CA1136743A - Laser gyro oscillation suppression - Google Patents
Laser gyro oscillation suppressionInfo
- Publication number
- CA1136743A CA1136743A CA000332843A CA332843A CA1136743A CA 1136743 A CA1136743 A CA 1136743A CA 000332843 A CA000332843 A CA 000332843A CA 332843 A CA332843 A CA 332843A CA 1136743 A CA1136743 A CA 1136743A
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
- H01S3/131—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation
- H01S3/134—Stabilisation of laser output parameters, e.g. frequency or amplitude by controlling the active medium, e.g. by controlling the processes or apparatus for excitation in gas lasers
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
- G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
- G01C19/58—Turn-sensitive devices without moving masses
- G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
- G01C19/66—Ring laser gyrometers
- G01C19/667—Ring laser gyrometers using a multioscillator ring laser
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- Engineering & Computer Science (AREA)
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- Optics & Photonics (AREA)
- Plasma & Fusion (AREA)
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- General Physics & Mathematics (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Lasers (AREA)
- Gyroscopes (AREA)
Abstract
LASER GYRO OSCILLATION SUPPRESSION
Abstract of the Disclosure A laser gyroscope having a multi-frequency ring laser resonator containing a gas laser energized by electric dis-charge through a gaseous laser medium from two anodes to a common cathode positioned outside the lasing passage and com-municating therewith through a narrow bore having a nonuniform constant magnetic field applied to the gaseous discharge in said bore to suppress high frequency gas discharge oscillations in the laser.
Abstract of the Disclosure A laser gyroscope having a multi-frequency ring laser resonator containing a gas laser energized by electric dis-charge through a gaseous laser medium from two anodes to a common cathode positioned outside the lasing passage and com-municating therewith through a narrow bore having a nonuniform constant magnetic field applied to the gaseous discharge in said bore to suppress high frequency gas discharge oscillations in the laser.
Description
~3~ 3 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 :1 .
~3~;7~;~
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 1~3~3 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 ~ ~ 3 ~
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.
~3~7~3 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.
, ~
' :13 ~3~;~43 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 - 6a -B
~13~74;~
, 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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~3~7~L3 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.
~SL3~3 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.
~8-~13~
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 ~3~
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, ~L3~
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 '11 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 ~3tj743 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.
`:
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 :1 .
~3~;7~;~
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 1~3~3 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 ~ ~ 3 ~
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.
~3~7~3 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.
, ~
' :13 ~3~;~43 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 - 6a -B
~13~74;~
, 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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~3~7~L3 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.
~SL3~3 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.
~8-~13~
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 ~3~
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, ~L3~
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 '11 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 ~3tj743 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.
`:
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A laser gyroscope comprising:
means for providing a reentrant optical path for the propagation of a plurality of waves having respectively dif-ferent 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 unidirectional magnetic field in a predetermined region of said discharge which is spaced from said optical path and which has substantially no effect on the portions of said discharge within said optical path.
means for providing a reentrant optical path for the propagation of a plurality of waves having respectively dif-ferent 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 unidirectional magnetic field in a predetermined region of said discharge which is spaced from said optical path and which has substantially no effect on the portions of said discharge within said optical path.
2. The laser gyroscope in accordance with Claim 1 where-in said magnetic field has different intensities in different portions of said discharge.
3. In combination:
a ring resonator for electromagnetic waves;
an amplifier positioned in the path of said electromagnetic waves comprising a gaseous medium;
means for energizing said amplifier comprising means for producing an electric discharge through said gaseous medium along said path between electrodes positioned outside said path;
means for producing a substantially constant magnetic field in at least a predetermined region of said discharge outside said path; and means for substantially shielding all portions of said discharge in said optical path from said magnetic field.
a ring resonator for electromagnetic waves;
an amplifier positioned in the path of said electromagnetic waves comprising a gaseous medium;
means for energizing said amplifier comprising means for producing an electric discharge through said gaseous medium along said path between electrodes positioned outside said path;
means for producing a substantially constant magnetic field in at least a predetermined region of said discharge outside said path; and means for substantially shielding all portions of said discharge in said optical path from said magnetic field.
4. The combination in accordance with Claim 3 wherein:
said magnetic field is provided in a region of said dis-charge adjacent one of said electrodes.
said magnetic field is provided in a region of said dis-charge adjacent one of said electrodes.
5. A laser gyroscope comprising:
a ring resonator having a reentrant path for electromagnetic waves defined by a plurality of reflectors;
means for amplifying said waves in said path comprising a gaseous mixture 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 portions of said wave at each of the frequencies resonant therein.
a ring resonator having a reentrant path for electromagnetic waves defined by a plurality of reflectors;
means for amplifying said waves in said path comprising a gaseous mixture 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 portions of said wave at each of the frequencies resonant therein.
6. The laser gyroscope in accordance with Claim 5 wherein said magnetic field is substantially shielded from the portion of said discharge in said path.
7, 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 predetermined region of said discharge outside said path; and means for substantially preventing said magnetic field from affecting said discharge within said optical path comprising means for substantially shielding said optical path from said magnetic field.
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 predetermined region of said discharge outside said path; and means for substantially preventing said magnetic field from affecting said discharge within said optical path comprising means for substantially shielding said optical path from said magnetic field.
8. A laser gyroscope comprising:
a ring resonator having a closed path of electromagnetic 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 substantially entirely outside said path to stabilize 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.
a ring resonator having a closed path of electromagnetic 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 substantially entirely outside said path to stabilize 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.
9. The laser gyroscope in accordance with Claim 8 wherein:
said discharge extends between electrodes which are positioned outside said path of said waves.
said discharge extends between electrodes which are positioned outside said path of said waves.
10. A laser gyroscope having a reentrant optical path for the regenerative propagation of a plurality of electromagnetic waves having respectively different frequencies through a gaseous wave amplifying medium in said path;
means for energizing said medium in said optical path com-prising a cathode and a plurality of anodes positioned outside said path;
means for stabilizing an electric discharge between said cathode and anodes comprising means for providing a substantially constant magnetic field in a region of said discharge with said magnetic field being substantially outside said optical path; and means for substantially shielding said optical path from said magnetic field.
means for energizing said medium in said optical path com-prising a cathode and a plurality of anodes positioned outside said path;
means for stabilizing an electric discharge between said cathode and anodes comprising means for providing a substantially constant magnetic field in a region of said discharge with said magnetic field being substantially outside said optical path; and means for substantially shielding said optical path from said magnetic field.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US93866978A | 1978-08-31 | 1978-08-31 | |
| US938,669 | 1978-08-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1136743A true CA1136743A (en) | 1982-11-30 |
Family
ID=25471772
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000332843A Expired CA1136743A (en) | 1978-08-31 | 1979-07-30 | Laser gyro oscillation suppression |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1136743A (en) |
| DE (1) | DE2935349A1 (en) |
| GB (1) | GB2029631B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1189600A (en) * | 1980-10-17 | 1985-06-25 | Raytheon Company | Dispersion equalized ring laser gyroscope |
| DE3103385A1 (en) * | 1981-02-02 | 1982-08-26 | Teldix Gmbh, 6900 Heidelberg | GAS LASER |
| EP0078540B1 (en) * | 1981-11-04 | 1985-05-22 | Kimmon Electric Co., Ltd. | Laser discharge tube |
| DE3237658A1 (en) | 1982-10-11 | 1984-04-12 | Teldix Gmbh, 6900 Heidelberg | Device for measuring rotary movements |
| US4656637A (en) * | 1985-02-14 | 1987-04-07 | Sundstrand Data Control, Inc. | Multiple ring laser gyro power supply |
| US4672624A (en) * | 1985-08-09 | 1987-06-09 | Honeywell Inc. | Cathode-block construction for long life lasers |
| DE3905549A1 (en) * | 1989-02-23 | 1990-08-30 | Marinescu Marlene | Gas-discharge arrangement |
| FR2645261B1 (en) * | 1989-03-30 | 1991-07-26 | Salaberry Bernard De | MIXING AND READING DEVICE FOR A LASER GYROMETER |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3651367A (en) * | 1968-06-21 | 1972-03-21 | Hitachi Ltd | Electrical discharge tube |
| US3588590A (en) * | 1969-04-07 | 1971-06-28 | Ibm | Gas discharge plasma tube having a multiturn primary winding |
| FR2081080B1 (en) * | 1970-03-03 | 1973-02-02 | Thomson Csf | |
| US3854819A (en) * | 1971-03-03 | 1974-12-17 | K Andringa | Laser gyroscope |
-
1979
- 1979-07-30 CA CA000332843A patent/CA1136743A/en not_active Expired
- 1979-08-07 GB GB7927466A patent/GB2029631B/en not_active Expired
- 1979-08-31 DE DE19792935349 patent/DE2935349A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| DE2935349C2 (en) | 1993-03-25 |
| GB2029631A (en) | 1980-03-19 |
| DE2935349A1 (en) | 1980-03-20 |
| GB2029631B (en) | 1982-08-25 |
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