READOUT APPARATUS FOR A LASER ANGULAR RATE SENSOR

Abstract: Two electrically isolated photodetectors (201, 221) are re­ sponsive to an interference pattern output of a ring laser angu­ lar rate sensor. The two photodetectors (201, 221) are con­ structed to have a periodic pattern of photosensitive zones (20). The photodetectors are positioned such that the periodic pattern is selectively oriented (⊖) relative to the interference pattern fringes so as to produce a pair of output signals out of phase with each other in the presence of rotation.

Description: The present invention relates to a readout apparatus for a laser angular rate sensor according to the preamble of claim 1. Examples of ring laser angular rate sensors are shown and described in U.S. patent 3,373,650 and U.S. patent 3,390,606. Such sensors include a pair of counter-propagating laser beams traveling along an optical closed-loop path including a plurality of mirrors. In order to obtain useful information, a small percentage of the counter-propagating laser beams is allowed to pass through one of the mirrors. The light beams are passed through a prism which combines the beams at slightly different angles and thereby creates a fringe pattern across a surface containing two photodetectors such as photosensitive diodes, each photodetector being much smaller than an individual fringe. Usually the photodectors are separated by one quarter of a fringe spacing so as to provide readout output signals in phase quadrature. When the ring laser is rotated about its input axis, the counter-propagating beam frequencies change slightly, one increasing and the other decreasing. The difference in beam frequencies results in a beat frequency which is indicated by the rate of movement of the fringe pattern across the photodetectors. The photodetector outputs are fed into a logic circuit to count the number of fringes passing across the photodetector. The number of fringes passing each detector is directly related to the actual angular rotation of the sensor. The two detector output signals are compared to ascertain sensor rotation direction. The prism optics of the readout determines the angle between the beams which directly forms the interference fringe pattern and the fringe spacing. In the prior art, the prism optics results in an angle between the beams in the order 25 arc seconds and produces only a single fringe. Two photodetectors are used which have photosensitive areas which are less than the width of a fringe spacing and the two photodetectors are separated by approximately one quarter width of a fringe spacing. To accurately achieve this arrangement, the angle between the beams which produce the fringe pattern must be held to tight tolerances. From US-A-4 514 832 and FR-A-2 379 805 ring laser read-out devices are known having more than two photodetectors with a mask being arranged in front of the photodetectors. As far as this mask has transmitting and nontransmitting regions those regions are co-linear with the fringe pattern they help to produce. It is therefore the object of the present invention to devise a readout apparatus which allows a greater tolerance of the angle between the beams and therefore lowers the cost of the optical system. This object is achieved according to the characterizing features of the independent claim. Further advantageous embodiments may be taken from the dependent claims. According to the present invention two photodetectors are arranged to monitor the intensity of an interference fringe pattern of a ring laser angular rate sensor. Each of the photodetectors form, in part, a single periodic pattern of a plurality of photosensitive zones and nonphotosensitive zones. The periodic pattern of photosensitive zones of the photodetectors has a pattern axis, parallel with the direction of periodicity of the pattern, and which is at a predetermined angle relative to the fringes. The angle between the pattern axis and the fringes is selected to produce photodetector intensity output signals which are out of phase with respect to each other in the presence of sensor rotation. The readout of the present invention allows a greater tolerance of the angle between the beams thereby dramatically lowering the cost of an otherwise expensive optical system. With respect to the figures of the attached drawing the subject invention shall be further described, where: Figure 1 is an illustration of a ring laser readout system for generating an interference fringe pattern. Figure 2 is a diagramatic representation of a dual photodetector readout of the present invention. Figures 3a and 3b are graphs of photodetector output signals in the presence of rotation. Figure 4 is an example of a periodic mask pattern. Figure 5 is another example of a periodic mask pattern. Figure 1 illustrates an optical system for producing an interference fringe pattern resulting from the combining of each of the laser beams in a ring laser angular rate sensor. Figure 1 shows only one of the reflection surfaces which defines in part the optical closed-loop path of a ring laser angular rate sensor. Sensor block 10 is of the type shown and described in U.S. patent 3,390,606. Fixed to block 10 is mirror/readout prism 12 including substrate 110 for transmitting light. Substrate 110 includes a first major surface 111 suitably polished and optically coated to provide a partially transmissive mirror 14 for reflecting a major portion of beam 16 impinging thereon in a direction opposite of beam 15. Similarly, mirror 14 reflects major portion of beam 15 in the opposite direction of beam 16 in a well known manner so as to provide counter-propagating laser beams within block 10. The angle between beams 15 and 16 depends upon the optical close-loop path chosen, i.e. triangular (60 degrees), rectangular (90 degrees), and the like. Substrate 110 further includes a second surface 115 suitably polished and optically coated for providing a mirror surface 18. Mirror 18 is positionally located so as to reflect that portion of beam 16 transmitted through mirror 14. Substrate 110 is constructed such that the surface 115 is at a selected angle relative to surface 111 to select the angle between the incident and reflected beams at the surface of mirror 18. The angle between the incident and reflected beams is herein referred to as the "beam angle". In turn, the reflected beam from mirror 18 is reflected from mirror 14 so as to travel at a slight angle relative to that portion of beam 15 transmitted through mirror 14. This latter mentioned angle is substantially equal to the aforementioned "beam angle". The transmitted portion of beam 15 through mirror 14 and the doublely reflected portion of beam 16 arrive at transmissive surface 116 so as to form an interference fringe pattern on the surface thereof. The "beam angle" between the incident and the reflected beams at the surface of mirror 18 substantially determines the interference pattern at the surface 116 of substrate 110. As the angle increases therebetween, the fringe spacing of the interference fringe pattern at the surface 116 decreases. In the prior art, the beam angle between the incident and reflected beams would be in the order of 25 arc seconds thereby establishing a relatively large fringe spacing. In the present invention, it is desirable to have a beam angle to minimize directing the reflected beam from mirror 18 back into the block 10 in the opposite direction of beam 16 to avoid lock-in and large enough to create a discernable plurality of interference fringes. This criteria is met when the beam angle is in the order of 1 to 60 arc minutes. A choice of 20 to 40 arc minutes provides a reasonable number of interference fringes in a relatively small space relative to the photodetectors as will be subsequently described. Also shown in Figure 1 is a dual photodetector sensor 25 coupled to surface 116 for being responsive to the interference fringe pattern passing therethrough. Although in the preferred embodiment, sensor 25 is ridgedly secured to surface 116, sensor 25 may be spatially removed from surface 116 but still be responsive to the interference fringe pattern transmitted through surface 116. In the present invention, a single mask pattern is placed in front of two photodetectors of sensor 25 as a means of selecting the spatial portions of the fringe pattern that each detector samples. To measure the direction of fringe motion, and hence the direction of rotation, the mask is tilted with respect to the fringe pattern. By using this technique, a phase difference between the two detector signals is obtained and then utilized to determine the direction of rotation. In Figure 2, thereshown is dual photodetector sensor 25 comprised of a first photodetector 201 having, by way of illustration, a rectangular photosensitive area on substrate 200. Also shown is a second photodetector 221 having a rectangular photosensitive area similar to that of photodetector 201. Photodetectors 201 and 221 are electrically isolated from one another and shown to have substantially equal photosensitive areas. As illustrated in Figure 2, the photosensitive areas are symmetrically adjacent to each other. The two photodetectors are masked by a single periodic mask pattern consisting of transparent zones 20 and nontransparent zones 22 having a periodic repetition of period "x" nominally equal to the fringe period or spacing λ. The mask pattern is intended to sufficiently cover photodetectors 201 and 221 resulting in each detector having the same periodic pattern of photosensitive zones 20. Associated with the periodic mask pattern is a pattern axis 265 which is parallel to the direction of periodicity of the pattern. In the present invention the pattern axis of the periodic mask pattern is intended to be at a selected angle relative to the interference fringe pattern. Further, the photodetectors 201 and 221 are positioned such that there exists a reference axis 266 prependicular to the pattern axis and which passes through photosensitive zones of each photodetector. The reference axis is established to geometrically define the orientation of the detectors and pattern relative to the interference fringes. Also shown in Figure 2 is an intensity versus displacement profile of interference fringe pattern at an angle ϑ relative to the pattern axis. The profile shows intensity (I) vs. displacement (d) on the surface 116 of substrate 110. Figuratively denoted in Figure 2 are maximum intensity, minimum intensity, and half-maximum intensity portions of the fringe pattern incident on the periodic mask pattern, these portions are denoted by double solid lines, single solid lines and dotted lines, respectively. For simplicity, it is assumed here that all spatial intensity variations in the region of the mask are due solely to the interference between output beams at surface 116 of substrate 110, i.e. each individual beam is assumed to have a uniform intensity distribution over the extent of the mask. For instance, all points on the maximum intensity contours (double solid lines) have equal intensity, all points on the minimum intensity contours (single solid lines) have equal intensity, etc. It is also assumed that detector 201 samples all the light intensity transmitted through the transparent zones of the lower half of the mask (below the detector split 211.). Likewise detector 221 samples all the light intensity transmitted through the transparent zones of the upper half of the mask. By way of the present invention, it is intended that the two photodetector output signals be in phase quadrature in the presence of pattern movement caused by sensor rotation. To achieve this, the angle ϑ should be: where L is the total width of the two photodetectors and X is the repetition period of the periodic mask pattern. Figure 3 illustrates the phase quadrature photodetector output signals in the presence of sensor rotation. The reader should observe that the toal integrated power on detector 201 is substantially maximized while the total integrated power on detector 221 is substantially at half maximum. The photodetector output signals as a function of time in the presence of rotation are shown in Figures 3A and 3B, for the cases of fringe motion to the right and to the left, respectively. In the case of fringe motion to the right, the output of detector 201 lags that of detector 221 by 90 degrees (denoted by a phase difference of -90 degrees). In the case of fringe motion to the left, the output of detector 201 leads that of detector 221 by 90 degrees (denoted by phase difference +90 degrees). The direction of fringe motion is then related to the direction of rotation. As explained above, a uniform beam intensity profile over the extent of the mask was assumed here for the sake of simplicity. In principle, this technique can be generalized to achieve a phase difference range of ± 180 degrees. The angle ϑ to achieve a 90 degree phase difference (or any other desired phase difference) will depend on the intensity distribution. For example, in many practical cases, involving Gaussian beam profiles, the mask size and detector sizes are large compared to the beam shot size. In these cases, when the energy of fringe pattern is concentrated closer to the detector split (assuming the beam is centered on the detector split) the angle ϑ must be in general greater than to achieve a 90 degree phase difference. It should be recognized that photodetectors 201 and 221 have an output signal representative of the integrated intensity of light impinging on the photosensitive areas exposed to the interference fringe pattern thereof. Such output signals and connection means are not shown in Figure 1. In the absence of the periodic mask pattern, the output signals of photodetectors 201 and 221 will be substantially equal. However, with the masked dark lines 20 and 22 in place, the two photosensitive areas have output signals which are out of phase with one another as already described. Figures 4 and 5 show further examples of periodic mask patterns which may be used with photodetectors 201 and 221 to provide the intended function as already described. In Figure 4, triangle shaped masked opaque zones are indicated by numerals 204' , 224', and diamond shaped photosensitive zones are indicated by numerals 206',226'. Similarly, in Figure 5, thereshown is another arrangement having a somewhat sinusuidal shaped periodic mask pattern for photodetectors 201'' and 221'' including opaque zones 204'', 224'', and photosensitive zones 206'', 226''. Like Figure 2, the periodic mask pattern of both Figures 4 and 5 are interposed between the photodetector photosensitive areas and the interference fringe pattern. As before, the pattern axis should be at the selected angle ϑ relative to the fringes to obtain the out of phase photodetector output signals. Below the pattern of Figure 5 is an intensity vs. distance profile 240 representative of photodetectors 201'' and 221'' of Figure 5. This profile may be provided by a periodic mask pattern consisting of a periodic pattern of shaded areas 241 as an alternative to periodic mask patterns of opaque and transparent zones. As before, the pattern axis of the periodic pattern should be at the selected angle to obtain the desired out of phase photodetector output signal. In accordance with the invention, any periodic mask pattern which causes the photodetector output signal to periodically vary as a function of the periodic movement of the interference fringe pattern is within the scope of the present invention. It should be recognized by those skilled in the art that a pair of signals out of phase with one another is sufficient for subsequent ring laser angular rate sensor readout electronics for determining both angular rotation, rate, and direction in a well known manner. It should be recognized by those skilled in the art that the dual photodetector sensor 25 including photodetectors 201 and 221 and accompanying periodic mask pattern having the intended output maybe manufactured by a variety of techniques. The example illustrated in Figure 2 is one in which the photosensitive area of each photodetector is masked by a material which permits only unmasked or selected photosensitive zones to be responsive to impinging light. Alternatively, the resulting photosensitive zones may be, by way of example, a serpentine pattern of photosensitive areas for each photodetector. Further, another alternative may comprise a semiconductor comprising a plurality of photosensitive zones interconnected within the body of the semiconductor. Thus, there are many combinations of a dual photodetector sensor 25 and mirror/readout reflector 12 which provide the intended function in a low cost manner. Namely, the dual photodetector sensor 25 may be an integrated circuit which can be optically cemented, and the like, to the surface 116 for obtaining the intended signals. On the other hand, surface 116 may be optically coated to provide the mask on the surface 116 and the dual sensor may then be optically cemented above the mask. The combination set forth by the present invention sets forth a unitary mirror/readout reflector and dual photodetector system which is easily aligned and allows for greater flexiblity and tolerances in the manufacturing process. A combination of these elements allows a low cost and lightweight design for ring laser angular rate sensors. It should also be noted that although the diagram illustrated in Figure 1 shows beam angles normally associated with triangular lasers, the principles of the invention are applicable to ring laser angular rate sensors having greater than three sides including four, five, and the like.

Claims: A readout apparatus for a laser angular rate sensor having counter-propagating laser beams (15,16) which exhibit a change in frequency dependent upon rotation rate and direction, comprising means (110) for optically combining a portion of each of said beams to form an interference fringe pattern having a plurality of fringes; first photosensitive means (221) for providing an output signal representative of the intensity of said interference fringe pattern impinging thereon; and second photosensitive means (201) for providing an output signal representative of the intensity of said interference fringe pattern impinging thereon; characterized in that said first photosensitive means (221) having a periodic pattern of photosensitive zones (20,22); and said second photosensitive means (201) having a periodic pattern (20,22) of photosensitive zones substantially identical to said first photosensitive means; whereat said first and second photosensitive means (201;221) being positioned relative to said interference fringe pattern such that there exists a pattern axis (265) in parallel to the direction of periodicity of said periodic pattern and at a selected angle (ϑ) relative to said fringes, and there exists a reference axis (266) perpendicular to said pattern axis and passing through at least one photosensitive zone of each of said photosensitive means. Apparatus according to claim 1, characterized in that said first and second photosensitive means (201,221) each include a photosensitive area having parallel masked and unmasked zones (20,22). Apparatus according to claim 2, characterized in that said masked and unmasked zones are provided by periodic masking means (22) interposed between said photosensitive means (201,221) and said interference fringe pattern. Apparatus according to one of claims 1 to 3, characterized in that said periodic mask pattern is a plurality of parallel opaque lines (22). Apparatus according to claim 4, characterized in that said periodic mask pattern has a period substantially equal to the period of said interference fringe pattern. Apparatus according to claim 5, characterized in that said periodic mask pattern is a plurality of parallel opaque lines (22) having a width substantially equal to one half of the period of said periodic pattern.

Inventors: KILLPATRICK, JOSEPH E., SANDERS, GLEN A.
Applicants: HONEYWELL INC.
Application year: 1986
Main classification: G01C19
IPC codes: G01C19, H01S3, G01P9