Note: Descriptions are shown in the official language in which they were submitted.
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1 LASER GYRO MODE LOCKING REDUCTION SCHEME
_ _
2 FIELD OF THE INVENTION
3 This invention relates to ring lasers used as
4 gyroscopes wherein the difference ~etween resonank frequencies
of counterpropagating radiant energy or light waves is a measure-
~ ment of rotation of the structure in which said propagating
7 waves are traveling.
8 BACKGROU~D OF THE I~VE~TIO~
. . _ _
9 Ring laser gyroscopes utilizing counterpropagating
laser beams are well known. These devices are used for measuring
11 rotation o~ the ring laser gyroscope by combining portions of the
12 counterpropagating modes to generate a beat frequency representa-
13 tive of the differences in frequency between the opposing modes.
1~ Incidentally, the term "mode" is used herein interchangeably with
the word "wave", and means a resonant traveling wave of radiant
16 energy propagating within a ring laser cavity. As the ring laser
17 body is rotated about an axis having a component perpendicular
18 to the ring laser plane, the frequency of waves propagating in
19 one direction within the cavity will increase while the frequency
of waves propagating in the opposite direction will decrease.
21 This change in frequency between the counterpropagating modes
22 results in a change to the beat frequency proportiQnal to the
23 rate of rotation. By monitoring the beat signal, information
24 is obtained about rate of rotation of the ring laser.
Eowever, for the ring laser gyroscope to function
26 at low rates of rotation, frequency locking or "lock-in" must
27 be overcome. This phenomena occurs when two oppositely traveling
28 ___-- ~3
' ~ :
:
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~ 25~
1 waves in a resonant cavity with slig'htly different frequencies
2 are pulled -toward each other to combine in a single fre~uency
3 standing wave. The net result is that for low rates of rotation
of the ring laser, where fre~uency differences between the two
opposing modes are very small, the waves are pulled toye~her
~ such that beat frequency does not change and the gyroscope is
7 insensitive to small rates of rot.ation. The effects of lock-in
8 are described in detail in Laser Applications, edited by Monte
9 Ross, Academic Press, Inc., New York, New York, 1971, in the
article entitled, "The Laser Gyro," by Frederick Aronowitz,
11 pages 133-200.
12 It is well known that the principal cause of lock-in
13 coupling is the mutual scattering of energy from each of the
14 beams into the dlrection of the other. This mutual scattering,
or backscatter, is explained in detail in Aronowitz, supra,
16 pages 148-153. Briefly, the difference frequency between two
1~ counterpropagating waves in a ring laser is governed by the
18 equation
19 ~ = a + b sin ~
where ~ is the instantaneous phase difference between the
21 counterpropagating waves, a is proportional to the rate of
22 rotation of the ring laser, and b is proportional to the magnitude
: 23 of backscattered energy. In the case where a is smaller than b,
24 the beat frequency will be e~ual to zero and the ring laser will
'25 be locked in. In order to have a gyroscope output which is
26 representative of rotation of the ring laser body, a must be
27 greater than b.
28 _____ -4-
.
5~3~5
1 One way of eliminating lock-in i5 to m~chanically
2 oscillate the ring laser body. By osc.illating, or dithering, the
~ laser struc-ture, a rotation rate is superimposed on the gyxoscope
4 such that most of the time a is g,reater than b ~nd -khe ~~ects
of b are minimiæed or eliminated. A gyxo employing mechanical
dither is discussad in myU.S~ ~atent No. 4,1,15,'004 en-t~-tle* "Co~n~r-
7 balanced Oscillatin~ Ring Laser .Gyro" which w.as ~ssued"Sept~x~'~9, ,~918 to,~sJ. Hutc,hings and Virgil E. Sanders and which is assigned to Litton Systemsl ,~c.
9 , Another method of minimizing t~e effects of,lock-in
which has been suggested is the directional dither of the
11 magnetic field of a Faraday cell digposed within a ring laser
12 pathO Within the ring laser cavi~, lineaxly polarized laser
13 waves are converted to circularly'polarized light whose vector
,1~ rotates in ~he same'direction as the windin~s in the Faraday cellO
O~5 The circularly polarized light waves are ac~e~ upon by the
~6 magnetic field as they pass through the Faraday cell an~ an
17 increase or decrease in optical path leng~h occurs, depending
'~8 upon the direction o~ ~he field and the direction whic~ ~he waves
19 are traveling. After,leaving the Faraday cell, ~he circularly
polarized light is converted back to linearly polarized light. :'
21 By oscillating the current in the Faraday cell windings, the
2~ magnetic field oscillates accordingly and varies the optical path
23 lengths of the opposite propagating ~aves in a ~onreciprocal
2~ manner. This also can be used to make a larger than b in the
2~ above e~uation such that the effects o~ ~ock-in are minimizedO
26 ~This magnetic dithering using a Faraday cell is explained in
27 Arono~Jitz, supra, pp. 157 t~rough 1590
28 ' _5_
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GCD 78 5
l The above antilock-in techniques are passive, i.e., they
2 are not dependent upon active laser gain media. Also, with ~hese
3 methods the effects seen by waves propagating in one direction
4 in the laser path are equal and opposite to the ef~ect~ on the
waves traveling in the opposite direction.
6 SUMMARY OF THE INVENTION
7 For purposes of this discussion,the two opposing
8 resonant modes in a ring laser cavity which are combined to yield
9 rotational information are referred to as "primary modes". It
is an object of this invention to minimize lock-in between
ll primary counterpropagating modes in a ring laser cavity by
12 introducing additional modes into the ring laser cavity. These
13 additional modes, or secondary modes, oscillate at frequencies
14 different than the primary modes and couple with the primary
modes through the laser gain medium to produce an antilock-in
16 effect.
17 For example, in one embodiment of the invention, four
18 oscillating resonant modes are generated within the ring laser
l9 cavity. These four modes may be generated by detuning the laser
cavity such that the two primary modes operate at a frequency
21 slightly off-center from the center of the laser gain curve while
22 two weaker secondary modes oscillate at frequencies on the gain
23 curve only slightly above threshold. Threshold i5 define~ as
24 that area on the gain curve where a resonant mode begins to be
amplified in the laser gain medium. The secondary modes couple
26 through the active gain medium with the two strong modes to
2~ produce a dither effect on ~ . This dither effect produced
28 ~ 6-
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~ 395
1 on ~ as a result of coupling the weak and strong modes
2 counteracts the lock-in component of the ecfuation and reduces
3 or eliminates it.
4 Another embodiment of the inven-~ion includes providing
a perturbing oscillation from an external laser source. Two
6 laser beams can be injected into the ring laser cavity of a
7 two mode ring laser. The injected modes, one traveling in each
8 direction, experience gain from the laser medium and thus couple
9 with two primary modes generated in the ring laser. These injecte
modes, which have different frequencies than the primary modes,
11 couple with the primary modes to accomplish a dithering effect
12 in the difference frequency. The dithering reduces or eliminates
13 coupling between the two counterpropagating primary modes and ~
14 thereby reduces or eliminates lock-in accordingly.
An additional embodiment of the invention includes
16 using a portion of one of the counterpropagating waves of the
17 ring laser as an ext~rnal source. In this case, w~ere two primary
18 modes in the ring laser have sufficient gain to oscillate,
19 a portion of one mode is extracted from the ring laser cavity
through a partially reflective mirror. The extracted portion
21 is doppler shifted to alter its resonant frequency and attenuate
22 it, and then injected back into the ring laser. This doppler
23 shifted mode, having a slightly different frequency, recombines
24 with the original primary mode and causes a dikhering ~hich
reduces lock-in.
26 It is also an object of the invention to provide means
27 for combining portions of the principal modes to obtain siynals
28 _____ -7-
. ~
,
.,
' '
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representatlve of the ra-te and direction of rotation of the
laser gyroscope. Also, the invention includes apparatus for
monitoring and optimizing the cavity length of -the laser
gyroscope so that the resonant modes oscillate a-t the desired
freqency on the gain curve.
Therefore, in accordance with the pre~ent lnventlo~
there is provided a ring laser gyroscope comprisiny: a ring
laser body including reflective surfaces which define a closed
loop optical path, means for generating and maintaining at
least two counterpropagating in the optical path, whereby
frequency differences between the primary modes are
representative of angular motion experienced by the ring laser
body, means for generating and maintaining at least one
secondary resonant traveling mode propagating in the path
and coupling with at least one of the primary modes, whereby
the effects of lock-in are diminished or eliminated, and
means for processing the frequency differences between the
primary modes to generate signals representative of angular
motion of the ring laser body.
Other objects, features and advantages on the invention
will become apparent from consideration of the detailed
description and the drawings.
DESCRIPTION OF THF DRAWINGS
_ . _ . ... _ . _
Figure 1 shows a first embodiment of the invention
where the cavity length control circuitry adjusts cavity
length such that two strong primary modes and two weaker
secondary modes are generated in the gain medium.
- 8 -
csm/p~
- Figures 2 and 3 illustrate how the optical frequency
of the resonant cavity may be tuned so that the resonant waves
in the cavity operate at desired points on the laser gain curve.
Figure 4 illustrates how detuning a laser cavity to
allow weaker secondary modes to couple with stronyer prirnary
modes will reduce locking between two opposiny primary modes~
Figure 5 is a second embodiment o:E the invention where
secondary modes are generated by an outside laser source and
injected into the resonant cavity to couple with the opposing
primary modes.
- Figure 6 shows a third embodiment of the invention
wherein a portion of one primary mode is extracted from a ring
laser resonant cavity, doppler shifted in fre~uency, and then
xe-injected back into the cavity to couple with a primary mode.
8a -
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GCD 78--$
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1DETAILED DESCRIPTION OF T~ VENTION
2~s discussed above, the difference frequency or beat
3 freauency which results from combining the two primary opposing
resonant modes within a ring laser cavity is governed by the
expression
6 ~ = a ~ b sin ~
where ~ is the instantaneous phase difference between the
8 opposite traveling wave~, a is proportional to the rotation rate
9 of the ring laser gyroscope, and _ is proportional to the
magnitude of the backscattered energy. The second term on the
ll right hand side of the equaltion (b sin ~ ) represents the couplinc
12 which results from backscatter. For small rates of rotation, a
13 is smaller than b and ~ goes to zero. In this situation, the
14 ring laser gyroscope is locked-in and does not yield an output
representative of the actual rotation. Thus, at small but finite t
16 rotation rates the ring laser does not ~function well as a gyro-
17 scope.
18 By physically doing something to the ring laser such
l9 that beat freauency is perturbed sinusoidally, an additional
time varying term is added to the above equation to modify it
21 to read
22 ~ - a + b sin ~ + c cos ~t
23 In the new equation, c and ~ represent the amp~itude and ,
24 frequency respectively of the perturbation imposed on the
difference freauency ~ .
26 Solving this new equation for ~ ~t), a good
27 approximation results in the equation
2~ _____ _9
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I GCV 78-5
~ 5~
1¦ ~ (t) = at ~ ab JO (~) cos (at)
21 If the values of c and ~ are chosen such that JO = zero
¦ this equation reduces to
--41 ~ at
51 and the lock-in term of the original difference frequency equation
~¦ is eliminated. In the following discussion of the invention, such
an additional perturbing effect on the dif~erence frequency is
~ accomplished ~y introducing additional modes or frequencies into
9 the ring laser cavity to couple with the primary resonant modes.
The effect of these additional perturbing waves, or secondary
11 modes, is described by the addition of the term c cos ~t as
12 explained above. By controlling the magnitude and frequency of
13 the secondary modes, the terms c and ~ may be manipulated to
14 diminish lock-in in the ring laser gyro.
FIGURE 1 shows a ring laser gyroscope 2. The laser
16 body 4 is made of quartz and a sealed cavity 6 within the laser
17 body is filled with 90% helium and 10% neon. Two anode3 8 and 10
18 and two cathodes 12 and 14 are attached to the cavity 6. Th~
19 gas mixture in the areas of the cavity between the cathode 12
and anode 8 and cathode 14 and anode 10 respectively is
21 electrically charged to provide a gas plasma which serves as
22 the amplification medium for generating and amplifying the
23 resonant laser modes within the cavity 6. Three dielectric
24 mirrors 16, 18 and 20 are located at the three corners of the
triangular shaped resonant cavity 6. These mirrors comprise
26 multiple layers of dielectric coatings which are well known in
27 the art.
28 ~ 10-
29 ____ -
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l GCD 78~5
~ 5~
1 ¦ M~-ror 20 is a partially reflective mirror which allows
2 ¦ ~ @mall ~ e~tage of the rlng laser waves which strike it to
¦~Y~ th~ yh t~he mirror. Portions of the kwo primary coun-ter-
~ tl~ modes, which travel in the cavity 6 alony the pa~h
0 ¦ rg~e~e~ by line 22, pass through the mirror and are combined
6 ¦ in ~ ~rl.~m ~tructure within the combiner and photodetector
¦ a~mbl~ 23 -~Q form a fringe pattern. This fringe pattern is
~¦ ~@ggiVQ~ ~y ~hotosensitive detectors and the signals generated
- ~¦ ~h@~@ n ~E@ ~ransmitted along leads 24 to standard data reduction
9¦ a~ lo~ie gircuitry 26 which determine rate and sense of rotation.
~¦ A mQre ~@~iled discussion of combining counterpropagating waves
,~21 ~nd pro~sing the information obtained therefrom is included
in AronQwitz, supra, pages 139 through 141.
~¦ ~Fter beam frequency is controlled by varying the
~51 eavity length, i.e., the distance that the laser modes travel
¦ i~ eomplç~ing one full loop around the path 22. It is generally
7 ¦ desire~ to adjust or tune the cavity length such that the modes
¦ whi~h may resonate within the cavity are in the center of the
1~ ¦ i~tensity ~istribution curve (gain curve) for the particular
2Q ~ er galn medium. In order to adjust the cavity length,
21 ¦ ~irro~ 16 is attached to the laser body 4 in such a manner that
22 ¦ it ~n mQve in and out. Attached to the back of mirror 16 is a
231 ~$~k of piezoelectric elements. Cavity length control is
~¦ ~compl~hed by oscillating, or dithering, the mirror 16 by
2~1 ~pplyl~ an AC voltage to the piezoelectric elements 28. As
~61 ~he mirror 16 is oscillated at a given frequency, the intensity
~71 ~i~nal generated in the photodetector assembly 23 varies
2~ 1 ~
~9l ~ t
I GCD 7~-5
I ~ 3S
1 ¦accordingly and is transmitted along lead 30 to standard closed
2 ¦loop cavity length control circuitry 32. This circuitry
3 ¦ determines where the resonant modes in the cavity are locaked
41 along the gain curve and adjusts the nominal cavity Length by
~¦ increasing or decreasing the DC electric signal provided to the
61 piezoelectric elements 28 along lead 34. A thorough discussion
71 of this type of circuitry is contained in NASA Report No.
~¦ CR-132261, "Design and Development of the AA1300AbO2 Laser Gyro,"
91 by T. J. Podgorski and D. N. Thymian, 1973, pages 10 and 11.
~0¦ For the embodiment of the invention shown in FIGURE 1,
1¦ dithering of the difference frequency between the primary
12¦ counterpropagating modes in the cavity is accomplished by detuning
1~¦ the cavity length. For example, in FIGURE 2 is shown the laser
4¦ gain curve 44, i.e., the intensity distribution of light emitted
~5 ¦ in the laser gain plasma versus the optical frequency of such
16 ¦ emitted light. As is well known in the art, only certain
17 ¦ frequencies may resonate, i.e., be amplified, within the rin~
18 ¦ laser cavity. The frequency spacing between these resonant
19 ¦ modes is determined by the speed of light (c) divided by the
20 ¦path length tL), or the distance a wave makes in co~pleting one
21 ¦ full loop around the laser path.
22 ¦ In FIGURE 2, lines 36 and 38 represent the clockwise
23 ¦ and counterclockwise modes respectively which exist at a given
2~ ¦ ~requency when the ring laser cavity is tuned to the center of
25 ¦ the gain curve 44. 1ines 40 and 42 and lines 46 and 48 represent
26 ¦ ~he nearest modes on the optical frequency scale which could
27 ¦ also exist within the cavity, except that no gain medium is
2~ ~ 12-
29 ~
..
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'~ 5
1 provided which will amplify these other modes within the cavity 6.
2 The intensity level represented by dashed line 50 designates
3 the threshold, or the level above which the laser gain medium
4 will amplify the resonating waves within the cavity.
For the embodiment of the invention in FIGURE 1, detuning
~ of the cavity length is accomplished ~y adjusting the DC componerlt
7 of the electric signal on the piezoelectric elements 28 so that
8 the cavity length is detuned to cause the principal modes 36
9 and 38 to be moved from the center of the gain curve. Sufficient
detuning must be done to allow secondary resonant waves which
11 oscillate above threshold to be introduced into the resonant
12 cavity 6. FIGURE 3 shows how the cavity length is adjusted so
13 that resonant modes 36 and 38 are moved off the center of the gain
14 curve 44 sufficiently to allow secondary waves 40 and 42 to
15 oscillate slightly above threshold on the gain curve.
16 Secondary mode 40, which propagates in the cavity in
17 the clockwise direction, will now couple with stronger primary
18 mode 36, which propagates in the cavity 6 well above threshold
19 and in the same direction. This will cause a dither effect on
20 the ~ term in the difference frequency equation. In the same
21 manner the counterclockwise secondary mode 42 combines with
22 primary mode 38 to accomplish a dither effect. The effects of
23 the perturbing modes 40 and 42 are governed by the term c cos ~ t
24 in the above equation. By adjusting the intensity along the gain
25 curve of modes 40 and 42 as well as the requency at which they
26 oscillate, c and ~ in the equation can be controlled to dimish
27 the effects of lock-in, as explained in the above discussion.
28 FIGURE 4 is a graph illustrating how detuning affected
~ _13-
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GCD 78~5
. '~ 5 .
1 the locking frequency between pximary modes 36 and 38 in a parti-
2 cular experiment. Detuning is defined as tuning the path length
of the ring laser such that the optical frequency of the primary
modes is changed from the center of the gain curve. Note in
FIGURE 4 that lock-in was practically eliminated in one case when
6 the cavity path length was detuned to where the primary modes _ _
7 were 150 megahertz from the center of the gain curve.
8 Another embodiment of the invention is shown in FIGURE 5.
~ This embodiment includes a two mode ring laser similar to the
ring laser gyroscope shown in FIGURE 1. A sealed cavity 52 is
11 provided which contains 90% helium and 10,' neon which, when
12 electrically excited between anodes 54 and cathodes 56, comprises
13 the laser gain medium. Portions of two primary counterpropagating
14 waves in the cavity are processed through a partially transparent
dielectric mirror 58 into a combiner and photodetector assembly 60
16 where signals are generated and transmitted to a data reduction
17 and logic circuitry 62. An AC signal generated in the cavity
18 length control circuitry 66 is supplied to a piezoelectric
9 stack 68 which dithers mirror 70, and thereby oscillates the
20 cavity length of the gyroscope. Intensity signals from the
21 combiner and photodetector assembly 60 are transmitted along
22 lead 64 to the cavity length control circuitry 66. Variations
23 in the intensity signal due to the oscillations of the piezo-
24- electric stack 68 are processed in the length control circuitry 66.
25 The DC component of the signal transmitted to the piezoelectric
26 stack along lead 72 is adjusted to optimze the cavity length
27 for maximum intensity of the counterpropagating waves therein~
28 _____ -14-
: i
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Contrary to the example discussed above relative to FIGURE 1,
the cavity length is adjusted so that the resonant modes are
operating substantially at the center of the gain curve.
In the embodiment of the invention in FIG~RE 5,
perturbing secondary waves at frequencies difEerent rorn the
primary resonant modes in the laser yyro are introduced from
an external source. The external source in this case is a
two mode linear laser 74. Two separate modes generated in the
linear laser 74 travel colinearly to the dispersive element 76.
Such dispersive elements are well known in the art and may
comprise a grating for diffracting different frequencies
different amounts. After passing through the dispersive
element 76 one secondary mode 78 is diffracted towards
dielectric mirror 80 where it is reflected towards the
partially transmitting mirror 82. In passing through mirror
82, mode 78 enters the ring laser cavity 52 in the clockwise
- direction and couples with the clockwise primary mode generated
in the cavity.
The secondary mode 84 is deflected by the dispersive
element 76 towards mirror 86 and then through mirror 82. It
enters the cavity 52 traveling in the counterclockwise direction
and couples with the counterclockwise primary mode.
Again, the perturbing effect of the secondary modes
introduced into the cavity is represented in the difference
frequency equation by the term c cos ~t. The difference
frequency between secondary modes 78 and 84 is represented by ~.
The amplitude portion c is proportional to the magnitude of
signals 78 and 84 and the magnitude of the difference frequencies
mb/~ 15 -
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GCD 78-5
~5l~3~5
1 between the secondary and primary modes in the cavity. The
2 terms c and ~ may therefore be manipulated to diminish lock-in
3 by controlling the transmittance of mirror 82 and the fre~uency
4 and magnitude of the signals generated in the linear laser 74.
FIGURE 6 shows a third embodiment of the invention.
~ This embodiment includes a triangular ring laser gyroscope similar
7 to the ring lasers shown in FIGURES 1 and 5. Cavity length
8 control circuitry ad]usts the piezoelectric stack to maximize
9 intensity of the ring laser gyroscope output. The two opposing
modes propagating in the cavity along path 22 have frequencies
11 tuned substantially to the center of the laser gain curve 44 of
12 FIGURES 2 and 3.
lX In the device shown in FIGURE 6, one perturbing
14 secondary mode is introduced in the ring laser cavity and couples
with the counterclockwise propagating primary mode. To obtain
16 the secondary mode, a portion of the counterclockwise mode in
17 path 22 passes through the partially transmitting dielectric
18 mirror 88. This transmitted wave 102 then passes through a
19 directional isolator 90. Such directional isolators are well
known in the art and operat~ to change the angle of polarization
21 of the traveling waves passing through it~ The ~ode 102 then
22 strikes the dielectric mirror 92, which is attached to a
%3 piezoelectric stack 94. An AC voltage is supplied at a selected
24 frequency to the piezoelectric stack 94 from oscillation
circuitry 104, causing mirror 92 to oscillate. This oscillation,
26 in turn, doppler shifts the frequency of mode 102 so that after
27 it is deflected from dielectric mirror 98 and reintroduced
26~ _____ 16-
~ J
~ 39S
1 throu~h the partially transmitting mirror 88 into the ring
2 laser path, its frequency is change~ relative to the primary
mode from which it was extracted. This doppler shif~ed mode,
4 upon xeentering the path 22, couples wikh the counterclockwise
primary mode to produce the antilock-in dithering effect on
6 discussed ahove.
7 The magnitude oE the doppler shifted signal 102 ~hich
8 reenters the cavity is represented in the difference frequency
9 e~uation by c. The c term may be controlled by controlling the
magnitude of 102. Ways to control this magnitude include
11 controlling the transmittance of the partially transmitting
12 dielectric mirror 88. The ~ term in the difference frequency
~3 equation corresponds to the frequency of oscillation ~
14 transmitted to the piezoelectric stack,94. This term may be
easily controlled by simply varying or controlling the frequency
16 of oscillation generated in the circuitry 104. Therefore, by
17 controlling the magnitude and fre~uency of oscillation of mode 102
18 when it reenters the laser cavity and couples with the counter-
19 clockwise primary mode, the effects of lock-in may be substantially
diminished.
21 Incidentally, a polarizer 96 disposed in the path of
22 mode 102 effectively allows beams of one sense of polarization
23 to pass through while blocking out beams having different
2~ polarization. Polarizer 96 is adjusted to allow beams 102 to
pass through. Since the direction isolator 90 has changed the
26 sense of polarization of mode 102, portions of the clockwise
2~ propagating principal mode which pass through mirror 88 hav~
28 _____ -17-
29 ~
"~
¦ GCD 78-5
I J~5~3~5
1 ¦different polarization and will be blocked out by the polarizer ~6.
2 ¦ Changes may be made to the above described embodiments
3 ¦of my invention and still be within its scope and ~pirik.
4 ¦Examples of such changes include, but are not limited to, using
5 la rectangular shaped ring laser path, using means other than
6 ¦piezoelectric stacks for oscillating dielectrlc mirrors, using
~¦ alternate cavity length control apparatus, using no cavity
8¦ length control device, and using different means for combining
gl and processing primary counterpropagating beams to obtain
10¦ rotational information.
11 I
12
13 I .
14
16
1~ l
18 I .
19 l
21
22
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28
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