GB2224612A - Synchronous multichannel grating demultiplexer receiver - Google Patents

Abstract

A synchronous multichannel demultiplexer receiver for synchronous wavelength multiplexed optical signals includes an optical wavelength demultiplexer, means for separately regenerating (19) the multiple channel signal outputs of the multiplexer, means for extracting clock timing signals (18) from one of said channels outputs and means for applying said extracted clock timing signals to all said regenerating means whereby said multiple channel signals are regenerated synchronously. <IMAGE>

Description

SYNCHRONOUS MULTICHANNEL GRATING DEMULTIPLEXER RECEIVER.

This invention relates to a synchronous multichannel demultiplexer receiver for use in broadband optical networks and distributed switching systems.

There have recently been several proposals for high capacity distributed switching systems that make use of the very large bandwidth and high connectivity of passive optical star networks. See, for example, A.

Oliphant "Progress in the development of a digital optical routing system for television studio centres", International Broadcasting Conference IBC88, Brighton, Sept. 1988, IEE Conference Publication Ni. 293 pp 90-94, also D.P. Payne and J.R. Stern "Single mode optical local networks", in Conf. Proc. Globecom '85 1985, Houston, paper 39.5.

These systems use wavelength division multiplexing to broadcast many independent channels across the whole network. They offer capacities which are orders of magnitude larger than electronic (time multiplexed) networks, complete flexibility of interconnection configuration, service transparency and almost limitless capacity for future upgrades. These advantages are attractive for distributed switching of video signals, broadband overlay for fibre optic telecommunication to domestic subscribers, high capacity inter-exchange communication, high capacity switching networks for multiprocessor computers and ultrahigh-speed telecommunications packet switching.

Ultimately, multichannel coherent transmitters and receivers will allow thousands of independent gigHertz bandwidth channels to be distributed and switched between thousands of nodes. Each independent channel uses a separate wavelength and is broadcast to all the nodes on the passive network. Switching is achieved by using either fixed wavelength transmitters and tunable receivers or vice versa. In either case, the major problem limiting the number of channels in such system is the tolerance of the control of transmitter and receiver wavelengths with respect to the defined channels. This is illustrated in Figure 1, showing how the tolerance on transmitter wavelength sets the minimum channel width and the shaper and tolerance of the receiver spectrum defines minimum channel spacing.For a large number of channels the transmitter and receiver components must both have very tight tolerance on operating wavelength and must operate over the widest possible wavelength range. Consideration of the development timescales of coherent system components and likely system requirements in the 1990's suggests that a simpler system based on direct detention with up tp about 50 separate channels and a few hundred nodes would find widespread application.

A simple optical wavelength demultiplexing arrangement using integrated optics has already been described by W.S. Lee, S.W. Bland and A.J. Robertson, Monolithic GaInAs/InP photodetector arrays for high density wavelength division multiplexing (HD-WDM) applications, "Electron. Lett., 1st Sept., 1988, Vol 24 No. 18 pp 1143-1145, and is illustrated in Figure 2.

Wavelength multiplexed signals from a source, e.g. an optical fibre, are directed onto a grating reflector and the reflected signals are focussed by a lens onto a photodetector array. This arrangement is capable of receiving all the wavelength multiplexed channels simultaneously.

The applicants' co-pending application Neo.9826 .. discloses a synchronous multi wavelength optical network in which signals from a plurality of nodes are time to arrived synchronously at a central star coupler from whence all the signals are broadcast simultaneously to all the nodes. Consequently at each node all the wavelength multiplexed channels are received synchronously and each node therefore requires a synchronous demultiplexing receiver.

According to the present invention there is provided a synchronous multichannel demultiplexer receiver for synchronous wavelength multiplexed optical signals, the receiver including an optical wavelength demultiplexer, means for separately regenerating the multiple channel signal outputs of the multiplexer, means for extracting clock timing signals from one of said channel outputs and means for applying said extracted clock timing signals to all said regenerating means whereby said multiple channel signals are regenerated synchronously.

In one embodiment of the intention the receiver includes a space switching matrix for selection of demultiplexed channels, whereby one of the selected channels is selected for connection to said means for extracting clock timing signals.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the importance of commonly defined channels for multi channel wavelength multiplexed optical networks (already referred to above), Figure 2 illustrates schematically a multi channel demultiplexer, Figure 3 illustrates schematically an optoelectronic integrated circuit for a synchronous multichannel demultiplexer receiver, Figure 4 illustrates schematically a monolithic InP/InGaAs optoelectronic integrated circuit arrangement for a multichannel demultiplexer receiver, and Figure 5 illustrates a layout of a multichannel demultiplexer receiver module.

The basic concept of the multichannel demultiplexer is illustrated in Figure 2. A single mode optical fibre 10 carries the multiple wavelength optical input signal. A wavelength - space demultiplexer formed by a diffraction grating 11 and lens 12 directs each separate channel wavelength to a separate detector 13.

The detectors 13 are in a linear monolithic array 14 in the focal plane of the lens so the detectors and the width of each detector accurately defines the centre wavelength, channel spacing and channel width respectively of the receiver.

In order to select and regenerate the signals transmitted on each wavelength channel the receiver makes use of conventional electronic circuitry. A functional diagram of the electronic circuitry is shown on Figure 3. The array 14 of photodetectors 13 is followed by an array of electronic preamplifiers 15, a space switching matrix 16 for selection of input channels and finally a set of receiver circuits 17 which includes clock extraction decision circuitry 18 and retiming 19 to regenerate the required signals to standard digital form. It should be noted that unlike most other tunable receivers (including coherent receivers) the multichannel demultiplexer receiver is capable of receiving simultaneous signals on any or all of the input channels. With appropriate design it can, for instance, select any 4 out of 50 input channels for TV channel distribution to the domestic subscriber.

Since, for most applications, the receiver will be operating in the 1200-1600 nm transmission band of silica fibre, the detector array will in general use the InP/InGaAsP materials system. The more complex electronic processing (such as clock extraction and regeneration) is most likely to be silicon based. In order to obtain maximum receiver sensitivity a separate high gain electronic preamplifier 20 will be used with each detector. In receivers where it is necessary only to regenerate one or a few of the input channels simultaneously, it is most efficient to use an electronic analogue space switching matrix 16 after the preamplifiers 20 but before the complex clock extraction decision and retiming circuitry. This minimises the overall receiver complexity.

Extrapolation of the recent rapid rate of progress in the monolithic integration of high performance III-V transistors with GaInAs detectors suggests the best overall receiver performance will be obtained by the use of a small scale monolithic optoelectronic integrated circuit (OEIC) to carry out the preamplification and space switching functions prior to silicon regeneration circuitry. Figure 3 shows this.

The use of the OEIC allows the detectors 13 to be placed very close together (perhaps at only 20 um to 40 um centre to centre spacing) without incurring the space, capacitance and crosstalk penalties of wire bond pads and bond wires. The integration of each low capacitance detector 13 with its own front-end preamplifier 20 will maximise receiver sensitivity and provide a low impedence drive to the space switch 16. The likely evolution of such OEICs from today's 8-16 channel detector arrays to 50 channel wavelength switched receiver chips is shown in Figure 4. The physical dimensions of the 50 channel OEIC and its grating demultiplexer are remarkably small. Operating close to the diffraction limit each detector 13 could for instance have a width of 10 um on 40 um centre-centre spacing. The detector array would be 2 mm long.For a channel wavelength spacing of 2 nm a lens of 6 mm diameter and 30 mm focal length is required together with a blazed grating of 600 lines/mm. These small dimensions suggest that it will be practical to incorporate all the components of the receiver into a module not much larger than current single channel hybrid transmitter and receiver modules. A possible layout of such a module using prisms 21, 22 to fold the optics more conveniently is shown in Figure 5. The OEIC 23 is coupled to a single mode fibre input 24. The incoming optical signals are directed by prism 21 mounted beside the OEIC up to the prism 22 mounted above the OEIC and thence through lens 25 to an inclined optical reflecting diffraction grating 26. The reflected optical signals are reflected back through the lens 25 and via prism 22 are deflected down onto the array of photodetector incorporated in the InP OEIC 23. The demultiplexed signals, now in the electrical domain, are then fed to a second silicon integrated circuit 27 incorporating all the complex signal processing corresponding to the preamplifier 15, the matrix 16, receiver circuitry 17, clock extraction circuitry 18 and retiming and regeneration circuits 19. The complete module is'mounted in a package 28 with pin-connections 29.

Claims (5)

CLAIMS.

1. A synchronous multichannel demultiplexer receiver for synchronous wavelength multiplexed optical signals, the receiver including an optical wavelength demultiplexer, means for separately regenerating the multiple channel signal outputs of the multiplexer, means for extracting clock timing signals from one of said channel outputs and means for applying said extracted clock timing signals to all said regenerating means whereby said multiple channel signals are regenerated synchronously.

2. A receiver according to claim 1 including a demultiplexed channels, whereby one of the selected channels is selected for connection to said means for extracting clock timing signals.

3. A receiver according to claim 1 or 2 wherein said demultiplexer comprises a diffraction grating, an array of photodetectors and lens means for focussing optical signals onto the grating and the diffracted optical signals onto the array.

4. A receiver according to claim 1, 2 or 3 including a separate high gain preamplifier for each channel whereby the signal outputs of the array are each separately amplified.

5. A synchronous multichannel demultiplexer receiver substantially as described with reference to the accompanying drawings.