Ring Laser Gyroscope Beam Combiner for Separating and Combining Circulating Laser Beams for Power and Fringe Pattern Detections

The present invention relates to ring laser gyroscopes and more particularly to a prismatic beam combiner for a ring laser gyroscope that is also used to direct the power of the laser beams to a power monitoring means for controlling the pathlength of the ring laser gyroscope.

In a ring laser gyroscope (RLG), to obtain accurate inertial rotation information, a constant pathlength has to be maintained. To do so, U.S. Pat. No. 4,320,094 teaches the use of an electronic circuit for calculating the respective power of the circulating light beams in the cavity of an RLG. The power thus calculated is then used to modulate a piezoelectrically activated movable mirror assembly for maintaining the pathlength constant. However, due to the fact that the signals used for calculating the power are the same signals used for generating the RLG fringe pattern, noises are inherently added into the calculated power.

The present invention contributes to the substantial elimination of noises from a calculated power signal by using a prismatic beam combiner that has additional facets thereon for emitting the circulating laser beams. Power monitoring beams, for example PIN diodes, measure the intensities of the respective emitted laser beams and directly feed the measured power to a circuit for modulating the movable mirror assembly, thereby maintaining a constant pathlength. By thus directly measuring the power of the laser beams, the electronic components needed in the prior art for converting a fringe signal into a power signal are no longer necessary. Hence, noises picked up by these components, and added to the resultant power, are eliminated. As a consequence, a substantially noise free signal may be obtained from the present invention.

An additional advantage of the present invention includes the obvious fact that electronic components for converting fringe output signals to power signals are no longer needed.

Prior to the present invention, for a monolithic gyro assembly having three ring laser gyroscopes each measuring a single axis, there is ho room for the necessary power detectors. Thus, another advantage of the present invention resides in the fact that since the present invention beam combiner is also being used to measure the intensities of the laser beams, such over crowding problem is eliminated.

The above-mentioned objects and advantages of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic disassembled view of a prior art monolithic gyroscope assembly having three ring laser gyroscopes;

FIG. 2A is a combination of a cross-sectional view of one RLG and a prior art circuit for effecting the control of its pathlength;

FIG. 2B is a diagram showing the power output in relation to the tuning of the pathlength;

FIG. 3A is a cross-sectional view of a prior art beam combiner of an RLG;

FIG. 3B is an isometric representation of a prior art beam combiner prism shown in FIG. 3A;

FIG. 4 is an isometric view of the combination beam combiner and intensity emitter of the present invention;

FIG. 5A is a cross-sectional view of the FIG. 4 polyhedron beam combiner drawn along line B--B;

FIG. 5B is a cross-sectional view of the FIG. 4 polyhedron beam combiner having power monitoring means drawn along line C--C;

FIG. 5C is a top plan view of the FIG. 4 illustration; and

FIG. 6 is a schematic, using the FIG. 5 beam combiner, for illustrating the simplified pathlength control circuitry.

Referring to FIG. 1, a schematic illustration of a monolithic three RLG gyro assembly is shown. Mirrors 1 to 6 are respectively positioned in the center of each cube body face. The cube is machined such that a small diameter bore connects adjacent mirrors. A closed optical cavity is defined between four mirrors which are placed in one plane and are interconnected by bores. Thus, a total of three optical cavities are formed between the mirrors as follows: 2, 5, 4, 6; 1, 5, 3, 6; and 1, 2, 3, 4. Each cavity defined by these bores is filled with a helium-neon gas mixture which causes lasing due to well known principles.

As shown, there are three mutually orthogonal beam paths in FIG. 1. Each of the beam paths serves to detect angular rotation about a separate axis. These beam paths may be defined by the optical cavities which are mentioned above between mirrors 2, 5, 4 and 6; 1, 5, 3, and 6; and 1, 2, 3 and 4. In essence, each mirror is incorporated in the beam path of two orthogonal planes. Otherwise stated, each mirror is utilized for two orthogonal RLG's respectively associated with two separate axes.

Further shown in FIG. 1 are piezoelectric transducers 8, 10 and 12, coupled to mirrors 2, 3 and 5, respectively. The operation of these piezoelectric transducers is well known and it is further known that these transducers can modulate the corresponding pathlengths of the respective RLG's (by flexing the corresponding mirrors) such that a constant pathlength is maintained for each RLG in the gyro assembly. See U.S. Pat. No. 4,320,974, having the same inventor and assignee as the present invention.

As is usual in RLG's, each RLG has laser light waves propagating in opposite directions. Any frequency difference in these two light waves represents an inertial rotation. In order to avoid the "lock-in" effect, the monolithic assembly is dithered about axis 20, equally shared by the three RLG's.

Also shown in FIG. 1 are beam combiners 14, 16 and 18, associating respectively with mirrors 1, 4, and 6. These beam combiners would generate fringe patterns, one for each axis, for the gyro assembly. As is well known, by measuring the fringe patterns, the inertial orientation of the gyro assembly, can be ascertained. Such an assembly, has been fully described in U.S. Pat. No. 4,477,188, having as one of the co-inventors the instant inventor and assigned to the same assignee as the present invention.

FIG. 2A shows in simplified form the teaching of U.S. Pat. No. 4,320,974, which pertains to the controlling of the pathlength of an RLG. As shown in FIG. 2A, an RLG, which can be represented by any one of the three available RLG's of the FIG. 1 gyro assembly, has connected thereto a simplified circuitry for controlling its pathlength.

Take for example the RLG defined by cavity 1, 2, 3 and 4. There is shown a simplified electric circuitry having two outputs from beam combiner 18 and two inputs to transducer 8 for maintaining the pathlength of this RLG. As taught in the '974 patent, the output signals from beam combiner 18, in addition to being used for generating a fringe pattern, are amplified and squared before being summed by an adder 22. The inertial rotation of the RLG, in the meantime, is calculated in the processor. The summed power, represented by A, is then demodulated and integrated before being fed to transducer 8. The demodulator is driven by a reference oscillator, which frequency is also used to capacitively drive transducer 8, which is constantly modulated and moved so as to maintain a constant pathlength for the RLG.

The laser beam has characteristics of a bell-shaped curve, shown in FIG. 2B. A correctly tuned laser beam rests on the peak of the curve. But if there is mistuning, the power output of the laser beam is lower, for example at the point indicated by 24. Thus, by modulating mirror 2 via transducer 8, point 24 can be moved along the curve such that the intensity of the laser beam is brought to the maximum. Changes in the pathlength of the RLG due to for instance thermal expansion causes the pathlength transducer to move in such a way as to maintain the pathlength.

The beam combiner used for emitting the opposite propagating light waves in FIG. 2A is shown isometrically in FIG. 3B. Referring to FIG. 3A, a cross-sectional view of beam combiner 18--plus photodetector 26, which is used for intercepting the combined beams--is shown. In brief, the two light beams, represented as CW and CCW, propagate toward a coating 28 on mirror 4. The coating allows approximately 0.0001 of the light to pass through. The CCW beam is transmitted to beam combiner 18 and redirected against a second coating 30, which is approximately 50% transmissive. At the same time, the CW beam is directed past coating 30, through beam combiner 18 and converge with the transmitted CCW beam, on photosensitive element 32 of photodetector 26. The combined beams can converge or diverge with an angular separation of approximately 100.degree. arc therebetween. As is well known, these combined beams create the fringe pattern, from which the inertial rotational information of the gyro

assembly is derived. As shown in FIG. 3A, it should be appreciated that beam combiner 18, as is well documented, is light transmissive.

Referring now to FIG. 4, there is shown a schematic representation of a polyhedron which corresponds to beam combiner 18 of FIGS. 3A and 3B. To simplify matters, the polyhedron will henceforth be referred to as beam combiner 34, albeit it should be appreciated that this beam combiner performs more functions than a beam combiner, such as 18 of the prior art. Beam combiner 34 has facets 36, 38, 40 and 42. Facet 40 extends downwardly from the top parallel side of facet 36. The remaining facets 38 and 42 also extend from facet 36, but at the respective nonparallel sides thereof. As can be seen, one side of each of facets 38 and 42 is in abutting relationship with the nonparallel sides of facet 40. Also shown is a rectangular facet 44, situated on one side of combiner 34, adjacent to facets 42 and 36. A similar facet is located on the other side of facet 36 and is not shown. Similarly, a front rectangular facet abutting facet 40 is also not shown. It must be appreciated that the FIG. 4 configuration is shown for illustration purposes only and a differently configured prism providing for sufficient number of facets for both power monitoring means and a fringe detector can also be used, as long as the angles of incidence of the prism are such that the beams from the two pairs of counter propagating laser beams circulating about the two beam paths orthogonal to the prism can be respectively combined and separated. See for example FIGS. 2A and 5 where mirror 4 is impinged by the CW and CWW beams from beam path 1, 2, 3, 4; and FIG. 5B where mirror 4 is impinged by the CW and CWW beams from beam path 2, 5, 4, 6. More specifically, in FIG. 5A, beam combiner 34 is seen cross-sectionally along line B--B; and the CW and CWW beams from beam path 1, 2, 3, 4 (according to the FIG. 2A configuration) are combined and directed to photosensitive element 32 for effecting the fringe patterns for the axis of the gyroscope assembly about beam path 1, 2, 3, 4. In FIG. 5B, the necessity for requiring low incidence angles is due to the need for total internal reflection. Such reflection occurs without the losses that afflict reflections at small angles of incidence, which must be aided by reflective coatings on the surfaces. Total internal reflection, on the other hand, may not need such coatings.

FIGS. 5B and 5C show respectively a cross-sectional view and a plan view of the beam combiner of the present invention wherein corresponding power monitoring devices have been added. Referring to FIG. 5C, there is shown a pair of power monitoring diodes 46 and 48 mounted on facets 42 and 38, respectively. Fringe photodetector 26 is mounted on facet 40. The power monitoring diodes may be PIN photodetectors. Since the construction and operation of these power monitoring diodes are well known, suffice it to say that by using these diodes, the power of the laser beams can be directly measured, thereby eliminating the need for the components outlined with respect to the discussion of the FIG. 2A circuit.

Referring now to the cross-sectional view of FIG. 5B, it can be seen that the CW and CCW light waves propagating in the RLG having beam path 2, 5, 4, 6 are separated by beam combiner 34 and fed to power monitoring diodes 46 and 48, respectively. Thus, beam combiner 34 not only combines the CW and CCW beams from beam path 1, 2, 3, 4, for the fringe detector, it also separates the beams from beam path 2, 5, 4, 6, and directs them to power monitoring diodes.

FIG. 6 shows in simplified form the circuitry for modulating the pathlength of an RLG. Take for example RLG 1635. It must be recognized that two beam combiners and two piezoelectric transducers are used with each RLG. Bearing that in mind, it can be seen that the circuitry connects beam combiner 34 to transducer 12. As shown, instead of having squaring circuits, the respective power from the light waves is separately measured by preamplifiers 50 and 52 before being summed by an adder 54. As is readily apparent, the elimination of the squaring circuits not only saved components from the circuit, it also eliminated possible noise distortions that may be picked up. Moreover, by directly measuring the light intensities, a more accurate power output is obtained.

Operationally, as in the previously described FIG. 2A circuit, the summed power is sent to a demodulator and subsequently is integrated and fed to transducer 12 which, upon receipt of the signal sent by the reference oscillator, moves mirror 5 to maintain the pathlength of the RLG despite temperature variations caused possibly by the environment or from the gyro assembly being heated by nearby electronic components. It should be appreciated that the remaining two RLG's of the gyro assembly operate in similar fashion to that of RLG 1635. And since each of the mirrors is associated with two RLG's, beam combiner 14 of mirror 1 is used to output light intensities of RLG 1234.

In a gyro assembly having three RLG's, all three RLG pathlengths have to be maintained at respective constant lengths simultaneously. U.S. Pat. No. 4,585,346, by the same inventor an assigned to the same assignee, teaches how this is done. Since the beam combiner of the present invention can have mounted thereon the fringe detector as well as the power monitoring diodes, all of the necessary fringe detectors and power monitoring diodes can be fitted right on the monolithic gyro assembly, thereby allowing for a more compact assembly.

Insofar as the present invention is subject to many variations, modifications and changes in detail, it is intended that all matter described in this specification and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Accordingly, it is intended that the invention be limited by the spirit and scope of the appended claims.