data and a bias voltage for operating point control. The intensity-modulated light from the optical modulator 56 is split by the optical coupler 20 through the band-pass filter 18, one of the split lights is sent to an optical transmission line (not shown), and the other is converted into an electric signal by the light receiver 22. You. The modulation signal is supplied from a modulation circuit 66.

  The electric signal from the light receiver 22 is amplified by the amplifier circuit 24 and supplied to the ABC control circuit 60, the LD drive circuit 36, and the wavelength control circuit 62.

  The ABC control circuit 60 is for performing ABC, and the LN drive circuit 58 supplies a controlled bias voltage to the optical modulator 56 based on the output signal.

  The wavelength control circuit 62 is for AFC, and includes a synchronous detection circuit, a low-pass filter, a PID controller, and the like.

  The LD drive circuit 36 controls the bias current of the laser diode 30 based on a signal provided from the wavelength control circuit 62. In this example, the electric signal from the light receiver 22 amplified by the amplifier circuit 24 is also directly supplied to the LD drive circuit 36, and APC is performed.

  Since ABC and APC perform synchronous detection, a circuit for synchronous detection can be shared. For example, by sharing the optical coupler, the light receiver, and the amplifier circuit and making the frequency of the low-frequency signal used in the synchronous detection different between ABC and AFC, both controls can be performed independently.

  In this embodiment, the timing control circuit 64 controls the ABC, AFC, and APC in a time-division manner, and the low-frequency oscillator is also used in common.

  FIG. 7 shows an example of a timing chart of the time division control. In this embodiment, AFC, ABC, and APC are sequentially switched in this order by switching by the timing control circuit 64, so that a common control circuit is used. The time sequence of each control is, for example, 1 to 10 seconds.

  Next, several embodiments of the optical transmission system of the present invention will be described. FIG. 8 is a block diagram showing a first embodiment of the optical transmission system of the present invention.

This optical transmission system has a plurality of optical transmitters of the present invention that output signal light, and the signal light from each optical transmitter is added by an optical multiplexer and sent to one or more optical transmission lines. Is done. In this embodiment, an optical star coupler 70 is used as an optical multiplexer.
Each optical transmitter is provided with a transmission light source unit 68 (# 1, # 2,..., #N) for outputting signal light of wavelengths λ1, λ2,. Each of the transmission light source units 68 (# 1, # 2,..., #N) accommodates a laser diode that is directly modulated or a combination of a laser diode that outputs light of a constant intensity and an external modulator. ing.

  Each optical transmitter has a band-pass filter 18 (# 1, # 2,..., #N), an optical coupler 20 (# 1, # 2,. , #N, an amplifier circuit 24 (# 1, # 2,..., #N) and a temperature control circuit 28 (# 1, # 2,. n) is provided.

  The control circuit 72 (# 1, # 2,..., #N) to which the electric signal is supplied from each light receiver performs AFC of each transmission light source unit. Is included, each control circuit 72 (# 1, # 2,..., #N) also includes a circuit for ABC. Further, each control circuit may be configured to perform the function of the APC.

  In this optical transmission system, different wavelengths .lambda.1, .lambda.2,..., .Lambda.n are assigned to the respective transmission light source units, so that this system can be directly applied to WDM.

  By the way, in the embodiment of FIG. 8, the band-pass filter is provided independently for each optical transmitter. However, when the band-pass filter is used as a wavelength reference, it is preferable that there be a plurality of independent references. is not. For example, when the characteristics of each band-pass filter have temperature dependency, if the environmental temperature of each filter greatly varies, the accuracy of maintaining the wavelength interval may be degraded.

  Therefore, in the following embodiment, each filter element of a space division type optical filter array having a plurality of input ports and the number of output ports corresponding to the number of input ports is used as each bandpass filter.

  FIG. 9 shows an example of a space division type optical filter array. The optical filter array 180 has a single substrate SB, and the bandpass filter elements 18 (# 1, # 2) having transmission center wavelengths of wavelengths λ1, λ2,. ,..., #N) are formed.

  Reference numeral 74 (# 1, # 2, ..., #n) indicates an input port of each filter element, and reference numeral 76 (# 1, # 2, ..., #n) indicates an output port of each filter element. Is shown.

  In the embodiment of FIG. 6, the light from the laser diode 30 is modulated by the optical modulator 56, and then the modulated light is transmitted through the band-pass filter 18. However, the spectrum of the signal light due to the modulation is widened. If it is large, the side of the spectrum is cut by the narrow band-pass filter 18 and the code error rate may increase during demodulation. After passing through the filter 18, modulation by an optical modulator may be performed.

  In this case, although not shown, the arrangement of each part in the optical transmitter is in the order of the laser diode 30, the bandpass filter 18, the optical modulator 56, and the optical coupler 20.

  A specific example of the space division type optical filter array will be described with reference to FIGS. FIG. 10 is a diagram showing a first embodiment of the optical filter array.

  This optical filter array is formed by forming a plurality of waveguide type diffraction gratings 78 (# 1, # 2,..., #N) having different lattice constants on a waveguide substrate 76. Both ends of each waveguide type diffraction grating 78 (# 1, # 2,..., #N) form an input port and an output port of the optical filter array, respectively.

  FIG. 11 is a view showing a second embodiment of the optical filter array. This optical filter array includes a substrate 80 having a plurality of V-grooves 80A, and a plurality of fiber grating optical filters 82 (# 1, # 2,..., #N) seated in each V-groove 80A and having different lattice constants. And

  In this example, the V-shaped grooves 80A are formed in parallel with each other in order to simplify the assembly of the device. Both ends of each fiber grating type optical filter 82 (# 1, # 2,..., #N) constitute each input port and each output port of this optical filter array.

  FIG. 12 shows a third embodiment of the optical filter array. Each input port of the optical filter array is a plurality of input optical fibers 84 (# 1, # 2,..., #N) arranged in parallel with each other, and each output port is an input optical fiber 84 (# , #N) and a plurality of output side optical fibers 86 (# 1, # 2,..., #N) arranged in parallel to each other.

  Then, in order to form an optical path composed of a parallel light beam between each input port and the output port, the optical path is formed near the excitation end of the input side optical fiber 84 (# 1, # 2,..., #N). Have lenses 88 (# 1, # 2,..., #N) disposed therein, and a lens 90 near the excitation end of the output side optical fiber 86 (# 1, # 2,. (# 1, # 2,..., #N) are arranged.

  Reference numeral 92 denotes a transparent substrate provided so as to intersect with the above-described optical paths. The transparent substrate 92 is, for example, a parallel glass flat plate.

  On the transparent substrate 92, an interference film 94 whose thickness continuously changes in the arrangement direction of each optical path is formed. As the interference film 94, for example, a dielectric multilayer film formed by alternately laminating a plurality of TiO2 as a high refractive index layer and SiO2 as a low refractive index layer is used.

  In order to continuously change the thickness of the interference film 94 as described above, when this interference film is formed by, for example, vapor deposition, if the distance from the vapor deposition source to one end 92A and the other end 92B of the transparent substrate 92 is different. You should leave it. That is, the transparent substrate 92 accommodated in the vapor deposition device is inclined.

  The reason why the thickness of the interference film 94 is continuously changed in this way is to make the transmission center wavelength in each filter array different.

  The advantages of using the above-described space division type bandpass filter array are as follows.

  (1) Since each filter element is formed on the same substrate, if there is temperature modulation of the transmission wavelength, it can be managed in a unified manner, and only one control circuit is required and the stability is improved. .

  (2) When the portions that determine the characteristics of the filter element are formed of the same material on the same substrate as in the examples of FIGS. 10 and 12, even if the transmission center wavelength varies over time, The variation width and the variation direction of each filter element have the same tendency, and the relative relationship (for example, wavelength interval) between the wavelengths is substantially constant even if the absolute value of the transmission center wavelength varies.

  (3) By forming an array, it is possible to reduce the size and cost as compared with a case where bandpass filters are individually formed.

  FIG. 13 is a block diagram showing a second embodiment of the optical transmission system of the present invention. This embodiment is different from the first embodiment in FIG. 8 in that a space-division band-pass optical filter is used as a wavelength reference for each transmission light source unit 68 (# 1, # 2,..., #N). Is characterized by the fact that

  In this embodiment, the optical filter array 180 has another filter element 18 'in addition to the filter elements 18 (# 1, # 2,..., #N) for each transmission light source unit. To stabilize the absolute wavelength. The filter element 18 'is a band-pass filter having a transmission center wavelength of λABS. An absolute wavelength reference light source 100 is connected to an input port of the filter element 18 ', and an optical coupler 20' is connected to an output port of the filter element 18 '.

One of the lights split by the optical coupler 20 'is sent to a transmission path (not shown) via an optical star coupler, and the other of the split lights is converted into an electric signal by a light receiver 22'. The converted electric signal is amplified by the amplifier 24 'and supplied to the control circuit 94.

  A Peltier element (electronic cooling element) 98 is provided in close contact with the filter element 180, and the Peltier element 98 is driven by a drive circuit 96. The driving circuit 96 controls the absorption and emission of heat in the Peltier element 98 based on the signal supplied from the control circuit 94, thereby locking the transmission center wavelength of the filter element 18 ′ with respect to the absolute wavelength reference light source 100. be able to. Note that synchronous detection can be applied to the control circuit 94 in the same manner as the control circuits 72 (# 1, # 2,..., #N) for each transmission light source unit.

  According to Takahashi et al., "Wavelength Temperature Stability of Narrow Bandpass Filters Using Ion Assist," IEICE Spring Conference 1994, C-280, appropriate selection of thermal expansion coefficient of substrate material used for interference filter Thereby, the temperature dependence of the transmission wavelength can be suppressed. Therefore, required wavelength stability can be obtained without strict temperature control for the filter array.

  Conversely, by appropriately selecting the thermal expansion coefficient so that the temperature dependence of the transmission wavelength becomes large, the transmission wavelength can be controlled by changing the temperature of the filter array.

  For example, according to the above-mentioned literature, when the coefficient of linear thermal expansion of the substrate material is almost zero, the temperature dependence of the transmission center wavelength is 0.015 nm / ° C, and the wavelength change of 0.3 nm due to a temperature change of ± 20 ° C. Variable becomes possible. This value is sufficient to tune the absolute wavelength reference optical filter. In order to make the latent thermal expansion coefficient of the substrate material substantially zero, for example, quartz (0.4.times.10@-7) may be used.

  FIG. 14 is a diagram showing a third embodiment of the optical transmission system. In this embodiment, the optical branching at the optical coupler for each transmission light source unit is omitted, the signal light from each transmission light source unit added by the optical star coupler 70 is extracted, and this is converted into an electric signal by the light receiver 22. are doing. Since transfer is performed after all the signal lights are multiplexed, it is necessary to perform individual recognition for each transmission light source unit. This recognition is performed by the time division control circuit 102 in this embodiment.

  The time-division control circuit 102 receives the electric signal from the light receiver 22 amplified by the amplifier 24, and performs a Peltier element driving circuit 96 for controlling the absolute wavelength reference and a laser diode driving circuit 104 for each transmission light source unit. Are switched in a time sharing manner.

  Instead of performing time-division control, different low-frequency signals for synchronous detection may be assigned to each transmission light source unit and each absolute wavelength reference light source.

  The optical filter array and its control circuit of FIG. 13 can be applied to the receiving side as shown in FIG. FIG. 15 is a block diagram showing a first embodiment of the optical receiving system.

  The signal light supplied from an optical transmission line (not shown) is distributed by the optical tree coupler 106 and supplied to each input port of the optical filter array 180. Each of the output ports of the optical filter array 180 is connected to an optical receiver 108 (# 1, # 2,..., #N) that receives signal light having a wavelength of λ1, λ2,. Is done.

  Further, a light receiver 22 'is connected to an output port of the filter element 18', for example, to receive light from the absolute wavelength reference light source 100 of FIG.

  Each of the optical receivers 108 (# 1, # 2,..., #N) outputs data and clock of # 1 channel, # 2 channel,.

  The signal from the light receiver 22 'is amplified by the amplifier 24' and supplied to the control circuit 94. The Peltier driving circuit 96 controls the Peltier element 98 based on the output signal of the control circuit 94.

  According to this embodiment, the transmission center wavelength of the filter element 18 'can be made to follow the wavelength of the absolute wavelength reference light source 100 (see FIG. 13) in the same manner as in the embodiment of FIG. A stable receiving operation can be performed. In this embodiment, the filter element 18 'is used corresponding to the absolute wavelength reference light source 100. However, the filter element 18' may correspond to one of the transmission light source units.

  Next, an embodiment of an optical transmitter capable of stabilizing the wavelength without using synchronous detection will be described. In this embodiment, a space division type optical filter array is used to stabilize the wavelength.

  FIG. 16 is a block diagram showing a third embodiment of the optical transmitter. The signal light output from the transmission light source unit 68 is split into two by the optical coupler 110 and supplied to two input ports of the optical filter array 180. One of the two filter elements of the optical filter array 180 is a main filter element 112 connected to the transmission line, and the other is a control filter element 114.

  The transmission center wavelength of the main filter element 112 is at the set wavelength λ of the transmission light source unit 68, while the transmission center wavelength of the control filter element 114 is set at λ + Δλ slightly shifted from λ.

  λΔ is set to about half width at half maximum in the characteristics of the filter elements 112 and 114. That is, when the light from the transmission light source unit 68 transmits through the main filter element 112 at the maximum transmittance, the power transmitted through the control filter element 114 is set to be about 50%. This is shown in FIG.

  FIG. 17 is a diagram showing characteristics of the optical filter array 180 of FIG. The vertical axis represents the transmittance, and the horizontal axis represents the wavelength. The solid line represented by reference numeral 130 represents the characteristic of the main filter element 112, and the broken line represented by reference numeral 132 represents the characteristic of the control filter element 114.

  The peak wavelength in the characteristic represented by reference numeral 130 is λ, and the peak wavelength in the characteristic represented by reference numeral 132 is λ + Δλ.

  Therefore, by performing control such that the intensity of the light output from the control filter element 114 matches the stable point SP corresponding to the wavelength λ in the characteristic represented by reference numeral 132, the main operation is performed without performing the synchronous detection. The wavelength of the output light of the filter element 112 can be stabilized at λ.

  In order to perform such control, a control circuit 120 is provided in the optical transmitter of FIG. The output light of the control filter element 114 is converted into an electric signal by the photodiode 116, and the electric signal is amplified by the amplifier 118 and supplied to the control circuit 120.

  The control circuit 120 compares the signal level from the amplifier 118 with the setting reference 124, the error amplifier 126 amplifies the output signal of the comparison circuit 122, and the output level of the error amplifier 126 becomes 0 or a constant value. And a light source drive circuit 128 for driving the transmission light source unit 68 as described above.

  FIG. 18 is a block diagram showing a fourth embodiment of the optical transmission system of the present invention. This embodiment is characterized in that a plurality of optical transmitters shown in FIG. 16 are provided, and an optical filter array of each optical transmitter is formed on a common substrate.

  The signal light output from each transmission light source unit 68 (# 1, # 2,..., #N) is split by an optical coupler 110 (# 1, # 2,..., #N), respectively. Each of the branched lights is supplied to an input port of the optical filter array 180.

  The optical filter array 180 includes main filter elements 112 (# 1, # 2,..., #N) having transmission center wavelengths at wavelengths λ1, λ2,..., Λn, respectively, and wavelengths λ1 + Δλ, λ2 + Δλ, .., .Lambda.n + .DELTA..lambda., And control filter elements 114 (# 1, # 2,..., #N) each having a transmission center wavelength.

  The output ports of the main filter elements 112 (# 1, # 2,..., #N) are connected to the optical star coupler 70, respectively, and one or more optical transmission lines are connected to the output side of the optical star coupler. You.

  The output ports of the control filter elements 114 (# 1, # 2, ..., #n) are connected to the photodiodes 116 (# 1, # 2, ..., #n), respectively.

  Then, in order to control the wavelength of the transmission light source units 68 (# 1, # 2,..., #N) of each channel, the amplifiers 118 (# 1, # 2, # 2) as in the optical transmitter of FIG. , #N) and a control circuit 120 (# 1, # 2,..., #N).

  In this embodiment, in order to absolutely stabilize the transmission center wavelength in the optical filter array 180, the main filter element 112A having the transmission center wavelength at the wavelength λ ABS and the control filter having the transmission center wavelength at the wavelength λ ABS + Δλ are used. Element 114A is provided integrally with another filter element.

  The light from the absolute wavelength reference light source 100 is branched by the optical coupler 110 'and supplied to the main filter element 112A and the control filter element 114A, respectively, and the output light of the main filter element 112A is transmitted through the optical star coupler. The output light of the control filter element 114A is supplied to the photodiode 116A.

  In order to make the transmission center wavelength of the main filter element 112A coincide with the wavelength of the absolute wavelength reference light source 100, an amplifier 118A, a control circuit 120A, a Peltier element driving circuit 96 and a Peltier element 98 are provided according to the embodiment of FIG. ing.

  According to this embodiment, in an optical transmission system to which WDM is applied, wavelength stabilization can be performed without using synchronous detection. Further, since the optical filter array is used as a wavelength reference, the same effects as described above are produced.

  FIG. 19 is a block diagram showing a fourth preferred embodiment of the optical transmitter according to the present invention. This embodiment is different from the embodiment shown in FIG. 16 in that the optical filter array 180 has another control filter having a transmission center wavelength at wavelength λ−Δλ in addition to the control filter element 114 having a transmission center wavelength at wavelength λ + Δλ. In that it has a filter element 114 'for use.

  The output light of the transmission light source unit 68 is branched into three by an optical coupler 110 ', and each of the branched lights is supplied to a main filter element 112 and control filter elements 114 and 114'.

  Output light from the control filter element 114 is converted into an electric signal by a photodiode 116, and the electric signal is amplified by an amplifier 118 and input to an arithmetic circuit 134. The output light of the control filter element 114 'is converted into an electric signal by a photodiode 116', and the electric signal is amplified by an amplifier 118 'and input to an arithmetic circuit 134. Then, the output signal of the arithmetic circuit 134 is supplied to the control circuit 120.

  FIG. 20 shows the characteristics of the optical filter array of FIG. Reference numeral 130 indicates the characteristic of the main filter element 112, reference numeral 132 indicates the characteristic of the control filter element 114, and reference numeral 132 'indicates the characteristic of the control filter element 114'. The symbol SP indicates a stable point.

  The difference Δλ between the transmission center wavelengths of the control filter elements 114 and 114 ′ and the transmission center wavelength of the main filter element 112 is set to, for example, about half width at half maximum of each filter element. That is, when the light from the transmission light source unit 68 passes through the main filter element 112 at the maximum transmittance, the transmittance of the control filter elements 114 and 114 'is set to be about 50% of the above-described maximum transmittance. You.

  Now, when the wavelength of the transmission light source unit 68 slightly fluctuates to the longer wavelength side, the output of the control filter element 114 increases, while the output of the control filter element 114 'decreases. When the wavelength of the transmission light source unit 68 slightly fluctuates to the short wavelength side, the output of the control filter element 114 decreases, and the output of the control filter element 114 'increases.

  Thus, the output of the control filter element 114 and the output of the control filter element 114 'are in a complementary relationship. Therefore, in consideration of such symmetry, the output of the transmission light source unit 68 is reduced by performing subtraction on the outputs of the control filter elements 114 and 114 'and performing control so that the operation result becomes zero. The light will pass through the main filter element 112 at the maximum transmittance. Therefore, a circuit that performs subtraction on two inputs is selected as the arithmetic circuit 134.

  According to this embodiment, there is an advantage that even if the optical output of the transmission light source unit 68 fluctuates, the control wavelength is not affected. That is, even if the power of the light input to the two control filter elements 114 and 114 'fluctuates, the wavelength at which the output of the arithmetic circuit 134 becomes 0 as a result of the subtraction becomes constant. However, since the loop gain in the entire control system fluctuates, it is desirable to set the loop gain so as to withstand the fluctuation in the optical output.

  FIG. 21 is a block diagram showing a fifth preferred embodiment of the optical transmission system of the present invention. In this embodiment to which WDM is applied, a plurality of optical transmitters shown in FIG. 19 are used.

  The features of the present embodiment in comparison with the system of FIG. 18 are the same as those of the optical transmitter of FIG. 19 in comparison with the optical transmitter of FIG. 16, and a description thereof will be omitted.

  FIG. 22 is a block diagram illustrating a configuration example of an optical receiver that does not use synchronous detection. A signal light supplied from an optical transmission line (not shown) is amplified by an optical amplifier 136 and then branched into three by an optical coupler 110 '. Each of the split lights is input to each filter element 112, 114, 114 'of an optical filter array 180 similar to the optical transmitter of FIG.

  The output light of the main filter element 112 is supplied to an E / O conversion circuit 140, where it is converted into an electric signal containing transmission data.

  The output light of the control filter element 114 is converted into an electric signal by a photodiode 116, amplified by an amplifier 118, and input to an arithmetic circuit 134. The output light of the control filter element 114 'is converted into an electric signal by a photodiode 116', and the electric signal is amplified by an amplifier 118 'and input to an arithmetic circuit 134.

  Then, the control circuit 142 controls the Peltier element 98 based on the output signal of the arithmetic circuit 134, thereby stabilizing the wavelength.

  The control circuit 142 compares the output signal of the arithmetic circuit 134 with the reference signal from the setting reference 124, the error amplifier 126 that amplifies the output signal of the comparator 122, and the output level of the error amplifier 126. A drive circuit 138 for supplying a drive current to the Peltier element 98 so as to be 0 or constant.

  FIG. 23 is a block diagram showing a second embodiment of the optical receiving system. In this embodiment, the wavelength of the optical filter array 180 is stabilized according to the principle of the optical receiver of FIG. 22, and the stabilized optical filter array 180 is used to receive the WDM signal light. Specifically, it is as follows.

  The system of this embodiment is characterized in that the control circuit 142 does not use synchronous detection (see FIG. 22), as opposed to the control circuit 94 in FIG. 15 that uses synchronous detection. That is, a main filter element 112A having a transmission center wavelength at a wavelength λABS, a control filter element 114A having a transmission center wavelength at a wavelength λABS + Δλ, and a control filter element 114′A having a transmission center wavelength at a wavelength λABS−Δλ are provided. It is provided integrally with the filter elements 18 (# 1, # 2,..., #N), and the branched light output of the optical tree coupler 106 is supplied to each filter element.

  The output light of the main filter element 112A can be effectively used in this optical receiving system as an absolute wavelength reference. Further, the output light of the control filter elements 114A and 114'A can be used to stabilize the wavelength without performing synchronous detection.

  Some of the devices or systems that do not use synchronous detection described above have a feature that the configuration of the optical filter array is complicated, but the control signal of the light source or the optical filter array does not affect the reception characteristics. In the case of synchronous detection, a small fluctuation is given to the control target in order to obtain the control signal, and the phase of the signal obtained is compared with the phase of the signal obtained from the control target. Of the transmission signal, and the transmission signal may be degraded. On the other hand, when the synchronous detection is not used as in the embodiment of the present invention, there is no need to give a minute change, and there is no possibility that the transmission signal is deteriorated.

  Therefore, the embodiment not using such synchronous detection is suitable when each filter element of the optical filter array is a narrow band-pass filter.

  When constructing an optical communication system by combining the optical transmission system and the optical reception system described above, when an optical filter array is used in both the optical transmission system and the optical reception system, these optical filter arrays have characteristics. It is desirable that they match well. For that purpose, for example, when manufacturing the optical filter array shown in FIG. 12, by cutting the base material of the optical filter array created by the same manufacturing process, and applying each piece to the transmission side and the reception side , The characteristics of the filter array can be matched.

  As a light source for WDM, a semiconductor laser (laser diode), particularly a distributed feedback laser diode that oscillates in a single longitudinal mode, is usually used. In this laser diode, the oscillation wavelength is almost determined by the period of the grating formed inside the element. However, due to the problem of manufacturing deviation, the oscillation wavelength is usually about several nm in the same lot, and about tens of nm between different lots. Vary. For this reason, in order to procure a plurality of laser diodes compatible with WDM, a sorting operation is required, which is expensive.

  FIG. 24 is a diagram showing a basic configuration of a light source for WDM in which it is easy to obtain a desired oscillation wavelength. In this example, the optical filter array 180 includes a plurality of optical filter elements 144 (# 1, # 2,..., #N) having band rejection (notch) characteristics.

  An optical filter having a band rejection characteristic has a reflection characteristic of a bandpass type. In this example, each filter array 144 (# 1, # 2,..., #N) reflects light having a wavelength λ1, λ2,.

  A gain medium 146 (# 1, # 2,..., #N) is optically connected to an input port of each filter element 144 (# 1, # 2,..., #N). I have. As the gain medium 146 (# 1, # 2,..., #N), a semiconductor laser (semiconductor laser amplifier) having an AR coating (anti-reflection coating) or a rare-earth-doped optical fiber can be used.

  According to this configuration, laser oscillation can be caused at the reflection peak wavelength of each filter element, so that a light source having a desired wavelength can be easily obtained, and the wavelength interval and the absolute wavelength can be centrally managed. become.

  FIG. 25 is a block diagram showing a sixth embodiment of the optical transmission system of the present invention. This system is characterized by using the WDM light source of FIG. 24 instead of the transmission light source units 68 (# 1, # 2,..., #N) in comparison with the system of FIG. .

  The output light of each filter element 144 (# 1, # 2,..., #N) is modulated by an optical modulator 148 (# 1, # 2,. The light is transmitted to one or a plurality of optical transmission lines (not shown) via the optical path 70.

  According to this embodiment, since the wavelength of the optical filter array 180 is stabilized by the above-described method using the absolute wavelength reference light source 100, it is not necessary to manage the wavelength of each light source, and the optical transmission system is not required. The configuration can be simplified.

  FIG. 26 is a block diagram showing a seventh embodiment of the optical transmission system of the present invention. This embodiment uses filter elements 18 (# 1, # 2,..., #N) having bandpass characteristics instead of the filter elements having band rejection characteristics in comparison with the system of FIG. It is characterized in that laser oscillation is caused by configuring a ring resonator.

  Light output from the first excitation end of the gain medium 146 (# 1, # 2, ..., #n) passes through the filter array 18 (# 1, # 2, ..., #n). The light is branched by the optical coupler 150 (# 1, # 2,..., #N). One of the split lights passes through the optical isolators 152 (# 1, # 2,..., #N), and passes through each of the gain media 146 (# 1, # 2,. Is returned to the second excitation end.

  The other of the lights branched by the optical couplers 150 (# 1, # 2,..., #N) is modulated by the optical modulators 148 (# 1, # 2,..., #N). The light is transmitted to an optical transmission line (not shown) via the optical star coupler 70.

  In this embodiment, since all the filter elements of the optical filter array 180 are band-pass filters, the manufacture of the optical filter array 180 is easy.

  FIG. 27 is a block diagram showing an eighth embodiment of the optical transmission system of the present invention. This system has bandpass filter arrays 18 (# 1, # 2,..., #N) having transmission center wavelengths at wavelengths λ1, λ2,. are doing.

  The light from the coherent light source 154 that oscillates in a wide band (white) is branched into n by the optical star coupler 156, and each branched light is divided into each filter element 18 (# 1, # 2,..., #). n). The light output from the output port of each filter array is modulated by each of the optical modulators 148 (# 1, # 2,..., #N). It is sent to the transmission path.

  For example, Mori et al., “Ultra-wideband pulse light source using LD-excited supercontinuum”, and a light source that oscillates in a wide band are disclosed in 1993 Autumn Meeting B-920 of the Institute of Electronics, Information and Communication Engineers, 1993.

  By the way, in a transmission system using an optical amplifier, it is desirable to remove ASE components in a wavelength region other than the wavelength of signal light as much as possible. If the signal light is input to the optical amplifier with the extra ASE component added, the ASE component is also amplified, and the energy of the pump light for amplifying the signal light is wasted, and the amplification factor and noise characteristics are degraded. I do.

  In the case of WDM, as shown in FIG. 28, an ASE component can be reduced by using an optical filter that selectively transmits light having a wavelength corresponding to the wavelength of signal light. In the figure, reference numeral 158 represents the spectrum of signal light arranged at regular intervals on the wavelength axis, and reference numeral 160 represents the characteristic of an optical filter having transmission center wavelengths arranged at regular intervals corresponding to these signal lights. .

  When the wavelengths of the signal light are arranged at equal intervals, a Fabry-Perot interferometer can be used as the optical filter. When the wavelengths of the signal light are arranged at irregular intervals, a grating type filter can be used.

  When the optical filter is inserted in the middle of the transmission line or the like, it is assumed that the wavelength of the transmission light source is controlled. When the relative wavelength interval of the transmission light source is kept constant, it is desirable that the transmission characteristics of the optical filter follow the fluctuation of the absolute wavelength of the transmission light source.

  For this purpose, it is preferable to branch the filter output and perform control so that the optical output is maximized. Specifically, it is as follows.

  FIG. 29 is a diagram illustrating a configuration example for tracking control of an optical filter. Reference numeral 162 represents an optical amplifier inserted in the middle of the optical transmission line.

  An optical filter 164 including a Fabry-Perot interferometer or the like is arranged downstream of the optical amplifier 162. The output light of the optical filter 164 is split by the optical coupler 166, and one of the split lights is sent to the optical transmission line.

  The other of the split lights is converted into an electric signal by the photodiode 168, and the electric signal is amplified by the amplifier 170 and supplied to the control circuit 172. The control circuit 172 controls the optical filter 164 so that the supplied signal level becomes maximum.

  The system of this embodiment is desirably applied when the wavelength of the signal light is constant at intervals of λ1, λ2,..., Λn. That is, in this case, by controlling so that the total optical output of all the signal lights is always maximized, the transmission center wavelength of the optical filter can be matched with the wavelength of the signal light.

  When a dielectric film interference filter is used as the narrow band optical filter array, the cutoff characteristic of the transmission wavelength becomes almost a Lorentz function. FIG. 30 shows the cutoff characteristics of the interference filter.

  From this figure, assuming that the optical power is the same at each wavelength, when the wavelength interval is 1 nm (about 125 GHz in the 1.55 μm band), the crosstalk (leakage) of adjacent wavelengths is 20 dB or less (20 dB or less) as an example. In order to make the following value a required value that does not affect the reception quality), one interference filter having a transmission band of 3 dB and 0.3 nm is insufficient, and an interference filter having a transmission band of 3 dB and 0.5 nm is used. It should be understood that two or more sheets should be used.

  The required crosstalk value of the optical filter applies to the above-mentioned optical filter for wavelength stabilization on the transmission side, and also applies to the wavelength selection optical filter on the reception side.

  As described in Japanese Patent Application Laid-Open No. 5-281480, “Wavelength tunable filter and method of manufacturing the same”, an interference filter in which dielectric films are stacked in multiple layers can change the transmission wavelength by changing the thickness of the stacked dielectric films. Can be changed. Therefore, in order to obtain a predetermined transmission wavelength with good reproducibility, it is necessary to accurately control the thickness of the dielectric film.

  In order to obtain a narrow band pass filter having a 3 dB transmission wavelength range of about 1 nm or less, a large number of dielectric films, for example, 20 layers or more, must be formed. There are difficulties to get. For example, the transmission wavelength may be shifted by about several nm for each production lot.

  However, it is known that the in-plane distribution of the transmission wavelength on the same substrate is relatively stable. For this reason, as shown in FIG. 31, for example, in the case where the film thickness of the dielectric multilayer film 940 formed on the substrate 920 is continuously changed, the wavelength range of the transmission light source can be changed even in filters having different manufacturing lots. There is always a region that covers the desired transmission wavelength and covers it.

  Therefore, this portion is cut out from the base material of the interference filter shown in FIG. 31 to form an optical filter array as shown in FIG. 12, whereby an optical filter array having desired characteristics can be obtained with high yield.

  FIG. 32 is a diagram showing a first specific example of the optical filter array of FIG. In this example, optical fibers 84 (# 1, # 2,..., #N) serving as input ports are arranged at equal intervals in parallel with each other, and optical fibers 86 (# 1 , # 2,..., #N) are also arranged at equal intervals in parallel with each other.

  FIG. 33 is a diagram showing characteristics of the optical filter array of FIG. The vertical axis represents the transmission center wavelength, and the horizontal axis represents the number of channels of each port. In this example, since the optical fibers are arranged at equal intervals, the transmission center wavelength is equal at each channel.

  FIG. 34 is a diagram showing a second specific example of the optical filter array of FIG. This embodiment is different from the first embodiment shown in FIG. 32 in that the input side optical fiber 84 (# 1, # 2,..., #N) and the output side optical fiber 86 (# 1, # 2). , #N) are not arranged at equal intervals.

  By doing so, the transmission center wavelengths of the optical filter array can be made unequally spaced, so that the effect of four-wave mixing can be avoided when performing WDM (F. Forghieri et al, "Reduction of four- wave-mixing cross talk in WDM systems using unequally spaced channels ", OFC / IOOC '93 Technical Digest FC4).

  Alternatively, an equal-length optical filter array is created at a wavelength interval sufficiently smaller than the wavelength interval of the signal light in the WDM, and only the input / output ports corresponding to the desired transmission center wavelength are selected and used to obtain the desired equal-interval or Irregularly spaced optical filter arrays can also be obtained.

  If the wavelength reproducibility is good due to the manufacturing technology of the interference filter, the optical fibers of the input and output ports are set at equal intervals as shown in FIG. 35, and the film thickness distribution of the dielectric multilayer film 94 'is changed. By changing, it is also possible to give a desired transmission center wavelength for each filter element. FIG. 36 shows the characteristics of the optical filter array of FIG.

  FIG. 37 is a diagram showing a fourth embodiment of the optical filter array. In this embodiment, a plurality of optical waveguides 178 (# 1, # 2,..., #N) are provided on the waveguide substrate 176, and the grooves 176A are formed on the waveguide substrate 176 so as to block these optical waveguides. Are formed, and an interference filter 182 having a transmission center wavelength that differs depending on the position is inserted into the groove 176A.

  Reference numeral 184 (# 1, # 2,..., #N) denotes an optical fiber connected to the input port of each optical waveguide, and reference numeral 186 (# 1, # 2,. 4 illustrates an output side optical fiber connected to an output port of an optical waveguide. A branch portion is formed on the optical waveguide 178 (# 1, # 2,..., #N) on the downstream side of the interference filter 182, and a photodiode array 184 is provided at the output end of each branch waveguide. Connected.

  FIG. 38 shows a fifth embodiment of the optical filter array. In this embodiment, a plurality of optical waveguides 190 (# 1, # 2,..., #N) are provided side by side on a waveguide substrate 188, and a region 192 where a grating type filter is formed and an optical star A coupler region 194 is provided.

  If the transmission center wavelength cannot be set accurately for the grating filter, the number of optical waveguides must be equal to or greater than that required for WDM, and the grating constant of the grating section of each optical waveguide should be slightly different. By selectively using only the optical waveguides that match the wavelength of the optical filter array, the yield of the optical filter array can be improved.

  In this embodiment, since the optical star coupler region 194 is formed, the insertion loss increases when there is an optical waveguide that is not used. However, this problem is solved by performing the loss compensation using the optical amplifier. Can be solved.

  In the above description, the optical filter array is arranged one-dimensionally for simplicity, but it is naturally possible to adopt a two-dimensional arrangement. For example, when an absorption line of a molecule or an atom is used as an absolute wavelength reference, a halfway wavelength that does not match the wavelength arrangement of the signal light (for example, a case where the wavelength comes between signal light wavelength intervals) is expected. . In such a case, a desired wavelength reference can be used by spatially separating a position for transmitting light from the absolute wavelength reference to the optical filter array from a position for transmitting each signal light. Specifically, it is as follows.

  FIG. 39 is a diagram showing a sixth embodiment of the optical filter array. In this example, optical fibers 84 (# 1, # 2,..., #N) serving as signal light input ports are arranged in a line, and are shifted from these arrangement directions to serve as absolute wavelength reference input ports. An optical fiber 84 'is arranged. The symbol P (# 1, # 2,..., #N) indicates the position where the signal light beam enters the optical filter array, and the symbol P 'indicates that the light beam from the absolute wavelength reference is applied to the optical filter array. It represents the position of incidence.

  When creating an optical filter array using a dielectric film, the diameter of a light beam passing through the optical filter array is an important parameter. If the diameter of the beam passing through the optical filter array is larger than the distance between the optical fiber arrays, it causes crosstalk with other signals. Also, if the beam diameter is large compared to the change in the transmission wavelength of the optical filter having the transmission wavelength distribution, the 3 dB transmission wavelength range will be widened.

  For example, as shown in FIG. 32, in an optical fiber array in which the transmission center wavelength changes according to the position, as a specific example, the dielectric films are laminated so that the transmission center wavelength changes by 10 nm when the position changes by 10 mm. In such a case, if the transmission center wavelength interval is 1 nm and a filter array is formed at equal intervals, it is required to align the optical fibers at 1 mm intervals.

  When designing this optical filter, the 3 dB transmission width when the film thickness distribution is constant is set to 0.05. In order to prevent this characteristic from deteriorating, it is required that the light beam diameter be sufficiently smaller than 0.5 mm corresponding to 0.5 nm. For example, a light beam diameter of about 50 μm or less is required.

  If the beam diameter is large, the transmission center wavelength changes in the beam, which is wider than the 3 dB transmission width of the optical filter main body, and the transmission characteristics and blocking characteristics are deteriorated.

  In order to solve this problem, it is desirable that the focal length of a lens provided near the excitation end of the optical fiber be made sufficiently small to narrow the beam diameter. When the beam diameter is 50 μm and the interval between the fiber arrays is 1 mm, the ratio between the beam diameter and the interval between the optical fiber arrays is sufficiently large, and there is almost no influence of crosstalk on other channels.

  An embodiment for solving such a technical problem will be described with reference to FIG. FIG. 40 is a diagram showing a seventh embodiment of the optical filter array. FIG. 40A is a plan view of the optical filter array, and FIG. 40B is a side view.

  A plurality of V-shaped grooves are formed on the substrate 196 in parallel with each other, and the input side optical fibers 198 (# 1, # 2,... The output side optical fibers 200 (# 1, # 2,..., #N) whose mode field diameters are enlarged at the ends are also arranged opposite to the end faces. An optical filter 202 is inserted between these fibers. Reference numeral 204 denotes a Peltier element for controlling the temperature of the substrate 196. Here, the reason for using an optical fiber having an enlarged mode field diameter at the end is to reduce the insertion loss of the optical filter 202 having a finite thickness. For the manufacturing technology of such fiber, see M. Kihara et al. "Loss Characteristics of Thermally Diffused Expanded Core Fiber", IEEE Photonics Technology Letters Vol4 No.12 pp1390-1391, Chuzenji et al. Isolators, "1992 IEICE Autumn Conference C-229. Hereinafter, such a fiber is referred to as a TEC fiber.

  FIG. 41 shows the relationship between the coupling loss and the distance between the end faces when a pair of TEC fibers are arranged to face each other and an optical medium having a refractive index (n) of 1.5 is filled between the end faces, and the mode field diameter (MFD) is shown. It is a graph shown as a parameter. It can be seen that when the distance between the end faces is 1000 μm and the mode field is expanded to 40 μm, the insertion loss is suppressed to about 0.5 dD.

  FIG. 42 is a graph showing the relationship between the mode field diameter after propagation and the mode field diameter at the end of the fiber. When the mode field diameter at the end of the fiber is enlarged to 40 μm, the beam diameter is about 50 μm even when the beam propagates at 1000 μm, and it can be seen that the condition satisfies the condition not to deteriorate the transmission width described above.

  The present invention includes the following supplementary notes.

(Supplementary Note 1) A space division type optical filter array having a plurality of bandpass filter elements having different passband center wavelengths,
Multiple input ports,
A plurality of output ports provided to face each of the input ports;
A transparent substrate provided so as to intersect with a plurality of optical paths formed between the respective input ports and the respective output ports;
An optical filter array comprising: an interference film laminated on the transparent substrate and having a thickness that continuously changes in the arrangement direction of the optical paths.

FIG. 2 is a block diagram of a WDM transmission system. FIG. 2 is a block diagram illustrating a basic configuration of an optical transmitter. FIG. 2 is a block diagram illustrating a first embodiment of the optical transmitter. FIG. 3 is a diagram for explaining the principle of synchronous detection. FIG. 11 is a block diagram illustrating a conventional example of an optical transmitter. FIG. 6 is a block diagram illustrating a second embodiment of the optical transmitter. It is a timing chart of time division control. FIG. 2 is a block diagram illustrating a first embodiment of the optical transmission system. It is a figure showing a space division type optical filter array. FIG. 2 is a perspective view showing a first embodiment of the optical filter array. It is a perspective view showing a 2nd example of an optical filter array. FIG. 11 is a plan view showing a third embodiment of the optical filter array. FIG. 9 is a block diagram illustrating a second embodiment of the optical transmission system. FIG. 13 is a block diagram illustrating a third embodiment of the optical transmission system. FIG. 2 is a block diagram illustrating a first embodiment of the optical receiving system. FIG. 13 is a block diagram illustrating a third embodiment of the optical transmitter. FIG. 17 is a diagram illustrating characteristics of the filter array of FIG. 16. FIG. 14 is a block diagram illustrating a fourth embodiment of the optical transmission system. FIG. 14 is a block diagram illustrating a fourth embodiment of the optical transmitter. FIG. 20 is a diagram illustrating characteristics of the filter array of FIG. 19. FIG. 13 is a block diagram illustrating a fifth embodiment of the optical transmission system. It is a block diagram showing an example of an optical receiver. FIG. 9 is a block diagram illustrating a second embodiment of the optical receiving system. FIG. 2 is a diagram illustrating a basic configuration of a light source for WDM. FIG. 16 is a block diagram illustrating a sixth embodiment of the optical transmission system. FIG. 16 is a block diagram illustrating a seventh embodiment of the optical transmission system. FIG. 16 is a block diagram illustrating an eighth embodiment of the optical transmission system. FIG. 3 is a diagram illustrating an example of characteristics of an optical filter in WDM. FIG. 3 is a diagram illustrating a configuration example of tracking control of an optical filter. It is a figure showing the cutoff characteristic of an interference filter. It is a figure showing an example of an interference filter. FIG. 13 is a diagram illustrating a first specific example of the optical filter array in FIG. 12. FIG. 33 is a diagram illustrating characteristics of the optical filter array of FIG. 32. FIG. 13 is a diagram illustrating a second specific example of the optical filter array in FIG. 12. FIG. 13 is a diagram illustrating a third specific example of the optical filter array in FIG. 12. FIG. 36 is a diagram illustrating characteristics of the optical filter array in FIG. 35. It is a perspective view showing a 4th example of an optical filter array. It is a perspective view showing a 5th example of an optical filter array. It is a perspective view showing a 6th example of an optical filter array. It is the top view (A) and side view (B) which show the 7th Example of an optical filter array. 4 is a graph illustrating a relationship between a coupling loss of a TEC fiber and a distance between end faces. 9 is a graph showing the relationship between the MFD after propagation and the MFD at the end of the fiber.

Explanation of reference numerals

Reference Signs List 16 transmission light source 18 bandpass filter 20 optical coupler 22 light receiver 24 amplifier circuit 26 control circuit

Claims (5)

A plurality of optical transmitters for outputting signal light,
An optical multiplexer for adding the signal light from each of the optical transmitters and transmitting the signal light to one or more optical transmission paths,
Each of the above optical transmitters,
A light source,
A band-pass filter for transmitting light from the light source;
Light splitting means for splitting the light passing through the bandpass filter into at least two split lights;
A light receiver for converting one of the split lights into an electric signal having a level corresponding to the intensity thereof;
Control means for receiving the electric signal and controlling the wavelength of the light source so that the intensity of the split light is constant,
An optical transmission system for wavelength division multiplexing transmission, wherein another one of the split lights is transmitted as the signal light.   The optical transmission system according to claim 1, wherein each of the bandpass filters is a filter element of a space division type optical filter array having a plurality of input ports and a number of output ports corresponding to the number of the input ports. The optical filter array includes a waveguide substrate having a plurality of waveguide type diffraction gratings having different lattice constants,
3. The optical transmission system according to claim 2, wherein both ends of each of the waveguide diffraction gratings form the input port and the output port, respectively. The optical filter array includes a substrate having a plurality of V-grooves, and a plurality of fiber grating optical filters having different lattice constants seated in the respective V-grooves,
3. The optical transmission system according to claim 2, wherein both ends of each of said fiber grating type optical filters form said input port and output port, respectively. Each of the input ports includes a plurality of input side optical fibers arranged in parallel with each other,
Each of the output ports comprises a plurality of output-side optical fibers arranged in parallel with each other to face each of the input-side optical fibers,
A plurality of optical paths are formed between each of the input side optical fibers and each of the output side optical fibers,
The optical filter array includes a transparent substrate provided so as to intersect with each of the optical paths, and an interference film laminated on the transparent substrate and having a thickness that continuously changes in the arrangement direction of the optical paths. The optical transmission system according to claim 2.

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