GB2245116A - Telecommunications reflective optical links - Google Patents

Abstract

A telecommunication system having a main unit having a light transmitting device and means for modulating information on to the transmitted light at a first frequency, an optical link coupling the main unit to at least one subscriber unit, the subscriber unit including a quantum well reflective modulator (QWRM) for demodulating the input light signal in order to retrieve the transmitted information, and for reflecting at least a portion of the light received at the subscriber unit and at the same time modulating information on to the reflected light at a second frequency. <IMAGE>

Description

TELECGUNICATIONS REFLECTIVE OPTICAL LINKS Field of the Invention This invention relates to telecormunications systems employing fibre optics wherein a remote (subscriber) unit signals to a main unit by reflecting optical power back to the main unit.

Background Art Optical links based on the utilisation of a remote unit which signals bck to the main unit by redirecting optical power back to the main unit suitably modulated by the remote unit are known: See International Application W087/02531. Cost-effective and practical solution are required for point-to-multipoint and point-to-point communication networks, particularly in subscriber loops and customer premises networks (CPN's).

In regard to subscriber loops, there is currently considerable interest in realising cost effective architectures for the introduction of fibre into the subscriber loop, as, once installed, fibre can provide a 'future proof' communications path for narrowband and broadband interactive services as well as broadband distributive services (such as cable TV - CATV).

The major problems to be overcome are related to the 'up front' cost of providing the fibre infrastructure which may, for current regulatory requirements, initially be only able to carry telephony services. Architectures based on 'passive optical networks' (e.g. the BT 'TPON' configuration) have attracted considerable attention as these provide cost reduction through head end (main unit) and fibre infrastructure sharing among a number of subscribers. To further reduce costs 'fibre to the curb' architectures have been widely proposed, again using passive optical networks. These stop short of providing fibre direct to the subscriber, by housing the remote unit in the general proximity of the subscribers (in a street cabinet, footway box, pedestal etc.) and providing copper drop cables for telephony and possibly co-ax for CATV.The remote unit is shared between a number of subscribers, saving costs, particularly on expensive optoelectronic devices.

A secondary, although ultimately very important problem with the current architectures described above is the provision of electrical power to the remote unit and provide this for several hours following a power failure to meet emergency requirements.

Current architectures are relatively power hungry, demanding large batteries or substantial power feeding cables.

It is also necessary to provide a moderate degree of supervisory services over the link, to detect faults, phone off hook, loss of optical link etc. The supervisory signalling should make minimal demands on power or circuits in the remote unit.

Summary of the Invention It is an object of the present invention to provide a telecommunications system employing optical links wherein the subscriber (remote) units are simple low cost devices and wherein a large degree of multiplexing is permitted within the optical links.

The present invention is based on the use of a quantum well reflective modulator. Such devices are described for example in Electronic Letters, 1989, 25, pages 984-985, Whitehead et al. Such devices have the ability to reflect optical light and to superimpose modulation on to the reflected light The present invention is based on the concept of a telecommunication system having an optical link with a main unit having a light transmitting device and means for modulating information on to the transmitted light at a first frequency, an optical link coupling the main unit to at least one subscriber unit, the subscriber unit including a quantum well reflective modulator for demodulating the input light signal in order to retrieve the transmitted information, and for reflecting at least a portion of the light received at the subscriber unit and at the same time modulating information on to the reflected light at a second frequency.

In a preferred form, the main unit has a frequency multiplexing means of modulation wherein a number of messages may be simultaneously modulated on to the transmitted light by subcarrier multiplexing. The subscriber units coupled to the main unit have frequency selective filters so as to respond only to one wavelength.

In addition each subscriber unit will reflect light back with modulations within a second band of frequencies. The main unit will have frequency selective means for demodulating the various received messages at different wavelenqths.

A principal advantage of the arrangement is that each subscriber unit may be very simple and inexpensive since in a preferred form it employs a quantum well reflective modulator which only requires a very small power supply. A further advantage resides in the use of frequency multiplexing which permits simultaneous upstream and downstream signalling i.e. full duplexing and furthermore allows communication of a multiplicity of units over the same optical link.

In addition, supervisory services may be provided for detection of faults, phone off-hook, thus leaving optical link etc.

which supervisory signalling can be made at a frequency different from the traffic frequencies.

A further important advantage of the invention is that CATV signals can be multiplexed on to the same optical link and may be detected in the same receiving arrangement.

Brief Description of the Drawings A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a proposed configuration of the invention; Figure 2 is a modification of the reflecting and modulating arrangement within a subscriber unit; Figure 3 is a schematic view of a means of multiplexing a CATV signal to a telecoms system In accordance with the invention; Figure 4 is a schematic view of a means of mounting a QWRM for temperature stabilisation; Figure 5 is a schematic representation of a switch unit for the optical network; and Figure 6 comprises schematic representations of various switch coupling units for the optical network.

Description of the preferred embodiment Figure 1 shows schematically the proposed configuration of the invention. The operation is as follows. A number of subscriber remote units are connected via a passive optical network (PON) to a shared main unit. Main unit to subscriber signalling is achieved by modulating a light source in the main unit with a signal for each subscriber, the channels being identified by the use of subcarrier multiplexing (SCM). For a single ISDN circuit per subscriber (144kb/s)1MHz FM bandwidth per subscriber is required, giving 32'Hz bandwidth requirement for a 32 way PON. The subscriber receives the signal on a quantum well reflective modulator (CS)/detector device, and demodulates his downstream channel using conventional SCM techniques.

The upstream signalling is achieved by the subscriber applying a modulation voltage to the Qh'RM using SC;4 techniques to distinguish his upstream signal from all of the SCM downstream signals and from upstream signals from other subscribers. A key feature of the invention is that the upstream and downstream signalling is simultaneous, avoiding the ranging problems inherent in burst mode or bit interleaved architectures.

The main unit receives a composit optical signal from all active subscribers, and demodualtes these into the individual subscriber signals using SCM techniques. Analogue or digital signalling can be used for both upstream and downstream directions.

A complete bidirectional point-multipoint signalling architecture may thus be realised with a very simple remote unit.

The potential production cost of the QWRM is very low (comparable to a photodiode), with low tolerance coupling to single mode optical fibre as the QWRM functions as a mirror. Power consumption in the remote unit should be minimal as a semiconductor laser is not used, and circuitry is simple and relatively low speed.

Additional features which may be included to enable/improve operation of the remote unit include: (1) Reduction of Crosstalk/Intermodulation.

The remote unit relies on simultaneous reception and reflection/remodulation of optical signals. This may cause saturation and the generation of intermodulation products in the received signal. This may be minimised by: appropriate choice and separation of the up and down stream SCM frequency bands; use of a dummy matched QWRM differntially driven to reduce the common mode interfering signal at the input node of the receive amplifier; use of a tuned circuit to filter out the upstream SCM frequency band in the input of the receive amplifier (Figure 2).

(2) Wavelength Matching of the Qh'RM and the Downstream Lasser.

The current art of the QWRM device indicates that operation is efficient only over a few 10's of nm of optical bandwidth, implying that temperature stabilisation of the Qht may be necessary.

This could be simply achieved by mounting the QWRM with a small heater on a thermally insulating block and stabilising the device temperature above the maximum design ambient for the remote unit.

Power consumption cooling will not then be required (Figure 4).

(3) Provision of Supervisory Circuits.

The main unit has automatic measurement of the link loss by analysis of the return signal level, which is clearly identified as coming from a specific remote unit by the SCM carrier frequency. An additional low frequency, low bandwidth tone/data stream, again identified as coming from a specific subscriber by electrical frequency allocation could be added to the modulator independent of the other upstream signalling. As the bandwidth could be very low for this channel minimal optical power (and hence modulation depth) would be requird, giving a supervisory return circuit for only a few mW of power in the remote unit.

(4) Provision of CATV signals. (Figure 3) The QWRM device can be designed to be an efficient modulator at one wavelength and an efficient detector only at a slightly shorter wavelength. This is achieved by specific design of the mirror stack/quantum well resonator. A downstream CATV signal could thus be efficiently detected in the same receive diode, and amplified either through a comon input amplifier or an additional input amplifier with tuned circuits on each input to minimise noise figures in the interactive and distributive SCM bands. FM-SCM techniques would preferably be used for th CATV signals, with input into an unused port on the PON as proposed for the TPON/BPON architecture. Additional CATV or broadband interactive services could be added by provision of a wavelength division multiplex overlay through the passive optical network, with the WDM tap off prior to the QWRM.

(5) Improvements in Optical Budget.

The cost effectiveness of the proposed configuration is dependent on maximising the number of subscribers which can be fed off a common passive optical network. This depends on the available optical budget in the system. Our initial calculations indicate that between 32 and 64 subscribrs could be accommodated with an initial launch power of l W. The budget could be further improved by designing the passive optical splitters to feed into a multimode fibre for the final link back to the main unit. This would enable a substantial part of the splitting loss in the upstream direction to be eliminated. A device to do this could be realised by fabricating the splitter in a planar waveguide process, coupling the 'unused' single mode ports on the main unit side of the splitter into a common multimode slab guide which could then be efficiently coupled into a multimod optical fibre.

Advantages of the invention thus include: - Combination of use of reflective modulator with Passive Optical Network and Subcarrier Multiplexing.

- Use of QWRM in mode which allows simultaneous upstream and downstream signalling.

- Use of supervisory overlay with QWRM to monitor link and remote unit status.

- Provision of CATV overlay using the same QWRM, with the CATV signal being carried at a shorter optical wavelength.

- Use of a multimode fibre in the upstream direction from the PON splitter, which is designed with a matching output guide, to improve optical budget.

Claims (6)

1. A telecommunication system having a main unit having a light transmitting device and means for modulating information on to the transmitted light at a first frequency, an optical link coupling the main unit to at least one subscriber unit, the subscribr unit including a quantum well reflective modulator (QWRM) for demodulating the input light signal in order to retrieve the transmitted information, and for reflecting at least a portion of the light received at the subscriber unit and at the same time modulating information on to the reflected light at a second frequency.

2. A system as claimed ir Claim 1 including a pair of quantum well reflective modulators arranged to receive a differential modulating signal and connected in common to output means for a demodulated signal for reducing near end electrical intermodulation.

3. A system as claimed in Claim 1 including means for stabilising the temperature of the QWRM.

4. A system as claimed in Claim 1 arranged as a frequency multiplex system including means for modulating a multiplicity of sub-carrier frequencies onto the transmitted light, each frequency carrying information, and/or supervisory information at a fre iency lower than said first frequency and/oP a CATV signal at a frequency lower than said first frequency.

5. A system as claimed in Claim 1 including a passive optical network including a switch element for multiplexing a plurality of input signals on respective monomode fibres onto a single multimode fibre.

6. Telecommunications systems substantially as described with reference to the accompanying drawings.