WO1988002103A1

WO1988002103A1 - Low cost ring laser angular rate sensor

Info

Publication number: WO1988002103A1
Application number: PCT/US1987/002433
Authority: WO
Inventors: Theodore J. Podgorski
Original assignee: Honeywell Inc.
Priority date: 1986-09-22
Filing date: 1987-09-22
Publication date: 1988-03-24

Abstract

A ring laser angular rate sensor (10) is constructed from a solid block (11) with mirror assemblies (19, 21, 22) joined to the block (11) by welding together a thin metallic film (226) deposited on the block and mirror assembly prior to welding.

Description

LOW COST RING LASER ANGULAR RATE SENSOR

BACKGROUND OF THE INVENTION The present invention relates to a novel construction for a ring laser angular rate sensor and more particularly, to an assembly for fixing mirrors to a laser block, which is less costly to manufacture than prior art constructions.

Ring laser angular rate sensors are well known and are particularly described in U.S. Patent 3,373,650, issued to Killpatrick, and U.S. Patent 3,390,606, issued to Podgorski, both of which are assigned to the assignee of the present invention. The above-referred to patents are incorporated herein by reference thereto. Ring laser angular rate sensors of the type referred to commonly utilize a block of material that is dimeneionally stable, both thermally and mechanically. The block usually includes a plurality of interconnected gas containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any polygonal path. At each intersection of a pair of interconnected tunnels is a mirror mounted on the block. This arrangement of mirrors and interconnected tunnels forms an optical closed-loop path. Further, at least one anode and one cathode are each mounted on the block and in communicatlon with the gas. Each of the components, including the mirrors, anode, and cathode, must be sealed to the block to form a gas tight seal. The block is usually filled with a lasing gas such as a mixture of helium and neon. A sufficiently large electric potential is applied between the anode and cathode to cause a discharge current therebetween which results in the production of a pair of counter-propagating laser beams within the block along the defined optical closed-loop path.

Associated with ring laser angular rate sensors is a source of error usually referred to as "lock-in." The source of error is thought to be predominantly caused by back scattering of light at each of the mirrors which form in part the optical closed-loop path which the counter-propagating laser beams traverse - As is well underotood by those skilled in the art, there are two widely used techniques applied together to minimize the lock-in error. The first technique consists of dithering the block as taught in U. S . Patent 3,373,650. Mechanically dithering the laser block reduces the source of error caused by lock-in to acceptable levels such that ring laser angular rate sensors have become commercially successful. The second technique consists of producing mirror assemblies structured so as to provide highly polished substrates having superior reflective coatings which achieve minimal laser beam scattering at the surfaces thereof. Development of the mirror assemblies over the years has made it possible for high performance ring laser angular rate sensors. Prior art mirror assemblies comprise a block of material suitably polished for a mirror substrate. The mirror substrate usually is the same material as the laser block material so that they have matched thermal coefficients of expansion. Commonly, the mirror assembly further comprises alternating layers of a high dielectric material, for example, titanium dioxide (TiO₂) and a lower dielectric material, for example, silicon dioxide (SiO₂). deposited on the mirror substrate by a deposition process such as e-beam deposition or an ion-beam sputtering process, or any other appropriate process to achieve high grade mirrors.

The mirror assemblies of the prior art are usually fixed to the laser block by what is referred to as an optical contact. The optical contact technique requires that the block and the mirror substratβ be highly polished so as to form an optical contact when the mirror substrate is pressed against the block. The joining of the laser block and the mirror block by optical contact is usually accomplished at room temperatures.

The mirror assemblies referred to above include a substrate in the form of a block of material having a reflective surface deposited thereon such as the titanium dioxide variety described above. Further, the mirror assemblies may include transducers for controlling optical path length, alignment, and the like. Mirror transducers may be like those shown in U.S. Patent 3,581,227, issued to Podgorski, and assigned to the assignee of the present invention, U.S. Patent 4,383,763, issued to Hutchings et al, U.S. Patent 4,160,184, issued to Ljung, and UK patent application GB 2,104,283 in the name of Litton Systems, Inc. The just above-referred to patents being incorporated herein by reference. Ring laser sensors of the kind referred to above further include a plurality of electrodes including anodes and cathodes of various constructions like that shown in U.S. Patent 4,007,431, issued to Abbink et al and herein incorporated by reference. These prior art ring laser angular rate sensors have been proven highly satisfactory in operation and are rapidly gaining wide-spread acceptance for certain applications. These prior art ring laser angular rate sensors, however, are costly to manufacture.

Ring laser angular rate sensors demand dimeneionally stable material for many of the parts, and specifically the laser block and the mirror assemblies. This is so, since a closed-loop optical path has only so much leeway in position relative to the tunnels of the cavity and size of the mirrors. The ring laser assembly tolerances are far more critical than simple linear (single line tube) lasers. The mirror assemblies usually include a substrate of material which has thermal and mechanical characteristics substantially similar to those of the block. Commonly, the mirror substrate and the block are of the same material. This is so since the mirror substrate and the block would then have identical thermal coefficients of expansion. In order not to introduce another material type between the block and the mirror substrate, mirror substrates are commonly bonded to the block by what is referred to as an optical contact. That is, the mirror mounting surface of the mirror substrate and end surfaces of the laser block are highly polished to provide an optical contact. Since the block and the mirror substrates are commonly of a quartz-like material, polishing of such surfaces is time consuming and expensive. Others have attempted to bond the mirror substrate to the laser-block by other techniques including epoxy, indium seals, and other materials, but such materials, as indicated earlier, introduce other problems which can deleteriously affect the sensor performance and/or life. Particularly, nonuniformly applied bonding materials between the mirror substrate and laser block may lead to poor or non-existing ring laser alignment within the block. Further, the materials may introduce particulate matter which may react with the lasing gas. All of these problems may cause deleterious effects on laser life and/or performance. Although bonding of the electrodes to the laser block is not as big a concern, f orming o f a g as t i gh t coal an d mate hi n g o f m ater ials is still important. Indium seals have proven satisfactory as a technique of bonding the electodes to the laser block.

SUMMARY OF THE INVENTION The object of this invention is the provision of a novel construction and method of assembly of a ring laser angular rate sensor which permits it to be inexpensively manufactured. Briefly, this invention contemplates the provision of a ring laser angular rate sensor constructed from a solid block with mirror assemblies joined to the block by a thin film metallic material, particularly gold.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a planned view of a ring laser angular rate sensor constructed in accordance with the teaching of this invention. Figure 2 is a planned view of the mirror block assembly in accordance with the invention.

Figure 3 is another arrangement of a mirror transducer assembly in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

Referring now to Figure 1, there is disclosed a pictorial representation of a gas filled ring laser angular rate sensor 10 comprising a block 11 made of a quartz like material such as Cervit, Zerodur, or the like or glasses such as BK-7 (letter number combinations are Schott Optical Commercial Designations). A plurality of three interconnected tunnels 13, 15, and 17 are bored within block 11 at angles to each other to form a triangular-shaped cavity. Mirror assemblies 19, 21, and 22 are bonded to end surfaces of block 11 which form the intersection of each of the tunnels 13, 15, and 17, respectively, in a manner as will subsequently be described. Each mirror functions to reflect light from one tunnel into the next thereby forming an optical closed-loop path.

A pair of anodes 27 and 29 are bonded to end surfaces of laser block 11 and adapted to communicate with laser tunnels 13 and 17, respectively, through interconnecting cavities 23 and 25, respectively. A quantity of lasing gas for plasma is adapted to be contained within the tunnels 13, 15, and 17, and other tunnels in communication therewithr The gas may be inserted into the block, cavities through one of the anode cavities used as a fill tube and one of the anodes which may also serve as a sealable port, e.g. anode 29.

A cathode 40 is bonded to an end surface of laser block 11 and in communication with the optical closed-loop cavity through interconnecting cavity 43. Cathode 40 is symmetrically located relative to anodes 27 and 29, and tunnels 13, 15, and 17. These symmetrical locations of the pair of anodes and cathode is intended to reduce gas flow effects which can adversely affect the performance of the rate sensor, as is well known. In operation, with a sufficiently large potential applied between the cathode and the anodes, a first discharge current flows from cathode 40 out into tube 15 toward mirror 21 and through tube 13 to anode 27. A second discharge current flows through cathode 40 out into tube 15 toward mirror 22 and through tube 17 to anode 29. These two discharge currents are usually controlled in intensity. The discharge current's function is to ionize the lasing gas and thereby provide a pair of counter-propagating laser beams within the closed-loop optical cavity in a well known manner. It frill be appreciated that ring laser angular rate sensors with a rectangular lasing path or other optical cavity configurations, including a cubic cavity, can be constructed in accordance with the teaching of this invention.

Each of the aforementioned mirrors perform functions in addition to redirecting the laser beams about the cavity. Mirror 19 may be constructed as to be partially transmissive for providing a readout beam signal to be directed toward a photosensitive means 50. Mirror 22 is preferably curved so as to aid in the alignment and focusing of the counter-propagating laser beams within the cavity. Lastly, mirror 21 may be in part a transducer for cavity path length control in a well known manner. A suitable readout device 50 is disclosed in U.S. Patent 4,152,072, issued to Hutchings and is incorporated herein by reference thereto.

The construction of the ring laser angular rate sensor described above and its performance are in accordance with the basic operating principles of prior art ring laser angular rate sensors. Referring now to Figure 2, an important contributor to reducing the construction costs in accordance with the teaching of this invention is the use of a fused metallic material to join each of the mirror assemblies 19, 21, and 22 to laser Block 11 as win now toe described.

In the present invention, a substrate of a mirror assembly and the portion of the laser block ll which the mirror is to be attached are each polished to provide a lapped surface or finely ground surface for subsequent coating by a thin film metallic material. The surface finish requirements of the laser block and substrate, in accordance with the present invention, is significantly less than that required by optical contact. Each of the piece parts, the substrate and the laser block portion which the mirror substrate is attached are coated with a thin metallic film. The piece parts are subsequently pressed together and welded together, by way of example, by ultrasonic welding. The molded parts provide a gas-tight seal with good Butt-Tensile strength .

The metallic rilm Bonding technique, in accordance with the present invention, obviates the need for creating highly polished end surfaces on the block and mirror substrate which are required by the optical contact technique for fixing the mirror assemblies to the block. Accordingly, block polishing and inopootion reauirements are significantly reduced and thereby significantly reduce the coat of the quite expensive optical contact technique.

In the preferred embodiment, the metallic film is comprised of gold or gold alloy film sputtered on the surfaces. The film being so thin that thermal coefficient mismatch between the film and the bonded parts is negligible.

In the embodiment of the invention illuatrateel in Figures 1 and 2 , the ring laser a ngul ar rate sensor block 11 is a solid block of dimeneionally stable material to which the interconnecting tunnels are machined therethrough. Figure 2 specifically illustrates the bonding of a mirror assembly 21 to laser block 11. Mirror assembly 21 is shown including substrate 222 formed from a dimeneionally stable material, preferably the same material as the block. An optical coating 224 of alternating layers of zirconium dioxide and silicon dioxide is deposited on surface 225 of substrate 222 by the ion-beam deposition process. A suitable ion-beam process is that substantially shown and described in U.S. patent 4,142,958, entitled, "Methods for Fabricating Multi-Layer Optical Films" issued to Wei et al, and is hereby incorporated herein by reference.

The dimeneionally stable material referred to above for laser block 11 and mirror substrate 222 may preferably be of the same material type, but it is not necessary they be the same within the scope of the present invention. Dimensionally stable materials are like those referred to above, namely quartz like material or glasses which are dimensionally stable in the presence of mechanical and thermal stresses. It is best, of course, that the materials be identical since it minimizes any mismatch of thermal coefficients. In Figure 2, an optical coating 224 is shown as only a spot having sufficient area to reflect impinging laser beams thereon. It is within the scope of the present invention that the entire surface 225 of substrate 222 may have the optical coating 224. Further, the optical coating 224 may still alternatively be a mirror chip on its own substrate bonded to substrate 222 in a suitable manner or in accordance with the teaching of the present invention. In Figure 2, mirror 21 is shown to be bonded to the end surface 51 of block 11 by a thin film metallic bonding agent 226. In the preferred embodiment, the thin film bonding agent is comprised of a thin film of gold or gold alloy 226 first deposited on the mirror substrate 222 by any of a variety of techniques including vacuum-sputtered deposition, painting and firing, thermal evaporation, and other deposition techniques. The same thin film metallic bonding agent is also deposited on substrate 222 by a suitable process as already described in order to provide thereon a very thin and uniform thin film metallic bonding agent 226. The thickness of the layer of the metallic coating should be as thin as possible in order to form a uniform gas-tight seal. Generally, the thin film metallic coating should be in a range from from less than one micron thick to about 20 microns. The greater the thickness, the greater will be the tendency to have thermal mismatch effect which can cause short laser life. Therefore, the thickness of the thin film metallic coatings on each of t he picoo-parts should be as l ittl e as needed to achieve the desired bonding strength. In bonding substrate 222 to laser block 11, the substrate 222 is pressed against laser block end surface 51 in mated alignment. End surface 51 should be suitably polished to provide good mating parts, but much less than that required for optical contact.

Block 11, metallic coating 226, and substrate 222 are preferrably fused together by ultrasonic welding which causes the metallic coating on each piece to fuse together and thereby fuse mirror substrate 222 and laser block 11 together and provide a gas-tight seal.

Figure 3 illustrates a transducer embodiment in which a piezoelectric devices 510 is suitably bonded to mirror substrate 222. The diagram of Figure 3, except for the addition of piezoelectric wafer 510, has therein the components of Figure 2 and so the same numeral designations are used in Figure 3 as used in Figure 2. Mirror coating 224 is preferably shown directly on substrate 222. The metallic bonding agent is shown between block 11 and substrate 222. Like Figure 2 , substrate 222 is bonded to laser block 11 in the manner described above. If substrate 222 is made very thin, the piezoelectric wafer will be sufficient to cause movement of mirror coating 224 in a direction perpendicular to the surface of the mirror coating. Although not shown, piozoolootric wafer 510 may be bonded to substrate 222 by a variety of techniques. Those skilled in the art will recognize that only preferred embodiments of the present invention have been disclosed herein and that the embodiments may be altered and modified without departing from the true spirit and scope of the invention as defined in the accompanying claims. Particularly, the metallic material used as a bonding agent may also be comprised of other metallic materials such as silver, copper, and the like, to provide the intended function. It should be recognized by those skilled in the art that all gas filled ring lasers are generally gas discharge devices, and that the mirror assemblies form end members for sealing the gas discharge device cavity. Further, the principals of the invention are applicable to any discharge device cavity configuration including a cavity for linear lasers in contrast to ring lasers.

Claims

CLAIMS The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. A ring laser angular rate sensor comprising: a block of dimensionally stable material; a plurality of interconnecting tunnels to form a closed-loop path within said block; a mirror assembly including a first substrate having a bonding surface, said substrate including means for reflecting a laser beam from one to another of said interconnecting tunnels; and a thin film metallic bonding agent in between said laser block and said mirror assembly bonding surface for bonding said mirror assembly to said laser block.

2. The sensor of claim l wherein said bonding agent is comprised of gold.

3. The sensor of claim 1 wherein the thickness of said film is less than 20 microns.

4. The sensor of claim 1 wherein said thin film is deposited by vacuum sputtering.

5. The sensor of claim l wherein said thin film is deposited by sputtering.

6. A method of making a ring laser angular rate sensor, wherein said sensor includes a laser block of dimensionally stable material supporting a plurality of interconnecting tunnels to form a closed-loop path within said block, and a mirror assembly including a substrate having a bonding surface and a means for reflecting a laser beam from one to another of said interconnecting tunnels, said method comprising: depositing a thin film metallic bonding agent on said mirror assembly substrate bonding surface and a mating surface portion of said laser block; pressing said mirror substrate bonding surface against said surface portion of said laser block; and welding together said thin film metallic bonding agent material on said substrate and said block to fix said mirror substrate to said laser block.

7. The method of claim 6 wherein said welding is by ultrasonic waves.

8. The method of claim 6 wherein said bonding agent is deposited by vacuum sputtering.

9. The method of claim 6 herein said bonding agent is comprised of gold.

10. The method of claim 9 wherein said bonding agent is deposited by vacuum sputtering.

11. The method of claim 6 wherein the thickness of said thin film is less than 20 microns.