Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Inv~ntion
Laser gyroscopes have a gas laser which ampliies electro-
magnetic waves passing around a common path of a ring defined,
for example, by reflecting mirrors. The amplification which
results from interaction of the waves with excited s~ates of
atoms can produce oscillations at one or more frequencies for
waves traveling in the clockwise direction around the laser as
well as counterclockwise around the laser.
With a two wave or frequency 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 substan~ially less than would be expected
due to the phenomena known as lock-in. It is believed that the
lock-in problem arises because of coupling between the waves
which may arise from a number of possible factors including
back scattering of laser energy from elements within the
laser path such as mirrors or a polarization dispersive struc-
ture or from scattering centers within the laser gain medium
itself.
The attempts to compensate for this problem included one
proposal in which the two beams are biased at zero ro~ation
away from the zero output level by the use of a Faraday rotator
which subjects beams propagating in different directions to
different delay times. f~awever, simply biasing the two beams
sufficiently far apart to avoid lock-in produced such a large
frequency difference between the two beams that the change in
frequency caused by ordinary amounts o~ rotation was rather
insignificant compared to the total frequency difference.
Thus, any small drift could obliterate the actual desired signal
output. Further attempts to achieve biasing included
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one in which the Faraday rotator was switched from one
direction to another -using a sy~netric ~C switching wave-
form. Such systems have proven somewhat difficult to imple-
ment since the symmetry of the AC switching waveform had
to be maintained to greater than one part in a million.
The most successful laser gyroscopes yet proposed and
constructed employ four waves of two pairs or beams each
propagating in opposite directions. Such systems are shown
and described in United States Patents Nos, 3,741J657 and
3,854,819 to Keimpe Andringa and assigned to the present
assignee, the specifications of those patents being herein
incorporated by reference. In such a laser system, circu-
lar polarization for the four waves is preferred. The pair
of waves propagating in the clockwise direction includes
both left and right-hand circularly polarized waves as does
the pair propagating in the counterclockwise direction.
Two biasing components are provided. A device such as
a crystal rotator produces a delay for circularly polarized
waves that is different in one sense or handedness of circu-
lar polarization than for the opposite sense and is also
reciprocal. That is, a wave traveling in either direction
through the crystal will be delayed by the same amount of
time. Secondly, a device such as a Faraday rotator is also
disposed in the wave path. Such a device is nonreciprocal
providing a different time delay for the two directions of
propagation. This is achieved by rotating the circular
polarization vector by diferent angles. The delay is
independent of the sense of polarization. The result of
these biasing operations produces four waves, two ~,Yith
frequencies above the peak of the gain curve of the laser
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medium and two below. The two above may for example both be
right-hand circularly polarized while the lower two are left-
hand circularly polarized At a zero rate of rotation, the
frequency difference between the left-hand circularly polarized
and the right-hand circularly polarized waves are equal '~hen,
for example, the system is rotated in one direction the righ~-
hand circularly polarized waves will move closer togethe-r in
frequency while the left-hand circularly polarized waves will
move apart. The opposite direction of rotation produces the
OppO5ite direction of change in frequencies. The actual rota-
tion rate is readily related to the difference between the
difference in right-hand circularly and left-hand circularly
polarized wave pairs.
In the laser gyroscope systems disclosed in the referenced
patents, a structure for adjusting the length of the path through
which the four waves propagate to maintain the frequency pairs
positioned symmetrically about the center maximum gain frequency
of the laser gain medium curve is described. Such symmetric
positioning is desired in order to minimize residual drift or
lock-in effects.
The gain of the waves passing through the lasing medium is
normally a fraction of a percent and must be sufficient to over-
come losses in the medium of the ring cavity such as reflection
losses at the mirrors and at windows of the gas laser. The gain
of the laser can be increased by increasing the discharge current.
However, discharge oscillations in the range from a few hertz per
second, dependent on power supply constants, to many megahertz
are encountered. The megahertz disaharge oscillations cannot be
prevented by power supply desi~n since they are predominantly a
function of the discharge path geometry and the internal negative
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resistance of the laser tube gas discharge. Such oscillations
cause variations in laser amplification so that the laser gyro-
scope output will be unstable and erroneous. As a result, ~he
laser amplifier in laser gyros needed to be relatively large
and operated at low current to prevent gas discharge oscillations
so that overall gain would be sufficient to overcome the losses
in the ring cavity. In addition, the amount of energy which
could be extracted from the ring cavity to drive the output
circuitry was generally severely limited, due to the minimal
amount of laser amplifier gain.
Summary of the Invention
In accordance with this inven~ion a laser gyroscope is
provided having a gaseous laser amplifier excited by an elec-
trical discharge through a gaseous medium between a pair of
anodes and a cathode which are positioned outslde the optical
path of the laser arnplifier and discharge oscillations are
suppressed by a constant magnetic field in the discharge path
adjacent the cathode.
More particularly, in accordance with this invention, the
laser gyro comprises a ring cavity having a laser amplifier and
containing a plurality of reflecting mirrors. One o~ the mirrors
is moved as a function of signals derived from a detector coupled
to the laser cavity to control the pathlength of the ring reso-
nator. The laser amplifier has two adjacent regions with the
electro excitation discharges going in mutually opposite
directions from two anodes to a common cathode communicating
with the junction between the two regions through a sidearm
tubular bore structure which is also filled with the gaseous
medium. A magnetic field provided for example by a permanent
magnet adjacent to cathode region and the sidearm bore suppresses
high frequency discharge oscillations in the laser gas medium.
As a result, the laser discharge current can be increased to a
point where the discharge operates stably in the transition
region of the voltage current discharge curve of the laser
amplifier without substantial oscillations.
This invention further provides that such a laser amplifier
system may be made to operate with a very small bore laser which
essentially restricts the laser amplification to a single mode
thereby ~urther increasing accuracy.
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This invention further discl,oses that a laser gyroscope using dis-
charge oscillations suppression may be operated with pathlength stabili~ation
which is inperturbed by power supply fluctuations and/or lnternal voltage
gradients variation. In addition, such a laser gyro may use structures that
avoid frequency locking at low rotation rates by having frequency splitting
means to provide a plurality of frequencies of opposite polari~ation senses
with one pair of said frequencies of different circular polarization senses
passing in a clockwise direction about said ring laser cavity and another pair
of said frequencies of different polarization senses passing in a counter-
clockwise direction around said ring laser cavity. ey subtracting frequencies
of the same polarization senses from each other in detectors and then subtract-
ing the resultant difference frequencies from each other first order effects
of temperature variation vibration and/or laser gain shifts can be further
reduced.
In accordance with the present invention, there is provided a laser
gyroscope comprising: means for providing a reentrant optical path for the
propagation of a plurality of waves having respectively different frequencies;
an amplifying medium in said path comprising a gas; and means for stabilizing
an electric discharge through said gas comprising means for providing a uni-
directional magnetic field in a predetermined region of said discharge which
is spaced from said optical path and which has substantially no affect on the
portions of said discharge within said optical path.
In accordance with the present invention, there is further provided
in combination: a ring resonator for electromagnetic waves; an amplifier posi-
tioned in the path of said electromagnetic waves comprising a gaseous medium;
means for energizing said amplifier compr:lsing means for producing an electric
discharge through said gaseous medium along sald path between electrodes posi-
tioned outside said path; means for produclng a substantially constant magnetic
field in at least a predetermlned region of sald discharge outslde said path;
and means for substantially shieldlng all portions of said discharge in said
optical path from said magnetic field.
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In accordance with the present :Lnvention, there ls further provided
a laser gyroscope comprising: a ring resonator having a reentrant path for
electromagnetic waves defined by a plurality of reflectors; means for amplify-
ing said waves in said path comprising a gaseous miY~ture having an electrical
discharge produced therethrough; means for producing a substantially constant
magnetic field in a predetermined region of said discharge outside said path;
said magnetic field having substantially no affect on said discharge in said
reentrant path; and means coupled to said ring resonator for extracting por-
tions of said wave at each of the frequencies resonant thereinO
In accordance with the present invention, there is further provided
in combination: a ring resonator having an optical path for electromagnetic
waves defined by a plurality of reflectors; an amplifier comprising a gaseous
medium positioned in said path; means for producing an electric discharge
through said medium along said path between electrodes positioned outside said
path; means for producing a substantially constant magnetic field in a prede-
termined region of said discharge outside said path; and means for substantial-
ly preventing said magnetic field from affecting said discharge within said
optical path comprising means for substantially shielding said optical path
from said magnetic field.
In accordance with the present invention, there is further provided
a laser gyroscope comprising: a ring resonator having a closed path of elec-
tromagnetic waves; a gaseous amplifying medium positioned in said path; means
for producing an electrical discharge in said path through said medium; means
for producing a magnetic field which is applied to said discharge substantial-
ly entirely outside said path to stabili~e said discharge means for shielding
said path from said magnetic field; and means coupled to said ring resonator
for extracting portions of each of the frequencies produced therein and for
determining the rate of rotation of said resonator.
In accordance with the present invention, there is further provided
a laser gyroscope having a reentrant optical path for the regenerative propa-
gation of a plurality of electromagnetic waves having respectively different
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frequencies through a gaseous wave amplifying medium in said path; means for
energizing said medium in said optical path comprising a cathode and a plural-
ity of anodes positioned outside said path; means eor stabilizing an electric
discharge between said cathode and anodes comprising means for providing a
substantially constant magne~ic field in a region of said discharge with said
magnetic field being substantially outside said optical path; ànd means for
substantially shielding said optical path from said magnetic field,
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Brief Description of the Drawings
FIGURE 1 is a block diagram of a laser gyroscope syste~
embodying the present invention;
FIGURE 2 is a graph of the voltage current relationship
of a laser ampli~ier shown in FIGURE 1;
FIGURE 3 shows a laser medium gain curve with the positions
of the frequencies of the four waves indicated thereon.
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ReferTing now to FIG. l, which is a block diagram of a
laser gyroscope system, there is shown a reentrant optical
cavity lO formed by a plurality of reflectors 12, 14, 16, and
18 which direct waves along a reentrant path 20 through laser
30. One of the mirrors 16 permits the transmission of a small
percentage, such as one-half percent of the waves incident
thereon, through the mirror to be received ~y a dual function
detector 22. Signals from the waves are detected through
photo diodes in detector 2Z~ One output is used for supplying
a signal processor 24 whose output is a frequency indicative
of the rate of rotation of optical cavity lO.
Another output of dual function detector 22 drives a
piezoelectric crystal ~6 supporting mirror 18 to adjust the
overall pathlength so that four frequencies Fl, F2, F3, and
F4 sho~n in FIG. 3 are positioned respectively on opposite
sides of the center frequency 28 of the gain cur~e of a laser
30. Frequencies Fl and F4 are wa~es which travel clockwise
around cavity lO while frequencies F2 and F3 are waves which
travel counterciockwise around cavity lO. These frequencies
are produced due to a Faraday rotator 32 positioned in path 20
which produces a different delay in the waves traveling in the
clockwise direction from those traveling in the cou~erclock-
wise direction and to a crystal rotator 34 which introduces
delays for circularly polarized waves which are different for
left-hand circular polarization than ~or right-hand circular
polarization. The principles o~ such a system for producing
four frequencies and for deriving outputs the~eof in a detector
system are well known and are described,for example,in greater
detail in Patent No. 3,741,657 issued June 26, 1913 to
Keimpe Andringa.
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In general~ by the use of means in detector 22 which
convert circularly polarized waves to linear polarization of
different orthogonal senses dependent on the senseof polari-
zation, portions of frequencies Fl and F2 are detected by one
photo diode and portions of F3 and F4 are detected by another
photo diode with the outputs being the ~ifferences between
F2 - Fl and F4 - F3 respectively. The diference in these
difference frequencies is counted in signal processor 24 to
produce an output indicative of the rotation of cavity 10.
In such a system, because the center frequency 28 is at
li~ht frequencies, any variation in the shape or position of
the gain curve 32 will cause va~iations in the output from
signal process~r 24. Since such gain variations may include
variations in the center frequency 28 due~for example,to
variations in the gas velocity in the central bore 34 of laser
30, errors in the output signal -from signal processor 24 can
occur. To reduce such errors laser amplifier 30 is excited
by a discharge between a cathode 36 and ~wo anodes 38 and 40
positioned on opposite sides of cathode 36 so that a discharge
occurs simultaneously between the cathode 36 trzveling along
the bore 34 in opposite directions through the gaseous laser
gain medium to the anodes 38 and 40. Such a laser dischar~e
permits light waves traveling along path 20 thr.ough windows
42 and 44 crossing the ends of bore 34 to be amplified suf-
ficiently to overc~me the losses in the waves traveling around
path 20 so that only those waves which travel around the path
come back in phase with themselves, build up, and appear as
:~ resonant frequencies at detector ~2. While frequencles both
: lower than Fl and higher than F4 would be in phase when they
returned they are below the unity gain level, where cavity
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losses equal laser gain as shown, ~or example,at 46 so that
these frequencies do not build up in the resonator 10.
By providing a regulated power supply 48 which maintains
the current substantially constant between the cathodes 36
and the anodes 3~ and 40, low frequency current fluctua~ions,
which are normally encoun~ered in a gas tube discharge, such
as the helium-neon laser 30,are avoided since the time constant
of such oscillations ls dependent on the external circuit con-
stants of the system and the gaseous discharge appears as a
negative resistance; sufficient positive resistance can be
introduced to damp such oscillations. However, attempts to
increase laser gain by increasing the discharge current
through the laser, high frequency oscillations occur which
external circuit parameters will not control. While the ampli-
tude of such oscillations may not affect normal gas tube dis-
charge uses, it has been found that such discharges can affect
the accuracy of the gyros relying on very small frequency
shifts to measure rotational rates of the gyro system.
In accordance with this invention there is disclosed the
discovery that such high frequency oscillations, for example
many megahertz, may be controlled and substantially suppressed
by positioning a magnet 50 adjacent the cathode 36. As illus-
trated ~erein, magnet 50 is a bar magnet supported on a mag-
netic shield 52 positioned between magnet 50 and the bore 34
of the laser 30.
I~hile the precise mechanism or suppression of such oscil-
: lations is not certain, it is believed that the effect of the
magnetic field is to ]engthen the mean-ree path for electrons
in the discharge adjacent the cathode thereby maXing the inter-
nal characteristics of the discharge appear as a less negative,
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or even positive, resistance in this region. It has been
found that the orientation of the magnet can assume a large
number of positions in the region of the sidearm bore or
neck of the glass envelope 54 of cathode 36. As shown herein
envelope 54 is glass and contains a cathode electrode 56
hollowed,or example,in a cup shape to reduce the density of
the current at the cathode surface thersby reducing cathode
emission noise, Envelope 54 has a relatively small diameter
or neck where it connects with a ceramic block 58 containing
the bore 34 and it is in this reduced region that the mag-
ne~ic fîeld of magne~ 30 has been found to be most effective
in suppressing high frequency oscillations which transfer
with laser gyro accuracy. In general, the magnetic field
created by the bar magnet 50 should vary in density and
direction throughout a region of the reduced cross sec~ion
of envelope 54 through which the discharge from electrode 56
flows into the bore 34. Thus, while in some regions a par-
ticular magnetic field intensity and/or orientation may be
ineffective to suppress discharge oscillation other regions
of the magnetic field having a different intensity and/or
orientation interacting with other discharge regions are
effective to suppress such oscillations. Under these conditions,
it has been found that the regulated supply 48 may be adjusted
over a wide range of currents while still maintaining good gain
characteristics on the laser 30 or alternatively as the laser
30 ages and the amount of gas in the laser changes stabil opera-
tion of the system may be obtained,
Referring now to FIG, 2 there is sho~n the discharge
voltage-current curve 60 of a gas device of the general shape
of the voltage-current discharge encountered in laser 30. The
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precise shape of the discharge curve 60 of Figure 2 will change
dependent on the si~e and spacing of the structural elements
of the laser 30 as well as the gaseous mixture and pressure
and is intended only for the purposes of explanation of the
invention.
The operating point 62 of ~he laser 30 may be, for
example, 700 volts and 2 1/2 milliamperes. The laser 30 will
have more gain as higher currents are used. ~lowever, as cur-
rent is increased the negative slope of curve 60 may increase
thereby increasing the discharge oscillation potential. If the
current is increased to a point where the curve 60 is in the
region labeled, "normal glow", the laser gain is reduced. Thus,
to obtain optimum operating conditions for the laser with the
cathode 36 outside the amplifying bore 34 it is desirable to
provide a stabilizing magnetic field in the cathode region.
The principles of this invention have been found to
suppress oscillations in a laser gyro amplifier using a stand-
ard helium-neon mixture in a range of pressures around 3 TORR.
Preferably local magnetic fields intensities in the cathode
discharge region having some values at least in portions of
the range from 10 Gauss to 1,000 Gauss are produced by
magnet 50. With a laser bore 34 having a diameter of 1 milli-
meter and a length of about 10 centimeters between the anode
electrodes 38 and 40.
This completes the description of the embodiment of
the invention illustrated herein. However, many modifications
thereof will be apparent to persons skilled in the art without
departing ~rom the spirit and scope of the invention. ~or
example, various typcs of laser gain structures can be used;
the system can be used with devices other than the ~araday
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rota~or 32 and crystal rotator 34 for producing the multiple
frequencies and other output structures can be used. Accor-
dingly, it is intended that this invention be not limited to
the details of the particular embodiment disclcsed herein
except as defined by the appended claims.
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