GB2147695A - Balancing of interferometric optical fibre sensors - Google Patents

Balancing of interferometric optical fibre sensors Download PDF

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Publication number
GB2147695A
GB2147695A GB08326608A GB8326608A GB2147695A GB 2147695 A GB2147695 A GB 2147695A GB 08326608 A GB08326608 A GB 08326608A GB 8326608 A GB8326608 A GB 8326608A GB 2147695 A GB2147695 A GB 2147695A
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GB
United Kingdom
Prior art keywords
path length
interferometer
path
sign
frequency
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
Application number
GB08326608A
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GB2147695B (en
GB8326608D0 (en
Inventor
Richard Edward Epworth
D P M Chown
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
STC PLC
Original Assignee
Standard Telephone and Cables PLC
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Standard Telephone and Cables PLC filed Critical Standard Telephone and Cables PLC
Priority to GB08326608A priority Critical patent/GB2147695B/en
Publication of GB8326608D0 publication Critical patent/GB8326608D0/en
Priority to AU33438/84A priority patent/AU3343884A/en
Priority to DE19843435650 priority patent/DE3435650A1/en
Priority to JP59207224A priority patent/JPS60149903A/en
Publication of GB2147695A publication Critical patent/GB2147695A/en
Application granted granted Critical
Publication of GB2147695B publication Critical patent/GB2147695B/en
Expired legal-status Critical Current

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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
    • 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

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Testing Of Optical Devices Or Fibers (AREA)

Abstract

An interferometric sensor system able to null automatically the path difference between the sensor and reference fibre paths (2,3). A low level modulation (6) is applied to the laser drive current at frequency f1 to provide source intensity and wavelength modulation. The (piezo electric) fibre stretcher (5) is fed with two signals, one of which dithers the path difference at a relatively lower frequency f2, over a range of path difference of say half a wavelength. At the half peak intensity points on either side of the fringe, the intensity of the fluctuation at f1 is measured. These are compared (9) to identify the sign of the path difference between the arms, and an error signal is fed back to the fibre stretcher to balance the path differences. <IMAGE>

Description

SPECIFICATION Balancing of interferometric optical fibre sensors This invention relates to a method and means for automatic balancing of interferometric optical fibre sensors.
A range of sensors is evolving which make use of the phenomenon of optical interference to detect minute changes in the length of a single mode optical fibre. Figure 1 shows the essential configuration. Coherent light from an optical source 1 is divided between two single mode paths 2,3 which form the two arms of an interferometer. The beams suffer a differential delay under the influence of the parameter to be measured, and are then recombined, when interference takes place. The optical intensity upon the photodetector 4 varies sinusoidally from full constructive to full destructive interference as the path difference between the arms varies by half one source wavelength, as shown in Figure 2.
In this way, changes in the quantity to be measured are detected as a corresponding change in the intensity at the photodetector.
The sensitivity of a practical sensor to the quantity to be measured will be limited by sources of noise within the measurement systems, normally the dominant of these is optical source noise. With zero interferometer path difference, only the intensity fluctuations of the source are detected; these are usually of a relatively low level. However, an imbalanced interferometer acts as an optical frequency discriminator whose sensitivity to source phase noise increases with path difference. Semiconductor laser diodes, being small and rugged, are the obvious choice of optical source for practical sensors. However, these devices give rise to noise in an interferometer system owing to fluctuations in the optical wavelength, and also as a result of partition noise between longitudinal modes of oscillation.In addition, because laser sources have a finite spectral width the 'degree of coherence' may vary with path difference. As a result of all the aforementioned phenomena, the noise measured at the optical detector of a two-path interferometric sensor using a semiconductor laser source is usually a strong function of path difference, as shown for example in Figure 3. We see that, as mentioned above, the noise is lowest at zero path differences, and varies widely with path difference.
In order then to obtain best performance from the sensor, it will be important to constrain operation to small path differences, less than typically a few microns. To match physically the length of the two long fibre paths to this accuracy is clearly quite impracticable, and so some means of automatic path length equalisation is called for. The length of one fibre arm can be matched to the other sensor arm by use of a fibre stretcher 5, Figure 1, e.g. by coiling on a cylinder of piezo-electric material. There then remains the problem of deriving a control signal for this fibre stretcher, and here the major problem is to decide the sign of the required phase shifter (i.e.
which is the longer arm?). Because the source coherence function and noise at the detector are such complexfunctions of path difference, they cannot readily yield unambiguously the sign of the path difference.
According to this invention there is provided a method of balancing an interferometric fibre optic sensor having a path length difference compensation means, the method including modulating the optical source of the interferometer, comparing the output signal level at the source modulation frequency on both sides of an interference fringe to determine the sign of the path difference, deriving an error conrol signal according to the sign of the path length difference and applying said error control signal to the compensation means to cancel out the path length differences in the interferometer.
The invention makes use of the twin effects on the laser emission which result from a low level sinusoidal modulation of the laser drive current. An incremental increase in the laser's drive current gives rise to a small increase in optical intensity and simultaneously an increase in the wavelength of emission. Now at an arbitrary path length difference the signal intensity incident upon the optical detector at the source modulation frequency is measured on both sides of an interference fringe. The effect of the source intensity modulation will, of course, be the same on both sides of the fringe, but as Figure 4 explains, on the side of the fringe further from zero path difference the demodulated source wavelength modulation is in phase with the intensity modulation and augments it.On the side of the fringe nearer to zero path difference, the demodulated wavelength modulation is in anti phrase with the intensity modulation which it partially cancel.
Thus, by comparing the signal level at the source modulation frequency on both sides of any interference fringe, a clear indication of the sign of the path difference is obtained and automatic equalisation of the path lengths can then be implemented. Once equalised it will be usual to phase lock against drift the path length difference by taking advantage of the slope of the coherence function on each side of a fringe.
The implementation chosen for this phase nulling scheme will, of course, be influenced strongly by the requirements and mode of operation of the sensor type. Figure 5 shows one possible generalised schematic of an interferometric sensor system able to null automatically the path length difference between the sensor and reference fibre paths.
The basic interferometer configuration is the same as that of Figure 1. A drive current modulator 6 applies a low level of modulation at frequency f, to the laser source 1 to provide source intensity and wavelength modulation of the optical input to the interferometer. The piezoelectric fibre stretcher 5 is fed with two signals via a driver circuit, 7, one of which is produced by a generator 8 at a lower frequency 2, and which causes the path length of the reference arm to dither over a range of path differences of, say, one half of a wavelength at the optical frequency. At the half peak intensity points on either side of the fringe the intensity of the fluctuation at fa in the interferometer output is measured by a detector and control circuit 9. The two intensity values are compared to identify the sign of the path length difference between the interferometer arms and an error signal is derived.
This error signal is fed back to the piezoelectric driver circuit 7 and is superimposed on the dither signal atf2 to effect balancing of the path lengths.

Claims (5)

1. A method of balancing an interferometric fibre optic sensor having a path length difference compensation means, the method including modulating the optical source of the interferometer, comparing the output signal level at the source modulation frequency on both sides of an interference fringe to determine the sign of the path difference, deriving an error control signal according to the sign of the path length difference and applying said error control signal to the compensation means to cancel out the path length differences in the interferometer.
2. A method according to claim 1 including modulating the path length difference compensation means with a signal at a frequency lower than the source modulation frequency, the amplitude of the compensation means modulation being such that the path length difference varies by an amount of less than one wavelength at the optical frequency.
3. A method of balancing an interferometric fibre optic sensor substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.
4. An interferometric fibre optic sensor having piezoelectric path length compensation means, including means for modulating the interferometer optical source at a first frequency f1, means for modulating the compensation means at a second lower frequencyf2, means for comparing the interferometer output signal level on both sides of an interference fringe to determine the sign of the path length difference between the interferometer arms, and means for deriving from said comparing means an error signal according to the sign of the path length difference, the error signal being superimposed on the compensation means modulation to effect balancing of the interferometer path lengths.
5. An interferometric fibre optic sensor arrangement substantially as described with reference to Figure 5 of the accompanying drawings.
GB08326608A 1983-10-05 1983-10-05 Balancing interferometer sensor Expired GB2147695B (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
GB08326608A GB2147695B (en) 1983-10-05 1983-10-05 Balancing interferometer sensor
AU33438/84A AU3343884A (en) 1983-10-05 1984-09-24 Balancing of interferometric optical fibre sensors
DE19843435650 DE3435650A1 (en) 1983-10-05 1984-09-28 FIBER OPTICAL SENSOR
JP59207224A JPS60149903A (en) 1983-10-05 1984-10-04 Method and device for balancing interferometer optical fiber

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB08326608A GB2147695B (en) 1983-10-05 1983-10-05 Balancing interferometer sensor

Publications (3)

Publication Number Publication Date
GB8326608D0 GB8326608D0 (en) 1983-11-09
GB2147695A true GB2147695A (en) 1985-05-15
GB2147695B GB2147695B (en) 1987-03-18

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Family Applications (1)

Application Number Title Priority Date Filing Date
GB08326608A Expired GB2147695B (en) 1983-10-05 1983-10-05 Balancing interferometer sensor

Country Status (4)

Country Link
JP (1) JPS60149903A (en)
AU (1) AU3343884A (en)
DE (1) DE3435650A1 (en)
GB (1) GB2147695B (en)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2172101A (en) * 1985-03-05 1986-09-10 Plessey Co Plc Optical sensing system
GB2173592A (en) * 1985-04-04 1986-10-15 Ericsson Telefon Ab L M Fibre-optic mach-zehnder interferometer
EP0450244A2 (en) * 1989-12-01 1991-10-09 Thomson-Csf Detection device with optical fibres
US5363191A (en) * 1989-12-01 1994-11-08 Thomson-Csf Fibre optic sensor array reading device
FR2708733A1 (en) * 1993-07-30 1995-02-10 Sextant Avionique Device for optical detection of the vibrations of a microstructure, with stabilised operating point
EP0851205A2 (en) * 1996-12-26 1998-07-01 Hitachi, Ltd. Optical interferometer and signal synthesizer using the interferometer
WO2002001265A2 (en) * 2000-06-27 2002-01-03 Oluma, Inc. Mach-zehnder interferometers and applications based on evanescent coupling through side-polished fiber coupling ports

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3718192A1 (en) * 1987-05-29 1988-12-08 Hommelwerke Gmbh DEVICE FOR MEASURING THE DISTANCE BETWEEN THE DEVICE AND A MEASURING AREA
DE4224744A1 (en) * 1992-07-27 1994-02-03 Abb Research Ltd Interferometer for detecting electric field generated vibration of metallic particles in gas insulated HV switchgear - feeds laser beam into sensing and reference optical fibre branches of Mach=Zehnder interferometer, has two photodetectors at output of combiner-divider and control loop for zeroing difference between detector voltages
JPH0644527U (en) * 1992-11-25 1994-06-14 株式会社ララ Urine bag
DE4407176A1 (en) * 1994-03-04 1995-09-07 Diehl Gmbh & Co Pressure measurement using fiber optics
JP4586033B2 (en) * 2007-03-12 2010-11-24 アンリツ株式会社 Optical heterodyne interferometer and optical path length difference measuring method thereof

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2172101A (en) * 1985-03-05 1986-09-10 Plessey Co Plc Optical sensing system
GB2173592A (en) * 1985-04-04 1986-10-15 Ericsson Telefon Ab L M Fibre-optic mach-zehnder interferometer
US4759627A (en) * 1985-04-04 1988-07-26 Telefonaktiebolaget L M Ericsson Fibre-optic interferometer
EP0450244A2 (en) * 1989-12-01 1991-10-09 Thomson-Csf Detection device with optical fibres
EP0450244A3 (en) * 1989-12-01 1991-11-21 Thomson-Csf Detection device with optical fibres
US5363191A (en) * 1989-12-01 1994-11-08 Thomson-Csf Fibre optic sensor array reading device
FR2708733A1 (en) * 1993-07-30 1995-02-10 Sextant Avionique Device for optical detection of the vibrations of a microstructure, with stabilised operating point
EP0851205A2 (en) * 1996-12-26 1998-07-01 Hitachi, Ltd. Optical interferometer and signal synthesizer using the interferometer
EP0851205A3 (en) * 1996-12-26 2000-01-05 Hitachi, Ltd. Optical interferometer and signal synthesizer using the interferometer
US6091495A (en) * 1996-12-26 2000-07-18 Hitachi, Ltd. Optical interferometer and signal synthesizer using the interferometer
US6587278B1 (en) 1996-12-26 2003-07-01 Hitachi, Ltd. Optical interferometer and signal synthesizer using the interferometer
WO2002001265A2 (en) * 2000-06-27 2002-01-03 Oluma, Inc. Mach-zehnder interferometers and applications based on evanescent coupling through side-polished fiber coupling ports
WO2002001265A3 (en) * 2000-06-27 2003-11-13 Oluma Inc Mach-zehnder interferometers and applications based on evanescent coupling through side-polished fiber coupling ports

Also Published As

Publication number Publication date
AU3343884A (en) 1985-04-18
JPS60149903A (en) 1985-08-07
GB2147695B (en) 1987-03-18
GB8326608D0 (en) 1983-11-09
DE3435650A1 (en) 1985-04-18

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