GB2153176A

GB2153176A - Improvements relating to optical fibre communication systems

Info

Publication number: GB2153176A
Application number: GB8400999A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-01-14
Filing date: 1984-01-14
Publication date: 1985-08-14
Also published as: GB8400999D0; GB2153176B

Abstract

An optical fibre communication system in which modulator means 2 is provided at the transmitter end of the system for modulating the relative phase between orthogonally polarised signals derived from a coherent light source 1 in accordance with data or information to be transmitted over an optical fibre communication path to the receiver 4 end of the system where the signals will be demodulated. In order to reduce noise resulting from delay differentials between polarised signals transmitted over the optical fibre path L, it is includes sections of polarisation maintaining optical fibre 3 which are angularly rotated through 90 DEG or thereabouts relatively to one another so that the overall delay for the respective polarised signals over the entire length of the communication path is substantially the same. <IMAGE>

Description

SPECIFICATION Improvements relating to optical fibre communication systems This invention relates to optical fibre comunication systems.

In such communication systems polarised amplitude or phase (or frequency) modulated coherent light signals may be transmitted over optical fibres terminating at an optical receiver including signal detecting/demodulating means.

In order to cope with the noise content of incoming signals and thermal noise of the receiver the signal-to-noise ratio of the receiver may be improved by arranging that optical output signal from a relatively high power local oscillator at the receiver is mixed with the modulated incoming light signals, the frequency of the local oscillator signal being the same as, or different from the carrier of the incoming light signals according to whether homodyning or heterodyning of the signal is required.

However, the basic problem which arises in such communication systems is that since the optical fibre communication path may extend over large distances (e.g. hundreds of kilometres) it is not possible in a simple homodyne receiver system to maintain phase tracking between the carrier signal launched into the optical fibre at the transmitter end of the communication system and the local oscillator signal in order to correct for fluctuations in the communication path length due to environmental changes. Accordingly, noise results in a homodyne system, whereas in the case of a heterodyne receiver system the intermediate or difference frequency which is generated by the mixing of the local oscillator signal and the incoming signal will contain phase noise.

The present invention seeks to at least alleviate this problem by providing an optical fibre communication system in which modulator means is provided at the transmitter end of the system for modulating the relative phase between orthogonally polarised signals derived from a coherent light source in accordance with data or information to be transmitted over an optical fibre communication path to the receiver end of the system where the signals will be demodulated. The communication path preferably includes sections of polarisation maintaining optical fibre which are angularly rotated through 90 or thereabouts relatively to one another so that the overall delay for the respective polarised signals over the entire length of the communications path is substantially the same.

In this way the delay differential between the polarised signals in one section of polarisation-maintaining optical fibre is compensated for by an opposite or reverse delay differential in another section. The phase differential between the polarised signals at the receiver end corresponds to phase modulations embodying the transmitted data or information.

In one method of carrying out the present invention the optical fibre communication path may simply comprise two contiguous optical fibre sections of equal length which are rotated through 90 relatively to one another about their common axis. In this way although the orthogonally polarised light components in the first section of optical fibre may be subjected to different delays due, for instance, to imperfect spatial coherence of the light generated by the light source, the differential delay between the polarised light components in the second optical fibre section will be effectively reversed due to the 90 angular displacement of this second section relative to the first optical fibre section (i.e. the orthogonal axes of polarisation in the second section are aligned in parallel with respect to the opposite axes in the first section). The compensating effect produced by the second optical fibre section affords equalisation of the delays experienced by the polarised light signals over the entire length of the optical fibre communication path.

For the purpose of modulating the relative optical phase of the orthogonal polarised signals in order to encode the data or information to be transmitted over the communication path as differential phase information a planepolarised laser light source may be arranged to launch light through an electro-optic modulator crystal into the optical fibre communication path so that equal intensities of light are launched into each of the two polarisation modes. By applying to the modulator crystal a voltage which varies in accordance with the information to be transmitted over the communication path a corresponding relative phase modulation is produced between the two polarised signals.

Alternatively, the transmitter end of the system may comprise two separate planepolarised laser light sources which are arranged to launch light through beam splitter means into the optical fibre communication path in the respective polarisation modes. The average frequencies of the light sources may be locked together by the use of frequency lock-loop techniques and the data or information to be transmitted may be generated as relative phase perturbations or transient frequency perturbations of the two light sources.

The receiver end of the system may comprise a local oscillator which produces an output signal which is mixed by mixing means with the received signal after polarisation of the former at 45= to the two orthogonal polarisation axes of the communication path optical fibre. In the case of a heterodyne receiver the frequency of the local oscillator will be different from that of the transmitted light signal and consequently the output from the mixing means (e.g. photo-diode) will contain the frequency difference between the transmitted and local oscillator signals but it will also contain phase and amplitude modulation dependent on the relative phase modulation between the polarised signals applied to the communication path at the transmitter end of the system and representing the data or information transmitted.In the case of a homodyne receiver the local oscillator will, by means of a conventional feedback loop, be frequency locked to the average frequency of the received signal and again the amplitude of the mixing means may be detected for deriving the transmitted data or information.

As an alternative to the receiver arrangement just described the two polarisation mode signals arriving at the receiver may, by the use of a Wollaston Prism for example, be directed on to separate mixer/deteors where, they are mixed with a relatively high local oscillator signal of different average frequency to the received signals. The respective outputs from the mixer/detectors may then be compared in phase by a phase comparator in order to obtain the data or information encoded into the polarised inputs at the transmitter end of the system.By way of example the present invention will now be described with reference to the accompanying drawings in which; Figure 1 shows a simple diagram of a known optical fibre communication system; Figure 2 shows a communication system according to the present invention; Figure 3 is a diagram depicting the delay differential which may be produced between polarised components in the system of Figure 2 and means for compensating for such delay differential; and, Figure 4 shows two alternative arrangements of optical fibre sections to provide full compensation over the entire communication path for differences in velocity of the polarised light components.

Referring to Figure 1 the optical fibre communication system illustrated comprises a coherent light source (e.g. semiconductor laser) 1 the output from which is suitably amplitude or frequency (or phase) modulated by optical modulating means 2 for the encodement into the light carrier signal of data or information to be transmitted over an optical fibre 3 (e.g.

polarisation-maintaining or single polarisation monomode fibre or stress-free conventional monomode fibre) to an optical receiver 4 including a local oscillator 8.

The output from the modulating means 2 would usually be polarised along one of the orthogonal axes of polarisation of the optical fibre 3.

The modulated signal transmitted down the optical fibre 3 will experience propagation delay. Therefore, due to the imperfect spatial coherence of both the light source 1 and oscillator light source, and any propagation delay changes due to acoustic or seismic stressing of the fibre or fibre thermal fluctuations the received signal and the local oscillator may be unintentionally phase displaced relative to one another at the receiver end of the system giving rise to noise.

In order to improve the signal/noise ratio the receiver 4 the received signal is mixed by optical mixing means 7 with the optical output signal of the local oscillator 8. The frequency of the oscillator signal may either be the same as, or different from, the frequency of the light source output signal according to whether the receiver 4 is a homodyne or heterodyne receiver.

The mixed signal then passes to a detector 5 of the receiver for demodulation of the encoded signal.

This system suffers from the introduction of phase noise in both the case of the heterodyne and homodyne receivers.

Referring now to Figure 2 of the drawing this shows an optical fibre communication system according to the invention. The system comprises a plane-polarised laser coherent light source 9 which launches light into an electro-optic crystal 10 at a suitable angle to permit launching of orthogonally polarised signals of equal intensity into an optical fibre communication path 11. A variable voltage V is applied to the modulator crystal 10 and this voltage which corresponds to the data or information to be transmitted over the fibre path 11 produces modulation of the relative phase between the outputs from the crystal 1 0. This data is accordingly transmitted as "differential phase" information.

In order to ensure that the relative phase between the polarised signals is preserved on exit from the entire fibre section without the intrusion of phase noise (which may arise with imperfectly coherent light sources due to any differential delays produced between orthogonal polarised signals transmitted over the optical fibres), the optical fibre communication path comprises in the present example two sections 1 2 and 1 3 of equal length. As can be seen from the enlarged fibre diagram the two sections are rotated through 90 about their axes relative to one another so that the opposite polarisation axes of the fibre sections are aligned. Such an arrangement serves to equalise the delays suffered by the respective orthogonally polarised signals so that the signals arrive in a similar relative phase condition at the receiver end of the optical fibre communication path to that initially launched. This equalisation of the delays is depicted in Figure 2 where it can be seen that the polarised components P3 and P4 meet at point M (i.e.

no phase differential in the absence of encoded data) after travelling distance L from the transmitter. Without this means of com pensation, there would be a propagation time difference X between the two polarisation components P1 and p2 which gives rise to phase noise with incoherent sources.

The output from the optical fibre 11 is then mixed by a photodetector (diode) 1 5 with the output of a local oscillator 1 6 polarised at 45 to the two polarisation axes of the fibre. The electrical output from the photodetector 1 5 will contain amplitude and phase modulation dependent upon the phase modulation injected by the electro-optic crystal 10 at the transmitter end of the system. This modulation is detected by a demodulator 1 7 which produces an output corresponding to the data or information transmitted over the optical fibre communication path 11.

As will readily be appreciated, although the optical fibre 11 is shown as consisting of two fibre sections many different combinations of fibre sections are possible to achieve equalisation of the delays suffered by orthogonal polarised signals over the full length of the communications path. Two such combinations are shown in Figure 4 of the drawings.

It will be apparent from the foregoing that by the use of differential phase modulation techniques, and by providing delay equalisation over the communication path, the aforesaid problems experienced with known systems due inter alia to imperfect spatial coherence of the light source are overcome.

By the additional use of heterodyne techniques in the receiver, the requirement for accurate phase tracking of the local oscillator is overcome.

Claims

1. An optical fibre communication system in which modulator means is provided at the transmitter end of the system for modulating the relative phase between orthogonally polarised signals derived from a coherent light source in accordance with data or information to be transmitted over an optical fibre communication path to the receiver end of the system where the signals will be demodulated.

2. An optical fibre communication system as claimed in claim 1, in which the communication path includes sections of polarisation maintaining optical fibre which are angularly rotated through 90t or thereabouts relatively to one another so that the overall delay for the respective polarised signals over the entire length of the communication path is substantially the same.

3. An optical fibre communication system according to claim 1 or 2, in which the communication path comprises two contiguous optical fibre sections of equal length which are rotated through 90 relatively to one another about their common axis.

4. An optical fibre communication system as claimed in any preceding claim, in which a plane-polarised light source is arranged to launch light through an electro-optic modulator crystal into the optical fibre communication path so that equal intensities of light are launched into each of the two polarisation modes, means being provided to apply to the modulator crystal a voltage which varies in accordance with information to be transmitted over the communication path so that corresponding relative phase modulation is produced between the two polarised signals.

f requency of the local oscillator is different from that of the transmitted light signal and consequently the output from the mixing means will contain the frequency difference between the transmitted and local oscillator signals but will also contain phase and amplitude modulation dependent on the relative phase modulation between the polarised signals applied to the communication path at the transmitter end of the system and representing the data or information transmitted.

9. An optical fibre communication system as claimed in claim 7, comprising a homodyne receiver in which the local oscillator is frequency locked to the average frequency of the received signal by means of a conventional feedback loop.

1 0. An optical fibre communication system as claim in any 8 claims 1 to 6, in which the two polarisation mode signals arriving at the receiver are directed on to separate mixer/detectors where they are mixed with a relatively high local oscillator signal of different signal which is mixed by mixing means with the received signal after polarisation of the output signal at 45 to the two orthogonal polarisation axes of the communication path optical fibre.

8. An optical fibre communication system as claimed in claim 7, comprising a heterodyne receiver in which the frequency of the local oscillator is different from that of the transmitted light signal and consequently the output from the mixing means will contain the frequency difference between the transmitted and local oscillator signals but will also contain phase and amplitude modulation dependent on the relative phase modulation between the polarised signals applied to the communication path at the transmitter end of the system and representing the data or information transmitted.

1 0. An optical fibre communication system as claimed in any of claims 1 to 6, in which the two polarisation mode signals arriving at the receiver are directed on to separate mixer/detectors where they are mixed with a relatively high local oscillator signal of differ ent average frequency to the received signals, the respective outputs from the mixer/detectors may then be compared in phase by a phase comparator in order to obtain the data or information encoded into the polarised inputs at the transmitter end of the sytem.

11. An optical fibre communication system as claimed in claim 10, in which the two polarisation mode signals are directed on to the separate mixer/detectors by a Wollaston Prism.

1 2. An optical communication system substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.