GB2145514A - Optical detecting and/or measuring systems - Google Patents

Abstract

The system comprises a laser 1 which generates coherent light pulses for transmission down first and second optical transmission arms 2 and 3 respectively. A pulse splitting means 4 is provided for dividing each of the pulses so that each pulse is transmitted down each of the arms 2 and 3. The first arm comprises a varying means 5, e.g. a piezoelectric or magnetostrictive element but preferably in the form of an integrated optic phase modulator, for shifting the frequency of the pulses transmitted along the first arm 2, and a delay means 6 for delaying the pulses transmitted along the first arm 2. A multi-way optical coupler 7 combines the pulses which emerge from the arms 2, 3 and preferably divides the combined output between a plurality of optical fibre sensors 8,9,10. The coupler 7 enables reflected and interfering pulses from the sensors 8,9 and 10 to be combined before being fed to a phase modulation detector 12 and processor 13. <IMAGE>

Description

SPECIFICATION Improvements relating to optical detecting and/or measuring systems This invention relates to optical detecting and/or measuring systems of the kind in which time-displaced pulses of slightly different frequencies are utilised for sensing strain or deformation (e.g. elongation or bending) in one or more optical fibres.

In our co-pending Patent Application No.

8220793 there is described an optical sensing system in which two-pulse signals comprising two pulses of slightly different frequency F and F + AF and of predetermined duration and time relationship are generated and transmitted along an optical fibre provided along its length with a number of equally spaced discontinuities which effectively divide the fibre into a plurality of discrete fibre elements so that a small proportion of each light signal transmitted along the fibre will be reflected back along the fibre from each of the discontinuities. The signal reflected from the second discontinuity is caused to interfere with that reflected from the first discontinuity (i.e. the pulse of frequency F of the second reflected signal is heterodyned with the pulse of frequency F + AF of the first reflected signal).This heterodyning produces a detectable electrical beat frequency signal the modulation of which will vary with changes in length of the first optical fibre element between the first and second optical fibre discontinuities. Any such strain or deformation of the first and subsequent fibre elements can thus be detected and measured.

Additionally, in the optical fibre interferometer forming the subject of our co-pending Patent Application No. 8207961 two-pulse signals of slightly different first and second frequencies W1 and W1 + AWl and of predetermined duration and time relationship are generated and transmitted along an optical fibre to an optical fibre sensor having a bypass path (e.g. optical fibre) the length of which is such that the pulses at the first and second frequencies are then combined to produce coincident pulses for which a frequency difference signal is provided by non-linear optical detector means. Phase detector means serves to detect any phase displacement of the frequency difference signal which occurs when acoustic pressure waves impinge on the optical fibre sensor.In both of these optical systems disclosed in co-pending Patent Applications Nos. 8220793 and 8207961 Bragg cells are utilised to produce the shifted frequency components F + AF and WI + IXW1 from light pulses of frequencies F and W1 derived from a pulsed laser light source.

Bragg cells are expensive and cause incomplete deflection of the laser light beam in the first order diffracted beam (i.e. less than 10096 deflection efficiency). Therefore, Bragg cells provide incomplete conversion of optical power from one frequency to the other.

According to the present invention the aforesaid difficulties encountered with Bragg cells are obviated by providing an optical system in which a pulse output of frequency F from a pulse laser source is divided between parallel optical transmission arms, as by the use of an optical fibre branch-coupling means or discrete optical elements (e.g. beam splitter and convex lenses), ome of the parallel arms embodying varying means effective for varying an optical path in order to produce a frequency shift AF in the pulse signal of frequency F transmitted thereto and optical fibre signal delay means for delaying said signal, and the other parallel arm comprising a bypass arm (e.g. optical fibre) and in which combining means is provided for combining signal outputs from the parallel arms.

The combined output which comprises twopulse signals of frequencies F and F-AF may for example be utilised to advantage in the optical sensing systems forming the subject of the aforesaid co-pending Patent Applications but the present invention is not limited thereto.

As is explained in our co-pending Patent Application No. 8207961 the provision of a delay between the signals F and F-AF by the signal delay means enables the pulses of different frequencies to be derived from the same coherent light pulse and if this delay is equal to the interval occurring between consecutive reflections from a multielement hydrophone array as described in our co-pending Patent Application No. 832271 6 the effect of phase noise and coherence length limitations associated with light sources with non-perfect monochromatic outputs, such as semiconductor lasers or multimode gas lasers, can be reduced.

In carrying out the present invention the varying means may comprise a piezoelectric crystal preferabiy of cylindrical form around which an optical fibre is wound. A sawtooth waveform drive signal of suitable frequency and in predetermined time relationship with the laser pulse of frequencey F derived from the pulsed laser light source as will hereinafter be more fully explained may be applied to the input of the piezoelectric crystal to produce successive expansions and contractions of the crystal which will cause the optical fibre to be successively stretched and relaxed so that the light pulse of frequency F being transmitted through the optical fibre wound around the crystal will be phase modulated (or delay modulated) to provide an output from the optical fibre of frequency F -AF.

The varying means may alternatively comprise magnetostrictive means for producing stretching of an optical fibre in response to changing magnetic fields.

The varying means may alternatively comprise a serrodyne frequency shifter in the form of an integrated optic phase modulator. The integrated optic phase modulator may comprise an optical waveguide diffused into a lithium niobate substrate. On either side of the waveguide an electrode is located so that a voltage applied across the electrodes causes the optical refractive index of the waveguide to change. Light passing through the waveguide will be phase shifted by the change in refractive index and hence, by applying a suitably varying voltage to the electrodes, the frequency of the light passing through the waveguide can be shifted.

The combining means of the present invention may in accordance with our co-pending Patent Application No. 8322716 comprise a multi-way optical coupler or divider so that the combined output of the aforesaid parallel transmission arms (i.e. time-displaced pulses of frequencies F and F-AF) is divided between a plurality of optical fibre sensors which are provided with a number of reflective discontinuities over different sections of their sensing lengths and which in combination correspond to a single optical fibre sensor having over its entire sensing length a series of equally-spaced discontinuities equal in number to the total discontinuities of the plurality of sensors.The multi-way divider also enables the reflected and interfering signals from the discontinuities in the optical fibre sensors to be combined before being fed to a phase modulation detector and processor. As fully explained in the last-mentioned co-pending Patent Application this arrangement using a plurality of optical fibre sensors provides significant optical loss advantages.

By way of example the present invention will now be described with reference to the accompanying drawing in which: Figure 1 is a schematic diagram of an optical detecting and measuring system embodying to the present invention eminently suitable for use in hydrophones; Figure 2 is a diagram showing the positions of discontinuities along the three optical fibre sensors used in the system of Figure 1; Figure 3 is a diagram of an integrated optic phase modulator which can be used as a varying means in the embodiment shown in Figure 1; Figure 4a is a graph showing a voltage waveform applied across electrodes of the modulator of Figure 3; and Figure 4b is a graph showing a frequency shift caused in light passing through the modulator of Figure 3.

Referring to Figure 1 of the drawings, 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 the present invention a varying means in the form of an 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 substitution 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 frequenccy 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 our co-pending Patent Application No.

8322716.

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 to a single sensor arrangement having seventeen discontinuities along the single sensor which high number of discontinuities will result in relatively high optical losses.

As in the optical detecting system described in our co-pending Patent Application No.

8220793 previously referred to parts of the time displaced pulse signals of slightly different frequencies reflected from discontinuities 11 will be caused to interfere with or heterodyned with the signals reflected from 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 coupler 7 to a detector 12 and processor 1 3 for detection and measurement of acoustic waves impinging on the fibre sensors 8,9 and 10.

The optical sensing shown in Figure 1 may, instead of having an optical fibre deforming means formed by the piezoelectric crystal 5, have a serrodyne frequency shifter in the form of an integrated optic phase modulator as shown in Figure 3. The modulator comprises a lithium niobate substrate 1 5 in which an optical waveguide 16 is diffused. A pair of electrodes 1 7 and 18 are located either side of the waveguide 1 6 and are connected to a variable voltage supply 19. When a voltage is applied between electrodes 1 7 and 18, an electric field is set up across the waveguide 1 6 which causes the change in the refractive index of the waveguide.Hence, light passing in the waveguide 1 6 is phase shifted by an amount dependent upon the voltage applied between the electrodes 1 7 and 1 8.

If, for example, the voltage applied is varied with time in a manner as illustrated in Figure 4a (i.e. in a triangular form as a function of time) the phase of the light passing through the waveguide also changes with time in the same manner, Such a phase change corresponds to a frequency shift which is illustrated in Figure 4b. When no voltage is applied the light emerging from the waveguide has a frequency F,. When the voltage waveform is applied to the electrodes 1 7 and 18, then the frequency of the light will vary between F, + AF to F,--F.

The modulator has several advantages compared with other frequency shifting means.

For example, it has a better performance at frequencies of the order of 100KHz and has less distortion.

Claims (8)

1. An optical system comprising first and second optical transmission arms for transmitting coherent light pulses of frequency F generated by a laser light source wherein the first optical transmission arm comprises varying means effective for varying an optical path in order to produce a frequency shift AF in the light pulses transmitted along the first arm and delay means for delaying the pulses transmitted along the first arm, and wherein the second optical transmission arm by-passes the first arm, and combining means is provided for combining pulse outputs from the first and the second optical transmission arms.

2. An optical system according to claim 1 comprising pulse splitting means for dividing each of the light pulses so that each of the light pulses is transmitted down both the first and the second optical transmission arms.

3. An optical system according to claim 2 wherein the pulse splitting means is an optical fibre branch-coupling means.

4. An optical system according to any one of the preceding claims, wherein the varying means comprises a serrodyne frequency shifter in the form of an integrated optic phase modulator.

5. An optical system according to claim 4, wherein the integrated optic phase modulator comprises an optical waveguide diffused into a lithium niobate substrate on either side of which optical waveguide is disposed an electrode so that variations in a voltage applied across the electrodes causes the optical refractive index of the waveguide to change.

6. An optical system as claimed in any one of the preceding claims, in which the combining means comprises a multi-way optic coupler or divider so that the combined output of the first and second transmission arms is divided between a plurality of optical fibre sensors which are provided with a number of reflective discontinuities over different sections of their sensing lengths and which in combination correspond to a single optical fibre sensor having over its entire sensing length a series of equally space discontinuities equal in number to the discontinuities of the plurality of sensors.

7. An optical system as claimed in claim 6, in which the multi-way coupler or divider is arranged to enable reflected and interfering signals arising from the discontinuities in the optical fibre sensors to be combined before being fed to a phase modulation detector and processor.

8. An optical system substantially as herein described with reference to the accompanying drawings.