COHERENT OPTICAL RECEIVERS

Abstract: In an optical coherent multiport receiver the in-phase and quadrature signal components are provided by employing a phase splitter to divide the optical signal powers of the input and local oscillator signals evenly between the in-phase and the quadrature detectors.

Description: COHERENT OPTICAL RECEIVERS This invention relates to optical receivers and in particular coherent optical receivers. Experience to date has shown that optical heterodyne detection can be readily achieved at low and moderate bit rates (currently less than a few hundre Mbit/s) but is increasingly difficult to realise at Gbit/s data rates where coherent transmission has most to offer in terms of improved receiver sensitivity over direct detection. The major reason for this difficulty is the large receiver bandwidth required to contain both the upper and lower sidebands of the modulation signal. The need for IF (intermediate frequency) frequencies several times the data rate is further reinforced when non-synchronous methods of IF demodulation, such as simple envelope detection are used. (Such schemes are attractive for systems where the lasers may have poor noise characteristics). When lasers with exceptionally low phase noise such as, for example, HeNe lasers or external cavity semiconductor lasers are used it is possible to overcome the requirement for a large receiver bandwidth by using homodyne detection. This is a scheme where the local oscillator is locked in phase, frequency and polarisation to the optical input signal. In this scheme the electrical output from the receiver is demodulated directly to base band. It is envisaged that homodyne detection will enable demodulation of Gbit/s signal using similar electronic circuits to those currently used in direct detection receivers. However, the practical implementation of homodyne detection poses some formidable problems. In particular, the need for optically locking the local oscillator signal in phase, frequency and polarisation to the received input signal has so far presented great difficulties. The implementation is made difficult also because it requires: a) Low phase noise lasers, and b) Either a well balanced two detector receiver with low dc drift, or the use of more complex Costas loop techniques. To bridge the gap between the chracteristics of heterodyne and homodyne detection, multiport detection has been proposed as a possible alternative; c.f, for example, N.G. Walker, J.E. Carroll, "Multiport optical detectors" IEE collquim "Advances in coherent optical devices and Technologies", 26 March 1985, London; and A.W. Davies, S. Wright "A phase insensitive homodyne receiver", ibid. In multiport detection the input signal is divided into two or more paths, and each path is combined with a separate local oscillator signal. The local oscillator signals are arranged to have a known fixed phase relationship to each other. However, the phase difference between the local oscillator signals and the input signal is not fixed. In practice the local oscillator laser will generally be offset frequency locked to the input signal. This maintains an average IF frequency which can be a fraction of the modulation bandwidth. The ideal multiport detection scheme would employ two local oscillators with a quadrature phase relationship between them. This arrangement should give the same receiver sensitivity as an equivalent single detector heterodyne receiver, i.e. 3dB worse than perfect homodyne detection. Operating a multiport detection receiver with more than two detectors produces further reduction in performance and considerably complicates the electronic circuits needed to process the electrical signals from the photodetectors. However, generation of the correct phase relationship required for implementation with only two detectors, between the two local oscillator signals of 90° for in-phase and quadrature detection, has been found to present significant problems. To avoid these problems it has been proposed by A.W. Davies (op/cit) to employ a three-phase local oscillator and use the special properties of a six-port fused optical fibre coupler to implement the system. The present invention aims to provide an improved multiport receiver capable of operating stably with two detectors. According to the present invention, optical multiport coherent detection is achieved by employing relative polarisation rotation between the optical local oscillator signal and the input signal to provide the required phase relationship for in-phase and quadrature detection. The present invention solves the problem of providing local oscillator signals in phase and in quadrature with corresponding input signal components by using, and where necessary controlling, the polar isationn properties of the input and/ or the local oscillator signal such that their respective signal powers can be evenly divided by polarisation splitting. It is important to note that, unlike hitherto, in the solution provided by the present invention the local oscillator signal, or the input signal, does not always necessarily divide individually into two signal components phase shifted by 90° of angle with respect to each other; instead, it is only the necessary instantaneous phase relations between input and local oscillator signal components which are continuously maintained. The invention may, for example, employ a linearly polarised input signal whose power is divided evenly into two orthogonally linearly polarised components, and a local oscillator signal which is circularly polarised and similarly divided. In this case, the input signal components are in phase, and the local oscillator signal components 90° out of phase. The invention will also work, however, if both the input signal and local oscillator signal are elliptically polarised, provided their relative polarisations are such that the instantaneous phase relation required for in-phase and quadrature detection can be achieved. In this case the components of neither the input signal nore the local oscillator signal have individually a phase difference of 90° of angle, but the instantaneous in-phase and quadrature relationship required for multiport detection can nevertheless be achieved. In an optical coherent receiver according to the present invention, which comprises an optical input for an information modulated signal, an optical local oscillator, and power dividing means for evenly dividing the input signal power and the local oscillator signal power between an in-phase and a quadrature detection path, the power dividing means comprise a polarisation splitter, and polarisation control means are provided to control the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur . Where the polarisation of the input signal is unsuitable or subject to fluctuations or variation with time, the polarisation control means need to be capable of ensuring a desired polarisation of the input signal. In order to ensure the appropriate relative polarisation between the input signal and the local oscillator signal, it will usually be necessary also to provide means for adjusting the polarisation of the local oscillator signal. The relative polarisation of the input signal and the local oscillator signal are conveniently controlled by comparing the in-phase and quadrature detection output signals. The comparison can, for example, be performed by a double balanced mixer circuit. According to another aspect of the present invention, a method of performing optical coherent detection comprises evenly dividing optical input signal power and optical local oscillator signal power between an in-phase and a quadrature detection path by polarisation splitting, and controlling the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur. Conveniently, a substantially linearly polarised input signal and a substantially circularly polarised local oscilaltor signal are employed. As an alternative, elliptically polarised input and local oscillator signals may be employed. The present invention will now be described further by way of examples illustrated by the accompanying drawings, of which:- Figure 1 is a schematic diagram of an optical coherent receiver in accordance with the present invention; Figures 2A, 2B and 2C are schematic diagrams of detection circuits for use with the receiver of Figure 1 , and suitable for ASK, DPSK, and PSK modulation schemes , respectively, and Figure 3 is a schematic diagram of an alternative circuit for polarisation control . Referring now also to the drawings, and in particular to Figure 1 , a digitally modulated optical input s ignal passes along fibre path 1 and is coupled into a pol arisation control device 2 which is itself controlled by control circuitry 28. The input pol arisation to the control device 2 may be of any arbitrary state. The control ler 2 is used to adjust the input polarisation to a known state such that the received signal is divi ded half/half between two detectors 5 and 6 by a pol arisation splitter 26. Most simply, the input signal wi l l be adjusted to be in a l inear state of pol arisation with appropriate orientation. The output of the pol arisation control ler 2 is then combined with the output of a local oscillator l aser 25 in a beam combiner device 3. The combiner 3 coul d be a partially reflecting mirror or a fibre combiner. The pol arisation of the local oscil lator signal is also chosen to have a sui table polarisation state as expl ained below. In the case discussed here, the state woul d be circular polarisation and this can be obtained by the use of a suitable retardation pl ate 4 interposed between the local oscillator laser 25 and the combiner 3. From the combiner 3, the combined input and local oscillator signal is coupled, by optical fibre, to a pol arising beam spl itter 26. This could be either a device built around suitable birefringent crystals such as a Glan-Thompson prism or alternatively a polarisation selective waveguide device could be used; one such device is described by M.S. Yataky et al in "An all fibre polarising beam splitter and spectral filter", IEE col. Adavances in conherent optical devices and technologies, London, March 1985. The outputs from the polarising beam splitter are coupled to two separate photo detectors 5 and 6. The electric outputs of the two detectors 5 and 6 are then processed by circuits 7 and 8 before addition in circuit 9. The detail of circuits 7 and 8 will depend on the type of modulation used, and Figures 2a to 2c illustrate some suitable examples. For amplitude shift key modulation (ASK) each of the circuits 7 and 8 may comprise, as shown in Figure 2a, an amplifier, filter 11 and non-linear device 12. The latter could be a square law detector or rectifier. In the case of differential phase shift key modulation (DPSK) the circuits 7 and 8 may consist of the following: a signal amplifier 13, a low pass filter 14, a signal splitter 15, two electrical paths 16 with a difference in length of propagation time equivalent to the bit period and a multiplier device 17. For synchronous demodulation of a PSK (phase shift keyed) input signal, the circuits 7 and 8 each contain an amplifier 18, an IF carrier recovery circuit 19 and a multiplier 20. The combined electrical output from the two channels 9 is then further filtered before regeneration 21 in the usual manner. To maintain correct operation of the receiver, an automatic frequency control loop (AFC) 22 is included to enable the local oscillator laser to track the average frequency of the input signal with a fixed offset. This offset is usually called the IF as in heterodyne detection. In addition it is necessary to maintain the correct polarisation states between the input signal and the local oscillator for proper operation. Control signals to the circuit 28 controlling the polarisation control device 2 may be obtained by detecting an electrical signal at twice the IF at point 9. This is achieved by using a band pass filter 23. Further control signals may be obtained by comparing the average signal level from the two receivers in comparitor/ integrator combination 24. The polarisation control system may be expected to use techniques such as dither modulation to aid its operation. Figure 3 shows the relevant portions of an alternative, and for its greater simplicity preferable, embodiment of a polarisation control circuit which may be substituted in the receiver of Figure 1. The control signal for polarisation control is now derived directly from the outputs of the in-phase and quadrature detection circuits 7, 8. In addition to their respective output signals being applied to the comparator/ integrator 24, the outputs of the detector circuits 7, 8 are applied to a double balanced mixer circuit 50. The output of the double balanced mixer is a sin(theta)cos(theta) product, which is zero when theta is 90° (or a multiple of 90°), and has a finite value otherwise. Hence, when the phases in the detector circuits are in quadrature, indicating that the polarisation relation between input and local oscillator are correct, the double balanced mixer 50 has a zero output. If, however, the phase relation is incorrect, the output of the double bal nced mixer becomes positive or negative, depending on the direction in which the phase difference departs fro m 90° of angle. An immediate advantage of this modified polarisation control circuit is the absence of any need for dither modulation as mentioned above. Thus it will be seen that the present invention provides a relatively simple technique for deriving stable local oscillator signals with quadrature phase relationships which promises better system performance than schemes using, for example, three or more local oscillator phases. The key feature of the technique according to the invention is that use is made of the properties of relative polarsation between the input and the local oscillator signals to obtain the relative phase differences required for detection. In particular, in the afore-described example the received signal is linearly polarised, and the local oscillator signal is circularly polarised to generate the required 90° phase difference between the two local oscillator signals. Instead, it would be possible, for example, to impose circular polarisation on the input signal and linear polarisation onn the local oscillator signal. In practice, the input signal will often be somewhat elliptically polarised from the attempt to split the input signal power exactly half/half between the two detectors, and the state of polarisation of the local oscillator signal will then have to be such as to maintain the appropriate phase relation between its components and those of the received signal. Other important aspects of the invention are the capability it provides to demodulate signals of various formats and the control systems necessary for stable operation. The principles should find particular appl ication to optical fibre systems working the wavelength regions 0.8 to 1.7 microns but of course could also be used throughout the optical spectrum with suitable components .

Claims: CLAIMS 1. An optical multiport coherent receiver wherein relative polarisation between the optical local oscillator signal and the input signal is employed to provide the required phase relationship for in-phase and quardrature detection. 2. An optical coherent receiver comprising an optical input for an information modulated signal, an optical local oscillator, power dividing means for evenly dividing the input signal power and the local oscillator signal power between an in-phase and a quadrature detection path, wherein the power dividing means comprise a polarisation splitter, and wherein polarisation control means are provided to control the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur. 3. An optical coherent receiver according to claim 2, wherein the polarisation control means comprise means for imparting a desired polarisation to the input signal. 4. An optical coherent receiver according to claim 2 or claim 3, wherein the polarisation control means comprise means for imparting a desired polarisation to the local oscillator signal. 5. An optical coherent receiver according to any preceding claim, arranged to operate with a substantially linearly polarised input signal and a substantially circularly polarised local oscillator signal. 6. An optical coherent receiver according to any of claims 1 to 4, arranged to operate with elliptically polarised input and local oscillator signals. 7. An optical coherent receiver according to any preceding claim in the relative polarisation of the input signal and the local oscillator signal are controlled by comparing the outputs of the in-phase and quadrature detection. 8. A coherent receiver according to claim 7, wherein said comparing is performed by a double balanced mixer circuit. 9. A method of performing optical coherent multiport detection wherein relative polarisation between the optical local oscillator signal and the input signal is employed to provide the required phase relationship for in-phase and quadrature detection. 10. A method of performing optical coherent detection comprising evenly dividing optical input signal power and optical local oscillator signal power between an in-phase and a quadrature detection path by polarisation splitting, and controlling the relative polarisation of the input signal and the local oscillator signal for in-phase and quadrature detection to occur. 11. A method according to claim 10, wherein a desired polarisation is imparted to the input signal. 12. A method according to any one of claims 9 to 11, employing a substantially linearly polarised input signal and a substantially circularly polarised local oscillator signal. 13. A method according to any one of claims 9 to 11, employing elliptically polarised input and local oscillator signals. 14. A method as claimed in any one of claims 9 to 13 wherein the relative polarisation of the input signal and the local oscillator signal is controlled by comparing the outputs of the in-phase and quadrature detection. 15. A method as claimed in claim 14, wherein said comparing is performed by double balanced mixing of the in-phase and quadrature detection output signals. 16. A method as claimed in any one of claims 9 to 14 for detecting a digitally modulated input signal.

Inventors: SMITH DAVID WILLIAM, SMITH, DAVID, WILLIAM
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H04B9
IPC codes: G01R15, G02B27, G02F1, G02F2, H04B10, H04L27