GB2189880A

GB2189880A - Optical sensor system

Info

Publication number: GB2189880A
Application number: GB08610535A
Authority: GB
Inventors: Christopher Lamb
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1986-04-30
Filing date: 1986-04-30
Publication date: 1987-11-04
Also published as: GB8610535D0; GB2189880B

Abstract

An optical sensor system comprises an optical fibre 13 arranged to be subjected along its length to fibre deforming forces during operation of the system and means, including a laser 1, for producing coherent light signals for transmission along the fibre, the fibre being divided lengthwise into discrete fibre elements S1, S2,..., Sn with spaced discontinuities R0, R1,....., Rn each of which is capable of reflecting a small proportion of the transmitted light back along the fibre, the reflected signals causing optical interference with each other or with a reference frequency (f) and the output being applied to a photo- detector 6 in order to produce difference signals indicative of the deforming forces interacting with the fibre. An acousto-optic modulator 7, as well as switching on and off the beam 2 from the laser 1, shifts the frequency of the beam 2 from (f) to (f+/- DELTA f) for transmission to the optical fibre 13. <IMAGE>

Description

SPECIFICATION Optical sensor system This invention relates to an optical sensor system for sensing strain or deformation, that is elongation or bending, of various members.

Although the present invention is particularly concerned with hydrophones and the sensing of changes in length of an optical fibre in such hydrophones due to the impingement thereon of acoustic waves, it should be understood that the invention is not limited to such application. Any interferometric sensor system using reflectometry might be suitable.

According to the present invention, there is provided an optical sensor system comprising an optical fibre arranged to be subjected along its length to fibre deforming forces during operation of the system and means for producing coherent light signals for transmission along said fibre, in which the fibre is provided along its length with a plurality of spaced discontinuities which effectively divide the fibre into discrete fibre elements so that a small proportion of each light signal being transmitted along the fibre will be reflected back along the fibre from each of the discontinuities whereby each reflected light signal after the first interferes with the previously reflected signal from the preceding discontinuity 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 square law photo-detection means of the system, in which a pulse signal of frequency (f+Af) is transmitted along the optical fibre so that small proportions of the pulses are reflected back at each fibre discontinuity, the reflected signals being subtracted from a reference frequency signal (f) to give difference signals indicative of the deforming forces interacting with the fibre.

The reference signal may be applied continuously to the photo-detection means possibly through a delay means effective to reduce the time difference between reference light and the reflected signals.

Preferably, the reference frequency signal is derived from a local oscillator. The reflected signals may be applied after demodulation to a bank of difference amplifiers arranged to provide the required difference signals.

The invention also comprises an optical fibre hydrophone including the optical sensor system.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows the optical system of a first optical sensor; Figure 2 is an optical system of a second optical sensor; Figure 3 is a circuit diagram of the electronic circuit required to derive a signal representative of a fibre deforming perturbation for each individual sensing element, and, Figure 4 is a timing diagram for a typical three-sensor array showing pulses present in different parts of the optical system.

As shown in the optical system diagram of Figure 1, a gas laser 1 produces a coherent light beam 2 which may be produced continuously or in a different embodiment this might be produced in a pulsed manner. The beam 2, which is at a frequency f, is directed through a low reflectivity beam splitter 3 by which part of the beam is diverted through a lens 4 and onto a photodiode 6. This input to the photodiode 6 forms a reference frequency signal (f) of the sensor system.

The remainder of the beam 2 passes through the beam splitter 3 and falls onto an acousto-optic modulator 7 which in this embodiment is a Bragg cell. The modulator 7 acts to switch and frequency shift the beam 2 in accordance with a pulsed RF input signal 8 derived from a local oscillator 9. The modulator thus switches the continuous team to make pulses of predetermined length and spacing. The modulator acts to shift the frequency of the incoming beam so that the pulses to be eventually returned from the sensor element can be compared with those pulses which have not passed through the element.

The modulator 7 then transmits the beam 2 through a second lens 11 and into a downlead 12 connected with an optical sensor array 13. The sensor array 13 comprises a number of lengths of optical fibre each of which constitutes a sensor element S. The lengths are connected in series to form a line of elements S" S2 53 ... Sn. At the beginning of the line of elements there is connected a semi-reflecting joint R0 and further joints R, R2, R3 ... Rn are connected at the end of each element along the whole line of the elements.

The elements in the line may be of differing lengths if necessary.

In operation of this system, the light beam 2 is switched and frequency shifted by the acousto-optic modulator 7 which is controlled by a pulsed RF input signal 8 from the oscillator 9. A single pulse of frequency (f+Af) is transmitted at regular intervals down the optical fibre in the optical sensor array 13. In the timing diagram of Figure 4, which depicts the pulses occurring in the sensor system after a starting time of T,, the pulses transmitted into the fibre are shown on the line 14.

Each pulse travels down the length of the fibres in the array 13. At each semi-reflecting joint R0 to R,, a proportion of the pulse signal is reflected back to the beginning of the array 13 and this proportion passes through the acousto-optic modulator 7 and is received by the photodiode 6. The light in each reflection has a phase shift which is dependent on the fibre sensitivty up to the point of reflection in the fibre. The reflected signals as they are received from succeeding splices R,, R1, R2 Rn are shown on the timing diagram at line 16. For simplicity in the Figure 4 diagram, this shows only reflected pulses that would be present if the sensor system included only four reflecting splices, RO to R3.

The photodiode 6 also receives an input from the light beam 2 since part of this beam is diverted by the beam splitter 3 into the diode to form a reference frequency beam.

This reference frequency beam is shown on the timing diagram at line 17.

The reference frequency beam acts as a phase reference signal of the interferometer and, at the photodiode, heterodyning takes place with the reflected signals from the array 13. The resulting output from the photodiode is shown on the timing diagram at line 18.

A circuit diagram of an electronic system used to interpret the photodiode output is given in Figure 3. In this diagram the train of pulses from the photodiode 6 passes through an amplifier 19, a demultiplexer 21, filters 22 and phase demodulators 23. The resulting demodulated outputs will contain the information about length and refractive index changes on the optical fibre traversed by each individual reflection. By feeding the output signals through time delays 24 into difference amplifiers 26 and subtracting the signals for consecutive reflections, the signal for each individual sensing element may be obtained. The inclusion of electronic subtraction means is necessary in order to compensate for pertur bations of the array occuring on parts of the fibre other than the elements such as parts upstream of a particular element and on other elements.

Figure 2 shows a modified optical system which was designed to avoid any problem which may occur due to the finite coherence length of the laser beam. Instead of the reference beam being obtained using a beam splitter, a zero order (unfrequency shifted) beam from the acousto-optic modulator 7 is used and this is passed down a delaying length 27 of the optical fibre to reduce the time difference between the reference light pulses and the sensor pulses. In an alternative embodiment, the delaying length 27 of the fibre may be replaced by an optical path in free space rather than an actual fibre length.

The sensor system of the invention has been found to have advantages over alternative systems in that: 1. Only a single heterodyne frequency is generated by the acousto-optic moduiator and this can be made higher (typically 40MHz) than the frequency presently used and this makes the filtering requirement considerably more simple.

2. The reference beam obtained from the main beam and hence the overall power levels can be a lot higher than can be obtained at present leading to an improvement in the heterodyne signal-to-noise level.

The system of the invention thus avoids a serious loss of light which is an advantage over some alternative systems.

3. The reference beam will be a known polarization which should allow better control of the polarization conditions in the system.

The foregoing descriptions of embodiments of the invention have been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, the invention is not limited in its use for an optical fibre hydrophone, and any interferometric sensor system using reflectometry might be suitable.

Claims

1. An optical sensor system comprising an optical fibre arranged to be subjected along its length to fibre deforming forces during operation of the system and means for producing coherent light signals for transmission along said fibre, in which the fibre is provided along its length with a plurality of spaced discontinuities which effectively divide the fibre into discrete fibre elements so that a small proportion of each light signal being transmitted along the fibre will be reflected back along the fibre from each of the discontinuities whereby each reflected light signal after the first interferes with the previously reflected signal from the preceding discontinuity 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 square law photo-detection means of the system, in which a pulsed signal of frequency (f+Af) is transmitted along the optical fibre so that small proportions of the pulses are reflected back at each fibre discontinuity, the reflected signals being subtracted from a reference frequency signal (f) to give difference signals indicative of the deforming forces interacting with the fibre.

2. A sensor system as claimed in Claim 1, in which the said reference frequency signal is applied continuously to the photo-detection means possibly through a delay means effective to reduce the time difference between reference light pulses and the reflected signals.

3. A sensor system as claimed in Claim 1 or 2, including a local oscillator arranged to produce the required reference frequency signal.

4. A sensor system as claimed in any one of Claims 1 to 3, including a bank of difference amplifiers to which the said reflected signals are applied after demodulation to provide the required difference signals.

5. An optical fibre hydrophone including an optical sensor system as claimed in any one of Claims 1 to 4.

6. An optical sensor system substantially as hereinbefore described with reference to the accompanying drawing.

CLAIMS New claims or amendments to claims filed on Superseded claims 1-6 New or amended claims:- 1-6

1. An optical sensor system comprising an optical fibre arranged to be subjected along its length to fibre deforming forces during operation of the system and a laser light source for producing coherent light signals for transmission along said fibre, in which the fibre is provided along its length with a plurality of spaced discontinuities which effectively divide the fibre into discrete fibre elements so that a small proportion of a light signal being transmitted along the fibre will be reflected back along the fibre from each of the discontinuities whereby each reflected light signal after the first interferes with the previously reflected signal from the preceding discontinuity, in which a pulsed signal of frequency (fjAf) is transmitted along the optical fibre so that small proportions of the pulses are reflected back at each fibre discontinuity, the reflected signals being applied to a photodetector device simultaneously with a reference frequency signal (frequency f) from said laser source whereby interference between the signals occurs at the photodetector, an electrical output from said photodetector being applied to demultiplexer, filter and phase demodulation means and then to a bank of difference amplifiers in which signals for consecutive reflections are subtracted to provide an output signal for each individual sensing element of the fibre this signal being indicative of the deforming forces acting on that element.

2. A sensor systems as claimed in Claim 1, in which the pulsed signal is formed by an acousto-optic modulator such as a Bragg cell.

3. A sensor system as claimed in Claim 2, in which the modulator is operated so as to provide a pulsed signal at frequency (flAf) when the laser source has a frequency of (f).

4. A sensor system as claimed in any one of Claims 1 to 3, in which said reference frequency signal is applied to the photo-detection means through a delay means effective to reduce the time difference between reference light pulses and the reflected signals.