DIGITAL INFORMATION TRANSMISSION SYSTEM AND METHOD

Abstract: The system includes a source of polarised electro-magnetic radiation (1) such as a laser whose output radiation is fed to a polarisation modulator (2). The modulator (2) is responsive to digital information to cause, in successive clock periods, changes in polarisation of the radiation. The polarised radiation is transmitted along optical fibre (4) to a receiver where it is coupled to another optical wave of fixed polarisation. The resultant IF radiation is detected by a detector (7) and the differential phase and/or amplitude of the detected IF radiation is determined. From this information, the original digital data is reconstituted.

Description: DIGITAL INFORMATION TRANSMISSION SYSTEM AND METHOD The invention relates to the transmission of digital information in which electro-magnetic radiation is modulated in accordance with the information. Various detection methods have been proposed in the past for demodulating the transmitted radiation including coherent detection and direct detection. Although coherent detection has major advantages over direct detection, it has the drawback of polarisation sensitivity. This problem could, in principle, be eliminated if a transmission medium could be developed which was polarisation holding. In the case of optical radiation, special optical fibres have been developed which are substantially polarisation holding but these have complex structures and much higher losses (and are more expensive) when compared to standard circular symmetric monomode fibres. Furthermore, since large quantities of standard optical fibre have already been installed, are being installed, and are planned for the telecommunications network, network which is initially using polarisation insensitive to direct detection, it is desirable to devise a transmission method and system which is compatible with these fibre coherent networks. So far two schemes have been proposed that will enable coherent detection to be used with standard fibre; these are active polarisation control and polarisation diversity. The former is capable of eliminating all polarisation penalties. However, an extra opto-mechanical or electro-optic device is required in either the local oscillator or signal path at the receiver. This complicates the receiver and could also result . in an insertion loss penalty. Polarisation diversity reception eliminates the need for extra optical control devices in the receiver but requires the addition of a polarising beam splitter and a second photodiode, amplifier chain and intermediate frequency (I.F.) demodulator. With polarisation diversity reception there can be up to 3dB receiver sensitivity penalty for certain combinations of input polarisation and local oscillator polarisation states when the outputs of the two I.F. demodulators are simply combined (although it may be possible to reduce this penalty to about ldB with more complex post demodulator processing) . These two methods of overcoming polarisation problems both result in a more complex receiver which although possibly acceptable in a long distance . high capacity point-to-point transmission link could introduce a significant cost penalty in a local wideband distribution scheme or LAN/MAN type application. A method of transmitting digital information comprises generating a polarised electro-magnetic wave; modulating the wave in successive clock periods by causing changes in polarisation of the wave in accordance with the information; transmitting the modulated wave; and detecting changes in polarisation of the transmitted wave to regenerate the information. The invention, instead of making use of the absolute polarisation of radiation, detects changes in polarisation state between adjacent bits at the receiver. The absolute polarisation does not matter. We have realised that although the output state of polarisation of a long transmission path using standard optical fibre fluctuates, it does so only slowly. For cable buried under the ground, where the temperature remains fairly stable, significant fluctuations may not occur over several hours. The polarisation modulation could be based on any of the following known effects: Kerr, Faraday rotation or TE to TM conversion. The detection of the transmitted wave may be achieved in any conventional manner. Preferably, however, the detecting step comprises combining the transmitted modulated wave with a detection signal of fixed polarisation to generate a wave with an intermediate frequency, and detecting changes in phase and/or amplitude of the intermediate frequency wave to regenerate the information. In some examples, the modulation step may comprise switching the polarisation of the wave between two values at for example 90°. Alternatively, a change in polarisation within a clock period may be achieved by ramping the polarisation between two polarisations. Although the invention is applicable to electro-magnetic radiation with a variety of wavelengths, it is particularly applicable to wavelengths in the optical domain. In this specification, the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. It is particularly advantageous if the method further comprises additionally modulating one or more of the phase, amplitude, and frequency of the polarised electro-magnetic wave in accordance with the digital information. This facility can be used in two ways. Firstly, double the amount of information can be sent on the transmitted wave thus doubling the transmission rate, or alternatively the digital information used to modulate the one or more of the phase, amplitude and frequency of the polarised electro-magnetic wave may be the same information which is used to modulate the polarisation of the wave. The latter possibility provides a way of reducing the chances of error in detecting the transmitted information. In accordance with a second aspect of the present invention, a digital information transmission system comprises a source of polarised electro-magnetic radiation; modulating means responsive to the information for causing in successive clock periods changes in polarisation of the radiation in accordance with the information; transmitting means for transmitting the modulated radiation; and detecting means for receiving the transmitted modulated radiation and for detecting changes in polarisation of the radiation to regenerate the information. The source of electro-magnetic radiation may comprise for example a laser. In one particularly convenient arrangement in which the radiation has a wavelength in the optical domain, the apparatus further comprises a birefringent medium; and means for modulating the frequency of the polarised electro-magnetic radiation in accordance with the information, the frequency modulated radiation being incident on the birefringent medium whereby corresponding changes in polarisation of the radiation are caused. This provides a particularly elegant arrangement which would eliminate the need for an external modulator and thus avoid the losses associated with such a modulator. The output polarisation from the birefringent medium, such as a short length of optical fibre, is dependant on the optical frequency of the source and can therefore be modulated as the source frequency is modulated. We believe that the invention is particularly applicable to overcoming the problem of long term polarisation stability in coherent transmission systems using monomode fibre. The invention enables considerable simplification of the detecting means such a heterodyne receiver. This will be of benefit in future wideband distribution schemes. There may also be some scope for using the invention in optical networks that have all optical sources centrally located. In this case it may be possible to provide polarisation modulators at remote terminals fed by continuous wave light from a central laser bank. Some examples of methods and systems in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 illustrates schematically one example of a transmission system; Figure 2 illustrates the receiver of Figure 1 in more detail; Figure 3 illustrates a second example of a transmission system; Figure 4 illustrates graphically a detection method; and. Figures 5 and 6 illustrate the waveforms of input signals, local oscillator signals, and IF signals in two different examples. The system shown in Figure 1 comprises a semiconductor laser 1 which generates a linearly polarised beam of optical radiation. The beam is fed to a polarisation modulator 2 of conventional form which is controlled via a data input 3. At successive clock periods, data is applied to the modulator 2 which causes either a change or no change in the polarisation of the incoming beam. For example, a binary digit "1" may cause a 90° switch in polarisation whereas a binary digit "0" will cause no change. The modulated radiation is then fed into a conventional monomode optical fibre 4 defining a transmission path. At a receiving station, the optical fibre 4 is connected to an optical coupler 5 having a second input connected to a local oscillator 6 constituted by a semiconductor laser which generates circularly polarised optical radiation. The optical coupler 5 combines the incoming modulated optical signal with the local oscillator signal and the resultant IF signal is fed to a detector 7. Information is contained in both the differential phase, ie. the change in phase between clock periods, and the differential amplitude of the IF signal, that is the change in amplitude between clock periods. The relative magnitude of the demodulated phase signal to the demodulated amplitude signal will depend on the relationship of the received state of polarisation to that of the local oscillator polarisation. For certain combinations of input signal to local oscillator polarisation there will be no useful amplitude information. Take, for instance, the case when the input polarisation is switching between two linear orthogonal states (Figure 5A) and the local oscillator is circular (Figure 5B) . With this combination although the IF amplitude will remain constant the IF phase will switch in sympathy with the input signal's polarisation (Figure 5C) . In contrast, consider the case, again with a circular local oscillator (Figure 6B) , where the input signal is switching between right circular and left circular (Figure 6A) . This time the IF envelope switches completely (Figure 6C) . Therefore to determine that a polarisation change has taken place it is necessary to process both the demodulated differential phase and envelope signals together. To give optimum performance in some cases it may better not to represent symbols by step changes in polarisation states but by some other function; for example a polarisation ramp. The detector 7 which includes a filter generates an output signal which is fed in parallel to a differential phase demodulator 8 and a differential amplitude demodulator 9. The output signals from these demodulators 8, 9 are fed to a micro-processor 10 which provides an output signal representing the original data. The micro-processor 10 could select between the signals from the phase demodulator 8 and amplitude demodulator 9 the signal with the largest magnitude or it could add the two signals to produce a resultant signal. Figure 2 illustrates one way in which the demodulators 8,9 could be implemented. The detector 7 includes a sensor 11 such as a photodiode whose output is fed to an amplifier 12 and then to a filter 13 and a further amplifier 14. The amplitude demodulator 9 is constituted by a conventional envelope detector 15 whose output is split and fed in parallel to the inverting and non-inverting inputs of a differential amplifier 16. The path length to the non-inverting input is longer than that to the inverting input so that a single clock period (or bit period) delay is applied to that bit enabling comparison of signal levels between adjacent bits to take place. In a similar manner the phase change between adjacent clock periods or bits is determined by splitting the path from the amplifier 14 into two 17, 18, delaying one path 18 by a single bit period, and multiplying the two signals in a double balanced modulator 19. The micro-processor 10 determines what weighting should be given to each of the two demodulated signals. In the simplest case it may be possible to take the signal which has the largest peak-to-peak level. A second example is illustrated in Figure 3. In this example the frequency of the optical radiation generated by the semiconductor laser 1 is modulated directly by the digital data. This frequency modulated radiation, of fixed polarisation, is fed to a short length of high birefringence fibre 20. Preferably, the frequency modulated beam is launched at ιr/4 to the birefringent axis of the fibre 20. The output polarisation from this short length of fibre 20 will be dependant on the optical frequency of the source and can therefore be modulated as the laser frequency is modulated. The beam output from the fibre 20 is then coupled to the main optical fibre 4. Frequency modulation of a semiconductor laser can be achieved directly by control of injection current or by acoustic wave interaction. In the simple case of a laser directly frequency shift keyed betwen f.. and f_ (where the difference between these optical frequencies is much greater then the data rate) it is only necessary to demodulate just one of the two frequencies to determine the symbol transmitted; this single filter detection of FSK gives the same performance as ASK. If it is now arranged that the frequency shift is sufficient for the two signals to have orthogonal polarisations we now have a choice of two signals that could be detected at the distant receiver and either signal containing the transmitted information. The local oscillator frequency at the receiver could be tuned to whichever signal presented the best polarisation match. Moreover, by careful selection of the IF frequency with respect to f. - f 2 it may be possible to site the signal associated with the orthogonal polarisation state near the image band of the detected signal. Therefore under this condition where the local oscillator frequency is positioned just off centre of f, - f 2 either signal would automatically appear in the receiver IF bandwidth, individually or together depending on the received polarisation state. In this case polarisation diversity may be possible without retuning the frequency of the receiver local oscillator laser.

Claims: CLAIMS 1. A method of transmitting digital information, the method comprising generating a polarised electro-magnetic wave; modulating the wave in successive clock periods by causing changes in polarisation of the wave in accordance with the information; transmitting the modulated wave; and detecting changes in polarisation of the transmitted wave to regenerate the information. 2. A method according to claim 1, wherein the detecting step comprises combining the transmitted modulated wave with a detection signal of fixed polarisation to generate a wave with an intermediate frequency, and detecting changes in phase and/or amplitude of the intermediate frequency wave to regenerate the information. 3. A method according to claim 2, wherein the detection signal is circularly polarised. 4. A method according to any of claims 1 to 3, wherein the electro-magnetic wave has a wavelength in the optical domain. 5. A method according to any of claims 1 to 4; wherein a change in polarisation within a clock period is achieved by ramping the polarisation between two polarisations. 6. A method according to any of the preceding claims, further comprising additionally modulating one or more of the phase, amplitude, and frequency of the polarised electro-magnetic wave in accordance with the digital information. 7. A method according to claim 6, wherein the digital information used to modulate the one or more of the phase, amplitude and frequency of the polarised electro-magnetic wave is the same information which is used to modulate the polarisation of the wave. 8. A digital information transmission system comprising a source of polarised electro-magnetic radiation; modulating means responsive to the information for causing in successive clock periods changes in polarisation of the radiation in accordance with the information; transmitting means for transmitting the modulated radiation; and detecting means for receiving the transmitted modulated radiation and for detecting changes in polarisation of the radiation to regenerate the information. 9. A system according to claim 8, wherein the radiation has a wavelength in the optical domain, the apparatus further comprising a birefringent medium; and means for modulating the frequency of the polarised electro-magnetic radiation in accordance with the information, the frequency modulated radiation being incident on the birefringent medium whereby corresponding changes in polarisation of the radiation are caused. 10. A system according to claim 8 or claim 9, wherein the radiation has a wavelength in the optical domain, the transmitting means including an optical waveguide. 11. A method as claimed in any of claims 1 to 7 for coherent transmission of digital information. 12. A coherent digital information transmission system as claimed in any of claims 8 to 10.

Inventors: SMITH DAVID WILLIAM, SMITH, DAVID, WILLIAM
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H04B14
IPC codes: H04B10, H04B14