GB2165115A - Optical data transmission systems - Google Patents

Abstract

An optical data transmission system comprises a laser 2 for launching polarised light into one end of a polarisation maintaining optical fibre 1, 4 on two orthogonally-related polarisation modes AA, BB and modulating transducers 5, 6, 7 operable in accordance with data to be transmitted along said optical transmission path to provide variable mechanical stressing of the optical fibre, which consequentially produces differential phase modulation between the two polarised light signals being transmitted along said fibre. Detector means 11 is responsive to such differential phase modulation for the detection of the data content transmitted. The data may be multiplexed by the use of different modulation frequencies F1, F2, F3. <IMAGE>

Description

SPECIFICATION Improvements relating to optical data transmission systems This invention relates to optical data transmission systems and relates more especially to such systems embodying so-called fibredyne data collection techniques.

Such fibredyne data collection techniques may be used in optical telemetry systems as described in an article entitled "Method of Phase-Modulating Signals in Optical Fibres: Application to Optical-Telemetry Systems" (Electronics' Letters 24th January 1974 Volume 10 No. 2) and in an article entitled "Fibredyne Data Collection System for Industrial Telemetry Application" (Fibre Optics: Short-haul and long-haul Measurements and Applications (1982) in Proceedings of the International Society for Optical Engineering (SPIE) pages 132-137 of Volume 355.

According to these known fibredyne systems data (e.g. measurement data) is effectively collected by light signals transmitted along an optical fibre waveguide by phase-modulating the signals in accordance with the energisation of one or more transducers in dependence upon the data to be transmitted along the optical fibre to optical detecting means (demodulator). The transducer (s) distort the optical fibre by dynamic mechanical stressing of the fibre in order to produce phasemodulation of the transmitted signals.

The systems described in the first of the above mentioned references use single mode or multimode optical fibres and the phase-modulated light emerging from the optical fibre is mixed with light from a reference coherent light source impinging on the active surface of a non-linear (e.g. square law) optical detector in order to recover the phasemodulated information by homodyning or heterodyning techniques. This system suffers from the disadvantage that the provision of the reference light source at the detector end of the system is often inconvenient.

In the case of the other reference referred to above a multi-mode optical fibre is used for the transmission path and the differential phase-modulation between different transmission modes is detected by arranging that part of the output speckle pattern which appears at the output end of the optical fibre and which is produced by multi-mode interference falls on to the active surface of one or more optical detectors. Although this systems obviates the need for a reference light source at the detector end of the system it has other serious disadvantages. For example, because of the large number of modes present in typical multi-mode optical fibres only a very small proportion of the total optical energy received at the detector end of the system may be effectively utilised.Furthermore, due to the relatively random field distribution of the different modes in the output speckle pattern referred to the amplitude, polarisation and phase of the interference signal can vary unpredictably and thereby cause fading of the output light signal. Whereas different processing methods have been devised to overcome this problem none of these methods has successfully enabled the full effective use of the received light signal to be achieved and all of these methods add significantly to the complexity of the system as a whole.

The present invention seeks to obviate the problems experienced with multi-mode optical fibre data transmission systems just above described without the need for providing a reference light source at the detector end of the system as described by the first described systems, by providing an optical data transmission system comprising a polarisation-maintaining optical fibre constituting an optical transmission path for the data, laser means arranged for launching polarised light into one end of the optical fibre in tow or thogonally-related polarisation modes, transducer means operable in accordance with data to be transmitted along said optical transmission path to detector means to provide variable mechanical stressing of the optical fibre which consequentially produces differential phase-modulation between the two polarised light signals being transmitted along said fibre, the detector means being responsive to such differential phase modulation for the detection of the data content transmitted.

In carrying out the present invention the transducer means may comprise a single transducer or it may comprise a plurality of transducers for producing differential modulation between the two polarisation mode signals at different frequencies.

The transducers may comprise piezo-crystals which may be arranged to receive electrical signals (e.g. acoustically derived signals) which produce distortion or straining of the optical fibre for producing differential phase modulation.

To produce the differential modulation between the two polarisation modes the polarisation-maintaining optical fibre may be encased within a jacket having a non-circular (e.g. asymmetrical) cross-section so orientated with respect to the orthogonal polarisation axes of the optical fibre as to facilitate the application of transducer output mechanical stresses to predetermined locations relative to the fibre for maximum efficiency and minimum fading of the received signals.

The differential phase modulation between the two orthogonally-related polarisation modes may alternatively be achieved either by winding the optical fibre with a known orientation around a cylindrical piezoelectric crystal transducer which expands and contracts in response to electrical signals applied to it so as to exert mechanical stresses on the fibre or by using transducer means which applies an omni-directional compression force to the optical fibre and relying upon the smaller differential phase modulation changes which result from the basic asymmetry of the fibre itself.

In applying the present invention to optical data transmission involving relatively long optical transmission paths differential propagation delays between the two polarisation modes may be eliminated by the use of a compensation polarisa tion-maintaining optical fibre in the manner fully described in our co-pending Patent Application No.

8314570 (F13065) to which attention is hereby directed.

By way of example the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a diagram illustrating one form of optical data transmission system (e.g. telemetry system) according to the invention; and Figure 2 is a diagram which illustrates the use of optical compensating means for eliminating differential propagation delays between polarisation mode signals in data transmission systems of the form shown in Figure 1 having relatively long transmission paths.

Referring to Figure 1 of the drawings the system illustrated comprises a polarisation-maintaining optical fibre 1 capable of supporting two principal fundamental polarisation modes which are orthogonally-related. The polarisation axes of the optical fibre 1 are indicated at A-A and B-B, respectively.

Polarised light polarised at 45Q to the polarisation axes A-A and B-B and derived from a laser light source 2 is launched into the input end 3 of the optical fibre 1 so that the two polarisation modes of the fibre are excited equally. The polarised light signals are transmitted along the optical fibre 1 towards the output end 4 of the fibre.

For the purpose of injecting data into the light transmitted along the optical fibre the optical fibre is arranged to be subjected to mechanical dynamic stresses which effectively distort or strain the fibre and thereby produce phase modulation of the light being transmitted along the fibre. In the present case such distortion produces differential phase modulation between the two polarisation modes.

The stressing of the fibre 1 is achieved by means of a number of transducers such as the transducers 5, 6 and 7 in the present example to which electrical signals of different frequencies F1, F2 and F3 may be applied to cause the transducers to subject the optical fibre to transverse mechanical stresses. These electrical signals embody the data required to be transmitted along the optical fibre.

In the present example the transducers 5, 6 and 7 are positively orientated with respect to the polarisation axes of the optical fibre 1 by arranging that the fibre 1 is encased within a generally rectangular sheath or jacket 8 providing flat surfaces 9 and 10 to either of which the transducers may be applied. In this way stressing of the optical fibre 1 along one or more of its polarisation axes A-A, B-B ensures maximum response and efficiency of the system and positively avoids fading of the signal at the detector end.

As has previously been indicated, the same end can be achieved in other ways or by making use of the basic assymetry of the optical fibre itself when using omni-directional compression of the optical fibre to provide the necessary differential modulation between the two polarisation mode signals.

The differential phase modulation at frequencies F1, F2 and F3 between the two polarisation modes will be detected by means of a polarisation analyser 11 which produces electrical output signals at the frequencies F1, F2 and F3 containing the data (e.g. measured data) transmitted along the optical fibre. The detection of the output from the optical fibre 1 by the polarisation analyser may be performed by sensing the relative output in the two polarisation modes but to avoid any loss of signal due to fading caused by thermal drift from the required phase quadrature condition the output from the optical fibre 1 may be first split into two paths in which a relative phase bias of 90 between the two polarisation modes is introduced and then the signals received over the two paths analysed. This method ensures efficient signal recovery under all conditions.

Referring now to Figure 2, this illustrates how in the case of data transmission systems according to Figure 1 having relatively long transmission paths the problems resulting from different propagation delays arising between the two polarisation modes can be eliminated. The principle of propagation delay compensation employed in the Figure 2 arrangement forms the subject of the co-pending Patent Application No. 8314570 previously referred to.

As will be seen from Figure 2, two polarisationmaintaining optical fibres 12 and 13 of equal length are spliced together at 14 after anglularly rotating their ends relative to one another through 90 so that the "fast" polarisation mode in the fibre 12 couples into the "slow" polarisation mode in the fibre 13 and vice versa. This arrangement allows the use of iess coherent light sources (e.g.

semiconductor lasers) than would otherwise be possible without experiencing phase noise problems and eventural loss of coherence over very long trnasmission paths which could arise due to relative path imbalance.

As will be appreciated from the foregoing description of the invention the problems of fading in known systems using multi-mode optical fibres and due to interference between the multiplicity of modes of the fibres and the need for providing at the detection end a reference light source are positively avoided.

Claims (8)

1. An optical data transmission system comprising a polarisation-maintaining optical fibre constituting an optical transmission path for the data, laser means arranged for launching polarised light into one end of the optical fibre in two orthogonally-related polarisation modes, transducer means opeable in accordance with data to be transmitted along said optical transmission path to detector means to provide variable mechanical stressing of the optical fibre which consequentially produces differential phase modulation between the two polarised light signals being transmitted along said fibre, the detector means being responsive to such differential phase modulation for the detection of the data content transmitted.

2. An optical data transmission system as claimed in claim 1, in which the transducer means comprises a plurality of transducers for producing differential phase modulation between the two polarisation mode signals at different frequencies.

3. An optical data transmission system as claimed in claim 2, in which the transducers comprise piezo-crystals arranged to receive electrical signals which produce distortion or straining of the optical fibre for producing differential phase modulation.

4. An optical data transmission system as claimed in any preceding claims, in which the differential modulation between the two polarisation modes is produced by encasing the polarisationmaintaining optical fibre within a jacket having a non-circular cross-section so orientated with respect to the orthogonal polarisation axes of the optical fibre as to facilitate the application of transducer output mechanical stresses to predetermined locations relative to the fibre for maximum efficiency and minimum fading of the received signals.

5. An optical data transmission system as claimed in any of claims 1 to 3, in which the differential modulation between the two polarisation modes is achieved by winding the optical fibre with a known orientation around a cylindrical piezo-electric crystal transducer which expands and contracts in response to electric signals applied to it so as to exert mechanical stresses on the fibre.

6. An optical data transmission system as claimed in any of claims 1 to 3, in which the differential modulation between the two polarisation modes is achieved by using transducer means which applies an omni-directional compression force to the optical fibre and relying upon the smaller differential modulation changes which result from the basic asymmetry of the fibre itself.

7. An optical data transmission system as claimed in any preceding claim, in which differential propagation delays between the two polarisation modes are eliminated by the use of a compensation polarisation-maintaining optical fibre in the manner disclosed in our co-pending Patent Application No. 8314570.

8. An optical data transmission system substantially as hereinbefore described with reference to the accompanying drawing.