Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
GCD-75-17
This invention relates to ring laser gyroscopes
wherein the difference between resonant frequencies of
counterrot~ting li~ht beams i~ a measurement of rotation
of the laser body. More specifically, thi~ invention has
to do with la~er gyroscopes of the so-called four-mode type,
wherein the term "mode" is u~ed to di~tinguish a beam from
all other beam3 by both different frequency and different,
actually opposite direction of propagation.
Ring laser gyroscopes utilizing counterrotating,
i.eO oppositely propagating light beams are well-known.
These devices are used for measuring rotation rate~ of the
laser body about an axis perpendicular to the plane of the
ring laser resonant cavity by detecting the beat frequency
which occurs due to a frequency difference between the
oppositely propagating beams resulting from the rotation.
; ~owever, for the ring laser gyroscopes to function at low
rates of rotation, frequency locking, frequently referred
to as "lock-in"~ must be overcome. This phenomenon occurs
when two beams of light waves propagating in opposite di
rections in a resonant cavity have their slightly different
frequencies "pulled" toward each other, so to speak, so
that they combine into a single frequency beam of standing
light wavec~ so that no useful output can be obtainedO To
avoid this phenomenon of "lock-in"~ the frequencies of the
oppositely propagating beams mu~t be sufficiently separated
one from the other, ~qo that the "pulling together" does not
o~cur. The effect~ of lock-in are described in d~tail in
Laser ~ , edited by Monte Ross, Academic Press,
Inc., New York, N~Y. 1971, pp. 141 to 143.
-2-
GCD-75-17
~3~7~3~
One of the ways which has been proposed for
eliminating lock-in in a ring laser resonant cavity is to
have two pairs of oppositely, i~e. clockwise and counter-
clockwi~e, propagating beams, with two oppositely polarized
beams in each pair propagating in the cavity simultaneously~
Thus, one pair consist~ of two right circularly polarized
light beams, one propagating in the clockwise direction and
the other in the counter-clockwise direction. The other
pair consists of two left circularly polarized beams which
are al90 propagating in opposite directions within the same
resonant cavity. Sucn a so-called "four-mode ring laser
gyroscope" configuration i9 described in detail in U.S~
Patent No. 3,741,57, issued Jurle 26, 1973, entitled "Laser
Gyroscope" byKeimpe Andringa, The structure and operation
of a four-mode laser gyroscope is briefly described below.
Disposed in the laser beam path, i.e. within the
; cavity, are reciprocally anisotropic and nonreciprocally
anisotropic optically dispersive elements. A reciprocally
- anisotropic dispersive element, such as an optical rotator
made of quartz crystal, i9 provided to cause different
delays, due to different optical indices, to right and left
circularly polarized light beams. Thi~ difference in opti-
cal index is known as natural optical activity and results
in an optical path length difference as seen by one and
~5 the other of a pair of oppo~itely circularly polarized
light beams propagating within the ~ame cavity, regardless
of the direction of beam propagation~ In addition, a
nonreciprocally anisotropic dispersive element, such as a
Faraday cell,is provided which presents different optical
indices for light beams propagatlng in opposite directions,
so that different delays are caused for beams propagating,
--3--
,:: . ~ ,
GCD 75-17
1 ~! 3 7 9 ;~
i.e. traveling, in the counterclockwise direction and
in the clockwise direction~ regardless of the sense of
circular polarization. This is understood to be the result
of different path lengths for beams propagating in opposite
directions. Therefore, the presence of these two types of
optically dispersive, anisotropic elements results in fre-
~ncy separation between each resonant mode, wherein a mode
is determined by frequency and direction of propagation and
sense of circular polarization~ such that all four modes
resonate at different frequencies.
Separation between the resonant mode frequencies
i9 to be understood to be the phenomenon of conditioning
the resonance characteristic of the cavity such that the
different modes "see" distinct cavity lengths. In still
~; 15 other terms, a phenomenon of discrimination against other
than a predetermined resonant frequency for each mode is
~- involved by establi~hing such resonant frequencyO It
~- can be seen that the effect achieved by the optically
dispersive elements is of the natur0 of a tuning phenomenon,
though such "tuning" is achieved without physically modi-
fying the dimensions of the cavity. As a result, the reso-
nant frequencies of the two be~ms traveling, i.e. propagat-
ing, in one direction are spaced between the resonant fre-
quencies of the two beam~ propagating in the opposite direc-
tion, with the two higher frequency modes, or beams7beingcircularly polarized in the same sense but propagating in
opposite directionsO Similarly, the two lower frequency
mode~, or beams, are circularly polarized in the same sense,
- which is oppo ite to the sense of polarization of the other
pair, and are al80 propagating in opposite direction3.
' -
GCD-75-17
13L~ 3 î ~
Each pair of such mutually oppositely propagating,
identically circularly pola~ized beams may be viewed and
understood to operate as a separate one of two distinct
la er gyro~copes. As the ring laser gyroscope system is
xotated-about an axis which is oriented perpendicularly to
the plane of the ring-3haped beam path, the frequency sepa-
ration, i.e. the frequency difference,between the higher
frequency beam~ will either decrease or increase, while the
frequency difference between the two lower-frequency beams
will be oppositely affectedO That is, it will either in-
; crease or decrease. The output beak signal resulting from
combining the two lower frequency beams is subtracted from
the output beat signal resulting from combining the two
higher frequency beams. The resulting differential frequency
:.,
~ 15 i8 a substantially linear representation of rotation of
:~,
the ring laser gyroscope system. Further, the direction of
rotation is determined by monitoring one of the pairs of
, ~
beams.
~; In order to prevent the undesirable results of
the phenomenon known as "hole burning", the four frequenciesassociated with the four resonating modes in the cavity must
be substantially ~eparated. The concept of hole burning
;~ involves the population depletion of available light-
:,
emitting atoms in the gas laser gain medium which can emit
radiant light at a given frequency. A laser beam sustained
in a laser cavity through stimulated emission depletes the
population of available light-emitting atoms about that fre-
quency and thereby resultq in a dip or "hole" in the laser
gain curveO This hole has a certain bandwidth such that, if
two separate beams are operating at frequencies very close
to each other, the bandwidths will overlap. This condition
.: , .;, ,: ; ,. :
GCD~75-17
3 ~3
may be described by the phrase "hole burning competition."
As a result, one of the re~onant mode~3 will dominate and
the intensity of the mode, or beam,operating at the adjacent
frequency will be sub~tantially reduced or even eliminated.
Hole burning is explained in detail in the text Gas Laser
~S~E~a~YEY by Douglas C. Sinclair and W. Earle Bell, Holt
Reinhart and Winston, IncO, New York, ~.Y. 19~9, pp~ 33-350
Accordingly, in order to su~tain lasing action in
all four modes in the laser cavity, the ~requency of each
mode must be sufficiently separated rom the other three
to prevent the effects of hole burning competitionO The
frequency spacing must be such that there is no significant
overlap between the hole~ burned into the gain curves by each
pair of ad~acent resonatiny modes.
To provide sufficient dispersion to avoid hole
burning effe~ts between the different beams, the convention-
ally used, reciprocally anisotropic crystal disposed in the
laser beam path, frequently referred to as a crystal rotator,
which exhibits the property of optical reciprocal anisotropy
must be undesirably large. Its size contributes to thermal
stresses which occur due to thermal gradients within the
instrument and temperature changes in the laser system,
frequently aggravated by difference~ between coefficients
of expansion of the crystal and the remaining laser body.
These stres~es increase linear birefringence in the crystal,
which increases coupling between different modes. Coupling,
a~ u~ed herein, de~ignates an interaction between different
beams propagating in the ~ame direction which result~ in
an error source for the output from the ring laser gyroscope.
GCD-75-?7
3~7~313
Typically, the element exhibiting nonreciprocal
ani~otropy is a Faraday rotator, also called a Faraday cell,
which can be created by winding a coil around the cry~tal
and pa3sing a DC current through the coilO The magnitude
of the effect achieved by virtue of the property of nonrecip-
rocal ani~otropy occurrinq in the cell which--in similarity
with the Faraday effect--can be visualized as a twisting
action upon the polarized light beams, i9 determined by the
length of the cell, the magnitude of the magnetic field,
lQ and the Verdet con~tant of the crystal material~ The Verdet
constant is defined as rotation per unit length per unit
magnetic field strength. It is a material property, i.e.
different materials have different Verdet con~tants associ-
ated with them.
In view of the above-discussed output inaccuracies
j due to thermal stresses, a conventional crystal is undesir-
ably large. Its length, however, is very small when used
in a Faraday cell. Then, in order to achieve the required
, .
nonreciprocal anisotropy, the magnetic field over the short
2Q length of the cry~tal must be relatively large, typically
ovex 1000 Gauqs. Such high field intensity is difficult to
control over the short length of the cryqtal element.
The purposes of this invention include reduction
in size of the crystal rotator, as well a~ reduction in
field intensity of ~he magnetic field in the Faraday cell.
GCD-75-17
~:31¢i3~7~;~
In accordance with the invention, the foregoing
objects are achièved through the utilization of the Zeeman
effect, i.e. the imposition of a magnetic field parallel to
the laser path over a so-called dual laser gain medium,
which phrase denotes a laser gain medium comprising two
distinct, separately operative media in the form of distinct
gas isotopes within a common resonant cavity. The Zeeman
effect resulting from the field causes frequencies of light
emitted by atoms in the gain plasma to be shi~ted in such
a manner that the frequency of light generated by an atom
is either increased or decreased~ Further, these atoms
are aligned with the magnetic field, so that all those
shifted up in frequency may emit light of one sense of cir-
cular polarization in one direction of propagation and of
the opposite sense of circular polarization in the opposite
direction of propagation. Those atoms which are shifted
down in frequency are affected in the same manner, except
that the s~nse of circular polarization is reversed for a
given direction of propagation~
~0 Thus, due to the Zeeman effect, the gain curve
for a given beam generated by the atoms of each isotope in
the laser gain medium, i.e. plasma, will be divided into
two gain curves. The result is that for one sense of circu-
lar polarization, hole burning or source depletion resulting
from a light beam propagating in one direction in the laser
cavity will not affect the gain curve for a light beam of
the same sense of circular polarization propagating in the
opposite direction. The use of a dual isotope laser gain
plasma results in the fact that the Zeeman effect produces
four gain curves. This Zeeman splitting of the gain curve~
--8--
. ~ ~
. :
GCD-75-17
7~3;~
~qubstantially increases the independence of the individual
modeA with respect to the effects of hole burning in the
; gain medium. This minimization of the effects of hole burn-
ing permits a ~ubstantial reduction in the separation be-
tween ~he mean frequency of the two beams of one sense of
circular polarization from the mean frequency of the two
beams of opposite sense of circular polarization.
Accordlngly, the reciprocally anisotropic disper-
sive element, normally a quartz crystal, which accomplishes
!
~eparation between right and left circularly polarized
light in the ring laser beam path may be substantialLy re-
duced in size, as compared to the size in known instruments
of the type contemplated, thereby to reduce thermal stresses
~, caused by temperature changes or temperature gradients in
-
the laser body.
Furthermore, the magnetic field imposed over, i.e~
applied to~the light source, which is the laser gain plasma,
; also acts as a nonreciprocally anisotropic dispersive ele-
ment, i.e. it assumes the function of a conventionally used
Faraday cell, discu3sed above. Because of the different
; Verdet constants and the increased length associated with
the la~er gain medium, the same amount of Faraday splitting,
which term is used herein to denote conditionin~ the re~o-
nant cavity such that different re~onant frequencie~ are e~-
tablished for the two opposite directions of propagation,
i~ achieved as with prior art foux-mode gyroscopes with sub-
stantially reduced magnetic field intensity.
GCD-75-17
~L~3~313
In accordance with a broad aspect of the inven-
tion, there is provided a ring la~er ~yro~cope which oper-
ates with four circularly polarized beam~ at four mutually
distinct frequencie~, with two oppositely circularly polar-
ized beàm~ propagating in one direction and the two otheroppo~itely circularly polarized beam~ propagating :in the
opposite direction, the ring laqer gain medium comprising
two different ga~ isotope~ a~ it~ active component.~, wherein
means for detecting beat fre~uencie~ resulting from combin-
; 10 ing the beam~ are provided and wherein mean~ receiving the
output from the detecting means, during operation, generate
output 3ignal~ which are repre~qentative of rotational dis-
placement of the ring laser gyroscope, there being provided
means for app~ying a magnetic field, whose direction ~ub-
stantially coincide~ with the beam directions, to the gain
medium which includes the two isotope~, thereby to genarate
lasing action at the four mutually distinct frequencie~.
In accordance with specific feature~ of one em-
bodiment of the invention, use is made of at least one coil
which, during operation, carries direct cuxrent, thereby to
generate the magnetic field, the coil coaxially surrounding
the laser tube containing the gain medium. Then, the mag-
netic field, in addition to generating la~qing action at the
four mutually distinct frequencies, may assume the function
of e~tablishing different re~onant frequencies for oppositely
oriented direction~ of beam propagation, regardles~ of the
~ense of circular polari~ation~ In addition, there may be
provided a reciprocally ani~otropic disper~ive optical de~
vice disposed within the path of the beams for esta~ hing
a re~onant frequency for the la~er beam~ which are circularly
-10
GCD-~5-17
3'~
polarized in one ~ense and for simultaneou~ly e~tabli~hing
a different re~onant frequency for the l~ser beam~ which
are circularly polarized in the oppo~ite ~ense, regardle~3
of the direction of beam propagation~ Suitably, the opti
cal device i3 an ani~otropic crystal, such as a quartæ cry~-
tal.
The invention will become better under~tood from
the followi~g detailed de~cription of one embodiment there-
of, when taken in conjunction with the drawings, wherein:
Figure I is a schematic illustration of a multi-
oscillator, i.e. four-beam ring laser
gyroscope, combined with a block diagram
of the necessary circuitry to process
the information generated,
Figure 2 is a graphic representation of the sepa-
rate gain curves of each i~otope in a
dual isotope gas laser gain medium, to-
gether with the combined gain curves of
the two isotope~,
Figure 3 i~ a graphic representation of the sepa-
rate gain curves in a dual isotope ring
laser system, with Zeeman frequency
splitti~g,
Figure 4 schematically illugtrates the establish~
ment of re~onance conditions for the four
frequencies associated with each mode of
the multi-03cillator ring la~er gyroscope
of Figure l; and
Figure 5 i~ a ~chematic illustration of the gyro-
scope output a~ a function of rotation
- rate of the ring laser of Figure 1~
GCD-75-17
~ ~ 3t~ ~
With reference to Figure 1, the four mode ring
laser gyroscope include~ a laser body 12 with a sealed
resonant la er cavity 23. The cavity 23, a~ illustrated,
provides a rectangular beam path, with mirrors 14, 16, 18,
and 20 at its four corners~ The ~ealed cavity 23 is filled
with a dual isotope gain medium, such as a helium-neon gas
mixture, where the i~otopes neon 20 and neon 22 are the two
active isotopes. In the portions of the cavity 23 between
the cathode~ 46 and anode~ 48, where the gaseous gain med-
ium is electrically excited, it becomes a light-emitting
laser plasma which su~tain3 the laser beams at the re~onant
frequencies.
Mirrors 14 and 16 are used solely for reflecting
the beams in the laser path 24~ Mirror 18 is æecured to a
piezoelectric element 20 which moves the mirror in and out,
this portion of the structure forming part of the cav.ity
length control sy~tem~ Mirror 22 i~ only partially reflec-
tive, thereby allowing a .small portion of the light incident
on it~ ~urface to pas~. The proportions of light beams pass
ing through the mirror 22 arP combined one with the three
other~ and processed to provide the de3ired rotational i~-
formation a~ the outputO Line 24 represent~ the ring laser
heam path for the four modes o circularly polarized light.
The ring la~ex gyroscope is equipped with a recip-
rocally anisotropic disper~ive element 26. Matural opticalactivity which occurs wi~hin element 26 upon the circularly
polarized light by separating one from the other, due to
distinct resonance conditions, the two opposite senses of
circular polarization is well-known in the art and may be
accomplished with a material such a~ quartz cry~tal oriented
such that the beams propa~a~e along its optic axis. Elements
-12-
. :,,. . :
GCD-75 17
~qJ 3~
28 are electric coils whi.ch, during operation, carry DC
current and thu3 prc)vide a magnetic field superimposed
over the plasma gain medium ~ection~ between cathode~ 46
and anodes 48. Coils 28 are wound around the entire sec--
tions between the cathode~ and anode~ to apply the magneticfield over substantially the entire gas plasma light source~
The magnetic fields ~et up by the coils 28 are typically
about 100 ~au~s and both are oriented in the ~ame direction
with respect to the laser path 24, 90 as not to cancel one
another,
Impo~ition of the magnetic field over the la~er
; beam path creates a condition related to the Faraday rota-
tion effect, in the form of nonreciprocally ani~otropic
disper~ion which, by distinct re~onant frequencies, differ-
entiate~ between the clockwise and the counterclockwise
propagating beams. Also, the field ~quperposed over the ex-
cited pla~ma provides Zeeman frequency splitting between the
light emitted from atom~ in the plasma, so that hole burning
e~fects in the gain curves for right and left circularly
polarized light beams will be subqtantially xeduced when
the lasing frequencies are close together~ The Zaeman ef-
fect is thoroughly explained in the text FundameDtals of
Optics by Francis A. Jenkins and Harvey E. White, McGraw-
Hill, New York, ~.Y. 1957, page3 588 hrough 595~
Line 3G repre~en~ that portion o the counter-
cloc~wi~e propagating beams in the multi-o~cillatox ~y~tem
which are allowed to pa~ through the partially reflective
mirror 22~ The~e beams strike mirror 34 and are reflected
- through beam splitter 38 onto a ~ingle photodiode 40. Line
32 representY that portion of the clockwise pro~agating
-13-
3Lil~q~a~ 79~
.
beams in the system which pass through mirror 22 and
strike mirror 36 where they are deflected to beam ;~
splitter 38 and made approximately colinear with
line 30. The four beams simultaneously striking
photodiode 40 generate several beat ~requencies due
to the difference in frequency between all of the
individual beams.
The beat frequencies between all of the
four beams propagating in four associated modes in
the cavity are detected in the photodiode 40~ as
described in applicant's U.S. patent No. 4~123,162,
issued October 31, 1978. The information generated
; from the beat frequencies between the four oscillating
modes is used for determination of the magnitude of
rotation of the ring laser system, as well as for `
cavity length control and for determination of the
direction~ i.e. the sense of rotation~ A detailed
description of how this information is used for
these purposes is provided in the above-mentioned
patent.
Cavity length control circuitry 42 provides
an AC signal along leads 44 to the piezoelectric
element 20. This AC signal moves mirror 18 in and
out, i.e, back and forth parallel to itself, with
this motion resulting in variation of the cavity
length of the ring laser. This varies the output
from the ring laser system as applied to photodiode
40 at the same frequency as the AC component in leads
44 and thereby provides feedback to the cavity length
control circuitry 42. This feedback is processed as
described in the above-mentioned patent to control
the DC component along leads 44 to optimize the
length of the ring laser cavity for maximum output~
mb/pl~ - 14 -
il~a3~-7~3 GCD 75-17
Cathodes 46 and anodes 48 are connected to a
power supply 52 via leads 50. The cathodes and anodes
provide an electrical field over the gaq laser gain medium
which is sufficient to maintain stimulated light emi~ion
from the gas atoms to ~u~tain the propagation of la~er beams.
The vol~age acro~ cathodes 46 and anodes 48 oscillate~ at a
constant frequency controlled by the power supply 52, to
vary the output generated in photodiode 40. Thi8 output
variation i8 prOCe9Sed in circuitry 52 for determination
of the direction of rotation of the gyro~cope system in ac-
cordance with the above-mentioned cepe~ g patent a~ e
~3n. The output from photodiode 40 is al90 fed to loyic
- circuitry 54 for determination of the magnitude o~ rotation
o~ the ring la~er, a is thoroughly discussed in the same
patent ~ e~t~
In discus3ing Figures 2, 3 and 4 frequent refer-
ence is made to portions of Figure l by way of explanation.
Fisure 2 shows the Doppler-broadened gain curves for a typi-
cal dual isotope la~er pla~ma as contained in the tubes sur-
rounded by coils 28 o~ Figure l. As mentioned above, the
gain medium comprises the two isotopes neon 2Q and neon 22,
Line3 62 and 64 represent the gain-Yersus-frequency curves
of neon 20 and 22, respectively. Line 66, repxesenting the
combined gain curve for the two isotopes, i5 the sum of
curves 62 and 64. The curve3 as ~hown in Figure 2 are rep-
resentative of a gain curve in a typical dual isotopa ring
laser pla~ma, without application of a magnetic field whi~h
would cause occurrence of the Zeeman effect.
Zeeman ~plittin ~ as described in undamentals of
~E~ , cause~ ~ach gain curve of Figure 2 to be ~plit
into two gain curves separated on~ from the other in fre-
- -15-
GCD-75-17
~3~3
quency space, a~ i9 ~hown in Figure 3O The magnetic field
as set up b~ DC curr.ent through coil~ 28 causes the light-
emit~.iny atoms in the la~er gain pla~3ma to be oriented such
that any given atom may Pmit right ci.rcularly polarized light
in one propagation direction or left circularLy polar.ized
light in the opposite propagation direction. Al so . the mag
netic field causes the frequency at which light-emitting
atoms may emit light to shift either up or down by an amount
determined by the magnitude of the field.
CUrYeS 72 an~ 74 in Figure 3 are the gain curves
resulting from the splitting of curve 62 in Figure 2. The
available light-emitting atoms represented by gain curve 72
may emit light which is left circularly polarized and propa-
gates in the clockwise direction or r.i~ht circularly polar-
ized and propagates in the counterclockwise direction. Con-
ver~ely, the atoms repre~ented by gain curve i4 may emit
right circularly polarized l.ight propagating in the counter-
clockwise direction and left circularl.y polarized light
propagating in the clockwi~e direction. Also, curves 76 and
78 represent the gain for both right and left, respectively~
polarized light propagating in the colmterclockwise direction
resulting from splitting of gain cuxve 64 and ].eft and right,
re~pectively, circularly polarized light propagating in the
clockwise directionO ~ha gain curve~ of Figures 2 and 3 are
shown ~or purposes of explanation and are not necessarily
drawn to s~ale. The effective Zeeman ~plitting of the gain
curves sub~tantia.l!y reduces the hole burning type coupling
between the various modes~ As mentioned above, necessary
- field magnitudes to accomplish Zeeman splitting ln this four--
mode ring laser gyrosco~e are t~pically around 100 Gauss or
less~
33 GCD-7 5-17
Figure 4 illustrates the function of the recip-
rocally and nonreciprocally anisotropic elements which re
Yults in frequency separation between the four beams associ-
ated with the four resonating mode~ in the ring laser cavity.
In frequency space, where incxeasing optical frequency is
repr,3sented by line 83, line 81 represents the mean resonant
frequency of the ring laser cavity. The reciprocally aniso-
tropic dispersion element 26 ~natural optical activity crys-
tal rotator) in the ring laser path causes frequency ~plit-
ting between leEt and right circularly polarized light, i.e.establishing different resonant frequency conditions, as
represented by line~ 92 and 90, re~pectively~ Further fre-
quency splitting.of the four resonating modes in the re~o-
nant cavity is accomplished by nonreciprocally anisotropic
dispersion, called Faraday splitting, see above, in the
plasma, as the magnetic field causes clockwi~e and counter-
clockwise propagating, p~larized light beams to experience
different optical indices. Lines 82 and 84 represent the
results of Faraday splitting of the left circularly polarized
laser beams represented by line 92. In the same manner,
lines 86 and 88 show the effects of Faraday splitting on
right circularly polarized laser beams repre~ented by line
90. At this point, it should be mentioned that lines 82 and
88 represent frequencies of clockwise propagating beams, as
shown in Figure 4~ The lower and upper frequency beam~ thus
propagate in the same direction in the la~er cavity. If the
magnetic field polarity is rever3ed by reversal of current
direction in coils 28, the direction of the extreme frequency
will be reversedO
-17-
GCD-75-17
~ ~ 3t~
As shown in Fi~ure 4, the two beams at frequen-
cies 82 and 84 at the left-hand side of the figwre whose
mode is characterized by left circularly polarized light
may be understood to pertain to one ~yroscope/ designated
by the legend GY~0 1, while the beam~ of frequencies 8~ and
88 whose light is right circularly polarized similarly form
another gyroscope, designated GYR0 2.
Typically, separation resulting from Faraday
splitting between the counterrotating beams in GYR0 1 and
GYR0 2 is from 500 kilocycles to 1 megahertz. In four-mode
ring laser gyroscopes not employing the Zeeman effect
frequencies 92 and 90 generally are required to be separated
by a distance greater than 200 megahertz, in order to avoid
overlap of hole burning in the gain curvesO The Zeeman ef-
fect, or the magnetic field causing it in the laser gain medium,
makes it possible for the curves 72, 74, 76 and 78 to co-
exist at much closer resonant frequencies for a dual iso-
tope laser gain medium. The increased independence of the
four resonant modes with xegard to hole burning permits the
extent of necessary natural optical activity splitting ~re-
ciprocally anisotropic dispersion) to be reduced, so that
the frequency separation between lines 92 and ~0 may be
as small as 10 megahertz. Accordingly, tne crystal element
26 may be reduced in size. Both the size of element 25 and
the magnitude of the magnetic field generated by current
through coils 28 are optimized, so that the modes associated
with frequencies 82, 84, 8~ and 88 minimally affect each
other.
- As the ring laser system is rotated about an axis
perpendicular to the plane of the laser path in the counter-
-18-
~ ~23 ~ 9~ GCD-75-17
clockwi~e direction, frequencie~ 82 and 88 will increa~e,
while frequencies 84 and 86 will decr~ase. Becau~e the
output from the gyroscope is a ~unction of the separation
between the frequencies of beams propagating clockwise and
counterclockwise in the laser cavity, the output from GYRO 1
will decrease while the output from GYRO 2 will increase.
Conversely, if the laser 3y~tem i~ rotated in a clockwise
direction, the outputs from GYROS 1 and 2 will increase
and decrease, respectively.
Figura 5 graphically illustrates the output from
the gyroscope as a function of rotation rate of the ring
laser system. Lines 94 and 9~ repre~ent the outputs for
GYROS 1 and 2, respectively, as a function of system rota-
tion in inertial space. The output signals from one gyro-
scope are substracted from output signals of the other gyro-
scope and processed in logic circuitry 54 of Figure 1 to
provide a linear net output and a doubled scale factor for
system rotation. Point A in Figure 5 represent~ zero rota-
tion for the laser system where the outputs of both GYRO 1
and GYRO 2 are approximately equal.
Other embodiments of, and modifications to, the
described ring laser system are within the ~cope of this
invention. For example, other means of output detection and
information processing may be employed, the nu~ber of reflec-
tive elements in the ring laser beam path may be changed,and the magnetic field, or fields, for Faraday and/or Zeeman
splitting may be implemented using a permanent magnet~ Also,
means might be employed within the laser cavity to convert
circularly polarized light into linearly polarized light
throughout the la~er c vity except in the plaqma area where,
during operation, Zeeman splitting occurs~
--19--
'-~
.. . .
SUPPLEMENTARY DISCLOSURE
A more specific manner of clefining ~he necessary
magnetic Eield intensity will now be described with
reerence to the additional drawings in which:
FIG, 6 shows separate gain vs. frequency curves
of each isotope in a dual isotope gas laser plasma,
together, with acceptable laser wave frequency separation
according to the prlor art;
FIG. 6A shows a gain vs. atom velocity curve of
the isotope corresponding to the left curve of FIG, 6D
showing the depletion of atoms of that isotope caused by
lasing of the four modes; ~ :
FIG. 6B shows a gain vs~ atom velocity curve of
the isotope corresponding to the right curve of FIG, 6,
showing the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG. 7 shows separate gain vs. frequency curves
of each isotope in a dual isotope gas laser plasma,
together with unacceptable laser wave frequency separation
according to the prior art;
FIG. 7A shows a gain vs, atom velocity curve of
the isotope corresponding to the left curve of FIG, 7,
showing the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG. 7B shows a gain vs, atom velocity curve of
- ~he isotope corresponding to the right curve of FIG, 7 9
showlng the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG, 8 shows separate gain vs, frequency curves
foreach isotope in a dual isotope gas laser plasma, showing
an insufficient amount of Zeeman frequency splitting,
accordlng to the prior art
~ .
mb ~ . - 20 -
3~3
FIG. 8A shows a gain vs. atom velocity curve of
the isotope corresponding to the lef ~ curves of FIG. 8~
showlng the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG, 8B shows a gain vs~ atom velocity curve of
the isotope corresponding to the right curves of FIG. 8,
showing the deple~ion of atoms of that isotope caused by :
lasing of the four modes;
FIG~ 9 shows separate gain vs. frequency curves
for each isotope in a dual isotope gas laser plasma,
wherein the magnetic field intensities in the two gain
sections are aiding, showing a proper magnitude of Zeeman
frequency splitting, according to this invention;
FIG~ 9A shows a gain vs~ atom velocity curve of ~
the isotope corresponding to the left curves of FIG, 9, ~ :
showing the depletion of atoms of that isotope caused by ~ :~
lasing of the four modes;
;~ FIG. 9B shows a gain vs, atom velocity curve of : : -
the isotope corresponding to the right curves of PIG~ 9,
s~owing the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG. 10 shows separate gain vs, frequency curves
for each isotope in a dual isotope gas laser plasma,
showing the effect of an excess of Zeeman frequency
splitting;
FIG. lOA shows a gain vs. atom velocity curve of
the isotope corresponding to the left curves of FIG, 10,
showing the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG, lOB shows a gain vs. atom velocity curve of
the isotope correspond~ng to the right curves of FIG, 10,
showing the depletion of atoms of that isotope caused by
lasing of the four modes;
f~
mbl~ - 21 -
~ . .. ...
3'~513
FIG. 11 is identical to FIG, 9 except that the
field intensities in the two gain sections are opposing;
FIG. llA shows a gain vs, atom velocity curve of
the isotope corresponding to the left curves of FIG, 11,
showing the depletion of atoms of that isotope caused by
lasing of the four modes;
FIG. llB shows a gain vs~ atom velocity curve of
the isotope corresponding to the right curves of FIG, 6,
showing the depletion of atoms of that isotope caused by
lasing of the four modes.
Figures 6, 6A, 6B, 7, 7A, 7B, 8, 8A, 8B, 9, 9A, 9B,
10, lOA, lOB, 11, llA and llB are included herein to compare
this invention with prior art apparatus and to define the
upper and lower limits of the intensity of the magnetic
Eield applied by coils 28 to the gain medium in this
invention.
Figures 6, 6A, 6B, 7, 7A9 7B, 8, 8A, 8B refer to
prior art mechanisms. Figures 6, 6A~ 6B, 7, 7A, 7B, for
example, could correspond to the,operation of the apparatus
of United States patents 3,741,657 and 4,no6,989 with
Figures 6, 6A, 6B representing proper operation with a long
crystal and adequate frequency separation of the modes to
avoid hole burning. Figures 7, 7A, 7B is an inoperative
version of such apparatus where a small crystal is used
and the frequency separation of the modes due to natural
optical activity splitting has been reduced from the order
of 400 Mhz to 10 Mhz, 10 Mhz was chosen to compare such
apparatus to the apparatus of this invention which does
have a natural optical activity splitting on the order of
10 Mhz while still avoiding hole burning,
Figures 8, 8A, 8B corresponds to the apparatus of
~his invention e~cept that the intensity of the magnetic
field applied to the gain medium is far too low. For
mb~ - 22 -
~.a~3~
example, in United States patent 3,973,851, issued August
10, 1976 to Ferrar, the ield was less than one Gauss~
Aside from the fact that the field intensity is too low
to prevent hole burning, it is also so low that the earth's
magnetic field would interfere with its operation for its
intended purpose which is to equalize the gain between
clockwise and counterclockwise propagation.
Figures 9, 9A, 9B correspond to the proper operation
of the apparatus of this invention. Note in Figures llA~
llB, the region of competition for atoms~ shown shaded in
the figures, is minimized,
Figures 10, 10A, 10B corresponds to the apparatus
of this invention except that the intensity of the magnetic
field is far too high.
Thus, by comparing the figures, the range of
acceptable field intensity to produce a Zeeman effect of
appropriate magnitude to allow relatively small natural
optical activity splitting without hole burning may be
dlscerlled.
Zeeman splitting, as described in Fundamentals of
Optics, supra, results in each gain vs, optical fre~uency
curve of Figures 4, and 5 to be split into two curves
shifted ln frequency space as shown in Figures 8, 9 and lO~
The magnetic field elements 28 cause the light
emitting atoms in the laser gain plasma to be oriented
such that any given atom may emit by stimulated emission
a right circularly polariæed light wave ;n one direction
or a left circularly polarized light wave in the opposite
direc~ion,
Figures 6, 7, 8, 9, 10 and ll show typical plots
of gain vs. optical frequency for the isotopes neon 20 and
neon 22. Obviously if other elements or isotopes were used,
their frequency range would be different, Actually these
b ' ~h` ' '
a^, ~
mb/~ - 23 -
~3~
curves are only the portion of a normal distribution curve,
where the galn exceeds one, and the laser will oscillate~
Figures 6A, 7A, 8A, 9A, lOA and llA are gain vs,
atom velocity distribution for neon 20 in the clockwise (+)
and counterclockwise (-) directions of the laser path,
The graphs show the total available atoms as a function of
velocity and how the various optical wave modes deplete
and compete for the various available velocities. The
` shaded region shows where competition occurs, and the dips
in the curves demonstrate the "holes" which are "burned"
in the distribution by the four modes of optical wave
propagation.
Figures 6B, 7B, 8B, 9B, lOB and llB are the
corresponding gain vs. atom velocity distribution for neon
22,
Figures 6, 6A, 6B and 9, 9A, 9B and 11, llA, llB
situations where hole burning is avoided, The remaining
figures show inoperative situations because of hole burning,
Figures 6~ 6A, 6B correspond to the prior art
without Zeeman effect, Figures 9, 9A, 9B, 11, llA, llB
correspond to the appara~us of this invention~
Consider now the prior art represented by Figures 6,
6A, 6B.
Curves 100; 102 are gain vs, optical frequency
curves for neon 20 and neon 22, respectively, The maximum
gains for these two gases occur 875 Mhz apart, and the
laser cavity is tuned to the mid frequency fO between those
points 104, 106. The natural optical activity splitting
must be large. Typically it is about 400 Mhz, and it must
be larger than about 200 Mhz, The Faraday separation
between clockwise and counterclockwise propagating optical
waves is on the order of 0.4 Mhz. Note in U, S, patents
3,741,657 and 4,006,989 the frequency splitting is about
mb[J~ - 24 -
. .. . :. : ..
;3~93
200 Mhz, the quartæ is about 4 mm long and the field
strength is 2000 Gauss~ Figures 6~ 6A, 6B show operation
where the reciprocal frequency splitting is 400 Mhz The
frequencies are labeled on the abscissa wherein "L" means
left polarized,"R" means right polarized, "CW" means
clockwise, and "CCW" means counterclockwise The
separation of RCW and RCCW~ and the separation of LCw and
Lccw are exaggerated.
Turning now to FIG. 6A, it is seen that the
available velocities depleted by the four modes are
sufficiently separated that they do no~ substantially
compete for atoms. The "holes" 108, 110, 112, 114 do not
substantially overlap. The velocity at points 108, 110
is proportional to the difference in frequency between
that of point 104 (FIG. 6) and points 116. The velocity
at poin~s 112, 114 is proportional to the difference in
frequency between tha~ of point 109 and points 118c The
regions of competition for atoms is m-lnimal as represented
by the shaded zones 120, 122~ 124, 126, 128
FIG 6B is a similar graph for neon 22, Note that
the hole positions are identical, but they correspond to
different modes because the frequencies of 116, 118 are
less than that of point 106, The velocity at points 130,
132 is proportional to the difference in frequency between
that of point 106 and points 116. The regions of competition
for atoms is minimal as represented by the shaded zones
138, 140, 142, 144, 146.
Thus, the apparatus used for Figures 6, 6A, 6B is
operative to minimize hole burnin~, and all four modes will
lase. To achieve this compensation, however, the crystal
is relatively long and the magnetic field is very strong.
At such high fields ~1000-2000 Gauss), field control is
very difficult.
'`~
mb~ - 25 -
~`3 ~
If the crystal were shortened in the apparatus
corresponding to Figures 6, GA, 6B, to provide a natural
optical activity splitting of, for exampLe, only 10 ~Ihz
(as in this invention), the non-reciprocal Fara~ay
separation could not occur in the crysta:L because the
crystal would be too short (on the order of 0,4 mm) to
concentrate sufficient magnetic Eield intensity in the
crystal, An external Faraday section would be needed to
obtain even minimal non-reciprocal separation, Figures
7, 7A, 7B correspond to such a situation.
In FIG, 7, the difference between frequencies
150 and 152 is on the order of 10 Mhz, The distance from
the frequency of point 104 and that of points 1509 152
are a]most the same, i,e,, 432,5 Mhz and 4b~2,5 Mhz,
Thus, the "holes" 154 and 156 9 and the holes 158, 160 are
almost on top of each other in Figure 7A for neon 20. The
competition for atoms between R cw and LCcw modes and
between L w and Rc~ modes is very strong, and only one
mode in each pair will lase. The shaded areas 162, 164
representing competition between two modes, is very great,
Similarly~ the difference between the frequency
corresponding to point 106 and that of points 150, 152
are also 442,5 Mhz and 432.5 Mhz, and hole burning occurs,
Notice that holes 17n, 172 and 174, 176 are almost on top
of eacll other, The R and L modes in neon 22 compete
ccw ccw
for atoms as shown by the shaded area 180, The L and
R w modes also compete for atoms as shown by the shaded
area 178~ Only one mode of each pair will ]ase,
Keeping the crystal short and the Faraday field
as in Figures 7, 7A, 7B, but applying only a small amount
of magnetic field to the gain medium produces Zeeman
splitting as shown in Fig. 8. The neon 20 gain vs.
frequency curve of Figures 6 and 7 is shifted up and down
mb/J~ - 26 -
~3'~
in frequency a small amount to produce two gain vs
frequency curves 200, 202 symmetrical about ~he crossover
polnt 204. Similarly, the neon 22 gain vs. frequency
curve of Figures 6 and 7 is shifted up and down in
frequency a small amount to produce two gain vs. frequency
curves 206, 208 symmetrical about crossover point 210.
The crossover points 204, 210 and 875 Mhz apart and
symmetrically positioned relative to Fo. The amount of
Zeeman shift is 1,8 Mhz per Gauss of applied field. ~ote
that with 1 Gauss maximum of U~ S~ patent 3,973,851, the
amount of Zeeman shift would be negligible, and it likely
would not be seen if drawn to scale in Figure 8, Curves
200 and 206, which have shifted downward, describe the
gain vs. frequency for the LCw and RCcw
202 and 208, which have shifted upward~ describe the gain
vs. frequency for the RCw and LCCW modes.
FIG 8A is a graph of the atom velocity distribution
of neon 20. The difference in frequency between that of
peak point 220 and the frequency of 226 is too close to
the difference in frequency between that of peak point 224
and that of 230. Consequently "holes" 240, 242 and holes
244, 246 are too close together, and only two modes will
oscillate.
FIG~ 8B shows the corresponding velocity distribu-
tion for neon 22. The frequency difference between that
of peak point 232 and 226 is too close to the difference
between that of peak point 234 and 230. The coupling
between modes is excessive, as shown by the cross-hatched
areas of Figures 8A and 8B, and only two modes will lase.
Figures 9, 9A, 9B show conditions for the optimum
adjustment of field intensity according to this invention.
In Figures 9A, 9B notice that the region of coupling of
the modes, as lndicated by the shaded regions, is minimized.
mb~ - 27 -
The "holes" of the four modes are sufficien~ly separated
so that they all will lase. Note that the competing
regions for gain atoms are substantially the same as in
Figures 6A, 6B,
Figures 10, lOA, lOB show conditions wherein the
apparatus of this invention is using an excessive field
intensity Note that the L and R "holes" in FIG lOA
c c.w cw
are too close together, they are closely coupled as
indicated by the large hatched area, and any one of those
two modes will lase. Similarly in FIG. lOB, the L w and
RCcw "holes" are too close together, and they are closely
coupled as indicated by the large hatched area, and only
one of the two modes will lase.
With the fields of coils 28 aiding as shown, the
fields not only produce Zeeman effect, but they also
produce sufficient non-reciprocal anisotropic Faraday
effect without additional Faraday cells,
With the fields of coils 28 in the two gain
sec~ions opposing, the Faraday effect is minimiæed, and
if the two gain sections are substantially identical~ and
if the field intensitles are substan~ially identical, the
Faraday effect is cancelled, and an additional non-recip-
rocal anisotropic element must appear in the loop. Note,
however, that the ~eeman effect i5 unchanged from Figures
9, 9A, 9B except that the modes are interchanged as shown
in Figures 11, llA, llB~
The minimum allowable magnetic field intensity
is above the value where the RCw and LCw mode pair and
the R and L mode pair are sufficiently coupled to
ccw ccw
extinguish one mode of each pair.
~ mb/~
~?3~313
The maximum allowable magnetic fleld lntensity ;~
is,below the value where the LCcw and RCw mode pair and
the R and L mode pair are sufficiently coup~ed to
c cw cw
extln~ulsh one mode of each pair~
" '
- . ~
~. ,
~ ;`
~ mb~ ~ - 29 -
