FIELD OF THE INVENTION This invention relates to a photodetection readout apparatus for use with ring laser gyroscopes.

BACKGROUND OF THE INVENTION Examples of ring laser gyroscopes are shown and described in U.S. Patent 3,373,650 issued to J. Killpatrick and U.S. Patent 3,390,606 issued to T. Podgorski. Such ring laser gyroscopes utilize counter propagating light beams traveling in an optical closed loop path established in part by a plurality of mirrors.

When the ring laser gyroscope is rotated about its input axis, the counter propagating beam frequencies change slightly. One increases, while the other decreases. The difference in beam frequencies results in a beat frequency which is determined by observation of a portion of each of the counter-propagating light beams.

To obtain useful rotation information, a small percentage of light from the counter propagating laser beams is allowed to pass through one of the mirrors. The light beams are then passed through a prism which combines the beams at slightly different angles and thereby creates a fringe pattern at and exiting from a surface thereof. Photodetectors such as photosensitive diodes detect the interference fringe pattern established by the optical beams of light exiting the surface of the prism. A pair of photodetectors are typically used with the photosensitive areas of each of the photodetectors being smaller than the fringe spacing and separated by 1/4 of the fringe spacing so as to provide output signals in phase quadrature. The spacing permits processing of the photodetection output signals to deteπnine rotation direction about the input axis of the gyro.

One example of a photodetection readout apparatus is illustrated in U.S. Patent 4,871,253 issued to Killpatrick et al. This patent employes a photodetecting readout apparatus which includes a pair of photodetector elements, each having photosensitive surfaces, generally co-planar. A mask comprising a periodic pattern of co-parallel alternating transparent and opaque stripes is interposed between the interference "beam" fringe pattern exiting the surface of the prism and the photosensitive surfaces of the photodetectors. Thus, light exiting the prism will impinge on the photodetectors through the mask. The mask is appropriately oriented at an angle relative to the interference fringe pattern so that the light exiting through the mask exhibits a moire fringe pattern. The photodetectors respond to the fringe movement of the moire fringes as a function of gyro rotation. If the aforesaid angle is properly

selected, the output signal of the photodetectors will be appropriately out of phase so that the direction of fringe motion and thus the direction of the sensor rotation can be deteπnined.

Heretofore, the photodetection readout apparatus, in accordance with the teaching of U.S. Patent 4,871,253, includes a pair of photodetectors contained within a package which comprises an enclosure in which the photodetectors are mounted. The enclosure further includes a transparent window parallel to, and in front of, the photosensitive surfaces of the photodetectors. A mylar mask having a periodic pattern of co-parallel transparent and opaque stripes is attached to the outer surface of the transparent window with an adhesive.

Unfortunately, this procedure has presented several problems. First, great difficulty is encountered in getting the mask to remain flat. Because an adhesive is used to attach the mask to the window, air bubbles, which distort the fringe pattern, become trapped between the mask and the window. Moreover, the ends of the mask also have a tendency to curl. Finally, not only is the mylar mask difficult to clean, but it also fogs when exposed to radiation. Further, imperfections of the mylar mask can cause light scattering leading to gyro performance errors.

SUMMARY OF THE INVENΗON The present invention eliminates the need for a mylar mask by depositing thin film black chrome lines directly on the transparent window of the photodetection package. This el___ninates the process of cementing a mask to the surface of the transparent window. This in turn decreases the amount of light scattered by the mask, resulting in a more precise measurement of sensor rotation. Moreover, the chrome mask is much more durable and can be easily cleaned. Further, the physical properties of a thin film black chrome mask deposited on the transparent window is well suited to space applications, since it is substantially unaffected by radiation and does not "fog" like mylar.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross sectional view of a ring laser gyroscope illustrating a readout apparatus.

Fig. 2 is a cross sectional view of the photodetection readout apparatus of the present invention. Fig. 3 is a top view of the photodetection readout apparatus of Fig. 2.

DETAILED DESCRIPΗON OF THE INVENTION Fig. 1 illustrates the employment of the photodetection readout apparatus 10 of the present invention as part of a ring laser gyroscope. Only one mirror surface of the ring laser gyroscope is shown in Figure 1. Laser block 30 provides a pair of counter-propagating laser beams 35 and 36 of the type described in U.S. Pat. No. 3,390,606.

. Also illustrated in Figure 1, prism 32 is fixed to block 30 for transmitting and combining light beams as will be described. Prism 32 includes a first major surface 111 suitably polished and optically coated to provide a partially transmissive mirror 34 for reflecting a major portion of beam 36, in a direction opposite of beam 35. Similarly a major portion of beam 35 is reflected in the direction opposite of beam 36. Prism 32 further includes a second surface 115 suitably polished and optically coated for providing a mirror 38. Mirror 38 is positioned so as to reflect the portion of beam 36 that is transmitted through mirror 34. In turn the reflected beam from mirror 38 is reflected from the back side of mirror 34 so as to travel at a slight angle relative to that portion of beam 35 that is transmitted through mirror 34. The doubly reflected portion of beam 36 and the transmitted portion of beam 35 arrive at transmissive surface 116 so as to form an interference fringe pattern on the surface thereof, herein referred to as the first spot beam. Photodetection readout apparatus 10 is fixed relative to transmissive surface

116, having the transparent window 16 juxtaposed to the transmissive surface 116. A mask in accordance with the present invention, and illustrated in Figure 2, is interposed between surface 116 and a pair of photodetectors 12a and 12b.

As a result of the mask, an effective "second spot beam" exits through the mask and transparent window, and impinges upon photosensitive surfaces 20a and 20b of photodetectors 12a and 12b, respectively, each receiving at least a portion of said second spot beam impinging thereon. The second spot beam exhibits the moire fringe pattern as taught in U.S. Patent 4,871,253.

A moire fringe pattern is an interference pattern characterized by an array of high and low intensity regions which move transversely to the fringe motion of the first spot beam. The fringes of this pattern move across the photosensitive surfaces 20a and 20b when the ring laser gyro is rotated. This movement of the high and low intensity light causes the electrically separate photodetectors 12a and 12b to vary their output accordingly. With the mask appropriately oriented relative to the interference fringes, the output from the photodetectors 12a and 12b may then be used to deteπnine the rotation direction of the ring laser gyro.

Referring to Fig. 2, thereshown is a side view of the photodetection readout apparatus 10 which contains photodetectors 12a and 12b. The photodetector package

comprises an enclosure 14 along with a transparent window 16. As illustrated, transparent window 16 is relatively parallel with the photosensitive surfaces 20a and 20b, although the geometrical relationship therebetween is not critical. - Photodetection readout apparatus 10 has a plurality of leadwires 22 extending from the enclosure 14. Leadwires 22 are electrically connected to photodetectors 12a and 12b to provide photodetector output signals responsive to the second spot beam. It should be noted that the combination of photodetectors 12a and 12b is an available component from UDT Corporation (part No. PIN7788-1 or PIN7788-2). Furthermore, enclosure 14 is a component available from NTK Corporation (part No. ITK03FL-0044A), and transparent window 16 may be fabricated by Kyocera

Corporation. The assemblage of such components being commonly employed in ring laser gyros by Honeywell Inc.

Referring now to Fig. 3, photodetectors 12a and 12b are configured within photodetection readout apparatus 10 such that the two photosensitive surfaces 20a and 20b are beside one another (their spacing not being critical). A plurality of parallel thin film black chrome lines or stripes 24 are deposited on transparent window 16. Preferably the black chrome stripes 24 are deposited on the transparent window 16 by a thin film vacuum deposition technique such as ion beam sputtering, e-beam evaporation, and the like, that are well known in the art. Black chrome is preferred over other chromes because of its low reflectivity.

The thin film black chrome stripes 24 form a periodic mask pattern to permit deteπnination of rotation direction as aforesaid. The pattern is chosen to substantially match the interference fringe pattern that substantially impinges on the transparent window 16 having the mask deposited thereon. It should be understood that periodic mask patterns, other than alternating opaque and transparent stripes may be chosen, and are within the scope of the present invention.

The thin film black chrome periodic mask pattern deposited on the transparent window 16 replaces the mylar mask heretofore employed. Of course, the elimination of the mylar mask also elitminates the bonding process that was required to attach the mask to the transparent window 16. Furthermore, thin film black chrome periodic mask pattern on the transparent window 16 is a more durable mask, and is easier to clean than previously used mylar. The glass surface of the transparent window 16 provides a much flatter surface, and allows the deposition of a precise periodic mask pattern, which diminishes scattering, and reduces reflections. This, in turn, increases overall gyro performance. The thin film black chrome periodic mask pattern deposited directly on the glass makes the mask resistant to radiation such that the mask will not fog when exposed to radiation as does a mylar mask.

While the present invention has been disclosed in connection with the preferred embodiment thereof, it should be understood that there may be other embodiments which fall within the spirit and scope of the invention as defined in the following claims.