GB2147759A

GB2147759A - Optical sensor

Info

Publication number: GB2147759A
Application number: GB08322716A
Authority: GB
Inventors: Dr John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1983-08-24
Filing date: 1983-08-24
Publication date: 1985-05-15
Also published as: GB2147759B

Abstract

An optical sensing system comprises two optical fibre sensors 2,3 of different lengths connected in parallel and arranged to be subjected at 5 to deforming forces; pulse laser 1 which transmits signals simultaneously along the optical fibres; optical coupler 7; three optical fibres 8,9,10 each with a plurality of equally spaced reflective discontinuities (11, Fig 2, not shown) which define along each fibre 8,9,10 sensor a plurality of fibre elements so that the fibres 8,9,10 in combination are equivalent to a fibre with discontinuities over a length equivalent to the longest fibre 10. A small proportion of each light signal transmitted along each fibre 8,9,10 will be reflected back along the fibre 8,9 or 10 from each of the discontinuities (11) whereby each reflected light signal after the first in each of the fibres 8,9,10 interferes with either the previously reflected signal from the preceding discontinuity of the same fibre, or a reference signal from fibre 3 to cause photodetector 12, to which all the sensors are coupled, to produce an electrical signal, the difference between successive electrical signals corresponding to successive discontinuities (11) in the fibres being dependent upon the length of the fibre elements 2,3. Changes in length of these elements 2,3 produced by deforming forces will result in changes in the electrical signals which can be detected at 13. Application to hydrophones is mentioned. <IMAGE>

Description

SPECIFICATION Improvements relating to optical sensing systems This invention relates to optical sensing systems for sensing strain or deformation of optical fibre sensors and is especially, although not exclusively, applicable to hydrophones and the sensing of changes in length of optical fibres embodied in such hydrophones due to the impingement thereon of acoustic waves.

More specifically, the present invention is concerned with optical sensing systems of the kind forming the subject of our co-pending Application No. 8220793 in which an optical fibre sensor subjected to acoustic waves or other fibre deforming forces is effectively divided up into a plurality of relatively short fibre elements by the provision of reflective discontinuities along the fibre from which a small proportion of signals transmitted down the optical fibre sensor are caused to be reflected and to interfere with reference or previously reflected signals during a sensing operation for the detection of acoustic or other deforming signals.

The present invention is based upon the realisation that optical power losses which occur in an optical fibre sensor having a multiplicity of discontinuities (spliced joints) defining a large number of serially-connected fibre elements can be significantly reduced by providing a plurality of optical fibre sensors of different lengths effectively connected in parallel and by arranging that the plurality of fibres in combination effectively afford equally spaced discontinuities over a path length equivalent to the longest sensor. For example, in the case of three optical fibre sensors of different lengths coupled at one end, the shortest optical fibre may be provided with equally-spaced discontinuities over its entire length.The next longest optical fibre will have at its coupled end a section without discontinuities corresponding in length to the total length of the first optical fibre sensor plus a distance corresponding to the spacing between discontinuities but the remainder of the fibre will be divided into fibre elements by equally-spaced continuities. The third and longest optical fibre will also have a section at its coupled end without discontinuities, such section corresponding in length to the length of the second optical fibre plus the spacing between continuities with the remainder of the third optical fibre being divided equally into fibre elements by a plurality of discontinuities.

By providing a plurality of optical fibre sensors equivalent to a single fibre sensor with discontinuities over its entire length the overall power losses are much reduced. Moreover, the multiple reflection effects which can build up in sensors having high numbers of discontinuities will be substantially reduced by the optical sensing system according to this invention.

The optical fibre sensors may be coupled through a multi-way coupler on the one hand to signal generating means for producing light pulse signals for transmission simultaneously along the optical fibre sensors and on the other hand to detector and processor means for monitoring the signals reflected from the discontinuities along the sensor in order to detect and or measure any phase modulation resulting from the impingement of deforming forces on the optical fibre sensors.

By way of example one embodiment of the invention as applied to an optical system of the form described in our co-pending PatentApplication No.

will now be described with reference to the accompanying drawing in which: Figure 1 is a schematic diagram of an optical detecting and/or measuring system suitable for use in hydrophones; and, Figure 2 is a diagram illustrating the form of the optical fibre sensors of the system shown in Figure 1.

Referring to the drawing the optical sensing system shown comprises a pulsed laser light source 1 which produces light pulses of frequency F which are applied to the respective parallel optical fibre arms 2 and 3 of pulse processing means through an optical fibre coupler 4. The arm 2 of the pulse processing means includes according to our copending Patent Application No. optical fibre deforming means constituted in the present example by a piezoelectric crystal 5 of cylindrical form around which is wound part of the optical fibre of arm 2. A sawtooth signal generator (not shown) has its output applied to the piezoelectric crystal 5 which causes the crystal to expand and contract and thereby stretch and relax the fibre wound therearound.It is arranged that at the time of arrival of the laser pulse of frequency F the length of the optical fibre in mechanical contact with the crystal is increasing linearly as a function of time so that the frequency of the light pulse emerging from the fibre deforming means will be reduced by a frequency AF dependent upon the input frequency F and the rate of change in length of the optical path due to deformation of the optical fibre.

The pulse of frequency F- AF emerging from the piezoelectric device 5 which is used in sustitution for a Bragg cell which are expensive and inefficient passes through an optical fibre delay coil 6 before it is fed to a multi-way (three-way in the present example) optical fibre directional coupler or divider 7. This coupler 7 serves to combine the pulse of frequency F - AF with the pulse of frequency F which reaches the coupler through the parallel arm 3 which simply comprises an optical fibre which by-passes the deforming means 5 and the delay coil 6. At each of the three outputs of the coupler 7 a pulse of frequency F will be shortly followed by a pulse of frequency F- AF. These time displaced pulses of slightly different frequencies are applied simultaneously to three optical fibre sensors 8,9 and 10 which may form part of a hydrophone sensor array.

These optical fibre sensors may for example have discontinuities 11 distributed in the manner shown in Figure 2. Such a multi-fibre sensing arrangement which reduces significantly the optical losses incurred in single fibre sensors including a high number of discontinuities forms the subject of the present invention.

As can be seen from Figure 2 the shortest optical fibre sensor 8 of the three sensors has seven discontinuities 11 equally spaced along its entire sensing length. The next longest sensor 9 has five discontinuities equally spaced over an end section of its length located at a distance from the coupler end (e.g. left-hand end as viewed in drawing) of the sensor equal to the length of the sensor 8 plus the spacing between discontinuities. The longest fibre sensor 10 also has five discontinuities equally spaced over a corresponding end section which is located at a distance from the coupler end equal to the length of the sensor 9 plus the distance between discontinuities. It will be appreciated that the three sensors in combination will be effective in monitoring acoustic signals over a path length equal to the length of sensor 10.Thus the sensors 8,9 and 10 effectively constitute a low loss equivalent of a single sensor arrangement having seventeen discontinuities along the single sensor which high number of discontinuities will result in relatively high optical losses.

Although the multi-way coupler will introduce power losses which may be equivalent to that arising at a number of discontinuities (eg from 3 to 20) the number of optical sensors and discontinuities will normally be sufficiently high for this power loss to be readily accommodated.

As in the optical detecting system described in our co-pending Patent Application No. 8220793 previously referred to parts of the time displaced pulse singals of slightly different frequencies reflected from discontinuities 11 will be caused to interfere with or heterodyned with the signals reflected for preceding discontinuities to produce a beat frequency signal which will be phase modulated by changes in length of the fibre elements between discontinuities due to impingement thereon of acoustic waves. The beat frequency signal which may be phase modulated is applied through the coupler7 to a detector 12 and processor 13 for detection and measurement of acoustic waves impinging on the fibre sensors 8,9 and 10.

Although in specific embodiment described optical fibre couplers 4 and 7 are used these could be replaced by discrete optical elements including beam splitters and suitable lenses.

Claims

1. An optical sensing system comprising a plurality of optical fibre sensors of different lengths effectively connected in parallel and arranged to be subjected along their lengths to fibre deforming forces during operation of the system and means for producing coherent light signals for simultaneous transmission along said optical fibre sensors, in which each of the optical fibre sensors is provided with a plurality of equally spaced reflective discontinuities which effectively define along each fibre sensor a plurality of discrete optical fibre elements so that the sensors in combination afford equally spaced discontinuities over a path length equivalent to the longest fibre sensor, and in which a small proportion of each light signal transmitted along each fibre sensor during operation of the sensing system will be reflected back along the fibre from each of the discontinuities whereby each reflected light signal after the first in each of the sensors, interferes with either the previously reflected signal from the preceding discontinuity of the same sensor, or a reference light signal of the same frequency or a frequency with a constant difference frequency to the said transmitted light signal to produce an electrical signal in photo detection means to which all the sensors are coupled, the difference between respective electrical signals corresponding to successive fibre elements in each of the optical fibre sensors being dependant upon the length of the fibre elements so that changes in length of these elements produced by the incidence of deforming forces will result in changes in the electrical signals which will be detected.

2. An optical sensing system as claimed in claim 1 in which the optical fibre sensors are coupled at one end thereof through a multi-way coupler to signal generating means for producing light pulses for transmission simultaneously along the optical fibre sensors and to detector and processor means for monitoring the signals reflected from the discontinuities along the sensors in order to detect and or measure any phase modulation resulting from the impingement of deforming forces on the optical fibre sensors.

3. An optical sensing system as claimed in claim 1 or claim 2, in which three optical fibre sensors of different lengths are coupled together at one end thereof, the shortest optical fibre being provided with equally spaced discontinuities over its entire length, the next longest optical fibre having at its coupled end a section without discontinuities corresponding in length of the total length of the first optical fibre sensor plus a distance corresponding to the spacing between discontinuities but the remainder of the fibre being divided into fibre elements by equally spaced discontinuities and the third and longest optical fibre also having a section at its coupled end without discontinuities, such section corresponding in length to the length of the second optical fibre plus the spacing between discontinuities, the remainder of the third optical fibre sensor being divided equally into fibre elements by a plurality of discontinuities.

4. An optical sensing system as claimed in any preceding claim, in which two-pulse signals each comprising pulses of slightly different frequencies (F and F + AF) and of pre-determined duration and time relationship are transmitted simultaneously along the optical fibre sensors so that small proportions of the pulses are reflected back at each of the sensor discontinuities, in which the signal reflected back from the second discontinuity of each sensor is caused to interfere or is heterodyned with that reflected from the first discontinuity to produce a detectable electrical beat frequency signal the mod ulatión of which will vary with changes in length of the first optical fibre element between the first and second optical fibre discontinuities of each sensor and signals reflected from the third, fourth and last discontinuities, of each fibre sensor as the case may be, will similarly interfere with those signals re flected from the preceding discontinuity.

5. An optical sensing system as claimed in claim 1, in which a single pulse light signal of frequency (F) is transmitted down each of the optical fibre sensors simultaneously for reflection from the fibre discontinuities whilst a two-pulse signal comprising consecutive pulses of slightly different frequencies, (F and F + AF) is utiiised as a continuous reference at the photo-detection means to beat with the reflected signals of frequency (F) and in which means are provided to electronically delay or store information from a preceding reflection in order to make a comparison between the phase relationships of consecutive reflections.

6. An optical fibre sensing system as claimed in claim 1, in which signals reflected from the optical fibre discontinuities are homodyned by arranging that one or two light pulses in pre-determined time relationship and of the same frequency are transmitted along the optical fibre sensors simultaneously and reflected signals from the respective discontinuities of each sensor except the first are caused to interfere with the signals reflected from the preceding discontinuities of the same sensor to produce amplitude modulated electrical signals in dependance upon the lengths of the optical fibre elements, the photo-detection means detecting and or measuring any changes in modulation due to deformation of the fibre elements.

7. An optical sensing system substantially as hereinbefore described with reference to the accompanying drawings.

8. Hydrophone equipment embodying an optical sensing system as claimed in any preceding claim.