GB2248928A - Optical fibre reflector - Google Patents
Optical fibre reflector Download PDFInfo
- Publication number
- GB2248928A GB2248928A GB9022739A GB9022739A GB2248928A GB 2248928 A GB2248928 A GB 2248928A GB 9022739 A GB9022739 A GB 9022739A GB 9022739 A GB9022739 A GB 9022739A GB 2248928 A GB2248928 A GB 2248928A
- Authority
- GB
- United Kingdom
- Prior art keywords
- fibre
- optical fibre
- reflector
- light
- optical
- 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.)
- Withdrawn
Links
- 239000013307 optical fiber Substances 0.000 title claims abstract description 44
- 230000003287 optical effect Effects 0.000 claims abstract description 14
- 230000008878 coupling Effects 0.000 claims abstract description 3
- 238000010168 coupling process Methods 0.000 claims abstract description 3
- 238000005859 coupling reaction Methods 0.000 claims abstract description 3
- 239000000835 fiber Substances 0.000 claims description 38
- 230000001427 coherent effect Effects 0.000 claims description 6
- 230000005540 biological transmission Effects 0.000 claims description 4
- 238000001514 detection method Methods 0.000 claims description 4
- 239000000853 adhesive Substances 0.000 claims description 2
- 230000001070 adhesive effect Effects 0.000 claims description 2
- 230000001419 dependent effect Effects 0.000 claims 1
- 238000010276 construction Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000003491 array Methods 0.000 description 2
- 241001640753 Fibrella Species 0.000 description 1
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000000586 desensitisation Methods 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000004927 fusion Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 229910052709 silver Inorganic materials 0.000 description 1
- 239000004332 silver Substances 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 238000001771 vacuum deposition Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35383—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using multiple sensor devices using multiplexing techniques
-
- 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
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/241—Light guide terminations
- G02B6/243—Light guide terminations as light absorbers
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
Abstract
An optical fibre reflector comprises a pair of fibres (10, 11) interconnected intermediate of their ends to permit optical coupling therebetween. One of the fibres (11) has a reflective termination (12) at one end and a light dispersive termination at the opposite end.
Description
OPTICAL FIBRE REFLECTOR
This invention relates to optical fibre ref lectors for use in sensing systems such as for example are used in the detection of acoustic pressure waves e.g. hydrophone applications or temperature sensing.
Reflectometric arrays of interferometric sensors are formed by constructing a line of fibre transducers separated by a semi reflecting element such as is described in British
Patent Specification No. 2126820B.
One of the problems experienced with such optical fibre sensors is the tendency for multiple reflections to occur within the optical fibre sensor causing cross talk between the sensing elements of the sensor. This is due to the reflecting elements being symmetrical. One way of overcoming this problem is by incorporating an asymmetric reflector as described in British Patent Specification No.
2209211A. The present invention results from a consideration of the same problem which has resulted in the current invention which provides an optical fibre reflector suitable for use in such sensing systems but also having other applications.
According to the invention there is provided an optical fibre reflector comprising a pair of fibres interconnected intermediate of their ends to permit optical coupling therebetween, one of the fibres having a light reflective termination at one end and a light dispersive termination at the opposite end.
Preferably, the fibre with the reflective and light dispersive terminations is of a diameter smaller than the diameter of the other fibre.
The invention also includes an optical sensing 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 optical fibre, in which the optical fibre is provided along its length with a plurality of equally spaced optical fibre reflectors as previously defined 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 reflectors whereby each reflected light signal after the first interferes with either the previously reflected signal from the preceding reflector or a reference light signal of the same frequency or a frequency with a constant difference frequency to the transmitted light signal to produce an electrical signal in square law photo-detection means of the system, the difference between respective electrical signals corresponding to successive fibre elements 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.
In order that the invention and its various other preferred features may be understood more easily, embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:
Figure 1 shows an optical fibre reflector constructed in accordance with the invention employing two equal diameter fibres,
Figure 2 shows another optical fibre reflector constructed in accordance with the invention but employing two fibres of different diameter,
Figure 3 shows a schematic diagram of a sensing system constructed in accordance with a second aspect of the invention and
Figures 4 and 5 show pulse diagrams relating to alternative systems for sensing optical fibre deformation.
The drawing of Figure 1 shows two optical fibres 10 and 11 which may be single or multimode and are of equal diameter. The fibre 10 in this example is shown to be straight, although this is not essential, and is intended to form the main transmission path for coherent light pulses.
The fibre 11 in this example is shown curved, although this is not essential, and forms a reflector. The second fibre adjoins the first fibre and is optically coupled thereto by for example fusion. One end of the fibre 11 is provided with a reflective mirror 12 which may be a discrete device or alternatively the end surface may be adapted to reflect light back along the fibre by for example silvering of the end surface e.g. by dipping in a silver solution or vacuum coating by sputtering. The other end of the fibre 11 is arranged to be dispersive so that significant light is not reflected. Such dispersion can be effected by forming that end by a bad break of the fibre and/or by applying an adhesive which solidifies thereon and is light dispersive.
In use, coherent light P in introduced at end A of fibre 10 is diverted at the optical juncture of the two fibres partly to output end C of fibre 10 (P out) and partly into section D of the fibre 11 (PC1). If we assume the light is shared equally then P out = PC1 = 1/2 Pin.
Assuming perfect reflection then PC1 is reflected back along section D and is diverted at the optical juncture of the two fibres partly towards end A and partly into section B of fibre 11. Assuming again that light is shared equally then the light which is directed back towards the input and into section B is equal to PC1 = Pin. This reflected light
7 signal is available for interferometric evaluation. The component of light entering section B of fibre 11 is dispersed at the termination.
Whilst the construction described in connection with
Figure 1 is useable in fairly simple applications where only a few reflecting elements are required, it will be appreciated that the light transmitted from the output end
C to the next reflector is reduced by 50% and such a reduction will occur at each reflector. The amplitude of reflections from each reflector are thereby reduced by a factor of 4 in each subsequent reflector. In more complex multiple reflector application e.g. hydrophone arrays such a reduction is not tolerable. This problem is overcome in the construction illustrated in Figure 2.
Referring now to Figure 2, this shows a similar arrangement to Figure 1 the only difference being that the fibre 10A is of larger diameter than the fibre llA. In such a construction coherent light Pin introduced at end A of the fibre 10A is diverted unequally at the junction so that a major proportion of the light is directed to the output end of the fibre 10A for receipt by a subsequent reflector in an array. If we assume that 90% of the light is diverted into output end C and 10% into section D then when the light is reflected back to the junction it is again split but not in the same ratio so that greater than 10% of the reflected light is diverted into the end A and less than 90% into section B where it is dispersed. Accordingly the component reflected into A is a higher fraction than in the case of two similar size fibres, thus reducing system losses.
Referring now to Figure 3 there is illustrated a sensing system incorporating optical fibre reflectors as previously described. The drawing shows a laser 21 which produces an output of coherent light of frequency F which is fed into an optical switch means 22 wherein a modulated pulse of frequency F + b F is produced which by the inclusion of delay means in the optical switch means lags behind the pulse of frequency F by a predetermined time interval T. This two-pulse light signal passes through a beam splitter 23 and is focused into an optical fibre 24.
Equispaced reflectors 25 to 31 such as described in connection with Figures 1 and 2 are provided along the optical fibre and these reflectors may, for example, be formed by suitable junctions along a single optical fibre.
The fibre is effectively divided by these reflectors into six sensing elements and variations in the lengths of these fibre elements, such as due to the impingement thereon of acoustic waves, can be detected and measured in the manner now to be described.
As each two-pulse light signal reaches the first reflector 25 a small proportion of the signal will be reflected back along the fibre 24 to the beam splitter 23 which directs the signal to a photodetector 32. The remaining part of the two-pulse signals travels on to reflector 26 at which a further small proportion thereof will be reflected back along the optical fibre 24 to the detector 32. This procedure continues until that part of the two-pulse signal remaining reaches the last of the reflectors 31 and a small proportion of this signal is again reflected back along the optical fibre to the detector 32.
A further two-pulse optical transmission is then made and the cycle repeated.
Referring now to Figure 4 of the drawing this shows by way of example reflections of the two-pulse signals from the reflectors 25,26 and 27. As can be seen from the drawing the reflection from the second reflector 26 in the present example is delayed with respect to the reflection from the first reflector 25 by time T.
T = 2L where L = the length of each optical fibre C element and CG = velocity of light in the optical fibre.
By the appropriate choice of length L the delay between the reflections is such that there is total coincidence or at least some overlap between the reflected pulse or frequency F of a later reflected signal with the pulse of frequency F + AF of the preceding reflected signal. The reflected pulses are heterodyned in the square law photodetector 32 to produce beat or modulated signals as shown and the phase modulation of these signals will vary in dependence upon variations in length of the optical fibre elements. Accordingly, by detecting and measuring the phase modulation of the beat signals by means of a phase detector 33 changes in length of the optical fibre elements and thus deformation forces acting on these elements can be measured.
Referring now to Figure 5 of the drawings this shows the pulse diagram of an alternative sensing system in which the pulsed laser will produce at predetermined intervals one or two closely spaced pulses of the same frequency which constitute the signals fed to the optical fibre 24 (Figure 3) without the intervention of the optical switch means 22 (Figure 3). Assuming single-pulse signals are transmitted to the optical fibre the signals reflected from the reflectors 25,26 and 27 will be as shown in Figure 5. The reflected signals are homodyned and the changes in amplitude of the electrical signals produced by changes in length of the optical fibre elements will be detected by the photodetector 32 (Figure 3). The phase detector 33 is not required for this embodiment.
When the embodiments described are used in a hydrophone the free end of the optical fibre including the reflectors 25 to 31 may be trailed through the water and provide a beamforming acoustic wave sensor array which responds to acoustic waves impinging on the optical fibre sensing elements to produce variations in the lengths thereof which will be measured in the manner described. As will be appreciated this enables a single optical fibre sensor to be used as a beamforming array instead of using a plurality of separate sensors which can be inconvenient and expensive. The simple and relatively cheap provision of a beamforming acoustic sensor array as described also has the advantage of requiring access to one end only of the optical fibre which facilitates trailing of the fibre behind a vessel and which is compatible with the desensitisation of that part of the optical fibre between the signal generating and phase detection means and the fibre sensing elements.
Claims (1)
- CLAIMS:1. An optical fibre reflector comprising a pair of fibres interconnected intermediate of their ends to permit optical coupling therebetween, one of the fibres having a light reflective termination at one end and a light dispersive termination at the opposite end.2. An optical fibre reflector as claimed in claim 1, wherein the fibre with the reflective and light dispersive terminations is of a diameter smaller than the diameter of the other fibre.3. An optical fibre reflector as claimed in claim 1 or 2, wherein the fibres form a substantially X shaped configuration.4. An optical fibre reflector as claimed in any one of the preceding claims, wherein the light reflecting termination is formed by a mirror.5. An optical fibre reflector as claimed in claim 4, wherein the mirror is formed on the end surface of the fibre.6. An optical fibre reflector as claimed in any one of the preceding claims, wherein the light absorbing termination is formed by a light absorbing adhesive provided on the end of the fibre.7. An optical fibre reflector substantially as described herein with reference to Figure 1 or 2 of the drawings8. An optical sensing 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 optical fibre, in which the optical fibre is provided along its length with a plurality of equally spaced optical fibre reflectors as claimed in any one of the preceding claims 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 reflectors whereby each reflected light signal after the first interferes with either the previously reflected signal from the preceding reflector or a reference light signal of the same frequency or a frequency with a constant difference frequency to the transmitted light signal to produce an electrical signal in square law photo-detection means of the system, the difference between respective electrical signals corresponding to successive fibre elements being dependent 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.11. An optical sensing system substantially as described herein with reference to Figures 3 to 5 of the drawings.11. An optical sensing system substantially as described herein with reference to Figures 3 to 5 of the drawings.
Priority Applications (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9022739A GB2248928A (en) | 1990-10-19 | 1990-10-19 | Optical fibre reflector |
| AU87308/91A AU8730891A (en) | 1990-10-19 | 1991-10-17 | Optical fibre reflector |
| PCT/GB1991/001820 WO1992007236A1 (en) | 1990-10-19 | 1991-10-17 | Optical fibre reflector |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9022739A GB2248928A (en) | 1990-10-19 | 1990-10-19 | Optical fibre reflector |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9022739D0 GB9022739D0 (en) | 1991-06-12 |
| GB2248928A true GB2248928A (en) | 1992-04-22 |
Family
ID=10683990
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9022739A Withdrawn GB2248928A (en) | 1990-10-19 | 1990-10-19 | Optical fibre reflector |
Country Status (3)
| Country | Link |
|---|---|
| AU (1) | AU8730891A (en) |
| GB (1) | GB2248928A (en) |
| WO (1) | WO1992007236A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU657660B2 (en) * | 1992-08-14 | 1995-03-16 | Litton Systems, Incorporated | Optically switched sensor array |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2734373B1 (en) * | 1995-05-19 | 1997-07-04 | Gaz De France | OPTICAL COUPLER WITH VARIABLE BYPASS RATE AND USE AS A SENSOR |
| CA2240550C (en) * | 1997-07-31 | 2003-12-02 | Litton Systems, Inc. | Tdm array of optical non-acoustic pressure sensors |
| US6522797B1 (en) * | 1998-09-01 | 2003-02-18 | Input/Output, Inc. | Seismic optical acoustic recursive sensor system |
| GB2442745B (en) * | 2006-10-13 | 2011-04-06 | At & T Corp | Method and apparatus for acoustic sensing using multiple optical pulses |
| GB2442746B (en) | 2006-10-13 | 2011-04-06 | At & T Corp | Method and apparatus for acoustic sensing using multiple optical pulses |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4545253A (en) * | 1983-08-29 | 1985-10-08 | Exxon Production Research Co. | Fiber optical modulator and data multiplexer |
| GB2152689B (en) * | 1984-01-11 | 1987-07-01 | Plessey Co Plc | Optical fibre sensing apparatus |
| US4673270A (en) * | 1985-06-28 | 1987-06-16 | Amp Incorporated | Channel add/drop filter-coupler |
| US4810052A (en) * | 1986-01-07 | 1989-03-07 | Litton Systems, Inc | Fiber optic bidirectional data bus tap |
| US4775216A (en) * | 1987-02-02 | 1988-10-04 | Litton Systems, Inc. | Fiber optic sensor array and method |
-
1990
- 1990-10-19 GB GB9022739A patent/GB2248928A/en not_active Withdrawn
-
1991
- 1991-10-17 AU AU87308/91A patent/AU8730891A/en not_active Abandoned
- 1991-10-17 WO PCT/GB1991/001820 patent/WO1992007236A1/en active Application Filing
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU657660B2 (en) * | 1992-08-14 | 1995-03-16 | Litton Systems, Incorporated | Optically switched sensor array |
Also Published As
| Publication number | Publication date |
|---|---|
| WO1992007236A1 (en) | 1992-04-30 |
| AU8730891A (en) | 1992-05-20 |
| GB9022739D0 (en) | 1991-06-12 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |