 |
Description  |
|
|
The present invention relates to a novel construction for a ring laser
angular rate sensor and 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. Pat. No. 3,373,650, issued to Killpatrick, and U.S. Pat.
No. 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 utilize a block of material that is substantially
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
communication 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 electrical 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.
Associated with ring laser angular rate sensors is a source of error
usually referred to as "lock-in." The source of error is 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 understood 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 a taught in
U.S. Pat. No. 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 became 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 the development of 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. The mirror assembly further comprises alternating layers of
titanium dioxide (TiO.sub.2) and silicon dioxide (SiO.sub.2) deposited on
the mirror substrate 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 requires that the block and the
mirror substrate 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 is accomplished at room temperatures.
These prior art ring laser angular rate sensors have 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 due, primarily, to the high cost of
polishing the laser blocks 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 block with a thermally formed gas tight 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a planned view of a ring laser angular rate sensor constructed in
accordance with the teaching of this invention.
FIG. 2 is a partial sectional view showing detail of a mirror sealed to the
laser block.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there is disclosed a pictorial representation of a
gas filled ring laser angular rate sensor 10 comprising a block 11 made of
a borosilicate, preferrably BK-7 glass (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 mounted on block 11 at 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 a closed-loop optical path.
A pair of anodes 27 and 29 are mounted on 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 therewith. The gas may be inserted into
the block cavities through one of the anode cavities used as a fill 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 cavity 43. Cathode 40 is
symmetrically located relative to anodes 27 and 29, and tunnels 13, 15,
and 17. These symetrical location 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 is emitted from cathode
40 out into tunnel 15 toward mirror 19 and through tunnel 13 to anode 27.
A second discharge current flows through cathode 40 out into tunnel 15
toward mirror 21 and through tunnel 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 will 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 preferrably curved
so as to aid in the alignment and focusing or 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 a co-pending patent application
entitled, "Readout for Ring Laser Angular Rate Sensors", by Killpatrick,
having Ser. No. 733,297.
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 FIG. 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 mirror 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 angular rate sensors
since the use of frit seals, generically referred to as a thermal seal,
eliminates the need for creating a highly polished surface on block 11
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 block of BK-7 glass to which the interconnecting
tunnels are machined therethrough. A substrate 222 for each mirror
assembly is also formed from BK-7 glass. 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. Pat. No. 4,142,958, entitled, "Methods for Fabricating Multi-Layer
Optical Films" issued to Wei et al, and is hereby incorporated by
reference herein by reference thereto.
In FIG. 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 of expansion for the laser block 11 and mirror
substrate 222. With compatible coefficients of expansion, a thermally
formed frit seal process can be used to join the mirror substrate 222 to
block 11. As will be appreciated by those skilled in the art, the frit
seal 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.degree. to
500.degree. C. for a substantial period of time. This elevated temperature
imposes dramatically the need for each of the parts to have a compatible
temperature coefficient of expansion.
The ion-beam sputtered deposition of the alternating layers of the
zirconium dioxide/silicon dioxide optical coating provides such a coating
which can tolerate the high temperatures required in implementing the frit
seal joining of the mirror substrate to the laser block. To frit seal a
mirror substrate to a laser block in accordance with FIG. 2, it is
necessary to achieve temperatures generally in excess of 450.degree. C.
The optical coating of alternating layers of zirconium dioxide and silicon
dioxide on the mirror substrates deposited by ion-beam sputtering, in
accordance with the invention with reference to FIG. 2, exhibit the
necessary high optical quality, high plasma stability, and high
temperature stability in excess of the 450.degree. C. temperature to
permit fabrication of the sensor via sealing the mirror substrate to the
laser block. Prior art techniques and materials do not have the
characteristics demanded in ring laser angular rate sensor applications
when materials ar subjected to the high temperature thermal sealing
process. Specifically, prior art e-beam deposition techniques of titanium
dioxide do not degrade with the frit seal annealing temperatures, but are
unstable in the plasma of the ring laser and degrade rapidly such that the
ring laser fails. Optical coatings of ion beam sputtered titanium
dioxide/silicon dioxide on a mirror substrate have an increase in
crystallinity when such substrates are thermally sealed to a block. The
increase of crystallinity causes the mirrors to degrade such that the
optical scatter increases resulting in poor performance of the sensor.
These ion beam titanium dioxide/silicon dioxide mirrors are amorphous and
exhibit no crystallinity in the as-deposited state. At temperatures in
excess of 250.degree. C., however, the titanium dioxide based mirrors
crystallize into a predominantly anatase structural phase of titanium
dioxide with sufficient large grains to degrade optical scatter.
In contrast to ion beam deposited TiO.sub.2 /SiO.sub.2 mirrors, ZrO.sub.2
/SiO.sub.2 mirrors have a crystallinity in the as-deposited state which
varies in grain size with deposition temperature but which does not change
with subsequent annealing up to 600.degree. C. temperatures. ZrO.sub.2
/SiO.sub.2 mirror coatings have been ion-beam deposited at ambiant ion
beam process temperatures (in the range of 150.degree. C.) exhibiting a
grain size sufficiently small as to not affect optical scatter at 633 nm.
More importantly, as the mirrors are subsequently annealed in preparation
for the fritting process, the grain size does not increase. Hence the low
scatter properties of ion beam sputtered ZrO.sub.2 /SiO.sub.2 mirrors are
preserved up to the temperatures necessary for fritting of the mirrors
onto the gyro block. Additionally, the stability of these ion beam
ZrO.sub.2 /SiO.sub.2 deposited mirror coatings also makes them free from
optical degradation in the gyro plasma.
The use of mirror assemblies having an optical coating of alternating
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 aforesaid optical coating may be thermally sealed to the laser block
to provide a low cost ring laser angular rate sensor.
* * * * *
|
|
 |
|
 |
Description |
|
