CA1272788A - Ion beam sputtered mirrors for ring laser gyros - Google Patents

Abstract

ABSTRACT

Disclosed is a ring laser angular rate sensor constructed from a solid block with mirror assemblies joined to the block with a thermally formed gas tight seal. The mirror assemblies include a coating of alternating layers of zirconium dioxide (ZrO2) and silicon dioxide (SiO2) which have been ion beam sputtered on mirror substrates composed of a material which suitably matches the thermal coefficient of the laser block.

Description

7~
- 1 - 6 ~ :L 5 9- 9 3 4 ION BEAM SPUTTERE,D MIRRORS E'OR
RING LASEIR GYROS
BACKGROUND OE' THE INVE~II'ION
The present invention relates to a novel construction for a ring laser angular rate sensor ancl more particularly to a construction employing novel low scattering mirrors in such sensors.
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.
Ring laser angular rate sensors of the type referred to utilize a block of material that is substantially stable, both thermally and mechanically. Irhe block usually includes a plurality of inter-connec-ted gas containing tunnels or passages which form a closed-loop path in the shape of a triangle, a rectangle, or any poly-gonal path. At each intersection of a pair of interconnected tunnels is a ~727~38

-2--mirror mounted on the block. This arrangement of mirrors and i~terconnected tunnels ~orms an optieal closed-loop path. Further, at lea~t one anode and one cakhode are each mounted on the block and in communication with the gas. Each o~ 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 electrical potential is applied between the anode and cathode to cause a discharge current therebe~wPen which results in the production of a pair o~
counter-propagating laser beams within the block.
Associated with ring laser angular rate sensors is a source o~ error usually referred to as "lock-in." The ~ource of error is predominantly caused by back scattering of light at each of the ~irrors which form in part the optical closed-loop path which the counter-propagating laser bea~s ~o travers~ As i~ well understood by those skilled in the art, there are two widely used techni~ues applied together to mi~imize the lock-in error. The first technique consists of dithering the block as taught in `~
~7~78~3

3--U.S. Patent 3,373,650. Mechanically d~thering the laser block .reduces the sou:rce of error caused by lock-in to acceptable levels ~uch that ring laser angular rate sensors became commercially successful.
The second technique consists o~ producing mirror assemblies structured so as to provide highly polished substrates having superior reflective coatings which achieve minimal las~r beam scattering at the surfaces thereof. Development of the mirror assemblies over the years has made ~t possible ~or the deYelopment of high performance ring laser an~ular rate sensors~
Prior art mirror assemblies comprise a block of material euitably polished for a mirror substrate.
The mirror substrate usually i9 th~ same material as the laser block material ~o that they have matched thermal coefficients of expansionO The mirror assembly further comprisss alternati~g layers of titanium dioxide (TiO2) and ~ilicon dioxide (SiO2) deposlted on the mirror s~bstrate by a variety of deposition techniques including, among others, e-beam deposition and ion-beam sputtering.
The mirror assemblies of the prior art are fixed to the laser block by what is referred to as an optical contact. This requiras that the block and the mirror substrate be highly polished so as to form an ~,7~'73!3~
6415~-~34 optical con~act when the mirror substrate is pressed against the bloclc. The joining of the laser hlock and the mlrror block ls accomplished a~ room temperatures.
These prior art ring laser angular rate sensors have proven hiyhly satisfactory in operatlon and are rapidly gainlng wide~spread accepkance Eor certain applica~ions. These prlor art ring laser angular rate sensors, however, are costly ~o manufacture due, primarily, to khe high cost of polishing the laser hlocks and mirror substrates.
SUMMARY_OF THE INVENTION
An object of this invention is a provision of a novel construction for 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 bloclc with a thermally formed gas tighk seal. The mirror includes a coating of alternating layers of zirconium dioxide and silicon dioxide which have been ion-beam sputtered on mirror substrates composed of a material which suitably matches the thermal coefficient of the laser block.
In accordance with ~he present invention there is provided a ring laser angular rate sensor ln which counter-rotating laser beams propagate, comprising in combination:
a block co~prised of borosilicate glass; a plurality of in~erconnecting tunnels within said block; a plurality of mirrors, each comprised of a borosilicate substrate and ion-beam sputtered alternating layers of zirconium dioxide and silicon dioxide 6~159-g3 ther~on, each of said mirrors being flxed to said block wlth a frit seal to boncl ~said substrate to said hloalc, and each oE said mlrrors located at an intersection of a palr of said interconnectlny tunnels to form a clos~d-loop resonant cavity wtthln saicl block.

~a ~L2:~8 !3 BRIEF_DES~RIPTION OF_THE ~RAWINGS
Figure 1 i~ a planned view o~ a riny laser angular rate sensor constructed ~n accordance with the teaching of this invention.
Figure 2 is a partial 6ectional view showing de~ail of a mirror ~ealed to the laser block.
DETAILED DESCRIPTION OF TEIE INVENTION
Referring now to Figure 1, there is disclosed a pictorial representation of a yas filled ring laser angular rate sensor 10 comprising a block 11 made of a borosilicate, preferrably BK-7 glas~ (letter number combination~ are Schott Optical Commercial Designations). R plurality o~ three interconnected tunnels 13, 15, and 17 are bored within bloc~ 11 at angles to eaah other to form a triangular-shaped cavity. ~irror as~emblies 19, 21, and 22 ar~ mounted on block 11 at ~he intersect.ion of each o~ the tunnels 13, 15, and 17, raspectively, in a manner as will ~ub~eguently be de~cribedO Each mirror ~unctions to re~lect light ~rom one tunnel into the next thereby forming a closed-loop optical path.
~ pair o~ anodes 27 and ~9 are mounted on block 11 and adap~ed to communicate with laser tunnels 13 and 17, respectively, through interconnec~ing cavities 23 and 25, respectively. A quantity of lasing gas ~or plasma is adapted to be contained , ~L~2~
--6~
within the tunnels 13, 15, and 17, and other tunnels in communication therewith. ~he gas may be inserted into the block cav.ities through one o~ the anode cavities used as a ~ill tunnel and one of the anodes which may also serve as a sealable port, e.g. anode 29.
A cathode 40 is mounted on block 11 and in communication with the optical closed~-loop cavity through interconnecting caYity 43. Cathode 40 is ~ymetrically located relative to anodes 27 and 29, and ~unnels 13, 15, and 17. These symetrical location of the pair of anodes and cathode is intended to reduce ga~ ~low ef~ects which can adversely a~fect the performance of the rate sens~r, as ls w~ll known.
In operation, with a sufficiently large potential applied be~ween th~ cathode and the anodes, a ~ir~t discharge current is emitted from cathode 40 out into tun~el 15 toward mlrror 19 and through tunnel 13 to anode 27. ~ ~econd di~charge current flow~
through cathode 40 out into tunnel 15 toward mirror 21 and through tunnel 17 to anode ~90 These two discharge current~ are usu~lly controlled in intensity. The discharge current'~ function is to ioniza the lasing gas and thereby provide a pair of 25 . counter-propagating laser beams within the closed-loop optical cavity in a well known manner. It will be -7- 6~1~9-934 appreciated that ring laser angular rate sensors with a rectan-gular laslng path or other optical cavity configurations, incLud-ing 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 provid~
ing a readout beam signal to be direc-ted toward a photosensitive means 50. Mirror 22 is preferrably 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 par-t a transducer for cavity path length control in a well known manner.
The construction of the ring laser angular rate sensor described above and its per~ormance are in accordance with the basic operating principles o-f 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 frit seal to join each of the mlrror assemblies 19, 21, and 22 to the block 10 containing the interconnecting tunnels. The frit seal is chosen in place of optical contacts generally used in the prior art ring laser angu-lar rate sensors since the use of frit seals~ generically referred to as a thermal seal, elimina-tes the need for creating a highly polished surface on block ll joining the mirror assemblies to a block by optical contact. In the preferred embodiment of the invention, the ring laser angular rate sensor block 11 is a solid ''3 block of BK-7 gLass to which the interconnecting tunnels are machined there~hrough. A substrate 222 ~or each mirror assembl~
is also formed from BK-7 glass. An optlcal coating 224 o~ alter-nating layers of zirconium dioxide and silicon dioxide is deposi-ted 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, enti-tled, "Methods for Fabricating Multi-Layer Optical Films" issued to Wei et al.
In Figure 2, the optical coating is shown as only a spot having sufficient area to reflect impinging laser beams thereon.
The choice of material for laser block 11 and mirror substrate 222 is dictated by the need to have compatible coefficients o~ expansion ~or the laser block 11 and mirror 6ubstrate 222. With compatible coe~ficients o~
expansion, a thermally ~ormed frit seal process can be used to ~oin the ~lrror ~ubstrate 222 to block 11. As will be appreciated by those 6killed in the art, the frit sea.l is formed with a solderable glass or frit material 226 in a process in which temperatures are raised to be in the range of 450~ to 500C ~or a substantial period of time. This elevated temperature 10 imposes dramatically the need ~or each of the parts to have a compatible temperature coef~icient of expansion.
The ion-beam sputtered deposition o~ the alternating layers o~ the zirconium dioxide/silicon 15 dioxide optical coating provides such a coating which aan tolerate the high temperatures required in implementing the frit seal ~oining of the mirror substrate to th~ laser ~lock. To frit saal a mirror substrate to a l~ser block in accordance with Fi~ure 20 2, it is necessary to achieve temperatures generally in ~G4es~ og 450C.
The optical coating of alternating layers of zirconium dioxide and ~ilicon dioxide on the mirror substrates deposited by ion-beam sputtering, in 25 accordance with the invention with re~erence to Figure 2, exhibit the n~cessary high optical quality, high plasma ~tabllity, and hiyh temperature etabil.lty e~ ss ~' in.aGc~ o~ the, 450C tempe.rature to permlt fabrication o~ the sensor via sealing the mirror substrate to the laser block. Prior art teehniques and materials do not have the characteristics demanded in ring laser angular rake ~ensor applications when materials are 51~j ected to the high temperature ~hermal ~ealing processt Specifically~ prior art e-beam deposition techniqu2s o~ titanium dioxide do not degrAde with the fri~ ~eal annealing temperatures, but ar~ unstable in the plasma o~ the ring laser and degrad~ rapidly such that the ring laser fails.
Optical coatings of ion beam sputtered titanium dioxide/silicon dioxide o~ a ~irror ~ubstrate have an increa~e in crys~allini~y wh~n such ~ubstrates are therm~lly sealed to a block. Tha lncrease o~
crystallinity cau~es the mirrors to degrade such that the op~ical scatter ~ncreAse~ resulting in poor performance o~ tha ~ens~r.
Thes~ ion beam titanium dioxide/silicon dioxide mirror~ are amorphou~ and exhibit no crystallinity in the as-depositad state. At temperatures in ~xcess o~ 250C, howev~r, the titanium dioxide ba~ed mirrors crystallize into a predominantly anatase structural phase o~ titanium dioxide with su~icient large grains to degxade optical scatter.

-:Ll In contrast to ion beam deposlted TiO2/Sio2 mirrors, Zro~/SiO2 ~irror3 have a crys~alllnity in the as-deposited state which varies in grain size with deposition temperature but which does not change with subsequent annealing up to 600C
temperatures. ZrO2/SiO2 mirror coatings have been ion-beam deposited at ambiant ion beam process temperatures (in the range of 150C) exhibiting a grain size sufficiently ~mall as to not affect optical scatter at 633 nm. More importantly, as the mirrors are subsequently anneal~d in preparation for the fritting process, the grain size does not increase.
Hence the low scatter propertie~ of ion beam sputtered ZrO2/SiO2 mirrors are preserved up to the temperatures neces ary or ~ritting of the mirrors onto the gyro block. Additionally, the etability of thece ion beam ZrO2/SiO2 deposlted mirror coatings also makes them free ~rom optical degxadation in the gyro pla~ma.
Th~ use of mirror assemblies having an op~ical coating o~ al~ernating layers of zirconium dioxide~silicon dioxide deposited by the ion-beam sputtering process do not degrade with annealing temperature and have excellent laser mirror properties. Therefore, the mirrors constructed with the a~oresaid optical coating may be thermally sealed to the laser ~lock to provide a low cost ring laser angular rate sensor.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A ring laser angular rate sensor in which counter-rotating laser beams propagate, comprising in combination:
a block comprised of borosilicate glass;
a plurality of interconnecting tunnels within said block;
a plurality of mirrors, each comprised of a borosilicate substrate and ion-beam sputtered alternating layers of zirconium dioxide and silicon dioxide thereon, each of said mirrors being fixed to said block with a frit seal to bond said substrate to said block, and each of said mirrors located at an intersection of a pair of said interconnecting tunnels to form a closed-loop resonant cavity within said block.

2. The sensor of claim 1 wherein said borosilicate glass is BK-7.