GB2248295A - Optical fibre coil assemblies - Google Patents
Optical fibre coil assemblies Download PDFInfo
- Publication number
- GB2248295A GB2248295A GB9020713A GB9020713A GB2248295A GB 2248295 A GB2248295 A GB 2248295A GB 9020713 A GB9020713 A GB 9020713A GB 9020713 A GB9020713 A GB 9020713A GB 2248295 A GB2248295 A GB 2248295A
- Authority
- GB
- United Kingdom
- Prior art keywords
- coil
- optical fibre
- pressure
- sensing
- force
- 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- Withdrawn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 38
- 230000000712 assembly Effects 0.000 title description 6
- 238000000429 assembly Methods 0.000 title description 6
- 239000000463 material Substances 0.000 claims abstract description 4
- 230000000694 effects Effects 0.000 claims description 12
- 230000003287 optical effect Effects 0.000 claims description 6
- 230000002093 peripheral effect Effects 0.000 claims description 2
- 230000003068 static effect Effects 0.000 claims description 2
- 238000001514 detection method Methods 0.000 abstract description 2
- 230000035945 sensitivity Effects 0.000 description 12
- 239000000835 fiber Substances 0.000 description 3
- 230000003321 amplification Effects 0.000 description 2
- 238000002955 isolation Methods 0.000 description 2
- 238000003199 nucleic acid amplification method Methods 0.000 description 2
- 230000001133 acceleration Effects 0.000 description 1
- 238000004026 adhesive bonding Methods 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 238000013016 damping Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 239000008393 encapsulating agent Substances 0.000 description 1
- 230000002706 hydrostatic effect Effects 0.000 description 1
- 238000005259 measurement 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
- 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
- G01D5/3537—Optical fibre sensor using a particular arrangement of the optical fibre itself
- G01D5/35377—Means for amplifying or modifying the measured quantity
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
- G01L11/02—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by optical means
- G01L11/025—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by optical means using a pressure-sensitive optical fibre
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R1/00—Details of instruments or arrangements of the types included in groups G01R5/00 - G01R13/00 and G01R31/00
- G01R1/02—General constructional details
- G01R1/06—Measuring leads; Measuring probes
- G01R1/067—Measuring probes
- G01R1/07—Non contact-making probes
- G01R1/071—Non contact-making probes containing electro-optic elements
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measuring Fluid Pressure (AREA)
- Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
Abstract
An optical fibre coil assembly for use in systems for the detection of acoustic pressure, temperature etc comprises a coil of optical fibre 1 supported by means of a hollow encapsulating structure 2 and force adjustment means 4 for substantially reducing the overall force which would otherwise act on the coil structure to produce a change of length of the coil when the coil is subjected to acoustic pressure etc. The plates 4,5 seal the ends of the hollow support 2, removing from the central region 3 any influence of external pressure and amplifying axial forces on support from pressures Pe. Outward radial pressure is produced by the axial pressures Pe, the magnitude determined by the size of the central hole and Poisson's ratio of the material. This outward pressure balances the inward acoustic pressure, counteracting any change in the coil diameter and thus its overall length. <IMAGE>
Description
Improvements Relating to Ontical Fibre Coil Assemblies
This invention relates to optical fibre coil assemblies and relates more specifically to such assemblies embodying optical fibre coils providing low acoustic pressure sensitivity.
In many optical fibre sensing systems an optical sensing coil is utilised in the detection and measurement of a particular measurand by causing a phase change in light transmitted therethrough which can be detected and measured.
The measurand may be acoustic pressure, as will be the case in hydrophone systems, whereas in other cases the optical fibre sensor may, for example, be designed to detect acceleration, temperature or electrical or magnetic fields.
In the case of such sensor coils or where optical fibre coils are used as reference or delay coils in optical interferometers, for example, it is often necessary to render the sensor or other coil insensitive to acoustic pressure. For instance, if the sensor is not to be utilised as an acoustic pressure sensor then an ambient acoustic pressure acting on the coil could produce light phase changes in the coil which are indistinguishable from phase changes produced by the effect (eg temperature, electric field, magnetic field) it is required to detect and measure.
For the sensor coil to produce signals uncorrupted by acoustic pressure induced phase changes the sensitivity of the sensor coil to acoustic pressure needs to be minimised.
In the case of the reference coil referred to above it is most important that light passing through the coil does not become phase modulated since this can reduce the sensitivity of the system and/or introduce noise into the output.
According to the present invention there is provided an optical fibre coil assembly comprising a coil of optical fibre supported by means of a hollow encapsulating structure or the like and force adjustment means effective for substantially reducing the overall force which would otherwise act on the encapsulated coil structure in order to produce a change in length of the optical fibre coil when the coil is subjected to acoustic pressure or other force by adjustment of the relative magnitudes of the forces acting on the coil in different directions so that the net , effect of these forces on the length of optical fibre is substantially zero.
In carrying out the present invention the force adjustment means may comprise rigid plates which are secured to the ends of the hollow encapsulated coil structure and which act as pressure amplifiers for amplifying the external forces acting on the ends of the encapsulated coil structure in the axial direction thereof so that inwardly acting transverse forces exerted on the optical fibre coil are substantially nullified.
By way of example the present invention will now be described with reference to the accompanying drawings in which:
Figure 1 is a diagrammatic representation of a conventional optical fibre coil assembly;
Figure 2 is a diagrammatic representation similar to that of Figure 1 but of an optical fibre coil assembly according to the present invention; and,
Figure 3 is a graph indicating the sensitivity of optical fibre coil assemblies constructed according to the present invention.
Referring to Figure 1, this shows diagrammatically a hollow cylindrical optical fibre coil 1 embedded in a cylindrical encapsulating structure 2 having a central hole 3 in order to provide an optical fibre sensor coil. The sensor coil in use as a hydrophone, for example, can be subjected to end pressures indicated by the pressure Pe in the axial direction of the coil, outwardly acting radial pressures Pi and inwardly acting radial pressures Po. These pressures exert straining forces on the optical fibre which result in a change in length of the coil so that light being transmitted through the sensor coil for sensing purposes will undergo a phase change. Moreover, a stress induced change in the refractive index of the optical fibre brought about by the pressure will also produce a phase change in the transmitted light.
As will be appreciated from the drawing, the pressure
Po will act to compress the fibre coil 1 so as to produce a negative phase change whereas the pressure Pi acts to expand the coil and thereby produce a positive phase change.
The axial pressure Pe acts orthogonally to the pressures Po and Pi but due to Poisson's Ratio effect the pressure Pe also produces a component of stress which acts outwards in the same plane as the pressures Po and Pi. This is depicted as the X plane in the stress diagram included.
The magnitude of this X plane stress component is proportional to Poisson's ratio and acts to oppose the pressure Pi and thereby reduce coil sensitivity which in an acoustic pressure sensing application (eg hydrophone) constitutes a disadvantage. In the case of hydrostatic pressure where the wavelength of sound impinging on the coil assembly is considerably longer than the diameter of the optical fibre coil 1 and the central hole 3 of the encapsulating structure 2 is open to the surrounding medium, the pressures Po, Pe and Pi are of equal magnitude and in these circumstances the previously referred to stress component of the pressure Pe in the direction X due to the
Poisson's Ratio effect has a magnitude of less than the pressure 1/2 Po. The net effect of the pressures is to compress the coil and produce a negative phase change.
It may here be mentioned that such acoustically sensitive coils have hitherto been acoustically desensitised by enclosing them in layers of damping or lossy materials, possibly including layers of air. However, such coils tend to be rather bulky and, more importantly, it is very difficult to render such coils insensitive to low frequency sound.
In accordance with one embodiment of the present invention an optical fibre coil assembly of the form shown in Figure 1 is rendered highly insensitive to even the lowest frequency sound by providing rigid end cover plates 4 and 5 sealingly secured, as by glueing, to the ends of the coil encapsulating structure 2. These end plates 4 and 5 serve as pressure amplifiers of the pressures Pe impinging on the ends of the optical fibre coil structure with the amplification factor of the amplifiers increasing with the size of the central hole 3. The end plates 4 and 5 also eliminate Pi acting on the coil from within the central hole 3 of the encapsulating structure so that only the pressures
Pe and Po exert pressure on the coil structure.
Since the pressure Pe by the amplification thereof can be made much larger than the pressure Pe in the Figure 1 coil assembly it is possible for a given Poisson's Ratio to provide a central hole of such dimensions that the stress component in the X plane or direction resulting from the pressure Pe is of the same magnitude but of opposite sense to the pressure Po. Consequently, the longitudinal strain on the optical fibre will be zero and will thus not produce any phase change in light being transmitted along the fibre.
Referring now to Figure 3, this shows the sensitivity of an optical fibre coil assembly according to Figure 2 as a function of E (Young's Modulus) and 0 (Poisson's
Ratio). The coil has a cylindrical optical fibre coil of diameter approx. 6cm embodied in an encapsulating cylinder of approx. 7.5cm diameter having a central hole size of approx. 1cam. The drawing shows that as Poisson's Ratio approaches the value which gives zero longitudinal strain of the optical fibre the sensitivity of the assembly decreases very rapidly. Although the minimum sensitivity point is located off the bottom of the graph shown in Figure 3 it will be clear that at least 70 dB of acoustic isolation can be achieved compared with typical hydrophone sensitivities.
Once the minimum sensitivity point has been reached the effect of pressure Pe in the X plane becomes larger than the pressure Po and begins to expand the optical fibre coil so that the sensitivity of the assembly rapidly increases again.
In practice Young's Modulus and Poisson's Ratio would be constant for a given coil encapsulant and the encapsulating structure hole size will be changed to give the required sensitivity. The graph plots shown in Figure 3 suggests that up to 70 dB of acoustic isolation can be achieved very simply. The effect relies on the pressure field being uniform on every face of the coil assembly which will be generally true if the hydrophone diameter is much less than half a wavelength (ie 75cm across at 1 kHz).
The same effect as is achieved by the use of the rigid end plates 4 and 5 in the Figure 2 coil assembly could be achieved by reducing the effect of the outer pressure Po exerted on the coil structure. This might be achieved by partially pressure relieving the outer curved peripheral surface of the cylindrical coil 1 although it may be difficult to find a suitable pressure release material performing well at low frequencies.
Although in the embodiment described with reference to the drawings the encapsulated coil structure is of cylindrical form it will of course be realised that the principle of the invention is also applicable to coil assemblies having different configurations.
As regards the application of the invention it may here be mentioned that a coil assembly constructed according to the invention may be applied to accelerometers in which the sensing coil is required to be sensitive to forces applied in one plane but not applied to those present equally in all three planes. Moreover, the invention may be applied to any optical fibre coil assembly which is required to sense an effect, such as temperature or magnetic field, other than acoustic pressure or static pressure changes.
The coil according to the invention also has application as a delay coil which provides path balancing in a balanced optical interferometer or a reference coil as used in optical sensing systems in order to reduce the optical phase noise produced in associated sensor coils.
Claims (8)
1. An optical fibre coil assembly comprising a coil of optical fibre supported by means of a hollow encapsulating structure or the like and force adjustment means effective for substantially reducing the overall force which would otherwise act on the encapsulated coil structure or the like to produce a change in length of the optical fibre of the coil where the coil is subjected to acoustic or other pressure by adjustment of the relative magnitudes of the forces acting on the coil in different directions so that the net effect of these forces on the length of the optical fibre is substantially zero.
2. An optical fibre coil assembly as claimed in claim 1, in which the force adjusting means comprises rigid plates secured over the open ends of the hollow encapsulated structure to amplify the forces impinging on the ends of the coil/support structure and to eliminate the force acting on the coil from inside the encapsulating structure.
3. An optical fibre coil assembly as claimed in claim 1 or claim 2, in which the coil/support structure is of cylindrical configuration.
4. An optical fibre coil assembly as claimed in claim 1, in which the outer peripheral surface of the coil has pressure release material applied thereto for reducing the effect of external radial pressure on the coil or at least partially achieving an equalisation of the forces acting on the coil/support structure which would otherwise produce a change in length of the optical fibre.
5. A sensing coil as claimed in any preceding claim forming part of an accelerometer.
6. A sensing coil as claimed in any of claims 1 to 4 forming part of a sensing system for sensing a parameter other than an acoustic pressure or static pressure.
7. A delay or reference coil for optical sensing systems as claimed in any of claims 1 to 4.
8. An optical fibre coil assembly substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9020713A GB2248295A (en) | 1990-09-22 | 1990-09-22 | Optical fibre coil assemblies |
| FR9111661A FR2667163A1 (en) | 1990-09-22 | 1991-09-20 | OPTICAL FIBER COIL ASSEMBLY. |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9020713A GB2248295A (en) | 1990-09-22 | 1990-09-22 | Optical fibre coil assemblies |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9020713D0 GB9020713D0 (en) | 1991-06-12 |
| GB2248295A true GB2248295A (en) | 1992-04-01 |
Family
ID=10682628
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9020713A Withdrawn GB2248295A (en) | 1990-09-22 | 1990-09-22 | Optical fibre coil assemblies |
Country Status (2)
| Country | Link |
|---|---|
| FR (1) | FR2667163A1 (en) |
| GB (1) | GB2248295A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2715730A1 (en) * | 1994-02-01 | 1995-08-04 | Thomson Csf | Fibre optic pressure sensor in coil form for hydrophones |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2169400A (en) * | 1984-11-13 | 1986-07-09 | Canadian Government | Fibre optic magnetic sensor |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2566907B1 (en) * | 1984-06-28 | 1986-07-11 | Flonic Sa | PRESSURE SENSOR |
| US4850098A (en) * | 1988-04-15 | 1989-07-25 | The United States Of America As Represented By The Secretary Of The Navy | Method and apparatus to enhance the sensitivity of cylindrical magnetostrictive transducer to magnetic fields |
| GB2208711A (en) * | 1988-08-16 | 1989-04-12 | Plessey Co Plc | Fibre optic sensor |
-
1990
- 1990-09-22 GB GB9020713A patent/GB2248295A/en not_active Withdrawn
-
1991
- 1991-09-20 FR FR9111661A patent/FR2667163A1/en active Pending
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2169400A (en) * | 1984-11-13 | 1986-07-09 | Canadian Government | Fibre optic magnetic sensor |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2715730A1 (en) * | 1994-02-01 | 1995-08-04 | Thomson Csf | Fibre optic pressure sensor in coil form for hydrophones |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2667163A1 (en) | 1992-03-27 |
| GB9020713D0 (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) |