GB2087545A - Interferometer - Google Patents
Interferometer Download PDFInfo
- Publication number
- GB2087545A GB2087545A GB8034178A GB8034178A GB2087545A GB 2087545 A GB2087545 A GB 2087545A GB 8034178 A GB8034178 A GB 8034178A GB 8034178 A GB8034178 A GB 8034178A GB 2087545 A GB2087545 A GB 2087545A
- Authority
- GB
- United Kingdom
- Prior art keywords
- interferometer
- frequency
- sensor
- 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.)
- Granted
Links
- 239000000835 fiber Substances 0.000 claims abstract description 16
- 230000003287 optical effect Effects 0.000 claims abstract description 13
- 230000001427 coherent effect Effects 0.000 claims abstract description 7
- 239000013307 optical fiber Substances 0.000 claims description 6
- 230000007613 environmental effect Effects 0.000 description 9
- 238000010586 diagram Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 230000005684 electric field Effects 0.000 description 2
- 238000000034 method Methods 0.000 description 2
- 238000012545 processing Methods 0.000 description 2
- 101150093857 Ccn4 gene Proteins 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 238000006073 displacement reaction Methods 0.000 description 1
- 230000036039 immunity Effects 0.000 description 1
- 238000009413 insulation Methods 0.000 description 1
- 230000003534 oscillatory effect Effects 0.000 description 1
- 230000006798 recombination Effects 0.000 description 1
- 238000005215 recombination Methods 0.000 description 1
- 238000012216 screening Methods 0.000 description 1
- 238000010561 standard procedure Methods 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
- 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
Landscapes
- Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- General Physics & Mathematics (AREA)
- Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
Abstract
An interferometer comprises generator means (21, 22, 23, 24, 25) for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate. A beam splitter (28) is arranged to route the alternating beam over two different paths simultaneously. A fibre optic sensor (29) is arranged to receive the light beam in one path. A combining device (30) is arranged to receive the light beam after passage through the sensor (29) and also the light beam in the other path which bypasses the sensor (29). The relative length of the two paths is arranged such that two different frequency components are combined contemporaneously. An optical detector (31) is responsive to the combined beams to produce a difference frequency component and a phase detector means is provided which produces a signal variable in dependence upon the phase of the difference frequency component. While the invention is not limited to a particular field one useful application is in the production of a hydrophone. <IMAGE>
Description
SPECIFICATION
Interferometer
This invention relates to interferometers and more particularly to an interferometer in which a fibre optic sensor can be located remotely from a generator of coherent light and from an optical detector for the sensed signal.
According to the invention there is provided, an interferometer comprising generator means for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate, a beam splitter arranged to route the alternating beam over two different paths simultaneously, a fibre optic sensor arranged to receive the light beam in one path, a light combining device arranged to receive the light beam after passage through the sensor and also the light beam in the other path which bypasses the sensor, the bypass length being arranged such that two different frequency components are combined contemporaneously, a non linear optical detector responsive to the combined beams to produce a difference frequency component and a phase detector means for providing a signal variable in dependence upon the phase of the difference frequency component.
In a particularly advantageous form of the invention the splitter and the combiner are positioned adjacent the sensor and remote from said generator means and the beam is coupled with the light splitter via an optical fibre. Such an arrangement enables the sensor to be located remotely from the generator means without substantial degradation of signal by environmental variations as temporally close frequency components experience similar changes and due to cancellation in the subtraction process these changes are not present on the difference frequency component.
Preferably, the bypass length is arranged so that consecutive periods of said two different frequencies are combined contemporaneously at the combiner.
The optical detector may also be positioned remotely from the combiner and be optically coupled therewith via an optical fibre line. Such an arrangement has the advantage that the sensor with its associated beam splitter and combiner can be separated from the processing and generating apparatus and can be linked therewith by just two optical fibre lines.
the generator means may comprise a Bragg cell having an input from a coherent light source at a first frequency and driven by a modulating frequency to provide a second frequency shifted relative to the first frequency.
The Bragg cell may be continuously modulated by the modulating frequency and said first and second frequencies may alternately be routed to the beam splitter by means of an optical switch operating at said predetermined rate.
Alternatively the frequency transmitted by the
Bragg cell may be alternated between said first
and second frequency by alternately connecting and disconnecting the modulating frequency to the cell.
In a particularly advantageous application of the invention a hydrophone comprises an interferometer as previously defined wherein the sensor is adapted to sense acoustic signals and phase displacements of the difference signal are representative of the acoustic signal.
In order that the invention and its various other preferred features may be understood more easily, an interferometer of known type and an interferometer constructed in accordance with the invention will now be described, by way of example only, with reference to the drawings in which:
Figure 1 is a schematic diagram of a standard
Heterodyne Mach Zehnder interferometer of known type,
Figure 2 is a schematic diagram of an interferometer constructed in accordance with the invention, and
Figure 3 is a schematic illustration of the signals passing through the sensor head of the interferometer of Figure 2.
The system described in Figure 1 comprises a high coherence laser 10 which supplies light at an angular frequency of WL to a beam splitter 11. The beam splitter directs the beams along two paths 12,13.
The light beam in path 12 is reflected by a mirror 1 4 into one end of a fibre optic light guide which is coiled and forms a fibre optic sensor 1 5.
The light beam after passing through the sensor 1 5 enters a beam combiner 16.
The light beam in path 13 enters a Bragg cell 1 7 which is modulated by a signal generator 1 8 of angular frequency WB to produce a band of frequencies centred on WL but spaced one from another by W,. A mirror 1 9 is arranged to direct the angular frequency component WL + W5 as a reference beam to the beam combiner 1 6.
The combined output of the beam combiner 1 6 is focused onto a photodiode 20 which is non linear and mixes the two signals to provide a difference frequency component W8 by optical heterodyning.
The sensor 1 5 is responsive to environmental changes and when these are oscillatory e.g.
pressure variations due to sound waves, then the light travelling through the sensor 1 5 is phase modulated by the sound field due to a combination of mechanical (changes in length) and optoelastic (changes of refractive index) effects on the fibre. Accordingly the difference frequency WB produced by the photodiode 20 is phase modulated by the acoustic signal and the information can be detected by standard phase modulation techniques in an f.m. receiver. The function of this system can be expressed mathematically as follows:--- Electric field of combined beam = E = A cos (wLt + A) + B cos (w, + WB)t Where the first term contains the phase modulation as seen by the sensing arm and the second term is due to the reference arm.
Output from the photodetector is proportional to EE*
i.e. cc (A2 + B2) + 2AB cos (wBt + A)
The second term contains the phase modulated
radio frequency carrier and also contains the
acoustic phase modulation, which can be
recovered by conventional techniques.
In some applications of such an interferometer
it is necessary for the fibre optic sensor to be
located remotely from the active parts of the
system i.e. the Bragg cell, laser, photodiode and
phase detector. One such application is use as a
hydrophone. Some possible ways of doing this
are.
Positioning the fibre optic sensor 1 5 remotely
from the rest of the interferometer and linking by
means of two fibre optic lines. In such an
arrangement however the cabies pick up noise
from the environment. Even with acoustic
screening of the lines the required signal can still
be lost if the lines are long enough.
Theoretically, environmental insulation could be
achieved by making both sensing and reference
paths through optical fibres both of which are
routed down to the hydrophone in a two core
cable or in two cables mechanically coupled.
Environmental noise would affect the two fibres equaily and so should cancel out. However, only a
slight difference between the two fibres would
result in excess noise and signal loss.
An alternative approach is to have the complete
system at the sensor coil and just the phase
detector remotely situated. Here there are
problems of supplying electrical power to the
interferometer, and of encapsulating the system
for protection from the environment, and of cost
related to the problem of replacement or repair in
the event of failure.
The arrangement of Figure 2 overcomes the
problem of loss of signal due to noise pick up.
Figure 2 illustrates a hydrophone system in which
a laser 21 provides a highly coherent source of
light at an angular frequency of W, which is
directed into a Bragg cell 22. The Bragg cell is
modulated at an angular frequency of WB by a
signal generator 23 to produce a band of
frequencies centred on WL but spaced one from
another by WB. An optical switch 24 is arranged to
select WL or WL + WB from the output of the
Bragg cell and is alternated therebetween at a
predetermined rate by a switch 25. In this way a
time division multiplex signal comprising
frequency shifted beams and unshifted beams are
produced. As shown by the inset waveform in
Figure 2. The duration of each signal portion is T.
This multiplexed signal is launched into a
downward fibre optic line 26 to a remote
hydrophone head 27 which comprises a beam
splitter 28 which directs the beam over two paths.
One of the paths is via a fibre optic sensor 29 of
coiled form to a combiner 30 and the other path is directly to the combiner over a line bypassing the sensor 29. The path length of the bypass line is arranged such that the combiner receives contemporaneously signals of the same frequency
WL 8 WL + WB and shifted and unshifted beam frequencies are superimposed (see Figure 3).
Preferably the differential delay in the paths from the beam splitter 28 to the combiner 29 via the sensor and via the bypass line is equal to the duration T of each signal portion. The delay is governed by the length L of the fibre line and the velocity V of light in the fibre.
The combined signals are routed to a sensing photodector 31 along an upward optic fibre line 32 having a square law characteristic which results in hetrodyning therebetween. In this way a difference frequency is obtained. The signal passing through the sensor 29 is phase modulated by acoustic signals due to mechanical and acoustic optic effects as previously described in relation to Figure 1 and the phase modulation can be detected by an f.m. receiver.
Environmental immunity of the downward line 26 is achieved as follows. The system effectively uses consecutive bursts of shifted and unshifted frequencies to form the heterodyne. As these are temporally short e.g. they effectively experience the same sound field with the result that phase changes caused by environmental noise are equal in both sections and effective relative modulation is zero.
In the upward line 32 the two aligned beams are accurately co-planar. By following the same path they are subject to the same sound fields and so again experience no overall relative modulation.
Expressing the operation in mathematical terms, the electric field after recombination of the beam is given by either: E1 = A cos (wLt + A1 + A2)
+ B cos (wL + WB)t + A1)
Where At is due to the environmental noise and A2 is due to the sound field (unshifted beam in sensor coil), or E2 = A cos (WLt + A1)
+ B cos ((WL + WB)t + A1 + A2) (shifted beam in sensor coil).
Now the detector output is either V cc Elm1* or
E2E2* i.e.. V cc (A2 + B2) + 2AB cos (WBt + A2) or ce (A2 + B2) + 2AB cos (WBt - A2) These are the signals in alternate pulses and can be demodulated using standard techniques.
The arrangement of Figure 2 has the following advantages
(a) Time division multiplexing of the downward going signals permits transmission over a single optical fibre line
(b) Environmental insepsitivity
(c) Low cost and simplicity.
Instead of modulating the Bragg cell 22 continuously and alternately selecting the angular frequencies W, a WL + WB by means of an optical switch, the modulating signal WB from the generator 23 may be gated at the input to the
Bragg cell at the predetermined rate so that the cell passes the frequencies W, and W, + WB alternately into the downward line 26.
Although the interferometer of the present invention has been described as applied to hydrophones it will be appreciated that it is applicable to other applications where rapidly alternating environmental conditions are to be sensed e.g. force and pressure and can be extended to radar and microwave processing applications. Such applications are intended to fall within the scope of this invention.
Claims (9)
1. An interferometer comprising generator means for producing a beam of coherent light which alternates between two different frequencies at a predetermined rate, a beam splitter arranged to route the alternating beam over two different paths simultaneously, a fibre optic sensor arranged to receive the light beam in one path, a light combining device arranged to receive the light beam after passage through the sensor and also the light beam in the other path which bypasses the sensor, the relative length of the two paths being arranged such that two different frequency components are combined contemporaneously, an optical detector responsive to the combined beams to produce a difference frequency component and a phase detector means for providing a signal variable in dependence upon the phase of the difference frequency component.
2. An interferometer as claimed in claim 1, wherein the splitter and the combiner are positioned adjacent the sensor and remote from said generator means and the beam is coupled with the light splitter via an optical fibre.
3. An interferometer as claimed in claim 1 or 2, wherein the relative path lengths are arranged so that consecutive periods of said two different frequencies are combined contemporaneously at the combiner.
4. An interferometer as claimed in any one of the preceding claims, wherein the optical detector is positioned remotely from the combiner and is optically coupled therewith via an optical fibre line.
5. An interferometer as claimed in any one of the preceding claims, wherein the generator means comprises an optical modulator cell having an input from a coherent light source at a first frequency and driven by a modulating frequency to provide a second frequency shifted relative to the first frequency.
6. An interferometer as claimed in claim 5, wherein the modulator cell is continuously modulated by the modulating frequency and said first and second frequencies are alternately routed to the beam splitter by means of an optical switch operating at said predetermined rate.
7. An interferometer as claimed in claim 5, wherein the frequency transmitted by the modulator cell is alternated between said first and second frequency by alternately connecting and disconnecting the modulating frequency to the cell.
8. An interferometer substantially as described herein with reference to Figures 2 and 3 of the drawings.
9. A hydrophone comprising an interferometer as claimed in any one of the preceding claims wherein the fibre optic sensor forms an acoustic sensor for the hydrophone.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8034178A GB2087545B (en) | 1980-10-23 | 1980-10-23 | Interferometer |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8034178A GB2087545B (en) | 1980-10-23 | 1980-10-23 | Interferometer |
Publications (2)
Publication Number | Publication Date |
---|---|
GB2087545A true GB2087545A (en) | 1982-05-26 |
GB2087545B GB2087545B (en) | 1984-09-19 |
Family
ID=10516847
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
GB8034178A Expired GB2087545B (en) | 1980-10-23 | 1980-10-23 | Interferometer |
Country Status (1)
Country | Link |
---|---|
GB (1) | GB2087545B (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2200986A (en) * | 1986-09-12 | 1988-08-17 | Cogent Ltd | Optical fibre measuring system |
US4765738A (en) * | 1986-01-08 | 1988-08-23 | Nec Corporation | Method and apparatus of measuring frequency response in an optical receiving system |
-
1980
- 1980-10-23 GB GB8034178A patent/GB2087545B/en not_active Expired
Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4765738A (en) * | 1986-01-08 | 1988-08-23 | Nec Corporation | Method and apparatus of measuring frequency response in an optical receiving system |
GB2200986A (en) * | 1986-09-12 | 1988-08-17 | Cogent Ltd | Optical fibre measuring system |
GB2200986B (en) * | 1986-09-12 | 1990-05-16 | Cogent Ltd | Optical fibre measuring system |
US4974961A (en) * | 1986-09-12 | 1990-12-04 | Jackson David A | Optical fibre measuring system |
Also Published As
Publication number | Publication date |
---|---|
GB2087545B (en) | 1984-09-19 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
JP2578601B2 (en) | Sensor system, sensor array and method for remotely sensing environmental changes | |
US5754293A (en) | Apparatus for the simultaneous acquisition of high bandwidth information in very long arrays containing large numbers of sensor elements | |
Cranch et al. | Large-scale multiplexing of interferometric fiber-optic sensors using TDM and DWDM | |
US4648083A (en) | All-optical towed and conformal arrays | |
US5345522A (en) | Reduced noise fiber optic towed array and method of using same | |
US4363114A (en) | Low noise remote optical fiber sound detector | |
US4748686A (en) | Coherence multiplexed optical position transducer | |
US6466706B1 (en) | Pulsed system and method for fiber optic sensor | |
EP0524252B1 (en) | Optical fibre loss detection | |
CA2589794A1 (en) | Optical system | |
AU4092100A (en) | Intrinsic securing of fibre optic communication links | |
CN102117525A (en) | Optical fiber intrusion detection system | |
KR840004581A (en) | Improved fiber optic detector for detecting small displacements on surfaces | |
WO2003014674A2 (en) | Two delay coil sagnac-based sensor array | |
Kersey et al. | Overview of multiplexing techniques for interferometric fiber sensors | |
CN115200691A (en) | Few-mode optical fiber distributed acoustic sensing system and signal processing method thereof | |
Peng et al. | Optical fiber hydrophone systems | |
GB2165118A (en) | OTDR for sensing distortions in optical fibres | |
CA2371576A1 (en) | Intrinsic securing of fibre optic communication links | |
JP3357623B2 (en) | Optical fiber interference type sensor, optical fiber interference type signal detection method, optical fiber interference type vibration sensor, optical fiber interference type vibration detection method, optical fiber cable contrast method, optical fiber core wire contrast method, and optical fiber communication method | |
GB2087545A (en) | Interferometer | |
US6515939B1 (en) | Pulse sampled optical fiber hydrophone array (U) | |
WO2003014774A2 (en) | Amplified tree structure technology for fiber optic sensor arrays | |
Dandridge et al. | Invited Paper Signal Processing For Optical Fiber Sensors | |
Kersey et al. | Tapped serial interferometric fiber sensor array with time division multiplexing |
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 |
Effective date: 19921023 |