Abstract:
A diffractive optical element is interposed between the exit beams from a ring laser gyro readout and the photodetectors for detecting such beams. The diffractive optical element permits the use of photodetectors much reduced in size in order to minimize gyro performance degradation due to radiation.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a ring laser angular rate sensor, usually referred to as a ring laser gyroscope. More particularly it relates to a readout apparatus for such a ring laser gyroscope. 
   A ring laser gyro is a laser apparatus having a ring type resonant cavity, or simply a ring resonator. The ring resonator is commonly constructed of a block having a plurality of interconnecting tunnels in the shape of a polygonal path such as, for example, a triangular or rectangular path. The laser beam is directed around the ring path by suitable mirrors appropriately positioned at the intersections of pairs of interconnecting tunnels. In ring laser gyros there are commonly two laser beams traveling in opposite directions relative to each other around the polygonal ring path. The positioning of the mirrors in the corners of the polygonal ring path direct the laser beams through the tunnels of the resonant cavity. At one of the corners, the mirror is partially transmissive whereby major portions of the counter-propagating beams are reflected, while small portions of each of the counter-propagating beams are transmitted through the mirror into a readout assembly. Some examples of ring laser gyros are shown and described in U.S. Pat. Nos. 3,373,650 and 3,467,472 issued to Killpatrick, and U.S. Pat. No. U.S. Pat. No. 3,390,606 issued to Podgorski, these being incorporated herein by reference. 
   The aforementioned readout assembly generally comprises a prismatic structure for combining those small portions of each of the counter-propagating beams to produce either an interference fringe pattern comprised of light intensity bands with directional movement, or alternatively a light intensity spot which varies in intensity between high and low values at a rate proportional to the frequency difference between the counter-propagating beams. The readout assembly may be either “block mounted” or displaced from the block. U.S. Pat. No. 3,373,650 illustrates a readout assembly which is displaced from the gyro block. In U.S. Pat. No. 3,373,650, the readout assembly is comprised of a corner prism which combines the small portions of the counter-propagating beams which are transmitted through the partially transmissive mirror, and recombines them to form an interference fringe pattern. 
   U.S. Pat. No. 4,582,429, issued to Steven P. Callaghan, U.S. Pat. No. 4,677,641 issued to Theodore J. Podgorski, and U.S. Pat. No. 4,712,917 issued to Bergstrom et al. all show block mounted readout assemblies which are all solid structures comprised of one or more prism elements. These prism elements combine those portions of the counter-propagating laser beams, transmitted through the partially transmissive mirror, to produce an interference fringe pattern or light intensity spot. These patents are also incorporated herein by reference. 
   The aforementioned Callaghan and Bergstrom et al. Patents disclose prismatic structures which combine the small portions of the counter-propagating laser beams to produce an interference light spot. The Podgorski Patent shows a unitary solid structure prism for producing an interference fringe pattern; and the Callaghan and Bergstrom et al. Patents show a solid structure prism comprised of a pair of prisms mounted to a mirror assembly substrate. All of the just referred to readout assemblies are all intended to be directly mounted to the ring laser gyro block. 
   Associated with the readout assemblies of the prior art is at least one photodetector which is generally responsive to either the interference fringe pattern or the interference light spot. 
   Readout assemblies known in the prior art for ring laser gyros are particularly susceptible to radiation-induced noise, i.e., unwanted signal or effect generally caused by nuclear radiation, which may result in poor sensor performance due to rotation rate information errors. The magnitude of the sensitivity to radiation-induced noise is thought to be directly related to the size of the photodetectors. The size of the detectors being determined by the size of the output light spot or interference fringe pattern. 
   BRIEF DESCRIPTIONS OF THE INVENTION 
   The object of the present invention is to produce a small light intensity spot or light interference fringe pattern so as to lower the required size of the photodetectors which respond to such patterns, and at the same time keep package size to a minimum. 
   In the present invention, an optical means receives the small portions of the counter-propagating waves which are transmitted through one of the reflecting means at one of the corners of the gyro. Exiting from the optical means is a light beam which is directed toward a photdetector through a diffractive optical element, thereby causing the light beam to converge at the photdetector. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a readout assembly in accordance with the present invention. 
       FIG. 2  illustrates in more detail the readout of FIG.  1 . 
       FIGS. 3   a  and  3   b  show a diffractive optical element. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings in more detail, there is shown in  FIG. 1  a schematic diagram similar to that shown in U.S. Pat. No. 4,582,429. Particularly,  FIG. 1  represents a readout corner of a ring laser gyro of the triangular type as shown in the aforementioned U.S. Pat. No. 3,390,606. 
   Illustrated in  FIG. 1 , a thermally and mechanically stable block  2  has formed therein a resonant cavity  4  (partially shown). Cavity  4  is sealed at each of the corners by an output substrate  6  which includes a partially reflecting mirror means  38  generally comprised of a plurality of alternating layers of two different dielectric materials, e.g., titanium oxide, and silicon dioxide. 
   For triangular ring laser gyros, there are three such corners with suitable substrates  6  sealing each of the three corners. Cavity  4  is filled with a suitable lasing gas which is excited by an appropriate voltage in accordance with well established principles for lasers. By an appropriate electrical exciting means, first and second laser beams  8  and  10 , respectively, are established to travel in opposite directions about the closed-loop or ring path of the assembly in a well known manner. 
   As is particularly illustrated in  FIG. 1 , substrate  6  includes a first surface  7  thereof having a partially transmissive mirror coating  38  thereon, whereby a portion of each of the two impinging laser beams  8  and  10  are transmitted through the surface  7  of substrate  6 ; and a larger portion of each impinging beam is reflected back into resonant cavity  4 . Those portions of beam  8  and  10  which pass through mirror  38  and substrate  6  are identified as beams  8 A and  10 A, respectively. 
   Coupled to substrate  6  are a pair of prisms  12  and  14  which are strategically located to direct the light paths followed by beams  8 A and  10 A, and combine portions of these beams so as to exit from the prismatic structure for subsequent photodetection. The prismatic structure illustrated produces light beams  80  and  85  being characteristic of selected functions of beams  8  and  10 , as will be subsequently described. 
   Prism  12  includes surfaces  16 ,  18 , and  20 . Surface  16  is perpendicular to surface  20 , and surface  18  is at a selected acute angle relative to surface  16 . Similarly, prism  14  includes surfaces  24 ,  28 , and  34 . Surface  24  is perpendicular to surface  28 , and surface  34  is at a selected acute angle relative to surface  24 . Preferably, prisms  12  and  14  are symmetrical. Further, in the embodiment illustrated, the acute angle selected is in the order of 30 angular degrees. 
   Surfaces  18  and  34  each have a wave reflecting coating thereon as identified by numerals  50  and  48  on surfaces  18  and  34 , respectively. Further, a beam splitter coating  36  is provided on either surface  24  or  16 . 
   Prisms  12  and  14  are positioned such that surfaces  20  and  28  are co-planar and positioned in contiguous juxtaposition with respect to surface  9  of substrate  6 . Further, surfaces  16  and  24  are fixed together by suitable means, e.g., optical contact bonding or an adhesive. 
   The arrangement of substrate  6 , prisms  12  and  14 , and wave reflecting coatings  48  and  50 , and beam splitter coating  36  is to provide light beams  80  and  85  exiting from surfaces  34  and  18 , respectively. These beams are sometimes referred to as “double beam signals” since they are a composite beam consisting of portions of beams  8  and  10  passing through partially reflecting mirror  38 . Beams  85  and  80  exit from prisms  12  and  14 , respectively, and impinge upon substantially identical detector assemblies  200   b  and  200   b  as particularly described in FIG.  2 . Exit beams  80  and  85  produced by the prismatic structure of  FIG. 1  are of the variety for creating an interference light spot, described earlier, which varies in intensity as opposed to the variety which creates an interference fringe pattern. 
   The path of the waves of the prismatic structure of  FIG. 1  will now be described. Laser beam  8  impinges upon partially transmitting mirror  38 . A portion  8   a  of beam  8  is refracted by substrate  6  and transmitted through surface  9  of substrate  6 . Beam  8   a , in turn is reflected by means  48  to impinge upon beam splitter  36 . Beam splitter  36  functions to split beam  8   a  into a reflected portion which becomes a component of beam  80 , and a transmitted portion which becomes a component of beam  85 . 
   Similarly, laser beam  10  impinges upon partially transmitting wave reflecting means  38 . A portion  10   a  of beam  10  is refracted by substrate  6  and transmitted through surface  9  of substrate  6 . Beam  10   a , in turn is reflected by means  50  to impinge upon beam splitter  36 . Beam splitter  36  also functions to split beam  10   a  into a reflected portion which becomes a component of beam  85 , and a transmitted portion which becomes a component of beam  80 . 
   The construction of prisms  12  and  14 , and particularly the location of wave reflecting means  48  and  50 , and beam splitter  36 , and the angular relationships between all of the prism surfaces and their relationship to the surface  9  of substrate  6  are such to cause beams  80  and  85  to be composed of co-linear and coexisting portions of beams  8   a  and  10   a . The combined beams  80  and  85  are therefore a function of the phase relationship between beams  8  and  10 . 
   The prismatic structure as just described is similar to that shown and described in U.S. Pat. No. 4,582,429. 
   A detector assembly  200  in accordance with the present invention will now be described with reference to FIG.  2 . Detector assemblies  200   a  and  200   b  are substantially identical, and each are separately responsive to the double beam signals  80  and  85  which have an intensity which varies as a function of the phase difference between beams  8  and  10 . Detectors  200   a  and  200   b  have output signals (not shown) which are directly related to the intensity of a light “spot” which varies in intensity due to the interference between the two combined beams, namely the double beam signal. The detector output signals therefore provide sensor rotation information in a well known manner. 
   If beam splitter  36  is constructed by way of a low absorption or a low loss optical coating such as a dielectric material, the reflected and transmitted beams of an impinging beam will be substantially complementary or 180° out of, phase with each other. Accordingly, the first and second double beam signals,  80  and  85 , will also be complementary or 180° out of phase with each other. The detector output signal may be differentially compared to provide rotation rate information. 
   Detector assembly  200  is comprised of a cylindrical substrate  201  having an aperture  263  with an inner bottom surface  205 . Aperture  203  is sealed by an optically transmissive substrate  207 . Fixed to substrate  207  is a second optically transmissive substrate  209  having a diffractive optical element  211  imbedded therein. A photodetector  213  is secured to surface  205 . 
     FIGS. 3   a  and  3   b  illustrate a Fresnel lens which forms the diffractive optical element  211 . Referring to the drawings, substrate  209  can be etched in a manner to form the Fresnel lens having concentric rings having an increasing circular fringe spacing, T, and fixed step height as particularly illustrated in  FIG. 3   b . Preferably, substrate  209  may be a very thin substrate of fused silica. It is also preferable that the beam be aligned with the center of the Fresnel lens. 
   The detector assembly  200  is intended to be fixed, for example, to surface  18  of prism  12  such that the double beam signal  85  is directed through the diffractive optical element  211  so that the double beam signal is focused by the diffractive optical element at a point somewhat behind the light sensitive surface of photodetector  213 . It is intended that the cross-section of the photodetector  213  match the cross-section of the resulting converging beam at the detector surface. 
   By use of the diffractive optical element, beams  80  and  85  are made to converge at a single focal point of the diffractive optical element. By virtue of the detector intercepting the resultant optical beam in front of the focal point, the detector size can be selected. 
   Preferably, the relationship between the double beam signal and the diffractive optical element is such that the double beam signal is incident on the diffractive optical element at a slight angle in order to minimize any retroreflection back into the ring laser. In  FIG. 2 , substrate  209  is shown to have non-parallel surfaces  209   a  and  209   b  such that surface  209   b  is at a slight angle relative to surface  18 . This cause the impinging beam to be refracted (not shown) by the optical elements  209  and  207 . 
   There of course, many alternative embodiments for the detector assembly employing the diffractive optical element in accordance with present invention. Specifically, the diffractive optical element, for example the Fresnel lens may be directly etched on substrate  207  acting as the hermetically sealing window. 
   Although the diffractive optical element in the form of a Fresnel lens has been illustrated, it should be understood to those skilled in the art that the diffractive optical element could also be realized by either etching or appropriate application of optical coatings on either substrate  209  or window substrate  207 . 
   There are, of course, many variations in the implementation of a diffractive optical element in order to achieve the intended function as described in the illustrated embodiment. Specifically, two confocal (optical elements having the same foci) diffractive optical elements could be utilized to implement a focal system (foci at infinity). These two confocal diffractive optical elements can be separated in order to match a collimated beam to the area of a photodetector. This assembly will be somewhat thicker and although somewhat more complicated. However, it provides an optical system which minimizes retroreflection back into the ring laser. 
   Still, alternatively, a single diffractive optical element could be used in a higher order, off axis mode. Such a diffractive optical element would be very sensitive to polarization, and this feature could be used to advantage by matching it to the polarization of the incident beams. 
   It should be understood therefore, that various changes and modifications may be made to the invention shown in the accompanying Figures and described herein as will be apparent to those skilled in the art, and are within the true spirit and scope of the present invention. It should be particularly noted than an interference fringe pattern may also be reduced in size in accordance with the principles of the present invention other than as illustrated in the drawings herein.