GB2172101A - Optical sensing system - Google Patents
Optical sensing system Download PDFInfo
- Publication number
- GB2172101A GB2172101A GB08505616A GB8505616A GB2172101A GB 2172101 A GB2172101 A GB 2172101A GB 08505616 A GB08505616 A GB 08505616A GB 8505616 A GB8505616 A GB 8505616A GB 2172101 A GB2172101 A GB 2172101A
- Authority
- GB
- United Kingdom
- Prior art keywords
- optical
- sensing system
- optical sensing
- light
- output
- 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.)
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- 230000003287 optical effect Effects 0.000 title claims abstract description 71
- 239000013307 optical fiber Substances 0.000 claims description 13
- 239000000835 fiber Substances 0.000 claims description 3
- 238000006073 displacement reaction Methods 0.000 abstract description 3
- 230000001427 coherent effect Effects 0.000 description 6
- 238000005259 measurement Methods 0.000 description 4
- 238000001514 detection method Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 2
- 230000000737 periodic effect Effects 0.000 description 2
- 230000002787 reinforcement Effects 0.000 description 2
- 230000003321 amplification Effects 0.000 description 1
- 230000001066 destructive effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000003199 nucleic acid amplification method Methods 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
- G01D5/35303—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
Abstract
The system comprises a light source 11, the light output from which is amplitude modulated by means of a high frequency modulating signal derived from modulator means 9, and the modulated signal being applied simultaneously to respective optical paths 14, 15 of different lengths. The depth of modulation of the combined light intensity output at the distant end of the optical paths is detected by detector means 8 and utilised in determining changes in the relative path lengths of the two optical paths, eg in response to strain, temperature, displacement. <IMAGE>
Description
SPECIFICATION
Improvements relating to optical sensing systems
This invention relates to optical sensing systems and relates more specifically to such systems for sensing changes in optical fibre and/or free optical path lengths due for example to variations in temperature or strain of optical fibres or displacement between predetermined relatively moveable elements in an optical path of the sensing system.
It is well known in optical interferometers such as the Mach Zehnder interferometer to use separate monomode optical fibre paths into which coherent light beams derived from a coherent light source are launched simultaneously (or split by a fibre splitting device after launching), the light beams subsequently being combined before detection by suitable optical detecting means.
The relative phases of the coherent light beams in the respective optical fibre paths of the interferometer determine the amplitude of the combined or superimposed light beams detected.
When these light beams are in phase a maximum-amplitude combined output is produced due to reinforcement, whereas a minimum amplitude combined output will be produced when the light beams are in anti-phase due to attenuation. Changes in the relative lengths of the optical fibre paths produce variations in the amplitude of the detected light output from the interferometer.
The present invention provides an optical sensing system generally of the form just described but in which the light beam from a light source is amplitude modulated by a modulating signal and applied simultaneously to respective optical paths of different lengths and in which the depth of modulation of the light output at the distant ends of the optical paths, as detected by detector means, is utilised for determining changes in the relative optical path lengths. The depth of modulation of the detected light output will depend upon the relative phases of the envelope modulation of the combined light beams.
The light source is preferably an incoherent light source so as to avoid undesirable optical interference effects.
The optical sensing system according to the invention achieves linear addition of the optical power in the two parallel optical paths so that if the modulated light beams emerging from the respective paths are in phase the output has the highest modulation depth, whereas if the modulated light beams are in anti-phase a minimum depth of modulation occurs. There will be zero modulation in the detected output signal if the two light beams have equal and opposite amplitude modulation.
By way of example of the present invention will now be described with reference to the accompanying drawings in which:
Figure 1 is a schematic diagram of a well-known Mach Zehnder interferometer;
Figures 2 and 3 are schematic diagrams of alternative optical sensing systems according to the present invention;
Figures 4 and 5 are modulation depth/optical path length differences and modulation depth/modulation frequency response curves appertaining to the optical sensing sytem of Figs. 2 and 3; and,
Figure 6 shows a specific exemplary arrangement for locking the modulation frequency of a modulator associated with the light source of the sensing systems of Figs. 2 and 3 to a maximum or minimum point on the modulation depth/modulation frequency curve and for measuring the average frequency change of the modulator in order to measure changes in the optical path length difference.
Referring to Fig. 1 of the drawings this shows a Mach Zehnder optical sensing interferometer comprising a coherent light source 1 for producing a coherent light output which is launched into a monomode optical fibre 2 which ensures good fringe visibility. The path of the coherent light output is divided by means of an optical fibre power splitter 3 into two optical paths constituted by respective monomode optical fibres 4 and 5 of different lengths 1, and 12. The relative phases of the respective light beams in the two paths will depend upon the difference between the path lengths 1, and 12.Consequently, when the two light beams are combined into a single monomode optical fibre 6 by means of an optical power combiner 7 the amplitude or intensity of the combined optical signal will depend on the relative phases of the two optical signals combined by the combiner 7. If the light beams at the combiner 7 are in phase with one another then a maximum amplitude combined signal will be obtained due to reinforcement but if the light beams at the combiner 7 are in anti-phase then a minimum amplitude combined signal will be produced due to mutual cancellation or destructive interference. Changes in the path length Ii and/or 12 resulting in changes in the path length difference produce amplitude changes in the combined optical signal.Such amplitude changes may be detected by means of an optical signal detector 8 for the measurement of changes in path difference.
Referring to Fig. 2 of the drawings, this shows one exemplary optical sensing system according to the present invention. As can be seen from the drawing, the sensing system is generally similar in form to the Mach Zehnder interferometer depicted in Fig. 1 but the optical sensing system of Fig. 2 utilises amplitude-modulated light beams. For this purpose the sensing system comprises an alternating signal source 9 which provides a high frequency modulating output signal preferably of between about 10 MHz to 3 GHz. The high frequency modulating signal is fed together with a DC biasing signal into an electronic summing device 10 the output from which is applied to a laser or LED light source 11 to amplitude modulate the light output therefrom.The amplitude-modulated light output signal is launched into an optical fibre 12 but the signal is divided by power splitter means 13 into two modulated light signals which are fed along optical fibre paths 14 and 15 of length Ii and 12. The signals in the respective paths 14 and 15 will comprise light signals which are amplitude modulated (envelope modulated) at a predetermined high frequency. These signals in the present example are combined by means of optical fibre power combining device 16 into a single optical fibre 17 prior to detection of the combined signal by an optical detector 18 which provides an output signifying any changes in the relative path lengths Ii and 12 of the optical fibres 14 and 15.
It may here be mentioned that use of the power splitter 13-and power combiner 16 which are relatively expensive to provide may be dispensed with in the sensing system according to the present invention since it is not essential for the optical light beams to optically interfere on the detector 18 since simple linear addition of detector currents is sufficient. Accordingly, as shown in Fig. 3 of the drawings a proportion of the light output from the light source 11 may be launched simulataneously into two optical fibres 19 and 20 of lengths It and 12 respectively and the far ends of these optical fibres are brought directly into close proximity to a detector 21 such that a significant proportion of the optical output of each is detected.
Reverting now to the operation of the sensing system according to the present invention as depicted in Fig. 2 or Fig. 3, if the relative path lengths 1I and 12 are changed (e.g. by heating or stretching one of the optical fibres 14 and 15 or fibres 19 and 20) changes in the modulation depth of the output signal will be detected by detector 18 or 21. However, in general it may be difficult to measure small changes in gath length difference reliably in view of the difficulty in making analogue amplitude measurements to high precision. A preferred way of detecting such changes is described later with reference to Fig. 6.
If the path lengths 1, and 12 of the optical fibres are made significantly different, the detected the detected signal intensity will be of the form
a similar medium in both paths, where w is the frequency of the sub-carrier (envelope) modulation, t is the time, v is the velocity of propagation of the light in the medium and 1o is a constant.
As the curve of Fig. 5 shows, changing the frequency of modulation produces a series of minima and maxima points on the modulation depth response curve. The frequencies at which these minima and maxima points occur afford a well-defined means of detecting the imbalance in the path lengths 1, and 12 of the sensing system and thus determining temperature or strain for example in the fibres.
For a minimum modulation depth
n radians, when n is an integer
For a maximum modulation depth
radians
Although the positions of the minima and maxima are convenient measurement parameters in that they are independent of the amplitudes of either of the optical signals in the two paths, care must be taken with their measurement as the rate of change of modulation depth with modulation frequency is actually zero at these points. In order to improve the accuracy of detection of a maximum or minimum modulation depth point, the phase of frequency of the modulating signal may conveniently be given a controlled periodic perturbation in order to operate on the steeper parts of the response curve in Fig. 5.
Referring now to Fig. 6 of the drawings this shows an optical sensing system as depicted in
Fig. 3, in which additional means are provided for locking the frequency of a voltage-controlled oscillator modulator 22 to provide a maximum or minimum modulation depth. When the modula oscillator modulator 22 to provide a maximum or minimum modulation depth. When the modulation is jittered by using a separate low frequency oscillator 23 the output from which is fed to the oscillator 22 through an analogue adder 24 the operating point is moved along the response curve in Fig. 5 producing a periodic change in the amplitude modulation.
As can be seen from Fig. 6, the jitter signal is also fed to a double balanced mixer or multiplier 25 which effectively compares the phase of the jitter signal with that of a signal derived from the output of the detector 21 amplified by a high-frequency amplifier 26 and passing through a band-pass filter 27. The mixer provides an output which after amplification by amplifier 28 and filtering by low-pass filter 29 is fed to the analogue adder 24.
The circuit arrangement of Fig. 6 locks the average modulation frequency of the voltagecontrolled oscillator 22 such that it corresponds to a maximum or minimum in the response curve of Fig. 5. Thus if the optical path length difference between the path lengths 1, and 12 is changed by a physical parameter to be measured by the sensor the average frequency of the voltage-controlled oscillator 22 will vary and thus provide an indication of the value of the parameter to be sensed. This variation of the oscillator modulation frequency may be measured by means of counting means 30.
It may here be mentioned that although in the specific examples described the optical paths comprise optical fibres it should be appreciated that free space optical paths could be utilised in applications for example where small displacements between elements located in one of the optical paths are required to be measured. In addition combination of optical fibre paths and free space paths may be used within the general spirit of the invention.
Claims (10)
1. An optical sensing system comprising a light source the light output from which is amplitude modulated by means of a high frequency modulating signal derived from modulator means, in which the modulated signal is applied simultaneously to respective optical paths of different lengths and in which the depth of modulation of the combined light intensity output at the distant end of the optical paths as detected by detector means is utilised in determining changes in the relative path lengths of the two optical paths.
2. An optical sensing system as claimed in claim 1, in which the optical paths comprise optical fibres preferably of significantly different lengths.
3. An optical sensing system as claimed in claim 2, in which the modulated light output from the light souce is launched into one end of a single output fibre and then launched at the far end of the fibre into the respective optical path fibres by means of an optical splitter device.
4. An optical sensing system as claimed in claim 2 or claim 3, in which the light at the ends of the optical fibres is combined by means of an optical combiner device which launches the combined light into a single optical fibre coupled to the detector means.
5. An optical sensing system as claimed in any preceding claim, comprising an incoherent light source.
6. An optical sensing system as claimed in claim 1 or 2, in which the light output from the light source is launched directly into respective optical fibres of the two optical paths with the remote ends of the optical fibres being located in direct close proximity with the detector.
7. An optical sensing system as claimed in any preceding claim, in which means is provided for varying the modulation frequency of the modulator means so that the modulation depth of the detected output signal is at a minimum or maximum the variation in frequency of the modulator being detected, as by frequency counting means, to give an indication of changes in the relative lengths of the optical paths.
8. An optical sensing system substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.
9. An optical sensing system substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.
10. An optical sensing system substantially as hereinbefore described with reference to Figs.
4,5 and 6 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08505616A GB2172101B (en) | 1985-03-05 | 1985-03-05 | Improvements relating to optical sensing systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08505616A GB2172101B (en) | 1985-03-05 | 1985-03-05 | Improvements relating to optical sensing systems |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8505616D0 GB8505616D0 (en) | 1985-04-03 |
| GB2172101A true GB2172101A (en) | 1986-09-10 |
| GB2172101B GB2172101B (en) | 1988-08-17 |
Family
ID=10575451
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08505616A Expired GB2172101B (en) | 1985-03-05 | 1985-03-05 | Improvements relating to optical sensing systems |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2172101B (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2205211A (en) * | 1987-05-11 | 1988-11-30 | Marconi Co Ltd | Signal processing device |
| GB2238112A (en) * | 1986-11-11 | 1991-05-22 | British Aerospace | Measurement of distortion |
| US5038618A (en) * | 1986-11-11 | 1991-08-13 | British Aerospace Public Limited Company | Measurement of distortion |
| WO1993017303A1 (en) * | 1992-02-26 | 1993-09-02 | Sensor Dynamics Limited | Apparatus for measuring two physical parameters |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0021199A2 (en) * | 1979-06-08 | 1981-01-07 | Kabushiki Kaisha Toshiba | Optical sensing system |
| EP0102102A1 (en) * | 1982-08-26 | 1984-03-07 | Shell Internationale Researchmaatschappij B.V. | A method and apparatus for tank gauging using diode lasers and optical fibres |
| GB2147695A (en) * | 1983-10-05 | 1985-05-15 | Standard Telephones Cables Ltd | Balancing of interferometric optical fibre sensors |
-
1985
- 1985-03-05 GB GB08505616A patent/GB2172101B/en not_active Expired
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0021199A2 (en) * | 1979-06-08 | 1981-01-07 | Kabushiki Kaisha Toshiba | Optical sensing system |
| EP0102102A1 (en) * | 1982-08-26 | 1984-03-07 | Shell Internationale Researchmaatschappij B.V. | A method and apparatus for tank gauging using diode lasers and optical fibres |
| GB2147695A (en) * | 1983-10-05 | 1985-05-15 | Standard Telephones Cables Ltd | Balancing of interferometric optical fibre sensors |
Non-Patent Citations (1)
| Title |
|---|
| WO A1 84/04385 * |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2238112A (en) * | 1986-11-11 | 1991-05-22 | British Aerospace | Measurement of distortion |
| US5038618A (en) * | 1986-11-11 | 1991-08-13 | British Aerospace Public Limited Company | Measurement of distortion |
| GB2238112B (en) * | 1986-11-11 | 1991-10-09 | British Aerospace | Measurement of distortion |
| GB2205211A (en) * | 1987-05-11 | 1988-11-30 | Marconi Co Ltd | Signal processing device |
| GB2205211B (en) * | 1987-05-11 | 1991-01-23 | Marconi Co Ltd | Signal processing device |
| WO1993017303A1 (en) * | 1992-02-26 | 1993-09-02 | Sensor Dynamics Limited | Apparatus for measuring two physical parameters |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2172101B (en) | 1988-08-17 |
| GB8505616D0 (en) | 1985-04-03 |
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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 |