GB2150687A - Fibre optic sensor - Google Patents
Fibre optic sensor Download PDFInfo
- Publication number
- GB2150687A GB2150687A GB08429812A GB8429812A GB2150687A GB 2150687 A GB2150687 A GB 2150687A GB 08429812 A GB08429812 A GB 08429812A GB 8429812 A GB8429812 A GB 8429812A GB 2150687 A GB2150687 A GB 2150687A
- Authority
- GB
- United Kingdom
- Prior art keywords
- fibre
- fringes
- light
- sensing
- physical parameter
- 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
- 239000000835 fiber Substances 0.000 title claims abstract description 50
- 238000000926 separation method Methods 0.000 claims abstract description 17
- 230000005540 biological transmission Effects 0.000 claims abstract description 16
- 239000013307 optical fiber Substances 0.000 claims abstract description 14
- 230000003287 optical effect Effects 0.000 claims abstract description 13
- 238000000034 method Methods 0.000 claims description 7
- 230000001902 propagating effect Effects 0.000 claims 1
- 238000005253 cladding Methods 0.000 description 2
- 101100005882 Mus musculus Cel gene Proteins 0.000 description 1
- 101100289046 Mus musculus Lias gene Proteins 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 101150091094 lipA gene Proteins 0.000 description 1
- YAFQFNOUYXZVPZ-UHFFFAOYSA-N liproxstatin-1 Chemical compound ClC1=CC=CC(CNC=2C3(CCNCC3)NC3=CC=CC=C3N=2)=C1 YAFQFNOUYXZVPZ-UHFFFAOYSA-N 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
Classifications
-
- 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
- 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
- G01D5/35303—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 a reference fibre, e.g. interferometric devices
-
- 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
-
- 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
- G01J2009/0261—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods polarised
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- General Physics & Mathematics (AREA)
- Optical Transform (AREA)
Abstract
A fibre optic interferometric sensor comprising a light source 2, an optical fibre 1, means 3, 5, 6, 7 for isolating one lobe from one distribution pattern for each transmission mode, and means 8, 9, 10, 11 for determining the fringe separation caused by interference of the two modes within the fibre. The optical path length is different for each mode and the difference can be altered by changing the fibre length or temperature. Thus by measuring the fringe separation, the temperature or other factor change affecting temperature or fibre length may be sensed. <IMAGE>
Description
SPECIFICATION
Fibre optic sensor
This invention relates to fibre optic sensors and particularly to a method of, and apparatus for, sensing changes in the temperature or length of an optical fibre by interferometric means.
Fibre optic sensors which sense changes in temperature, pressure, sound and stress due to deformation of an optical fibre are well known (see for example EP 0027540, EP 0082604, EP 0082615 and WO 79/00377). In all these cases, the deformation of the fibre by bending it in an undulating fashion produces a reduction of the light transmitted to the end of the fibre where the intensity variations are measured. This means that the intensity needs to be calibrated and any fluctuations of intensity from the source affect the performance of the sensor.
It is thus an object of the present invention to provide a fibre optic sensor which can be used to measure changes in any variable which can be made to affect the temperature or length of an optical fibre without the necessity for accurately measuring the intensity variations of the light transmitted by the fibre.
The invention is based on the fact that two modes of transmission within a fibre will interfere (for light of the same polarisation state) and produce interference fringes if the optical path length for each mode is different, since the modes have different propagation velocities. By altering the optical path length difference between the modes, the wavelength separation of the fringes changes and thus by detecting the changes in fringe separation, one can sense a change in any of the parameters which affect the optical path length.
Accordingly, the invention provides a method of sensing changes in a physical parameter by causing the parameter to affect the optical path length of each of two light transmission modes in a single optical fibre and determining the wavelength separation between fringes caused by interference between the two light transmission modes in the fibre.
Each transmission mode in a fibre has a number of distribution patterns of energy characteristic of that mode, each pattern consisting of either a set of concentric rings (in the simplest case a whole circle) or a set of lobes symmetrically disposed about a centre line. When the delay difference between modes is varied, the power at any particular wavelength oscillates between the alternate lobes or rings in each distribution pattern of each mode. Thus to see the interference fringes between two modes, one should preferably isolate the one alternate set of lobes or rings of any one pattern which are "in phase Preferably, therefore, the fibre is a dualmode fibre which propagates the LPol and the
LP,1 modes.The Lip,, mode, in general, consists of two distribution patterns each of a solid circular pattern but with the light being polarised perpendicularly in one pattern, compared to the plarisation direction in the other.
The Lip1, mode consists of, in general, four distribution patterns each having two generally semicircular lobes separated by a narrow dark band. The band separating the two lobes is in the perpendicular direction, in two of the patterns, to its direction in the other two patterns, and within each set of two patterns, the light in one pattern is polarised in a direction perpendicular to the direction of polarisation in the other pattern.
Preferably, therefore, the wavelength separation between fringes is determined by isolating one lobe from one distribution pattern for each transmission mode, dispersing the light from that one lobe, and detecting the position of the resultant fringes.
The invention further provides a sensor comprising a single sensing optical fibre, means for affecting the optical path length of transmission modes in the fibre and means for determining the wavelength separation between fringes caused by interference between two light transmission modes in the fibre.
The optical fibre is preferably a dual-mode fibre which propagates the Lip01 and the LP modes.
The wavelength separation determining means preferably comprises means for isolating one lobe from one distribution pattern for each of the two transmission modes, means for dispersing the light from that one lobe and means for dectecting the position of the resultant fringes.
The one lobe is conveniently isolated by coupling the light from a source into the fibre via an input fibre which is spliced offset to the dual-mode fibre so that only half of the distribution patterns for each mode is coupled into the dual-mode fibre. A second offset splice between the output of the dual-mode fibre and an output fibre is provided so that only one lobe from the distribution pattern is coupled to the output fibre and a polarisation filter is provided between the end of the output fibre and the light dispersing means so that only one distribution pattern for each transmission mode is passed to the light dispersing means.
The light dispersing means is conveniently a prism or diffraction grating and the fringe detecting means is preferably a photodiode array producing a signal representing the position of the fringes. Any change in the separation of the fringes thus produces a changing signal which is related to a change in the physical parameter affecting the optical path length in the optical fibre.
The invention will now be more fully de scribed by way of example with reference to the drawing which is a diagrammatic representation of one embodiment of a sensor in accordance with the invention.
The fibre 1 is a dual-mode fibre which is coupled to an appropriate light source 2 via an input fibre 3 which is spliced offset to the fibre 1 in order to remove two of the possible four distribution patterns of the LP11 mode.
Part of the length of fibre 1 is a contained within a sensor head 4 which is designed so that any desired physical parameter can affect the optical path lengths in the fibre.
The output end of the fibre 1 is connected via a further offset splice to an output fibre 5, the offset splice being used to transmit only one lobe of each of the two distribution patterns having different polarisation states of the
LP11 mode.
The output of the output fibre 5 is passed through a lens 6 to a polarising filter 7 which cuts out one of the two remaining lobes of the LP11 mode and one of the distribution patterns of the Lip01 mode. The transmitted light is then dispersed by a prism 8 and focussed by a lens 9 onto a photodiode array 10. The array 10 produces signals representative of the position of the fringes which pass to a circuit 11 which monitors the signals and relates any change in them to a change in the physical parameter affecting the sensor head 4.
As mentioned above, the Lip01 and PL1, modes interfere as they are transmitted down the fibre due to their different propagation velocities and the difference in optical path lengths. The separation between fringes changes as the difference in optical path lengths changes and so by monitoring the changes in fringe separation, this difference can be measured.
The difference in optical path lengths can be changed by changing the fibre length, the refractive index of the core or the V-value where:
where A is the wavelength of the light; a is the core radius; n is the refractive index of the cladding; and A is the refractive index difference between the core and the cladding.
By varying the temperature of the fibre all three, length, refractive index and V-value will be varied, although the length variations will be comparatively small. Thus by measuring the differences in fringe separation, temperature changes of the fibre can be sensed.
Alternatively, the length of the fibre can be varied, by any mechanical means, so that its length depends on some other parameter such as pressure, and fringe separation will then given an indication of changes of this parameter.
Claims (11)
1. A method of sensing changes in a physical parameter, which method comprises causing the parameter to affect the optical path length of each of two light transmission modes in a single optical fibre and determining the wavelength separation between fringes caused by interference between the two light transmission modes in the fibre.
2. A method according to Claim 1 wherein the fibre is a dual-mode fibre propagating the Lip01 and the PL11 modes.
3. A method according to Claim 2 wherein wavelength separation between fringes is determined by isolating one lobe from one distribution pattern for each transmission mode, dispersing the light from that one lobe and detecting the position of the resultant fringes.
4. A sensor for sensing changes in a physical parameter comprising a single sensing optical fibre, means for affecting the optical path length of transmission modes in the fibre due to changes in said physical parameter and means for determining the wavelength separation between fringes caused by interference between two light transmission modes in the fibre.
5. A sensor according to Claim 4 wherein said sensing optical fibre is a dual-mode fibre which propagates the Lip01 and the LP modes.
6. A sensor according to either Claim 4 or 5 wherein said wavelength separation determining means comprises means for isolating one lobe from one distribution pattern for each of the two transmission modes, means for dispersing the light from that one lobe and means for detecting the position of the resultant fringes.
7. A sensor according to Claim 6 wherein said isolating means includes an input optical fibre coupled offset to the input end of the sensing fibre, an output optical fibre coupled offset to the output end of the sensing fibre and a polarisation filter between said output optical fibre and said light dispersing means.
8. A sensor according to either Claim 6 or 7 wherein said light dispersing means is constituted by a prism or diffraction grating.
9. A sensor according to any one of
Claims 6, 7 or 8 wherein said detecting means is a photodiode array producing a signal representative of the position of the fringes.
10. A sensor according to Claim 9 further comprising means for relating the signal representative of the position of the fringes to changes in said physical parameter.
11. A method of sensing changes in a physical parameter substantially as hereinbefore described with reference to the drawing.
1 2. A sensor for sensing changes in a physical parameter substantially as hereinbefore described with referenece to the drawing.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB838332409A GB8332409D0 (en) | 1983-12-05 | 1983-12-05 | Fibre optic sensors |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8429812D0 GB8429812D0 (en) | 1985-01-03 |
| GB2150687A true GB2150687A (en) | 1985-07-03 |
| GB2150687B GB2150687B (en) | 1986-10-22 |
Family
ID=10552836
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB838332409A Pending GB8332409D0 (en) | 1983-12-05 | 1983-12-05 | Fibre optic sensors |
| GB08429812A Expired GB2150687B (en) | 1983-12-05 | 1984-11-26 | Fibre optic sensor |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB838332409A Pending GB8332409D0 (en) | 1983-12-05 | 1983-12-05 | Fibre optic sensors |
Country Status (1)
| Country | Link |
|---|---|
| GB (2) | GB8332409D0 (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1989008238A1 (en) * | 1988-02-24 | 1989-09-08 | Hughes Aircraft Company | Comb filter pressure/temperature sensing system |
| EP0359360A2 (en) * | 1988-08-31 | 1990-03-21 | Simmonds Precision Products Inc. | Optical fibre sensors and methods |
| EP0359705A2 (en) * | 1988-09-16 | 1990-03-21 | Centre Suisse D'electronique Et De Microtechnique S.A. | Bi-modal fibre-optical sensor with spatial demultiplexation |
| GB2224566A (en) * | 1988-06-30 | 1990-05-09 | British Aerospace | An optical sensor for determining changes in a dimension and/or a temperature of an object |
| EP0418202A2 (en) * | 1989-09-15 | 1991-03-20 | Centre Suisse D'electronique Et De Microtechnique S.A. | Optical sensor with intermodal interferometry |
-
1983
- 1983-12-05 GB GB838332409A patent/GB8332409D0/en active Pending
-
1984
- 1984-11-26 GB GB08429812A patent/GB2150687B/en not_active Expired
Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1989008238A1 (en) * | 1988-02-24 | 1989-09-08 | Hughes Aircraft Company | Comb filter pressure/temperature sensing system |
| GR1000329B (en) * | 1988-02-24 | 1992-06-25 | Hughes Aircraft Co | Comb filter pressure temperature sensing system |
| GB2224566A (en) * | 1988-06-30 | 1990-05-09 | British Aerospace | An optical sensor for determining changes in a dimension and/or a temperature of an object |
| US5009505A (en) * | 1988-06-30 | 1991-04-23 | British Aerospace Public Limited Company | Optical sensor for determining changes in a dimension and/or a temperature of an object |
| EP0359360A2 (en) * | 1988-08-31 | 1990-03-21 | Simmonds Precision Products Inc. | Optical fibre sensors and methods |
| EP0359360A3 (en) * | 1988-08-31 | 1990-11-28 | Simmonds Precision Products Inc. | Optical fibre sensors and methods |
| EP0359705A2 (en) * | 1988-09-16 | 1990-03-21 | Centre Suisse D'electronique Et De Microtechnique S.A. | Bi-modal fibre-optical sensor with spatial demultiplexation |
| EP0359705A3 (en) * | 1988-09-16 | 1990-11-28 | Centre Suisse D'electronique Et De Microtechnique S.A. | Bi-modal fibre-optical sensor with spatial demultiplexation |
| EP0418202A2 (en) * | 1989-09-15 | 1991-03-20 | Centre Suisse D'electronique Et De Microtechnique S.A. | Optical sensor with intermodal interferometry |
| EP0418202A3 (en) * | 1989-09-15 | 1991-10-16 | Centre Suisse D'electronique Et De Microtechnique S.A. | Optical sensor with intermodal interferometry |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8332409D0 (en) | 1984-01-11 |
| GB2150687B (en) | 1986-10-22 |
| GB8429812D0 (en) | 1985-01-03 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |