Note: Descriptions are shown in the official language in which they were submitted.
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1(~886S7
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~ Background of\the Invention
j 1. Field of the Inven~ion.
The invention relates to laser gyroscopes and particularly
: to laser gyroscopes which employ waves of four different fre-
quencies within the laser gyroscope cavity. More particularly,
the invention relates to output optical structures used for
extracting a portion of the beam circulating within the cavity
and producing therefrom output signals representing the differ-
ence in frequency between beam pairs having the same polari-
, 10 zation within the cavity.
2. Description~of-the Prior Art.
;~ In general, laser gyroscopes are devices which have two or
,,~` more waves circulating in opposite directions through a laser
medium so that rotation of the system wi 1 cause the round-trip
, . .
time for oppositely rotating waves to differ depending upon
the rate and amount of rotation. With a two-wave system, it
~' has been found that for low rates of rotation corresponding
to a small theoretical difference frequency the actual output
~ difference frequency is zero or substantially less than would
; 20 be expected due to the phenomena known as lock-in. It is be-
~ lieved that the lock-in problem arises because of coupling
,~f
- between waves which may arise from a number of possible factors
including back scattering of wave energy from elements within
` the path such as mirrors or a Faraday rotator or from scattering
:
- centers within the laser medium itself.
The earliest attempts to compensate for this problem in-
cluded one proposal in which the two beams are biased at
zero rotation away from the zero output level by the use of a
Faraday rotator which subjects beams propagating in different
directions to different delay times. Unfortunately, simply
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1~88657
biasing the two beams sufficiently far apart to avoid lock-in produced a
large frequency difference between the two beams, so large in fact that the
change in frequency caused by ordinary amo~mts of rotation was rather in-
significant compared to the total frequency. Thus, any small drift could
obliterate the actual desired signal output. Further attempts to achieve
biasing ~ncludqd one in which the Faraday rotator was switched from one
direction to another using a symmetric AC switching waveform. Such systems
have proven somewhat difficult to implement since the symmetry of the AC
:
switching waveform must be maintained to greater than one part in a million.
One of the most successful laser gyroscopes yet proposed and con-
structed employs four waves of two pairs each propagating in opposite direct-
~:,
ions. Such systems are shown and described in United States Patents Nos.
3,741,657, June 26, 1973 to Keimpe Andringa and 3,854,819 to Keimpe Andringa
.... ....
December 17, 1974 and assigned to the presenttassignee. In such laser systems,
: ~
circular polarization for each of the four waves is used. The pair of waves,
or beam, propagating in the clockwise direction includes both left and right-
hand circularly polarized waves as does that propagating in the counterclock-
wise direction.
Two biasing components are providedO A device such as a crystal
rotator produces a delay for circularly polarized waves that is different for
one sense or handedness of circular polarization than for the opposite sense
and is also reciprocal. That is, a wave of given polarization travelling inci
either direction through the crystal will be delayed by the same amount of
time. Secondly, a device such as a Faraday rotator is also
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1~813~;S7
disposed in the wave path. Such a device is nonreciprocal,
providing, for a wave of either polarization sense, a different
,~ time delay for the two directions of propagation therethrough.
.;
In any of these laser gyroscope systems, it is necessary to
extract a portion of each beam circulating within the laser
cavity to produce two output signals each one of which repre-
- sents the difference in frequency between wave pairs having
- the same sense of circular polarization within the cavity. In
order to accomplish this purpose, it is desirable at some point
within the output structure to combine these two beams in such
a manner as to produce two new beams, each including waves
having the same sense of polarization.
Previously known output structures for separating, com-
`- bining, and detecting the output signals were both mechanically
bulky and wasteful of signal energy and did not fully separate
the polarization states resulting in crosstalk at the detector
output. ~ecause of the waste of signal energy within the
output structure, larger proportions of output energy had to be
extracted from the cavity requiring higher gain from the laser
gain medium. The mechanical awkwardness of the structures made
such systems difficult to use in many applications. Moreover,
because the various components within the output optical structure
-~ were not within direct physical contact with one another, mis-
alignment problems between the various components often arose
as did drift problems. Also in many structures it was not
possible to use two diodes upon a single chip for the output
detectors so that the characteristics of the diodes will be
nearly identical.
. ~ Accordingly, it is an object of the present invention to
provide a laser gyroscope system having a mechanically rugged,
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1~886S'7
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compact, and efficient output structure.
It is further an object of the present invention to provide
such a system in which all the components of the OlltpUt structure
. may be directly mechanically coupled to eliminate misalignment
; and accompanying drift problems.
Moreover, it is an object of the present invention to pro-
vide such an output structure i31 which the available output
; signal energy is maximally utilized.
~ Further, it is an object of the present invention to pro-
`~ 10 vide a structure capable of separating completely the waves
` having inside the cavity the same polarization sense from the
.~ other pair of waves, thereby eliminating crosstalk between the
two signal outputs of the detec~ors thereby resulting in a
more stable and noise~free signal.
i;~ Also, it is an object of the present invention to provide
an output optic structure in which two diodes on a single chip
may be employed.
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Summary of the Invention
These, as well as other objects of the invention, may be
: -
- met by providing the combination of a laser gyroscope cavity
including a laser gain medium, a polarization dispersive
StTucture for producing waves o~ at least four frequencies,
a plurality of re~lecting means which ~orm a loop or closed
path for the waves with at leas~ one of the re~lecting means
being partially transmissive of the waves incident thereon, a
- beam splitter, and means for directing the portions of the waves
` 10 transmitted through the partially transmitting reflecting means
to a common position upon the beam splitter whereon the beam
; splitter produces both transmitted and reflected beams. As
used herein, the term "wave" applies only to a single electro-
magnetic wave propagating in one direction whereas the term
"beam" refers to two or more such waves propagating along the
; same path in the same direction. The combination may also
include means for converting the circularly polarized waves
within the output structure to linear polarization disposed in
the path of the reflected and transmitting waves from the beam
splitter, polarizing means disposed in the path of the waves
converted to linear polarization as they emerge from the polari-
zation converting means, and detecting means which receives the
waves as they emerge from the polarizing means. Preferably,
the waves are four in number with two of the waves in the cavity
circularly polarized with a first sense of polarization and two
of the waves within the cavity circularly polarized with a
second sense of polarization. A first one of the waves having
the first sense of polarization and a first one of the waves
having the second sense of polarization circulate around the
path in a first direction~ the other one of the waves having
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the first sense of polarization and the other one of the waves
having the second sense of polarization circulating around the
- closed path in the opposite direction. In preferred embodiments,
the partially transmitting reflecting means, the beam splitter,
the directing means, and the converting means are mechanically
coupled and mounted as a rigid unit. The detecting means and
; the polarizing means may further be included as part of the
`` unit. The partially transmitting reflecting means may be a
multilayer dielectric mirror including a transmitting substrate.
:
The converting means is preferably a quarter-wave plate whereas
the directing means is preferably a retro prism.
The invention may also be practiced by the combination of
; a laser gyro cavity including a laser gain medium, a polarization
dispersive structure for producing waves of at least four fre-
;1~ quencies, a plurality of reflectors positioned so as tc form
~ a loop or closed path in which propagate the waves, a trans-
:~^
', mitting substrate, a plurality of layers of dielectric material
upon a first surface of the substrate forming one of the re-
flecting means for the waves within the cavity, a beam splitter
`~ 20 disposed upon a portion of the second surface of the substrate at
a position from which emerges a first beam from the transmitting
substrate, a quarter-wave plate disposed over the beam splitter,
a prism having a first surface adjacent the quarter-wave plate
and having first and second surfaces at least portions of which
are reflecting opposite the first surface and inclined at an
angle thereto, first and second polarizers disposed adjacent a
fourth non-reflecting surface of the prism, and first and second
detectors positioned adjacent the first and second polarizers.
The first beam emerging from the substrate strikes the beam
splitter at a predetermined position and a second beam emerging
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from the substrate passes through the quarter-wave plate without
touching the beam splitter and is reflected from both the re-
flecting surfaces of the prism back to the same position upon
the beam splitter as was struck by the first beam. Transmitted
and reflected waves are produced thereby each containing waves
of all four frequencies. A fir,t transmitted wave and a first
reflected wave from the same position on the beam splitter pass
through the substrate and are reflected from the rear side of
the plurality of layers of dielectric material again through
the substrate to the quarter-wave plate and are finally re-
flected from one of the reflecting surfaces of the prism to
the firs~ polarizer and first detector. The first polarizer
is oriented so as to pass only 1~he preferred two of the four
.
waves contained in the beam at that point. A second transmitted
and a second reflected wave from the position on the beam
' splitter are reflected from the same one of the reflecting
~ ~.
surfaces of the pTism to the second polarizer and second de-
; tector. The second polarizer is oriented so as to couple the
~.~
other two waves to the second detector. In preferred embodi-
~ 20 ments, the layers of dielectric material, the substrate, the
- beam splitter, the quarter-wave plate, and the prism are
mechanically interconnected so as to form a rigid structure.
Further there may be provided second and third quarter-wave
plates with the second quarterwave plate being disposed be-
tween the prism and the first polarizer and the third quarter-
ware plate being disposed between the prism and the second
polarizer. Provision of the second and third quarter-wave
- plates has the additional advantage of reducing interference
or crosstalk in the beams caused by depolarization at the sur-
face of the dielectric layers.
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from the substrate passes through the quarter-wave plate without touching
the beam splitter and is reflected from both the reflecting surfaces of the
prism back to the same position upon the beam splitter as was struck by the
first beam. Transmitted and reflected waves are produced thereby each con-
taining waves of all four frequencies. A first transmitted wave and a first
reflected wave from the same position on the beam splitter pass through the
substrate and are reflected from the rear side of the plurality of layers of
dielectric material again through the substrate to the quarter-wave plate
and are finally reflected from one of the reflecting surfaces of the prism to
'; 10 the first polarizer and first detector. The first polarizer is oriented so as
" to pass only the preferred two of the four waves contained in the beam at that
point. A second transmitted and a second reflected wave from the position on
the beam splitter are reflected from the same one of the reflecting surfaces
of the prism to the second polarizer and second detector. The second polar~
izer is oriented so as to couple the other two waves to the second detector.
In preferred embodiments, the layers of dielectric material, the substrate,
the beam splitter, the quarter-wave plate, and the prism are mechanically
interconnected so as to form a rigid structure. Further there may be provided
second and third quarter-wave plates with the second quarterwave plate being
20 disposed between the prism and the first polarizer and the third quarter~wavep}ate being disposed between the prism and the second polarizer. Provision
of the second and third quarter-wave plates has the additional advantage of
reducing interference or crosstalk in the beams caused by depolarization at
the surface of the dielectric layers.
In accordance with the invention there is provided in combination:
a laser gyroscope cavity having a laser gain medium, a polarization dispersive
structure for prodùcing continuous waves of at least four frequencies, and a
plurality of reflecting means positioned to form a closed path for said waves;
at least one of said path defining reflecting means being partially transmit-
- 30 ting for allowing a plurality of beams of therwaves incident thereon to exit
said closed path; a planar beamsplitter; means for directing each of said beams
transmitted through said partially transmitting reflecting means through a
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11~8~3657.
solid dielectric medium along paths intersecting the plane of said beamsplit-
. ter, at least two of said beams intersecting the plane of said beamsplitter
at a common position and each producing transmitted and reflected beams; and
. first and second detecting means, said first detecting means being positioned
. .
:` to receive a first set of said beams which are transmitted and reflected by
. said beamsplitter at said common position and said second detecting means
~- being positioned to receive a second set of said beams which are reflected and
transmitted by said beamsplitter at said common position.
In accordance with another aspect of the invention there is pro~
10 vided in combination: a laser gyroscope cavity having a laser gain medium for
producing continuous waves of four frequencies, with a first two of said waves
~t travelling along in opposite directions along a path in said cavity and having
: a first sense of circular polarization and with a second two of said waves
. travelling along opposite directions in said path having a second sense of :
` circular polarization, and a plurality of reflecting means positioned to form
~.,
~; a closed path for said waves, at least one of said reflecting means being
v partially transmitting for allowing portions of beams of said waves incident
;
.: thereon to exit said closed path; an output detection system comprising first
. ~, .
and second detecting means; a beamsplitter lying in a plane intersected at a
common position by the paths of said beams; means for directing through a
solid dielectric medium more than one quarter of the energy in the waves of
said beams to said detection system; said first detecting means intercepting
waves derived from those beams which have circularly polarized waves with a
first polarization sense and opposite path directions in said cavity; and said
second detecting means intercepting waves from those beams which have circular-
ly polarized waves with a second polarization sense and opposite path direct-
ions in said cavity.
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1~8~3~5~7
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- Brief Description of the Drawings
: FIGURE 1 is a block diagram of a laser gyroscope system
.: in which the present invention is used to advantage;
' FIGURE 2 is a diagram of an output structure in accordance
~, with the invention with separately mounted optical components;
; -
:
FIGURE 3 shows a cross-sectional view of an output struc-
~, ture in accordance Wit]l the invention in which all output com-
.
.~ ponents are coupled in a mechanically rigid structure;
: FIGURE 4 shows a cross-sectional view of another embodi-
`~ 10 ment of the invention; and
FIGURE 5 is a cross~sectional view of still another em-
bodiment of the invention.
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; Descri~tion of-the Preferred Embodiments
., -- --
~ Referring ~irst to FIGURE 1 there is shown a block
.,
diagram of a laser gyroscope system in which the present
invention is used to advantage. Laser gyroscope cavity 5
` operates in the manner described above in the incorporated
patent specification and in the manner described above in
~ the background of the invention. Laser gyro cavity 5 in-
s cludes a closed or re-entrant path along which the four electro-
`` magnetic laser waves may propagate. The path includes laser
' 10 gain medium 10, mirrors 12 and 13, polarization dispersive
~,
structure 16 including crystal rotator 17 and Faraday rotator
~,
18, mirror 15, and output mirror 22. Because of the biases pro-
duced by crystal rotator 17 and Faraday rotator 18 there are
four electromagnetic waves of frequencies fl-4 propagating within
the closed path. Waves of frequencies of fl and f4 circulate
in the clockwise direction forming one beam within cavity S
while waves of frequencies f2 and f3 circulate in the counter-
clockwise direction forming a second beam. All four waves are
preferably circularly polarized with frequencies fl and f2
being circularly polarized with one sense and waves of fre-
quencies f3 and f4 being circularly polarized with the opposite
sense. The positions upon the gain curve for laser gain
medium 10 are shown in the diagram below of FIGURE 2.
The output signal from the system is desired to be a
digital number or other signal representing the total amount
of rotation experienced by laser gyroscope cavity 5 commencing
from a predetermined time period or, alternately, a digital
number or other signal representing the present rate of ro-
tation of laser gyroscope cavity 5. The rate of rotation is
computed in accordance with the formula:
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1~81~657
,) 8A ~( 4 3) ( 2 1)]
where Qis the rate of rota~ion about the sensitive axis of the gyro, L is the
total path length, A is the effective area enclosed within the path, and ~ is
the wavelength of the waves propagating within laser gyroscope cavity 5. The
amount of rotation is found by integrating the above equation with respect to
timeO
In order to produce the signal representing the amount of rotation
it is first necessary to derive signals representing the difference in fre-
. quency between the cavity waves of one circular polarization and the differ-
; 10 ence in frequency in the other circular polarization represented by f4-f3 and
f2-fl respectively. It is a function of the output optics structure 30 to
combine the beams of frequencies fl and f2 on one detector diode and f3 and f4
on a second detector diode without the presence of the two other waves upon
.,
either diode. Output processing circuitry 32 converts the signals represent-
ing the differences between the right and left-hand circularly polarized
signals to a digital number representing the amount of rotation in accordance
with the equation above. Output processing circuit 32 also operates upon the
amplitudes of the signals derived from the two output diodes and produces
therefrom an analog signal for operating piezoelectric transducer 20 to main-
tain the appropriate total path length within laser gyroscope cavity 5 suchthat the gain accorded to wave pairs is substantially equal. The operation
of output processing circuit 32 and piezoelectric transducer 20 are described
in United States Patent 4,108,553, August 22, 1978, Alfred A. Zampiello and
Bradley B Patch, JrO, assigned to the present assignee~
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1~8~365~7
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Referring next to the view of FIGURE 2 there is shown
~,.
schematically an output optics structure embodying the present
,: .
invention. Output mirror 22 forms one of the reflectors de-
fining the closed path within laser gyroscope cavity 5. Out-
put mirror 22 is mostly reflecting, reflecting preferably 99%
or more of the two beams incident thereupon. However, a small
proportion of each outpu~ beam is transmitted through output
mirror 22 emerging from the rear side thereof in the direction
1~
i~ of mirrors 40 and 41.
,~ 10 For the discussion which immediately follows we will
, .
consider the case that the transmission of a circularly polarized
wave through the output mirror 22 does not substantially alter
its polarization state. The transmitted counterclockwise beam
labeled CCW contains, for example, frequencies f2 and f3 of
left and right-hand circular polarizations respectively. As
indicated in the two small diagrams in FIGURE 2, the polarization
sense of each wave is opposite in successive legs of the cavity
path due to the well-known change in handedness upon reflection.
Thus, the clockwise beam labeled C~ then contains waves of
frequencies fl and f4 of right hand and left-hand circular
polarization respectively. The clockwise beam is reflected
from mirror 40 to a predetermined position on beam splitter 42.
The counterclockwise beam is similarly reflected by mirror 41
to the same position upon beam splitter 42. Each beam shone
: .
upon beam splitter 42 produces both transmitted and reflective
components, preferably of approximately equal magnitudes. The
beam splitter is angularly aligned so as to colinearly combine
beam transmitted through and reflected from the beam-splitting
surface. Hence, the combined beams transmitted and reflected
from beam splitter 42 each contain waves of all four frequencies.
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88657
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The transmitted portion of the clockwise beam and the re-
flected portion of the counterclockwise beam propagate along
the same path to quarter-wave plate 53. Similarly, the trans-
mitted portion of the counterclockwise beam and the reflected
. .; , . .
portion of the clockwise beam propagate together along the
!~ same path to quarter-wave plate 43. Thus, between beam splitter
;~
42 and quarter-wave plates 43 and 53 the right and left-hand
. circularly polarized frequency pairs have been combined and
are traveling together in the same directions along the same
paths. For example, béfore quarter wave plate 53, there are
present waves of frequencies fl and f2 with left-hand circular
polarization and of f3 and f4 with right hand circular polari-
zation. The same frequency waves will be present between beam
splitter 42 and quarter-wave plate 43 with the opposite sense or
handedness of circular polarization.
Quarter-wave plates 43 and 53 are oriented so that, for
example, left-hand circular polarization is converted to vertical
polarization and right-hand circular polarization is converted
to horizontal polarization. Polarizers 44 and 54 are provided
which pass only linear polarization. These are oriented or-
. .
thogonal to each other so that, for example, polarizer 44 passes
only horizontal polarization and polarizer 54 passes only vertical
polarization. With this arrangement, the beams between polarizer
54 and detector diode 55 will consist only of frequencies fl and
f2 with linear vertical polarization and the beams between polar-
izer 44 and detector diode 45 will consist only of frequencies
f3 and f4 with horizontal polarization. It is readily apparent
that waves of only two frequencies strike each detector diode
with waves of the other tWQ frequencies completely eliminated.
- 3- It is a distinct advantage with the present invention that
both transmitted and reflected beams from the combining beam
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splitter are utilized to form final output beams. In prior
art output optics devices, only one of the two output beams
from the beam splitter was utilized. Thus, utilizing the present
invention, a waste of half the output power is eliminated. Only
half the power as before need be extracted from the laser gyro
cavity thereby in turn reducing the gain required from the
laser amplifier and laser gain medium.
Referring next to FIGURE 3 there is shown an embodiment
of the invention in which all the output optical components
are mechanically interconnected in a mechanically rigid and
, . ~
compact structure. In this embodiment, a mirror substrate 104
which may be of clear transmitting glass, is coated on one
.,
smooth surface thereof with a plurality of layers of dielectric
material 102 to form a mostly reflecting mirror. This mirror
; forms one of the reflecting elements which define the closed
' path of laser gyro cavity 5. The number and construction of
dielectric layers 102 is chosen so that a small portion of the
; beams incident thereon are transmitted into mirror substrate
:
104. Typically, 1/2~ of the energy within the incident beams
is transmitted into mirror substrate 104.
Beam splitter 106, also formed of dielectric material, is
; positioned next to the opposite parallel surface of mirror
.,
substrate 104 from the plurality of dielectric layers 102.
Apertures are provided within beam splitter 106 so that beams
may pass therethrough as indicated in the diagram. In practice,
beam splitter 106 may be formed by depositing dielectric material
on the surface of quarter-wave plate 108 and subsequently etching
an annulus, for example by using ion beam etching techniques,
to form the required apertures. By providing the apertures
- 30 within beam splitter 106 in the form of an annulus, the struc-
ture consisting of quarter-wave plate 108 and beam splitter 106
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may be rotated to give the proper interaction between quarter-
,:
wave plate 108 and the beam incident thereupon so that for
,
example beams of left-hand circular polarization will be con-
~ verted entirely to vertical linear polarization and beams of
; right-hand circular polarization will be converted entirely
, .
- to linear horizontal polarization.
Positioned adjacent to ~he upper surface of quarter-wave
, plate 106 is the lower face of retro prism 110. Retro prism
~, 110 is preferably constructed of highly transmitting glass.
Opposite the surface of retro prism 110 in contact with
quarter~wave plate 108 are two additional surfaces of retro
,;; prlsm 110 to provlde the beam reflective angles shown in the
diagram. These surfaces are provided with reflecting coatings
` 112 and 114 which may either be a plurality of dielectric layers
,....
,; or a metallized coating.
Adjacent a fourth surface of retro prism 110 shown in
~, the left-hand side of FIGURE 3 are located polarizers 116
;. ~.
~- and 118. Detector diodes 120 and 122 are positioned opposite
. .
polarizers 116 and 118 so as to receive the linearly polarized
waves transmitted by the polarizers. Polarizers 116 and 118
~ are, as before, oriented orthogonal to one another. Diode
;, mount 124 secures detector diodes 120 and 122 and polarizer 116
; and 118 to the left surface of retro prism 110. The device
. .
~;- shown in FIGURE 3 operates in a similar manner to that shown
~' in FIGURE 2 to combine cavity waves of like sense of circular
polarization while rejecting waves of the other sense of circu-
lar polarization and to transmit the combined waves to detector
diodes without a large waste of signal power.
Operation of the device shown in FIGURE 3 may be described
; 30 with reference to Table I of the APPENDIX which indicates the
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1(1 88S,57
sta~e of polarization for the various waves at selected points
within the device. A superscript L as used therein indicates
left-hand circular polarization, R indicates right-hand circular
polarization, V indicates vertical linear polarization, and
H indicates linear horizontal polarization. As before, the
subscript indicates the frequency of the particular wave.
Quarter~wave plate 108 is orien~ed SUC]I that right-hand circular
polarization and left-hand circular polarization are converted to
~- horizontal and vertical linear polarization respectively, i.e.
its fast axis azimuth is ~45. Also1 it is to be noted that
horizontal and vertical polarizations are converted to right-
- hand and left-hand circular polarizations respectively upon
- passing through quarter-wave plate 108 in either direction. It
may be seen that all beams below quarter-wave plate 108 are
circularly polarized while those above quarter-wave plate 108
~; are linearly polarized. Since horizontal and vertical polari-
zation are the normal modes for reflection from surfaces having
normals lying in the plane of incidence, no crosstalk-inducing
depolarization occurs upon reflection from any of the surfaces
above quarter-wave plate 108.
The passage of a typical beam may be traced through the
device. At point A, which is within laser gyroscope cavity 5,
the counterclockwise circulating beam has frequencies f2 and
f3 which are left-hand and right-hand circularly polarized
respectively. Small portions of these beams are transmitted
into mirror substrate 104 through dielectric layers 102.
The polarizations of the incident beams are substantially con-
served due to the low angle of incidence relative to the normal
to dielectric layers 102.
Laser gyroscope cavity 5 is preferably constructed in
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such a manner that the angle be~ween the incoming beams is 30
or less. By maintaining such a low angle of incidence upon
dielectric layers 102, a high degree of maintcnailce of circular
polarization is achieved. For larger angles of incidence, the
.i ellipticity of beams emerging from the rear surface of
dielectric layers 102 increases rapidly. Wi-~h an increase in
ellipticity the signal power av~ilable at eacil ~ctector diode
is decreased because the polari%ers must eitiler be orie~ d to
.:
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completely eliminate the undesired components or to pass tne
maximum amplitude of the desired signals. Becausc the beams
are no longer power orthogonal for elliptical polarization,
the polarizers cannot be oriented so as to pass both maximum
amplitude of the desired signals yet eliminate all of the
undesired components. In the former case, crosstalk occurs
between the desired output signals while in the latter case a
~ decrease in signal amplitude results.
'~ At point B within mirror substrate ln4, the counter-
., .
clockwise components from the four waves circulating wi1:hin
laser gyroscope cavity are present. These are of frequencies
f2 and f3 of left and right-hand circular polarization respec-
:' tively. Passing through the aperture in beam splitter 106
and through quarter-wave plate 108, these waves are converted
to vertical and horizontal polarization respectively. After
being reflected from reflecting coatings 114 and 112 upon the
upper surfaces of retro prism 110 the same polariza~ions are
maintained but the beam is displaced a lateral distance such
that it coincides upon the upper surface of beam splitter 106
: at the same position as the beam labeled I containing the
components from the clockwise beam of frequencies fl and f4
of right-hand and left-hand circular polarizations respectively.
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" Transmitted and reflected components are produced from those
beams so that both the reflected and transmitted beams from
the common point of incidence upon beam splitter 106 contain
` waves of all four frequencies. Beam J has waves of frequencies
fl and f2 of horizontal polarization and waves of frequencies
f3 and f4 of vertical polarization, the latter two having been
converted from circular to linear polarization by quarter-
wave plate 108. In a similar fashion, beam D contains waves of
frequencies fl and f2 of left-hand circular polarization and
' 10 waves of frequencies f3 and f4 of right-hand circular polari-
~- zation. The sense of polarization is reversed upon reflection
from the backside of dielectric layers 102. Beam E passes
through the aperture in beam splitter 106 and is converted
back to linear polarization becoming beam F containing waves
of frequencies fl and f2 with horizontal polarization and
; waves of frequencies f3 and f4 of vertical polarization.
Beams F and J are reflected by re'flecting coating 112 to
polarizers 116 and 118. Polarizer 116 is oriented so as to
pass only horizontal polarization while polarizer 118 is
oriented to pass only vertical polarization. In this manner,
waves having frequencies fl and f2 are coupled to detector
120 and waves of frequencies of f3 and f4 coupled to detector
122.
Heterodyning between the two waves incident upon each
detector occurs within the detector producing an output signal
in the form of a sinusoid biased by a low frequency or DC com-
ponent. The frequency of each sinusoid is equal to the fre-
quency difference between the two waves incident upon the particu-
lar detector while the magnitude of each DC component, proportion-
al to the average amplitude of the output signal, is in pro-
: .
-17-
~ 8657
portion to the sum of the intensities of the two incident waves.
:
Further processing by output processing circuit 32 produces
' a digital signal indicative of the amoun~ of rotation and an
, analog signal used for operation of piezoelectric transducer
~; 20 to maintain the appropriate path length within laser gyro-
scope cavity 5.
~.,
It may be seen that with the i-nvention as described in
~:~ conjunction with FIGURE 3, a rugged and compact mechanically
rigid structure is provided. In the prior art, each individual
. . .
, 10 optical component was separately mounted upon an individual
~` frame which in turn was secured to an underlying substrate.
With the use of the invention, the entire output optic struc-
; ture is provided in a single rigid unit resulting in a large
, savings in space. This advantage is extremely important in
many applications in which space occupied by any component as
well as the total weight is to be minimized.
Referring next to FIGURE 4 there is shown a cross-sectional
view of another embodiment of the invention in which a compact
and rugged mechanical output optic structure is provided. The
embodiment shown in FIGURE 4 operates in much the same manner
as that shown in FIGURE 3 but with the beam splitter and quarter-
wave plate reversed in position and a differently shaped prism
130 is employed resulting in different beam paths. Also,
polarizers 116 and 118 and dstector diodes 120 and 122 are
- mounted upon the same surface of retro prism 130 of which a
portion is covered by reflecting coating 134. The operation
for the device shown in FIGURE 4 is specified by the entries
of Table 2 showing the frequencies and polarization states of
the waves present within the device. In this table, polarization
state labels + and - signify linear polarization orien1:ed at
'
-18-
,
:~ .
,''' ~
`:
: 1~88t;57`''
plus and minus some angle to the horizontal, typically 30 to
45
Transmission through multilayer dielec*ric coatings
; may alter the polarization state of circular-polarized waves,
rendering them elliptical. This effec~ arises from the differ-
;.. ; .
: ential transmission of electromagnetic waves whose principal
plane of vibration of electric field lies in the plane of in-
cidence ("p" polarization) compared to waves vibracing
- perpendicular to this plane ("s" polarization). The ratio of
transmissivities for the two polarizations depends on angle of
incidence and may be typically 1.2 to 5. This results in a
corresponding ellipticity of the transmitted waves and would
give rise to waves having ellip~ical polarizations rather than
linear polarizations. Thus the polarizers cannot block the
` two undesired waves at each diode. ~owever, with quarter-wave
plates 136 and 138 in FIGURE 4 the polarizations of the un-
desired waves are rendered linear in each case with polarizers
oriented so as to block the undesired polarization. Thus cross-
talk may be entirely eliminated. Furthermore, should there be
- 20 a polarization change upon reflection at the back of mirror 102,
this may be compensated simultaneously with the same quarter-
wave plate 138. As stated earlier, the desirability of elimi-
nating crosstalk follows from considerations of gyro output
; noise. The present invention allows elimination of crosstalk
even in the presence of different "s" and "p" transmissivities
of the output mirror 102 with only a small reduction in signal
power. In the embodiments shown in FIGURES 4 and 5 the quarter-
wave plate is to be oriented at 0 rather than at 45. In
this case the elliptically polarized waves exiting from mirror
-~ 30 102 are converted by the quarter-wave plate at once to linear
,. - 1 9 -
14~88657
, ...
~,
polarizations lying in two generally non-orthogonal planes.
Thus, if no further depolarizing elements are encountered by
the waves, the cross-talk may be eliminated by use of polarizers
116 and 118 without additional quarter-wave plates.
Referring further to the device shown in cross-section in
PIGURE 5, another embodiment of the inven~ion in a mechanically
rugged and rigid compact structure is shown. I~he pola-rization
states of various beams are shown in Tal~le 3 below. A prism
~-
shape as used in the device of FIGURE 4 is employed in this em-
10 bodiment. However, no quarter-wave plate is provided adjacent
beam splitter 106. However, individual quarter-wave plates
136 and 138 are provided adjacent polarizers 116 and 118. This
embodiment achieves the advantage that the quarter-wave plate
for each detector diode may be individually adjusted. This is
of use in situations where some degree of depolarization occurs re-
sulting in elliptically polarized waves due to differential
~ phase errors or differential reflectivity or transmissivity
; from or through the various reflective coatings and dielectric
~ layers, or from a non-ideal quarter-wave pla~e. Moreover, a
'` 20 quarter-wave plate could be provided adjacent beam splitter
,; . .
106 in the embodiment of ~IGURE 5 or, equivalently, individual
. quarter-wave plates be provided before polarizers 116 and 118
of the embodiment of FIGURE 4. Double quarter-wave plates
have the advantage of eliminating almost any aberration caused
by depolarization of the individual beams.
This completes the description of the preferred embodi-
, ments of the invention. Although preferred embodiments have
been described, it is believed that numerous modifications
and alterations thereto would be apparent to one having ordinary
- 30 skill in the art without depar1:ing from the spirit and scope
of the invention.
;,
~i -20-
~ ^~
: 1CE 88~5~7
: . .
TABLE 1
~':
i A: (fZLf3R)ccw(fl f4 )CW G:f3Vf4v
i,,~, . .
B: f2Lf3R H: (flRf4L)Cw(f2 f3L)ccw
.:...
~: .
C: f2Vf3H I: flRf4L
~; .
D; f2Lf3RflLf4~ J: flHf4Vf2Hf3V
" '
E: f2 f3LflRf4L K flHf2H
; F: f2Hf3VflHf4v
:
. .............................. .
",
, TABLE_2_
`; . A: tf2Lf3R)CCW(fl f4 )cw G:f3+f +
,....... ~ .
. . .
. B: f2Lf3R : H: (flRf4L)Cw~f2 f3 )ccw
.
.`~ C: f2 f3 I: flRf4L
.. . . .
;,t,;",'
, D: flRf2Rf3Lf4L fl f2 f3 f4
E: flLf2Lf3Rf4R K f +f ~
~ ' ' .
F: fl 2 -f3.+f4 t
.
,
: -21-
,
.
s ~
TABLE 3
~, . (f2 f3 ) C ~flLf4R~ cw G: f3Hf4H
~: , . . . . .
" ~
B 2L~3R ~ : (flRf4L) Cw ~f2 f3 ) ccw
,: . , ,
C; . f2Lf3R ~ I f ~f L
,. ,, ~: '
D flLf4Rf2Lf3R r flRf4Lf2~f3L
.. . .
E: flRf4Lf2R3L. ~ ~ ~: flV2v
`. F: flRf4Lf2Rf3L
~: . . .
,~; :,- . ,
,.: . .
~ ' ,. , ' .
.
`, : ., i, ., . .:
. . , ~,
. ` . . .. . . . .. .
. , , ', . , " '" , ,
, . .. . . . .
,~ . ., , . , , , " . . . .
.. . . .. . . . .
',. '' '; , ' ' ' ' .' - ". ' ' ' . .. .
.. . . ... . . . .
' : : ' ., .,:
,,
:.... . . .. . .
.~ ,. . . .. . .
, . ;. ' .
r , ~ ' .. . . ~ ~ , , '
' ~ ' ', '' ' , ' ,'' ' ~ ' ~ '
~, .,: ,' 1', ~ ",
Z 2
.` .
, .
: . : .
