CA1333758C - Readout for a ring laser gyro - Google Patents

Abstract

A symmetrical prismatic readout apparatus includes a pair of substantially identical prism elements positioned in back-to-back relationship with respect to each other to provide a symmetrical structure. The prism elements have bonding surfaces in which one includes a thin film platinum coating. The bonding surfaces are bonded together through optical bonding, without the use of adhesives. The platinum film forms a beam splitter. The two prism elements are similarly bonded to a substrate element, again by optical bonding, without the use of adhesives. Suitable beam splitter arrangements are provided to effect the desired readouts.

Description

READOUT FOR A RING LASER GYRO

BACKGROUND OF THE INVENTION
The present lnventlon relates to a rlng laser angular rate sensor, a so-called rlng laser gyro. More partlcularly lt relates to a readout apparatus for such a rlng laser gyro.
A so-called rlng laser gyroscope ls baslcally a laser apparatus havlng a rlng type resonant cavlty, typlcally trlan-gular ln conflguratlon. The laser beam ls dlrected around the trlangular path by sultable mirrors posltloned at each of the corners of the trlangular structure. In most cases there are two laser beams travellng ln opposlte dlrectlons relatlve to each other around the rlng. The posltlonlng of the mlrrors ln the corners of the rlng, or trlangle, dlrect the laser beams down the channels of the resonant cavlty. At one of the cor-ners, the mlrror must take the form of a so-called beam spllt-ter. There a portlon of each of the laser beams ls reflected lnto the resonatlng cavlty whlle another portlon of each of the beams ls transmltted through the mlrror lnto a readout assem-bly. Some examples of rlng laser gyros are shown and 2 1 3 3 3 7 ~ 8 64159-944 described in United States patents No. 3,373,650; 3,390,606;

3,467,472; and 4,152,071, all of which are assigned to the assignee of the present application. The readout apparatus for such ring laser gyros, as noted in the aforementioned patents, have been in the form of either a single beam readout arrangement or in the form of a so-called double beam readout arrangement, each having its own unique characteristics.
There has been previously suggested a readout assembly which features both single and double beam type readouts, the assembly having a symmetrical arrangement of two prismatic elements which are secured to each other at a common face by a suitable optical cement and both elements are similarly cemented to a substrate block. Although such structure has provided significant improvements in the readout capabilities of the associated ring laser gyro, it has been found that the use of such cements or adhesives has lead to instabilities, stresses and warpage of parts. Additionally, the use of such adhesives has further involved long and tedious assembly requiring fine adjustments as the adhesives cure.

SUMMARY OE THE INVENTION
In accordance with the present invention, a symmetrical prismatic readout apparatus which includes a pair of substantially identical prism elements positioned in back-to-back relationship with respect to each other to provide a symmetrical structure. Each of the prism elements has a first bonding surface. One of the bonding surfaces is coated with a thin film platinum coating to form a beam splitter. The pair of prism elements are bonded together at the first bonding surface through optical bonding, without the use of adhesives.
The two prism elements are similarly bonded to a substrate element, again by optical bonding, without the use of ~, ~, ~

3 13337~8 64159-944 adhesives. Suitable beam splitter arrangements are provided to effect the desired readouts.
In accordance with the present invention there is provided a readout optical apparatus for a ring laser gyro comprising:
an optically transparent substrate member having at least a first major surface;
a first prism element having at least a first and a second surface;
a second prism element having at least a first and a second surface;
said first surface of said first prism element being positioned in contiguous juxtaposition with respect to said first surface of said second prism element with second surface of said first prism element coplanar with said second surface of said second prism element, said first and second prism elements being bonded together at said first surface solely by optical bonding;

133375~
3a ~ 64159-944 a thin film platinum coating deposited on a selected one of said first surfaces of said first and second prism elements to form a beam splitter; and said coplanar second surfaces of said first and second prism elements being positioned in contiguous juxtaposition with said major surface of said substrate member and bonded thereto solely by optical bonding.

BRIEF DESCRIPTIONS OF THE DRAWINGS
A better understanding of the present invention may be had from the following detailed description when read in connection with the accompanying drawing, in which:
The single figure is a fragmentary diagram of a ring laser gyro having a readout structure embodying the present invention.

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133375g 64159_944 DETAILED DESCRIPTION
Referrlng, now, to the drawlngs ln more detail, there ls shown, ln the ~lngle flgure, a fragmentary dlagram repre-sentlng one corner, the readout corner, of a rlng laser gyro of the type baslcally shown ln Unlted States Patent No. 3390606 and ln Unlted States Patent No. 3581227. As in those patents, a thermally and mechanlcally stable block 2 has formed therein a resonant cavlty 4. The cavity ls sealed at each of the cor-ners by a reflectlve element substrate 6. In the exemplary embodlment, there are three such corners wlth sultable reflec-tor substrates seallng each of the three corners. The thus deflned cavity 4, accordlngly, comprl~es a closed loop cavlty.
The cavlty ls fllled wlth a sultable laslng gas ln accordance wlth well-establlshed prlnclples for a laser galn medlum. By sultable excltlng means, not here illustrated and formlng no part of the present lnventlon, ls provlded for lntroduclng lnto the gas fllled cavity 4 a flrst and a second laser beam 8 and 10, respectlvely. The two laser beams are arranged to travel ln opposlte dlrectlons about the closed loop or rlng of the assembly ln accordance wlth establlshed prlnclples. The reflector element 6 lncludes a substrate and havlng one ma~or face thereof constltutlng a so-called beam splltter whereby a portlon of each of the 2 lmplnglng laser beams are transmltted through the substrate 6, and a larger portlon ls reflected back lnto the cavlty 4.

_5_ 1333758 The prismatic readout structure includes a first prism element 12 and a second prism 14. The prism 12 is provided with a first and third surface 16 and 18 which are parallel with respect to each other. The second surface 20 is perpendicular to both of those two surfaces 16 an 18. A
fourth surface is formed at a predetermined angle with respect to the first surface. In the exemplary embodiment, the fourth surface 22 of the prism 12 was at an angle of approximately 45 with respect to the surface 18.

The second prism 14 is substantially identical in construction, having a first surface 24 and a third surface 26 which are essentially parallel, and a second surface 28 which is mutually perpendicular to the two surfaces 24 and 26. A fourth surface 30 is, again, at the predetermined angle with respect to the surface 24.
The positioning of the prisms 12 and 14 is such as to present a symmetrical arrangement, with the surfaces 20 and 28 coplaner and the surfaces 16 and 24 in contiguous juxtaposition. The coplaner surfaces 20 and 28 are positioned in contiguous juxtaposition with respect to the outer major surface of the substrate 6. A beam splitter coating 32 is applied to the fourth surface 22 of the prism element 12.
Similarly, a second beam splitter coating 34 is applied to the fourth surface 30 of the prism element 14.
The beam splitter coatings 32 and 34 may be of the so-called dielectric type, signifying that no energy is absorbed by the beam splitter, itself. A third beam splitter 36 is imposed at the interface between the surfaces 16 and 24 of the two prisms 12 and 14, respectively. As was previously noted, the inner surface 38 of the substrate 6 also comprises a beam splitter arrangement.
A first sensor 40 and a second sensor 42 are positioned as will be hereinafter discussed adjacent to the first prism element 12. A third sensor 44 and a fourth sensor 46 are similarly positioned adjacent to the prism element 14.
The path of the beam 8 reflected and/or refracted through the prismatic structure is indicted by a single headed arrow. Thus a portion of the beam 8 is reflected at the surface 38 of the substrate 6 and reflected back down the opposite leg of the cavity 4. A second portion of the beam 8 is refracted at the substrate 6 and passes through the prism 14 to the surface 30. At the surface 30 a portion of the beam 8 is refracted through a first exit port at the beam splitter 34 to impinge upon the sensor 46. A second portion of the beam 8 is reflected at the surface 30 towa~rd the interface 16, 24 between the two prisms 14 and 12. There it impinges upon the beam splitter 36. At the beam splitter 36, the beam 8 has a portion thereof reflected back into the prism 14, emerging from a second exit port at the surface 26 to impinge upon the detector or sensor 44. Another portion of the beam 8 is transmitted throught the beam splitter 36 into the prism 12 and emerging from a third exit port at the surface 18 to impinge the sensor 40.
Similiarly, the beam 10 impinges upon the beam splitter surface 38 of the substrate 6 where a portion of the beam 10 is reflected down the opposite leg of the cavity 4. A second portion of the beam 10 is refracted into the substrate 6 thence into the prism element 12 to impinge on the surface 22 at beam splitter 32. At the beam splitter 32, a fourth exit port, a first portion of the beam 10 is transmitted to the sensor 42. A second portion of the beam impinging of the beam splitter 32 is reflected back into the prism 12 to impinge upon the beam splitter 36 at the interface between the 2 surfaces 16 and 24. A first portion of the beam 10 impinging upon the beam splitter 36 is transmitted through that beam splitter into the prism element 14, emerging from the surface 26 thereof and impinging on the sensor 44. The second portion of the beam 10 impinging on the beam splitter 36 is reflected back into the prism 12 emerging from the surface 18 thereof and impinging on the sensor 40.
Following these paths on the single figure of the drawing it may be noted that the sensor 46 is arranged to receive only signals from the beam 8 while the sensor 42 ~ - 64159-944 recelves only slgnals from the beam 10. On the other hand, the sensor 40 recelves a comblnatlon of signals from the beams 8 and 10. Slmilarly the sensor 44 receives slgnals whlch are a comblnatlon of the superlmposed beams 8 and 10.
In the present lnventlon, beam splltter 36 ls pro-duced to have a somewhat hlgh optlcal beam (wave) ab~orptlon or loss. If the optlcal absorptlon ls of a sufflclent amount, a substantlally 90 phase shlft can be lmposed between the flrst and second double beam slgnals emerglng from each slde of beam splltter 36. In these clrcumstances, the output of detectors 40 and 44 can be phase compared to determlne dlrectlon. It ls analogous to two detectors g spatially separated for monitoring a linear interference pattern, a technique well known. Optical absorption in beam splitter 36 is accomplished by using a thin film of a platinum coating which provides an absorption type coating.
It should be noted that although a 90 phase shift between the beam emerging from splitter 36 is most desirable, any phase shift which can be utilized to determine direction is all that is required to be within the scope of the present invention.

In accordance with the present invention, the beam splitter 36 comprised of platinum is on the order 50 to 100 Angstroms in thickness. In practicing the present invention, the surfaces 16 and 20 of the prism element 12 and the surfaces 24 and 28 of the prism element 14 as well as the outer surface of the substrate 6 are polished optically smooth and flat. One of the surfaces 16 or 24 is coated with the thin platinum film. When the two prism elements are positioned with surfaces 16 and~24 in juxtapostion, they will be rigidly adherent together by operation of a so-called optical bond. Similarly, the now coplaner surfaces 20 and 28 of the two prisms 12 and 14 are placed in juxtapostion to the outer surface of the substrate 6. Here, again, the elements will be rigidly adherent through the so-called optical bond. Because the platinum coating of the beam splitter 36 is so microscopically thin, the platinum coating will not 133375~

interfere with the optical bonding between the surfaces 16 and 24 of the two prism elements 12 and 14 respectively.
As was herebefore noted, in the prior art devices which used the adhesive optical cement to bond the several elements together into a unitary structure, the cement itself changes dimensions as it cures. This change in dimension introduces instabilities, stresses and warpage of parts during the curing process. These tendencies necessitate a very tedious and time-consuming operation of adjusting the relative position of the components during the curing of the cement in order to maintain the optical viability of the system. Further, it occurs sometime that the amount of warpage or stress or instability is such that it cannot be adjusted during the assembly and results in the assembled system being rejected and unacceptable. In accordance with the present invention, the optical bonding, without intervening adhesive cement provides rigid bonding of the elements with no intervening medium to introduce the undesirable instabilities, stresses and warpage. This, in turn, simplifies the assembly procedure and greatly reduces the reject rate. With the prism elements being made of the same material as the reflector substrate, there is a perfect thermal matching of the components thereby obviating thermal stresses in the assembly.

1333758 1l Thus, there has been provided, in accordance with the present invention, an improved readout assembly for a ring laser gyro which obviates the shortcomings of the previous apparatus.

Claims (7)

1. A readout optical apparatus for a ring laser gyro comprising:
an optically transparent substrate member having at least a first major surface;
a first prism element having at least a first and a second surface;
a second prism element having at least a first and a second surface;
said first surface of said first prism element being positioned in contiguous juxtaposition with respect to said first surface of said second prism element with second surface of said first prism element coplanar with said second surface of said second prism element, said first and second prism elements being bonded together at said first surface solely by optical bonding;
a thin film platinum coating deposited on a selected one of said first surfaces of said first and second prism elements to form a beam splitter; and said coplanar second surfaces of said first and second prism elements being positioned in contiguous juxtaposition with said major surface of said substrate member and bonded thereto solely by optical bonding.

2. A readout optical apparatus as set forth in 1 wherein, said second surface of said first prism element is perpendicular to said first surface thereof and said second surface of said second prism element is perpendicular to said first surface thereof.

3. A readout optical apparatus as set fourth in 1 wherein said coating is on the order of 50-100 Angstroms in thickness.

4. A readout optical apparatus as set fourth in 1 wherein said substrate member includes a second major surface and having a beam splitter on said second major surface.

5. A readout optical apparatus as set forth in 4 wherein each of said first and second prism elements has a third and fourth surface with said third surface being parallel to said first surface, respectively, and with said fourth surface being at a predetermined acute angle with said first surfaces, respectively, and a beamsplitter on each of said fourth surface.

6. A readout optical apparatus as set forth in 5 wherein said predetermined acute angle is substantially 45°.

7. A readout optical apparatus as set forth in 5 wherein said apparatus defines first and second predetermined paths for laser beams to first impinge together on said beam splitter on said substrate, next separately on said beam splitters on said first and second prism elements, then together on said beam splitter at the interface of said first and second prism elements, establishing four exit ports for portions of said laser beams, and detector means positioned adjacent to each of said exit ports responsive, respectively, to energy from said portions of said beams.