GB2147758A - Optical detecting and/or measuring - Google Patents
Optical detecting and/or measuring Download PDFInfo
- Publication number
- GB2147758A GB2147758A GB08322715A GB8322715A GB2147758A GB 2147758 A GB2147758 A GB 2147758A GB 08322715 A GB08322715 A GB 08322715A GB 8322715 A GB8322715 A GB 8322715A GB 2147758 A GB2147758 A GB 2147758A
- Authority
- GB
- United Kingdom
- Prior art keywords
- optical
- optical fibre
- frequency
- sensing system
- discontinuities
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 230000003287 optical effect Effects 0.000 title claims abstract description 34
- 230000005540 biological transmission Effects 0.000 claims abstract description 5
- 239000013307 optical fiber Substances 0.000 claims description 48
- 239000013078 crystal Substances 0.000 claims description 13
- 230000008602 contraction Effects 0.000 claims description 3
- 238000010168 coupling process Methods 0.000 claims description 2
- 238000005859 coupling reaction Methods 0.000 claims description 2
- 230000002452 interceptive effect Effects 0.000 claims description 2
- 239000000835 fiber Substances 0.000 description 13
- 230000035559 beat frequency Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000005452 bending Methods 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000001427 coherent effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
- G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
- G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
- G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
- G01R15/24—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices
- G01R15/248—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using light-modulating devices using a constant light source and electro-mechanically driven deflectors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
- G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods
- G01J2009/0226—Fibres
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
- Optical Transform (AREA)
Abstract
In optical sensing system, light of frequency F from pulse laser 1 is divided between parallel optical transmission arms 2,3 and one of the arms embodies a deforming means 5 which produces a frequency shift DELTA F in the signal of frequency F, and a delay line 6. The other parallel arm comprises a by-pass arm. A combiner 7 is provided which combines the signals from the parallel arms after reflection by discontinuity-bearing optical fibres 8,9,10, and transmits them to photodetector 12 for processing 13. <IMAGE>
Description
1
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 timedisplaced 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 WI and WI + WI and of predetermined duration and time relation- ship are generated and transmitted along an optical fibre of an optical fibre sensor having a by-pass path (e.g. optical fibre) the length of which is such thatthe 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 + F and WI + WI from light pulses of frequencies F and WI 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 100% deflection efficiency). Therefore, Bragg cells provide incom- plete conversion of optical powerfrorn 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 GB 2 147 758 A 1 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), one of the parallel arms embodying optical fibre cleforming means effective for producing 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 by-pass 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 two-pulse signals of frequencies F and F -- AF may for example be utilised to advantage in the optical sensing system forming the subject of the aforesaid copending Patent Applications but the present invention is not limited thereto.
As is explained in our co-pending Patent Applica- tion 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 multi-element hyd rophone array as described in our co-pending Patent Application No. 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 outthe present invention the optical fibre deforming means may comprise a piezoelectric crystal preferably 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 frequency 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 fibre deforming means may alternatively comprise magnetostrictive means for producing stretching of an optical fibre in response to changing magnetic fields.
The combining means of the present invention may in accordance with our co-pending Patent Application No. comprise a multi-way optical coupler or divider so that the combined output of the aforesaid parallel transmission arms (i.e. timedisplaced 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 discon- tinuities equal in number to the total discontinuities 2 GB 2 147 758 A 2 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 modula- tion 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 according to the present invention eminently suitable for use in hydrophones; and, Figure 2 is a diagram showing the positions of discontinuities along the three optical fibre sensors used in the system of 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 the present invention 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 3 115 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 our co-pending Patent Application No.
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. This the sensors 8,9 and 10 effectively constitute a low loss equivalent to a single sensor arrangement having seventeen dis- continuities 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 pre- viously 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 frequen- cy 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 13 for detection and measurement of acoustic waves impinging on the fibre sensors 8,9 and 10.
Claims (8)
1. An optical sensing 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, one of the parallel arms embodying optical fibre deforming means effective for producing a frequency shift (A F) in the signal of frequency (F) transmitted thereto and optical fibre signal delay means for delaying said signal and the other parallel arm comprising a by-pass arm and in which combining means is provided for combining signal outputs from the parallel arms.
2. An optical sensing system as claimed in claim 1, in which the optical fibre deforming means comprises a piezoelectric crystal.
3. An optical sensing system as claimed in claim 2, in which the piezoelectric crystal is of cylindrical form around which an optical fibre is wound.
4. An optical sensing system as claimed in claim 2 or claim 3, in which a sawtooth waveform drive signal of suitable frequency and in predetermined time relationship with the laser pulse of frequency (F) derived from the pulse laser light source is applied to the input of the piezoelectric crystal to 3 GB 2 147 758 A 3 produce successive expansions and contractions of the crystal which will cause the optical fibre to be successively stretched and relaxed so thatthe light pulse of frequency (F) being transmitted through the optical fibre wound around or otherwise coupled to the crystal will be phase modulated or delay modulated to provide an output from the optical fibre of frequency (F - AF.
5. An optical sensing system as claimed in claim 1, in which the optical fibre deforming means comprises magnetostrictive means for producing stretching or contraction of an optical fibre in response to changing magnetic fields.
6. An optical sensing system as claimed in any preceding claim in which the combining means comprises a multi-way optical coupler or divider so that the combined output of the aforesaid parallel transmission arm is divided 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.
7. An optical sensing system as claimed in claim 6, in which the multiway coupler or divider is arranged to enable 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.
8. An optical sensing system substantially as hereinbefore described with reference to the accompanying drawings.
Printed in the U K for HMSO, D8818935,3185,7102. Published by The Patent Office, 25Southampton Buildings, London, WC2A lAY, from which copies may be obtained.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08322715A GB2147758B (en) | 1983-08-24 | 1983-08-24 | Optical detecting and/or measuring |
| GB08416306A GB2145514B (en) | 1983-08-24 | 1984-06-27 | Optical detecting and/or measuring systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08322715A GB2147758B (en) | 1983-08-24 | 1983-08-24 | Optical detecting and/or measuring |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2147758A true GB2147758A (en) | 1985-05-15 |
| GB2147758B GB2147758B (en) | 1987-08-05 |
Family
ID=10547769
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08322715A Expired GB2147758B (en) | 1983-08-24 | 1983-08-24 | Optical detecting and/or measuring |
| GB08416306A Expired GB2145514B (en) | 1983-08-24 | 1984-06-27 | Optical detecting and/or measuring systems |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08416306A Expired GB2145514B (en) | 1983-08-24 | 1984-06-27 | Optical detecting and/or measuring systems |
Country Status (1)
| Country | Link |
|---|---|
| GB (2) | GB2147758B (en) |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2166020A (en) * | 1984-09-29 | 1986-04-23 | Plessey Co Plc | Otdr-uses multiple frequencies to detect distortions in an optical fibre |
| GB2182223A (en) * | 1985-10-23 | 1987-05-07 | Stc Plc | Optical fibre reflectometer |
| GB2189880A (en) * | 1986-04-30 | 1987-11-04 | Plessey Co Plc | Optical sensor system |
| GB2192984A (en) * | 1986-07-25 | 1988-01-27 | Plessey Co Plc | Optical sensing arrangement |
| GB2242267A (en) * | 1989-02-10 | 1991-09-25 | Plessey Co Plc | Sonar detector |
| US5237632A (en) * | 1990-12-06 | 1993-08-17 | Gec-Marconi Limited | Optical fibre coil assemblies |
Families Citing this family (13)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4717253A (en) * | 1985-11-22 | 1988-01-05 | Massachusetts Institute Of Technology | Optical strain gauge |
| US4725728A (en) * | 1986-08-13 | 1988-02-16 | The United States Of America As Represented By The Secretary Of The Navy | Fiber optical time delay resonant oscillating strain gauge |
| US4974961A (en) * | 1986-09-12 | 1990-12-04 | Jackson David A | Optical fibre measuring system |
| GB2199135B (en) * | 1986-12-10 | 1990-11-07 | Plessey Co Plc | Improvements relating to optical sensing arrangements |
| GB2202046A (en) * | 1987-03-11 | 1988-09-14 | Plessey Co Plc | Optical fibre sensor arrangement |
| US5485296A (en) * | 1989-07-29 | 1996-01-16 | British Telecommunications Public Limited Company | Monitor unit for monitoring an optical waveguide |
| GB8917488D0 (en) * | 1989-07-31 | 1989-09-13 | British Telecomm | A monitor unit for monitoring an optical waveguide |
| GB0113766D0 (en) * | 2001-05-31 | 2003-02-26 | Bae Systems Plc | Improvements relating to optical delay lines |
| CA3175365A1 (en) | 2009-05-27 | 2010-12-02 | Silixa Ltd | Method and apparatus for optical sensing |
| AU2015200314B2 (en) * | 2009-05-27 | 2017-02-02 | Silixa Limited | Method and apparatus for optical sensing |
| AU2010359357B2 (en) | 2010-08-19 | 2014-02-27 | Halliburton Energy Services, Inc. | Optical pressure sensor |
| CN102564642B (en) * | 2012-02-21 | 2013-08-07 | 中国计量学院 | Fully-distributed optical fiber sensor for optical fiber Raman frequency shifter fused with Raman amplification effect |
| CN107045207A (en) * | 2017-06-07 | 2017-08-15 | 中国科学院半导体研究所 | Train of pulse produces the structure controlled with time domain pattern |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0021945A1 (en) * | 1979-06-29 | 1981-01-07 | Thomson-Csf | Monomode fibre-optic hydrophone functioning by the elasto-optic effect |
| EP0092369A2 (en) * | 1982-04-20 | 1983-10-26 | Sumitomo Electric Industries Limited | Light frequency change detecting method and apparatus |
-
1983
- 1983-08-24 GB GB08322715A patent/GB2147758B/en not_active Expired
-
1984
- 1984-06-27 GB GB08416306A patent/GB2145514B/en not_active Expired
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0021945A1 (en) * | 1979-06-29 | 1981-01-07 | Thomson-Csf | Monomode fibre-optic hydrophone functioning by the elasto-optic effect |
| EP0092369A2 (en) * | 1982-04-20 | 1983-10-26 | Sumitomo Electric Industries Limited | Light frequency change detecting method and apparatus |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2166020A (en) * | 1984-09-29 | 1986-04-23 | Plessey Co Plc | Otdr-uses multiple frequencies to detect distortions in an optical fibre |
| GB2182223A (en) * | 1985-10-23 | 1987-05-07 | Stc Plc | Optical fibre reflectometer |
| GB2189880A (en) * | 1986-04-30 | 1987-11-04 | Plessey Co Plc | Optical sensor system |
| GB2189880B (en) * | 1986-04-30 | 1989-12-28 | Plessey Co Plc | Optical sensor system |
| GB2192984A (en) * | 1986-07-25 | 1988-01-27 | Plessey Co Plc | Optical sensing arrangement |
| GB2192984B (en) * | 1986-07-25 | 1990-07-18 | Plessey Co Plc | Optical sensing arrangements |
| GB2242267A (en) * | 1989-02-10 | 1991-09-25 | Plessey Co Plc | Sonar detector |
| US5237632A (en) * | 1990-12-06 | 1993-08-17 | Gec-Marconi Limited | Optical fibre coil assemblies |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8416306D0 (en) | 1984-08-01 |
| GB2145514B (en) | 1986-12-17 |
| GB2147758B (en) | 1987-08-05 |
| GB2145514A (en) | 1985-03-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19920824 |