Note: Descriptions are shown in the official language in which they were submitted.
1333758 64159-944
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.
.
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
1333758
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
1333758
--7
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
- 1333758
~ - 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
1333758
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~
--10--
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.
