GB2029631A - Laser gyroscope - Google Patents

Abstract

A laser gyroscope has a multi- frequency ring laser resonator 10 including a gas laser 30 energized by electric discharge through a gaseous laser medium from two anodes 38 and 40 to a common cathode 36 positioned outside the lasing passage 34 and communicating therewith through a narrow bore 54 having a nonuniform but steady magnetic field applied by a magnet 50 to the gaseous discharge in the narrow bore to suppress high frequency gas discharge oscillations in the laser. This allows the lasing current and hence gain to be increased. <IMAGE>

Description

SPECIFICATION Laser gyroscope Laser gyroscopes have a gas laser which amplifies electromagnetic 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 states of atoms can produce oscillations at one or more frequencies of waves as counterclockwise around the ring.

With a two frequency system, it has been found that, for low rates of rotation corresponding a small theoretical difference frequency, the actual output difference frequency is zero, or substantially 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 polarisation dispersive structure or from scattering centres within the laser gain medium itself.

The attempts to compensate for this problem have included one proposal in which the two beams are biased at zero rotation away from the zero output level by the use of a Faraday rotator which subjects beams propagating in different directions to different delay times. However, simply biasing the two beamssuffiencientlyfarapartto avoid lock-in produces such a large frequency difference between the two beams that the change in frequency caused by ordinary amounts of rotations is rather insignificant compared to the total frequency difference. Thus, any small drift can obliterate the actual desired signal output. Further attempts to achieve biasing have included one in which the Faraday rotator is switched from one direction to another using a symmetric AC switching waveform.Such systems have proved somewhat difficult to implement since the symmetry of the AC switching waveform has to be maintained to greater than one part in a million.

The most successful laser gyroscopes yet pro posed and constructed employ four frequencies with two pairs of beams propagating in opposite directions. Such systems are shown and described in our United States Patent Specifications Nos. 3 741 657 and 3854819. In such a laser system, circular polarization for the four waves is preferrd. 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 which is different in one sense of circular polarization than forthe opposite sense but which is reciprocal. That is, a wave of given polarization travelling in either direction through the crystal will be delayed by the same amount. 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 different angles. The delay is independent of the sense of polarization. The result of these biasing operations produces four waves, two with frequencies above the peak of the gain curve of the laser 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 differences between the left-hand circularly polarized and the right-hand circularly polarized waves are equal. When (for example) the system is rotated in one direction, the right-hand circularly polarized waves will, move closer together in frequency while the left-hand circularly polarized waves will move apart. The opposite direction of rotation produces opposite changes in frequencies. The actual rotation rate is readily related to the difference between the differences in right-hand circularly and left-hand ciruclarly polarized wave pairs.

In the laser gyroscope systems disclosed in the referenced patent specifications a structure for adjusting the length of the path through which the four waves propagate so as to maintain the frequency pairs of positioned symmetrically about the centre maximum gain frequency of the laser gain medium curve is described. Such symmetric positioning is desired in orderto 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 overcome 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, dependent on power supply constants, to many megahertz are encountered. The megahertz discharge oscillations cannot be prevented by power supply design since they are predominantly a function of the discharge path geometry and the internal negative resistance of the laser tube gas discharge. Such oscillations cause variations in laser amplification so that the laser gyroscope output will be unstable and erroneous.As a result, the laser amplifier in laser gyros has had to be rather large and operated at low current to prevent gas discharge oscillations so that the overall gain will be sufficient to overcome the losses in the ring cavity. In addition, the amount of energy which can be extracted from the ring cavity to drive the output circuitry has generally been severly limited, due to the minimal amount of laser amplifier gain.

According to the present invention, there is provided a laser gyroscope having a closed optical path for the propagtion of a plurality of waves having different frequencies, an amplifying medium in the optical path comprising a lasing gas, and means providing a steady magnetic field in a predetermined region of the gas and such as to reduce the tendency for high frequency oscillations to occur in the gas discharge.

The laser gyroscope can comprise a ring cavity having a laser amplifier and containing a plurality of reflecting mirrors. In the preferred embodiment, one of the mirrors is moved as a function of signals derived from a detector coupled to the laser cavity to control the path length of the ring resonator. 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 gaseous medium.

A magnetic field provided for example by a permanent magnet adjacent to the cathode region and the sidearm bore suppresses high frequency discharge oscillations in the laser gas medium. As a result, the laser discharge operates stably in the transition region of the voltage current discharge curve of the laser amplifier without substantial oscillations.

Such a laser amplifier system can be made to operate with a very small bore laser which essentially restricts the laser amplication to a single mode, thereby further increasing accuracy.

A laser gyroscope using discharge oscillation suppression can be operated with path length stabilization which is unperturbed by power supply fluctuations or internal voltage gradient variations.

In addition, such a laser gyro can use structures that avoid frequency locking at low rotation rates by having frequency splitting means of the kind described above. By subtracting frequencies of the same polarization senses from each other in detectors and then subtracting the resultant difference frequencies from each other, first order effects of temperature variation, vibration and laser gain shifts can be further reduced.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a block diagram of a laser gyroscope system embodying the present invention; Figure 2 is a graph of the voltage current relationship of a laser amplifier shown in Figure 1; and Figure 3 shows a laser medium gain curve with the positions of the frequencies of the four waves indicated thereon.

Referring now to Figure 1, which is a block diagram of a laser gyroscope system, there is shown a reentrant optical cavity 10 formed by a plurality of refletors 12,14,16 and 18 which direct waves along a closed path 20 through a 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 by a dual function detector 22. One output is used for supplying a signal process or 24 whose output is a frequency indicative of the rate of rotation of optical cavity 10.

Another output of dual function detector 22 drives a piezoelectric crystal 26 supporting mirror 18 to adjust the overall path length so that four frequencies Fl, F2, F3 and F4 shown in Figure 3 are positioned respectively on opposite sides of the centre frequency 28 of the gain curve of a laser 30. Frequencies F1 and F4 are waves which travel clockwise around cavity 10 while frequencies F2 and F3 are waves which travel counterclockwise around cavity 10.

These frequencies are produced due to a Faraday rotator 32 positioned in path 20 which produces a different delay in the waves travelling in the clockwise direction from those travelling the in the counterclockwise direction and to a crystal rotator 34 which introduces delays for circularly polarized waves which are different for left-hand circular polarization than for right-hand circular polarization.

The principles of such a system for producing four frequencies and for deriving outputs thereof in a detector system are well known and are described, for example, in greater detail in the aforementioned U.S. Patent No. 3741 57.

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 sense of polarization, portions of frequencies F1 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 differences between F2 - F1 and F4- F3 respectively.

The difference 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 centre frequency 28 is at light frequencies, any variation in the shape or position of the gain curve 32 will cause variations in the output from signal processor 24. Since such gain variations may include variations i*z the centre frequency 28 due, for example, to vari ations 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 two anodes 38 and 40 positioned on opposite sides of cathode 36 so that a discharge occurs simultaneously between the cathode 36 travelling along the bore 34 in opposite directions through the gaseous laser gain medium to anodes 38 and 40. Such a laser discharge permits light waves travelling along path 20 through windows 42 and 44 crossing the ends of bore 34 to De amplified sufficiently to overcome the losses in the waves travelling around path 20 so that only those waves which travel around the path come back in phase with themselves, build up, and appear two resonant frequencies at detector 22.While frequencies both lower than F1 and higher that F4 would be in phase when they returned they are below the unity gain level, where cavity losses equal laser gain as shown, for 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 38 and 40, low frequency current fluctuations which are normally encountered in a gas tube discharge, such as the helium-neon laser 30, are avoided since the time constant of such oscillations is dependent on the external circuit constants of the system and the gaseous discharge appears as 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 amplitude of such oscillations may not affect normal gas tube discharge 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.

We have found that such high frequency oscillations, for example many meghertz, can be controlled and substantially suppressed by positioning a magnet 50 adjacent the cathode 36. As illustrated herein, magnet 50 is a bar magnet supported on a magnetic shield 52 positioned between magnet 50 and the bore 34 of the laser 30.

While the precise mechanism for suppression of such oscillations is not certain, it is believed that the effect of the magnetic field is to lengthen the mean-free path for electrons in the discharge adjacent the cathode thereby making the internal characteristics of the discharge appear as a less negative, 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, for example, in a cup shape to reduce the density of the current at the cathode surface thereby 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 th magnetic field of magnet 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 section of envelope 54 though which the discharge from electrode 56 flows into the bore 34.

Thus, while in some regions a particular 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 stable operation of the system may be obtained.

Referring now to Figure 2 there is shown the discharge voltage-current curve 60 of a gas discharge such as it encountered in laser 30. The precise shape of the discharge curve 60 of Figure 2 will change dependent on the size 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 the laser 30 may be, for example, 700 volts and 21/2 milliamperes. The laser 30 will have more gain as higher currents are used.

However, as current 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 labelled, "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 gryo amplifier using a standard helium-neon mixture in a range of pressures around 400 Pa. 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.

The laser bore 34 may have a diameter of 1 millimeter and a length of about 10 centimeters between the anode electrodes 30 and 40.

Various modifications may be made to the embodiment described. For example, various types of laser gain structure can be used; the system can be used with devices other than Faraday rotator 32 and crystal rotator 34 for producing the multiple frequencies and other output structures can be used.

Claims (9)

1. A laser gyroscope having a closed optical path for the propagation of a plurality of waves having different frequencies, an amplifying medium in the optical path comprising a lasing gas, and means providing a steady magnetic field in a predetermined region of the gas and such as to reduce the tendency for high frequency oscillations to occur in the gas discharge.

2. A laser gyroscope in accordance with Claim 1, wherein the magnetic field is a non-uniform field.

3. A laser gyroscope in accordance with Claim 2, wherein the strength of the field is non-uniform over the said region.

4. A laser gyroscope in accordance with Claim 2 or 3, wherein the direction of the field is non-uniform over the said region.

5. A laser gyroscope in accordance with any of Claims 1 to 4, wherein an electric discharge is produced through the gas along part of the optical path between electrodes positioned outside the path.

6. A laser gyroscope in accordance with Claim 5, wherein the said region is adjacent one the the electrodes.

7. A laser gyroscope in accordance with Claim 6, wherein the said one electrode is a cathode electrode.

8. A laser gyroscope in accordance with Claim 7, wherein the cathode electrode is positioned between two anode electrodes spaced along the optical path.

9. A laser gyroscope substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.