GB2028496A

GB2028496A - Interferometer gyro

Info

Publication number: GB2028496A
Application number: GB7927018A
Authority: GB
Original assignee: Rockwell International Corp
Current assignee: Boeing North American Inc
Priority date: 1978-08-23
Filing date: 1979-08-02
Publication date: 1980-03-05
Also published as: FR2434370B1; CA1136744A; DE2934192A1; JPS5530887A; GB2028496B; FR2434370A1

Abstract

The interferometer gyro includes a signal source 36 of light which is divided and introduced into a light path 34 in counter-rotating directions by a beam splitter 35. The light is removed from both counter-rotating directions after it has traversed the path by splitters 42 and 43 and the removed beams combined with a reference signal provided by a reference light source 50 and a beam splitter 51. The phase difference between the two combined signals is determined by detection at photodiodes 56 and 57 and a phase detector 61, to indicate the rotation of the optical path about which the light traversed. The signal and reference light beams can be produced from a single source by modulating with two frequencies, the difference between which is a frequency which is easily handled by the electronic circuitry. The path 34 can be formed by a coiled optical fibre instead of by mirrors 31, 32, 33. The apparatus may be constructed using integrated optics and electronics, Fig. 3. <IMAGE>

Description

SPECIFICATION interferometer gyro This invention relates to interometer gyros.

Known laser gyroscopes or gyros are of two types: the so-called ring laser gyro and the interferometer gyro. Both types of gyro are illustrated and described in United States Specification 4,013,365.

Briefly, an interferometer gyro can be represented by a structure described below with respect to FIGURE 1. In operation, light from a laser light source is divided, each portion of the divided beam being constrained to travel in a counter-rotating direction around a path. If the path experiences a rotation, the apparent length of the path in one direction travelled by one portion of the beam is ionger than the true path length while the apparent length of the path in the other direction is shorter than the true path length. This is the so-called Sagnac effect. The apparent path length changes result in phase shifts in the light travelling the paths in each direction from that which would be experienced if the path were merely at rest.The phase difference can be measured, and from this measurement, the rate of rotation of the path can be determined (and integrated to determine rotation).

In the prior art devices, the light beam portions which have traversed the light path in the counterrotating directions are combined after traversing the desired path. The combined waves add to or are subtracted from each other, depending on the particular degree of phase shift experienced during the traversal of the light path. The output, therefore, is merely light of variable intensity, the intensity variation indicating the rate of rotation of the optical path. It can be seen, therefore, that the prior art devices are singularly sensitive to the light level. If the light source generating the light which is divided and introduced into the signal path experiences a change in intensity, the output likewise will change in its intensity, which may be interpreted as a rotation of the light path.

Therefore, to assure accuracy of the gyro, especially in inertial guidance uses, particular steps must be taken to carefully monitor the intensity of the light source.

In light of the above, it is an object of the invention to provide an improved interferometer gyro, which is relatively insensitive to variations in the intensity of the source of light According to the present invention, there is provided an interferometer gyro comprising a source of light, an optical path circumscribing an area and into which first and second portions of the light are coupled to traverse the path in opposite directions, a reference light source with which the light coupled into the optical path is combined after it has traversed the path to produce a phase difference between the light after traversing the path in each direction combined with the light of the reference light source, and means for measuring the phase difference-to indicate rotation of the path.

This interferometer gyro can be fabricated using integrated optics and integrated electronics circuitry techniques.

The source of light and reference light source may be constituted by a common light source and means which modulate the light with the first and second frequencies.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic diagram of a ring laser interferometer gyro in accordance with the prior art.

FIGURE 2 is a schematic diagram iilustrating a ring laser interferometer gyro in accordance with one embodiment of the invention, and FIGURE 3 is a schematic diagram of a preferred embodiment of a ring laser interferometer gyro in accordance with the invention.

In the various figures of the drawings, light paths are indicated by dashed lines and electrical connections and mechanical elements are indicated by solid lines.

A typical laser gyroscope 10 of the prior art is shown diagrammatically in FIGURE 1 and includes three reflective surfaces 11, 1 2 and 13, carried upon a common inertial frame (not shown). Light from a laser light source is reflected from each of the reflective surfaces 11-13 in counter-rotating directions. This is achieved by allowing the light from the laser source 16 to impinge upon a beam splitter 1 7, which sends a first portion of the light beam from the laser 1 6 in the direction indicated by the arrow 1 8 and a second portion in the direction of the arrow 19.The light travelling in the direction of arrow 1 8 is reflected from, in order, surfaces 11, 12 and 13, then re-reflected from the beam splitter 17 to be detected by a detector 22. The second portion of the beam which travels in the direction of the arrow 19 is reflected, in order, from reflective surfaces 13, 12 and 1 to thereafter pass through the beam splitter 1 7 in the direction of the detector 22. The two beams, after travelling their respective counter-rotating paths, produce an interference fringe shift pr intensity change at the detector 22.

The interference fringe shift produced is of a form in which the intensity of the light detected at the detector 22 varies in accordance with the rate of angular rotation of inertial frame carrying the reflective surfaces 11-13. The rotation of the inertial frame is indicated by the circular arrow 24.

The interference pattern is produced by virtue of the apparent difference in path length seen by the counter-rotating beams of light due to the angular rotation of the inertial frame. The interferometer gyro of the prior art, however, suffers the disadvanteges above discussed, and it is to overcoming these disadvantages that the present invention is directed, as presently discussed.

In accordance with one embodiment of the invention, an inferferometer gyro 30 shown in FIGURE 2 includes a light path or ring 34 defined at three corners by reflective surfaces 31, 32, and 33, carried by an inertial frame. A beam splitter 35 is located on the frame at the fourth corner of the light path to complete the ring to divide light from a source of light, such as laser source 36, into two beams and to inject the light into the light path in counter-rotating directions, denoted by arrows 38 and 39. The ring 34 is defined by the reflective surfaces 31-33 and the beam splitter 35 and forms an optical path enclosing an area having an axis (perpendicular to the page) of sensitivity the rate of rotation of the optical path about which is desired to be measured.

Two beam splitters 42 and 43 are provided in the light path to intercept the counter-rotating light from within the light path to remove the counter-rotating light after it has completed, or.

substantially completed its travel around the ring 34. Thus, the portion of the light from signal source 36 travelling in the direction of the arrow 38 leaves the beam splitter 35 to be passed by beam splitter 42, then reflected, in order, from reflective surfaces 31, 32 and 33 to impinge upon the beam splitter 43 to be removed from the light path in the direction of the arrow 45. In like fashion, the portion of the light passing the beam splitter 35 in the direction of the arrow 39 is passed by the beam splitter 43, then reflected from the reflective surfaces 33,32, and 31, to be removed from the light path by the beam splitter 42, in the direction of the arrow 46.

In addition to the light from the light source 36, an additional or reference light source 50 is provided in the interferometer gyro 30. The light from the second light source 50 is divided by a beam splitter 51 into two portions, one of which is directed to a beam splitter 52 and the other of which is directed to a beam splitter 53.

The beam splitters 52 and 53, in addition to receiving the divided light beam from the source 50, are located in the light paths to receive the light deflected by beam splitters 42 and 43 respectively. The result, therefore, is that the light removed from the ring in the counter-rotating directions, traveling in the direction of the arrows 46 and 45, is combined and compared with the reference beams from the light source 50. This comparison is effected by detecting the combination of the light removed from the ring and the reference beams at respective photodiodes 56 and 57. Again, since the interference gyro produces light of changed phase due to the angular rotation of the light path, upon combination with the reference beam from the source 50, light of intensity dependent upon the rate of rotation will be detected upon photodiodes 56 and 57.The output from the photodiodes 56 and 57 is then applied to amplifiers 59 and 60; as electrical signals, and the phase difference determined by a phase detector 61. The output of the phase detector 61 therefore represents the phase difference of the light signals removed from the ring, and, therefore, represents the angular rotation of the ring. (In fact, since the reference beam is constant, and the light removed from the ring in each direction experiences the same phase shift, but with different directions or signs, the output from the phase detector 61 will represent twice the phase difference produced by virtue of the rotational rate of the ring.) In the embodiment shown in FIGURE 2, the signal light source 36 is referred to as a "signal" source and the reference light source is referred to as "LO" or local oscillator.This is to illustrate the analogous nature of operation of the interference gyro 30 to a hetrodyne electronic processing operation. That is, the light from the signal source is "modulated" during its traversal of the path around the ring, and is then "mixed" with a reference signal or local oscillator signal, in a fashion similar to what is ordinarily thought of in terms of hetrodyning one electronic signal with another.

Since a comparison is made between the phase differences between the light removed from each counter-rotating direction and the reference wave, the inter,erometer gyro embodiment shown in FIGURE 2 is essentially independent. For example, if the intensity of the light from the signal source 36 were to be decreased for some reason, the two outputs, each compared to a reference wave, produces a phase difference which will be essentially the same even though the intensity of the source were to be changed. Thus, the operation of the device need not be as closely monitored as that required of the prior art and, therefore, inertial grade interferometer gyros can be easily achieved.In addition, since two output signals are derived, any ambiguity which is exhibited by an intensity change at the output can be resolved to determine the direction of rotation of the device by developing quadrature signals, a technique known in the electronics and navigation arts.

A preferred embodiment of the ring laser interferometer gyro embodying the principles of the present invention is shown in FIGURE 3, and is designated generally by the reference numeral 70.

Embodiment of FIGURE 3 is shown and described as being of an integrated electronic circuit and integrated optics format, however, the principles described herein are equally achievable with discrete components, as will be apparent to those skilled in the art.

A source of laser light 71, which cannot be a helium neon gas laser or a distributed feedback gallium arsenide injection laser, produces an input beam which is incident upon a first beam splitter 72. The beam incident on the beam splitter 72 is split into two portions, one of which is directed in the direction indicated by the arrow 74 and the other of which is directed in the direction indicated by the arrow 75. The portion of the beam directed in the direction of the arrow 74 impinges upon an acousto-optic modulator 78, which can be, for example, a surface elastic wave zinc oxide (ZnO) interdigital transducer.It should be apparent to those skilled in the art that other types of optic modulators can be equally advantageously employed such as, for example, electro-optic modulators and the like.- The acousto-optical transducer 78 receives a signal from signal source 80 of, for example frequency w2. Thus, the light output from the acousto-optic modulator 78 is a light beam of frequency equal to the frequency w0 of the source 7 1 plus the frequency of the modulating frequency source w2. This signal is hereinafter referred to as the "signal frequency".

The light beam at the signal frequency from the acousto-optical modulator 78 is divided by a beam splitter 79 into two beams which are directed by beam splitters 81 and 82 into a light path 84 in counter-rotating directions. The light path 84 in the embodiment illustrated is defined by an optical fibre. The optical fibre can be of the type commercially avaiiable and, although a multimode fibre can be used, the fibre preferably should be a single mode fibre for the most efficient operation. The fibre can be of multiple turns to define a path about which the signal can be applied to travel in counter-rotating directions therearound. As in the case of prior art interferometer gyros, the sensitivity of the gyro is increased by increasing the number of turns of the fibre.It has been found, for example, that the optical fibre can include a number of turns to utilize a length of approximately one kilometer and exhibit satisfactory or suitable operation.

The signal beam, after traversing the light path 84, is removed and directed onto photo-detectors such as photodiodes 86 and 87, together with a reference beam presently to be described.

The reference beam above referred to is generated by the second portion of the signal source reflected from the beam splitter 72 along the path indicated by the arrow 75. That beam is directed to a second acousto-optic modulator 88, of similar construction to the acousto-optic modulator 78, and which receives a signal from a signal generator 89 at a frequency, for example, of w,. Thus, the output from the second acousto optic modulator 88 is a light beam of frequency w0 + w,. This reference beam is divided by a beam splitter 90 into two beams, each of which is directed to a respective one of photodiode 86 and 87 by beam splitters 92 and 93.Thus, the signal which is applied to photodiode 86 is w0 + w1 - and, likewise, the signal which is applied to photodiode 87 is w0 + w, + . The term 0 represents the phase difference undergone by the signal in in traversing the optical fibre 84 due to the rotation of the optical fibre about its axis of sensitivity (an axis perpendicular to the plane in which the optical fibre is configured).

The outputs from the photo diodes 86 and 87 are applied respectively to amplifiers 95 and 96, the outputs of which are compared by a phase detector 97 to produce an output indicative of the phase difference, or as above stated, two times the phase difference due to the sum and difference thereof developed in the optical fibre.

As above stated, the electronics of the signal generators 80 and 89, the amplifiers 95 and 96, as well as the phase detector 97, can be fabricated as a part of an integrated circuit which can be conveniently fabricated as a part of the optical circuit above described.

In addition, since the output of the interferometer gyro 70 appears as a voltage proportional to the phase difference between the two light beams, numerous methods exist for electronically processing the signal which may read out a function proportional to the phase developed. For example, a high frequency clock started by the phase in one channel could be developed, the count being stopped by the phase in the other channel at a zero crossing point. The high frequency clock could then be counted by a simple counter which would, as an output, yield a value equal to the time period representing the differential phase.

Claims

1. An interferometer gyro comprising a source of light, an optical path circumscribing an area and into which first and second portions of the light are coupled to traverse the path in opposite directions, a reference light source with which the light coupled into the optical path is combined after it has traversed the path to produce a phase difference between the light after traversing the path in each direction combined with the light of the reference light source, and means for measuring the phase difference to indicate rotation of the path.

2. An interferometer gyro according to chaini 1, wherein the optical path comprises a coiled optical fibre.

3. An interferometer gyro according to claim 1, wherein the optical path is formed by a plurality of mirrors arranged to defined the optical path.

4. An interferometer gyro according to claim 3, wherein there are three mirrors arranged at three corners of a rectangular optical path and wherein the first and second portions of light are introduced by way of a beam splitter arranged at the fourth corner of the rectangular path.

5. An interferometer gyro according to claim 4, further comprising second and third beam splitters arranged in the optical path to remove therefrom the counter-rotating light after the counterrotating light has traversed substantially the whole length of the optical path, first and second beam combiners, each located to receive the light removed from the optical path by a respective one of the second and third beam splitters, a fourth beam splitter dividing the light from the reference source into two portions, which are directed on to the first and second beam combiners respectively.

6. An interferometer gyro according to any of claims 1 to 5, wherein the said source of light is a laser light source.

7. An interferometer gyro according to claim 1 or 2, wherein the said source of light and the reference light source are constituted by a common light source, a first modulator which modulates the light from the common source with a first frequency to provide the light which is split into the said first and second portions, and a second modulator which modulates the light from the common source with a second frequency to - provide the reference light

8. An interferometer gyro according to claim 7, wherein each modulator comprises an acoustooptic modulator driven by electrical signal source.

9. An interferometer gyro according to claim 7 or 8, wherein the common light source is a laser.

10. An interferometer gyro according to claim 7, wherein the common source is a laser, and comprising an optical integrated circuit including a first beam splitter upon which light from the laser impinges to divide the light into two input beams, a first surface elastic wave interdigital transducer configured to add a first frequency to one of the input beams and forming the first modulator, a second beam splitter for dividing the output of the first transducer into two signal beams, an optical fibre providing the said path and having two ends arranged to receive respective ones of the two signal beams, a second surface elastic wave interdigital transducer configured to add a second frequency to the other input beams and forming the second modulator, a third beam splitter for dividing the output of the second transducer into two reference beams, beam combining means which combine the signal beams after the-signal beams have traversed the optical fibre with respective ones of the reference beams, and two signal generators whose outputs furnish the first and second frequencies added by the first and second surface elastic wave interdigital transducers.

11. An interferometer gyro according to any preceding claim, comprising two light detectors of which one detects the light after traversing the path in one direction combined with the reference light, while the other detects the light after traversing the path in the other direction combined with the reference light, to provide electrical signals to the means measuring the phase difference.

12. An interferometer gyro according to claim 11, wherein the detectors are photodiodes.

13. An interferometer gyro according to claim 12, further comprising a pair of amplifiers, each connected to a respective one of the photodiodes to produce an output representative of the intensity of the light received upon the respective photodiode, and a phase detector to which the amplifier outputs are applied.

14. An interferometer gyro substantially as hereinbefore described with reference to and as illustrated in Figure 2 or Figure 3 of the accompanying drawings.