GB2119083A - Optical fibre gyroscope - Google Patents
Optical fibre gyroscope Download PDFInfo
- Publication number
- GB2119083A GB2119083A GB08206713A GB8206713A GB2119083A GB 2119083 A GB2119083 A GB 2119083A GB 08206713 A GB08206713 A GB 08206713A GB 8206713 A GB8206713 A GB 8206713A GB 2119083 A GB2119083 A GB 2119083A
- Authority
- GB
- United Kingdom
- Prior art keywords
- frequency
- fibre
- light
- gyroscope according
- gyroscope
- 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- Withdrawn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 23
- 239000000835 fiber Substances 0.000 claims abstract description 58
- 230000003287 optical effect Effects 0.000 claims abstract description 12
- 230000002452 interceptive effect Effects 0.000 claims abstract description 8
- 230000010363 phase shift Effects 0.000 description 6
- 238000005259 measurement Methods 0.000 description 3
- 230000000630 rising effect Effects 0.000 description 3
- 238000001514 detection method Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/58—Turn-sensitive devices without moving masses
- G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
- G01C19/72—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers
- G01C19/723—Heterodyning fibre optic gyrometers
Abstract
Light from laser (3) introduced into a first end (A) of an optical fibre loop (1) travels to end (B) and after interference with a beam from the same laser (3), but frequency shifted by a Bragg cell (10) or the like, is detected at 11. A detector (13) functions similarly for a counter-rotating beam passing from end (B) to end (A). A signal processing device (16) generates from the outputs of the detectors (11,13) a signal representing any rotation of the fibre. It is said that various types of noise are eliminated or reduced in this system. Noise reduction is also provided by frequency modulating the laser light on a sawtooth pattern (via Bragg cell 17 or modulating the loser itself). In a second embodiment, Bragg cell 10 is omitted and such modulation of the laser frequency combined with the optical transit time in loop 1 provides the frequency difference between the interfering light beams. <IMAGE>
Description
SPECIFICATION
Optical fibre gyroscope
This invention relates to an optical fibre gyroscope.
Proposals have been made for gyroscopes in which the rotation of a body is detected by means of an optical fibre. In such gyroscopes beams of light are directed along the fibres in opposite directions and the interference between those beams is used to provide a measure of the rate of rotation of the fibre and hence of the body on which the fibre is mounted. However, known optical fibre gyroscopes perform far less well than would seem to be theoretically possible. A number of sources of extraneous noise in optical fibre gyroscopes have now been identified, and these include the foilowing:
1. Effects of vibration of optical components producing amplitude noise.
2. Low frequency noise in the photodetectors and lasers employed, arising from the fact that detection is carried out at low frequencies (typically less than 100 kHz).
3. Backscatter in the fibre producing phase modulation.
4. Thermal drift altering the relationship between the path traversed by the light which passes through the fibre in one direction and the path traversed by the light which passes through the fibre in the other direction.
5. Instability in the phase bias networks which are used in a number of the known optical fibre gyroscopes.
The point here is that the intensity of the interference pattern produced varies with rotation rate in a sinusoidal fashion. In order to obtain maximum sensitivity the phase shift produced by rotation of the fibre must be measured at a point approximately 90 in phase away from a maximum or minimum of the interference amplitude. To shift the point of measurement in this way a phase bias arrangement is used in most known optical fibre gyroscopes, and it is in this phase bias network that the instability just mentioned tends to occur.
It is an object of the invention to reduce or eliminate at least some of the above mentioned sources of noise, and, in preferred embodiments of the invention, to reduce or eliminate still further ones of the sources of noise.
According to the present invention there is provided an optical fibre gyroscope comprising a loop of optical fibre; means for introducing a beam of light into a first end of a fibre so that it travels from the first end of the fibre to the second end of the fibre, and a detector arranged to detect interference between the beam of light after it has emerged from the second end of the fibre and a beam which is frequency shifted with respect thereto; means for introducing a beam of light into the second end of the fibre so that it travels from the second end of the fibre to the first end of the fibre, and a detector arranged to detect interference between the beam of
light after it has emerged from the first end of the fibre and a beam which is frequency shifted with respect thereto; and means arranged to receive the outputs of the detectors and generate therefrom a signal representing any rotation of the fibre.
In the accompanying drawings:
Figure 1 shows diagrammatically a first embodiment of the invention; and
Figure 2 shows diagrammatically a second embodiment of the invention.
Referring first to Figure 1, this shows a loop of single mode optical fibre 1 having a first end which is denoted by A and a second end which is denoted by
B. The intermediate portion of the fibre is wound in a large number of turns on a ring 2, the total length of fibre being of the order of several hundred metres, say from about 200 metres up to 1,000 metres or more.
Light from a laser 3 is passed through a polariser 4 to a beam splitter 5 which splits it into two equal parts. One part passes to a further beam splitter 6 where it is again equally split. The part of the light which travels straight through the beam splitter 6 is directed by a lens arrangement, diagrammatically indicated at 7, into end A of the fibre loop 1. This beam then travels along the fibre loop from the end
A to the end B. After emerging from the end B it is collected by a lens arrangement 8 and directed at the beam splitter 9. The part of the light beam reaching beam splitter 6 which is reflected thereby passes through a Bragg cell or similar device 10, which causes the frequency of the light to be shifted by, typically, a few tens of MHz, say 80 MHz for example.
The light then passes to the beam splitter 9 where it interferes with light emerging from end B of the optical fibre 1 and passing through the lens arrangement 8. This interference takes the form of a signal at the frequency of the shift produced by the Bragg cell 10, say 80 MHz, for example. The interference signal is detected by a photodetector 11. The signal at the photodetector is phase shifted as a result of the optical path difference between the interfering beams.
At the beam splitter 5 half of the incoming light is reflected to a mirror T2 which in turn reflects the beam to the beam splitter 9. Here half of the beam passes straight through and is focussed by the lens arrangement 8 into the end B of the optical fibre 1.
After travelling through the optical fibre the beam emerges at end A and passes through the lens arrangement 7 to the beam splitter 6. Part of the beam incident on beam splitter 9 is reflected and passes through Bragg cell 10 where it undergoes the same frequency shift as is mentioned above. From there is passes to beam splitter 6 where it interferes with light emerging from end A of the optical fibre 1.
The interference signal at beam splitter 6 is detected by a photodetector 13. The interference signal at photodetector 13, like that at photodetector 11, is a signal at the frequency of the shift produced by the
Bragg cell, and, like the signal at photodetector 11, contains a phase shift due to the path difference between the interfering beams. The phase shifts at the two photo-detectors 11 and 13 are indentical, assuming, of course, that the gyroscope is not rotating.
The signals from the photodetectors 11 and 13 are amplified by respective pre-amplifiers 14 and 15 and passed to an electronic signal processing device 16 which generates an output representative of the difference in phase between the incoming signals. If the gyroscope is not rotating the output of the device 16 is zero. Suppose, however, that the gyroscope begins to rotate at an angular velocity 9. It is to be understood that this rotation is rotation of the whole gyroscope and it is this rotation which the gyroscope is to detect. Light proceding from end A of the fibre towards end B will sense that end B is receding from it, whereas light proceding from B to A will see end A advancing towards it.There is thus a difference in the time spent in the fibre by the two beams of light, and this difference in time varies lineariy with the rotation rate Q of the fibre loop. This results in a phase shift in one sense in the light travelling through the fibre loop in one direction and a phase shift in the opposite sense in light travelling through the fibre loop in the opposite direction. Thus, the signal detected bythe photodetector 11 and amplified by the amplifier 14 is phase shifted in one sense, and the signal detected by the photodetector 13 and amplified by the amplifier 15 is phase shifted in the opposite sense.The signal processing device 16 thus produces an output which is representative of the difference in phase shifts of the clockwise and counter-clockwise paths through the optical fibre, and thus representative of the rate of rotation Q.
The gyroscope thus far described has advantages as regards a number of the sources of extraneous noise listed above. Noise source 1 is eliminated since what is processed is a frequency modulated signal rather than an amplitude modulated signal.
Noise source 2 is also eliminated since detection is carried out at a relatively high frequency, of the order of tens of MHz rather than a few tens of kHz.
Noise source 4 is substantially reduced. The optical system used means that thermal drift involving optical paths to the left of lens arrangements 7 and 8 in Figure 1 will not affect the measurement result.
Thermal drift can only affect the measurement if it occurs in the region of the fibre loop itself, and then, as practical matter, only if there is a changing thermal gradient across the fibre. Noise source 6 is of course eliminated since no phase bias network is used.
This leaves noise source 3 to be considered. Figure 1 shows additional components for dealing with noise source 3. These comprise an optical modulator 17 driven by a source of oscillating voltage 18 controlled by a control member 19. The optical modulator may, as shown, be a device such as a
Bragg cell which modulates the frequency of the light after its emission by the laser, or it may be a device which modulates the emission frequency of the laser itself. The frequency of the modulated light follows a saw tooth pattern composed of rising ramps alternating with sharply falling edges, or some similar waveform, for example, a pattern of falling ramps and rising edges. The difference between the maximum and minimum frequency may be of the order of tens or hundreds of MHz or even more. The way this arrangement is used to reduce the noise produced by backscatter will now be described.There are two main sources of backscatter in the gyroscope of Figure 1. The first, and more important, of these noise sources is backscatter at the ends A and B and reflections from lenses and other components of the launching optics. The second source of backscatter is inhomogeneities and imperfections within the loop itself.The arrangement just described deals with both sources of backscatter, except for a very small part of the second source of backscatter attributable to the very centre of the loop, typically the central few centimetres of the loop. Considering now, by way of example, what happens at end A of the fibre loop 1, light travelling from end A to the beam splitter 6 can arise from two sources. The first is light which has passed through the fibre loop from end B. This is the light which is desired to detect.The second, and undesired, source of light is that which has been backscattered from end A. The problem is how to distinguish between these two contributions. The optical modulator 17 enables one to do this. Because the frequency of light entering the system is continuously altering light backscattered from end A will be at a different frequency to light which has passed through the fibre loop and emerged at end A. Except in the region of a falling edge in the laser frequency the backscattered light will always have a lowerfrequency than light which has passed through the fibre loop. In a typical case the frequency difference would be of the order of a few hundred kHz. By filtering the signal produced through a narrow band filter operating at the above mentioned frequency of a few hundred kHz the desired signal can be separated out from that produced by backscatter.
Exactly the same situation applies to the situation at the other end of the fibre loop;
Figure 2 shows a second embodiment of the invention. This comprises an arrangement of beam splitters, photodetectors, lenses, amplifiers, laser, polariser and fibre loop substantially equivalent to those shown in Figure 1 except for a slight change in geometry. These components are represented by the same reference numerals as are used for the corresponding components in Figure 1. However, the Bragg cell 10 is omitted. Instead an electronic circuit 20 is incorporated between the signal processing device 16 and the laser 3 which modulates the frequency of the laser in a sawtooth fashion, or the like, so that its frequency varies in the same way as is done by the optical modulator of Figure 1. At beam splitter 9 there is interference between light arriving directly from beam splitter 5 and light at a lower frequency which has passed via beam splitter 6 through the fibre loop, travelling from end A to end
B. The difference in frequency arises because of the optical path difference which exists between the interfering beams and because the frequency of the laser is continuously changing. This assumes the modulation to be of a pattern composed of rising ramps and falling edges. A similar situation exists at beam splitter 6.
As in the case of Figure 1, the interference signals at the beam splitters are detected by the photodetectors 11 and 13, and there is a difference between those two signals when the gyroscope rotates. This difference is detected by the signal processing device 16. Besides forming a difference signal the signal processing device 16 also serves to feed back to the electronic circuit 20 the frequency of the interference signal detected by one of the photodetectors, say for example photodetector 11. The circuit 20 then uses that frequency to control the modulation of the laser frequency, so that the interference frequency is locked to a predetermined value.The circuit 20 is shown as one which mod slates the emission frequency of the laser itself, but one could alternatively use a device such as a Bragg cell to modulate the frequency of the light after emission by the laser. It may be desired, for the sake of improving the symmetry of the system for the signal processing device 16 to flip periodically between using the output of the photodetector 11 for the locking signal and using the output of the photodetector 13.
CLAIMS (filed on 7.3.83)
1. An optical fibre gyroscope comprising a loop of optical fibre; means for introducing a beam of light into a first end of the fibre so that it travels from the first end of the fibre to the second end of the fibre, and a detector arranged to detect interference between the beam of light after it has emerged from the second end of the fibre and a beam which is frequency shifted with respect thereto; means for introducing a beam of light into the second end of the fibre so that it travels from the second end of the fibre to the first end of the fibre, and a detector arranged to detect interference between the beam of light after it has emerged from the first end of the fibre and a beam which is frequency shifted with respect thereto; and means arranged to receive the outputs of the detectors and generatetherefrm a signal representing any rotation of the fibre.
2. A gyroscope according to claim 1, wherein the beam emerging from a given end of the fibre and the beam interfering with it are both derived by splitting a common beam.
3. A gyroscope according to claim 2, wherein the said common beam in respect of the said first end of the fibre and the said common beam in respect of the said second end of the fibre are both themselves derived from the output of a single beam emitter.
4. A gyroscope according to claim 3, wherein the frequency shift in both the frequency shifted beams is produced by a single frequency shifting element.
5. A gyroscope according to claim 4, wherein the frequency shifting element is a Bragg cell.
6. A gyroscope according to any one of claims 3 to 5, wherein the output of the beam emitter is frequency modulated.
7. A gyroscope according to claim 6, wherein the outputs of the detectors are filtered to eliminate or reduce components thereof arising from backscattered light.
8. A gyroscope according to claim 6 or 7, wherein the frequency modulation of the beam emitter output is of a sawtooth form.
9. A gyroscope according to claim 3, wherein the output of the beam emitter is frequency modulated, and the said interferences result from differences in the optical paths of the interfering beams.
10. A gyroscope according to claim 9, wherein the frequency modulation of the output of the beam emitter is arranged to be controlled by the interference frequency detected by one or both of the detectors, whereby to lock the said interference frequency to a predetermined value.
11. A gyroscope according to claim 10, wherein the said modulation is alternately locked to the outputs of the two detectors.
12. An optical fibre gyroscope substantially as herein described with reference to the accompanying drawings.
**WARNING** end of DESC field may overlap start of CLMS **.
Claims (12)
1. An optical fibre gyroscope comprising a loop of optical fibre; means for introducing a beam of light into a first end of the fibre so that it travels from the first end of the fibre to the second end of the fibre, and a detector arranged to detect interference between the beam of light after it has emerged from the second end of the fibre and a beam which is frequency shifted with respect thereto; means for introducing a beam of light into the second end of the fibre so that it travels from the second end of the fibre to the first end of the fibre, and a detector arranged to detect interference between the beam of light after it has emerged from the first end of the fibre and a beam which is frequency shifted with respect thereto; and means arranged to receive the outputs of the detectors and generatetherefrm a signal representing any rotation of the fibre.
2. A gyroscope according to claim 1, wherein the beam emerging from a given end of the fibre and the beam interfering with it are both derived by splitting a common beam.
3. A gyroscope according to claim 2, wherein the said common beam in respect of the said first end of the fibre and the said common beam in respect of the said second end of the fibre are both themselves derived from the output of a single beam emitter.
4. A gyroscope according to claim 3, wherein the frequency shift in both the frequency shifted beams is produced by a single frequency shifting element.
5. A gyroscope according to claim 4, wherein the frequency shifting element is a Bragg cell.
6. A gyroscope according to any one of claims 3 to 5, wherein the output of the beam emitter is frequency modulated.
7. A gyroscope according to claim 6, wherein the outputs of the detectors are filtered to eliminate or reduce components thereof arising from backscattered light.
8. A gyroscope according to claim 6 or 7, wherein the frequency modulation of the beam emitter output is of a sawtooth form.
9. A gyroscope according to claim 3, wherein the output of the beam emitter is frequency modulated, and the said interferences result from differences in the optical paths of the interfering beams.
10. A gyroscope according to claim 9, wherein the frequency modulation of the output of the beam emitter is arranged to be controlled by the interference frequency detected by one or both of the detectors, whereby to lock the said interference frequency to a predetermined value.
11. A gyroscope according to claim 10, wherein the said modulation is alternately locked to the outputs of the two detectors.
12. An optical fibre gyroscope substantially as herein described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08206713A GB2119083A (en) | 1982-03-08 | 1982-03-08 | Optical fibre gyroscope |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB08206713A GB2119083A (en) | 1982-03-08 | 1982-03-08 | Optical fibre gyroscope |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2119083A true GB2119083A (en) | 1983-11-09 |
Family
ID=10528854
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08206713A Withdrawn GB2119083A (en) | 1982-03-08 | 1982-03-08 | Optical fibre gyroscope |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2119083A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0585954A1 (en) * | 1992-09-04 | 1994-03-09 | CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. | Optical fiber gyroscope |
| US5440389A (en) * | 1991-09-02 | 1995-08-08 | Robert Bosch Gmbh | Optical sensor for rotational movements having an optical running period element |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2028497A (en) * | 1978-08-23 | 1980-03-05 | Rockwell International Corp | Interferometer gyro |
| GB2028496A (en) * | 1978-08-23 | 1980-03-05 | Rockwell International Corp | Interferometer gyro |
-
1982
- 1982-03-08 GB GB08206713A patent/GB2119083A/en not_active Withdrawn
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2028497A (en) * | 1978-08-23 | 1980-03-05 | Rockwell International Corp | Interferometer gyro |
| GB2028496A (en) * | 1978-08-23 | 1980-03-05 | Rockwell International Corp | Interferometer gyro |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5440389A (en) * | 1991-09-02 | 1995-08-08 | Robert Bosch Gmbh | Optical sensor for rotational movements having an optical running period element |
| EP0585954A1 (en) * | 1992-09-04 | 1994-03-09 | CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. | Optical fiber gyroscope |
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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) |