GB1594047A - Ring laser gyroscopes - Google Patents

Description

(54) RING LASER GYROSCOPES (71) We, LITTON SYSTEMS, INC., a corporation of the State of Delaware, U.S.A., having an office at 360 North Crescent Drive, Beverly Hills, California 90210, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to ring laser gyroscopes. More particularly the present invention relates to an arrangement for determining the rate and the direction of rotation of a ring laser operating with four beams.

It is well known that two counterrotating laser beams, i.e. beams which propagate in opposite directions, may be established in a ring laser. This class of devices is described in a text entitled "Laser Applications" edited b Monte Ross, Academic Press, Inc., New York, N.Y. 1971, especially pp. 134 to 200 (relating to "The Laser Gyro").

Four-mode (i.e. four beam) laser gyroscopes can also be constructed by employing optical crystals and Faraday effect devices, also called Faraday cells, to shift the frequency of the laser beams to establish four beams with difference frequencies.

According to the present invention there is provided an arrangement for determining the rate and the direction of rotation of a ring laser operating with four beams at four mutually distinct frequencies, two of which beams propagate in a first direction through the laser cavity and are circularly polarised in opposite senses, and the other two of which beams propagate in a second, opposite, direction and are circularly polarised in opposite senses, the arrangement comprising: servo means for regulating the length of the laser cavity so as to substantially equalize the intensity of each of the four beams; a photodetector arranged to receive light energy simultaneously from said laser at said four mutually distinct frequencies and to produce an electrical output signal; means for superimposing onto the laser plasma current a phase reference signal of periodic changes of the dithering type and processing means coupled to said photodetector for determining from said electrical output signal and from said phase reference signal the rate, and the direction of rotation, of the laser.

The photodetector is preferably a single photodiode having a characteristic such that the electrical output signal contains sum and difference frequencies of said four frequencies.

A low pass filter is preferably provided with the arranged to pass only difference frequency components of the photodetector output signal. These component signals are then preferably applied for determining the rate and the direction of rotation of the ring laser.

For determining the direction of rotation of the ring laser there is provided means for superimposing a phase reference signal of periodical changes of the ditherintyp onto the laser plasma current, and the direction of rotation of the ring laser may be determined from the phase reference signal and a rectified, low frequency component of the photodetector output signal.

According to a preferred embodiment there is provided means for regulating the length of the laser cavity so that substantially 100% modulation of one of two frequency difference signals from the photodetector, by the other component is obtainable, i.e. so that the difference between the two frequency difference signals is servoed to a minimum.

The means for regulating the laser cavity length may be a transducer which is suitable periodically swept, i.e. caused to oscillate, to vary the lasing mode frequencies of the gyroscope. When equal magnitude beat frequencies for the two two-mode lasers constituting the four-mode ring laser is detected from the heterodyned output of the four signals (indicated by 100% modulation), the transducer control voltage is fixed at that level.

It will be seen that an arrangement according to the present invention can be constructed to substantially simplify and reduce the noise levels in biasing and detection schemes associated with ring lasers. Such an arrangement is particularly applicable to inertial guidance systems to determine rotation data and therewith the resultant orientation of an airplane or the like.

In a particular practical embodiment of the present invention a simple detection arrangement for the four optical frequencies passing through one of the mirrors of the multi-oscillator ring laser gyroscope is provided. These four optical frequencies, i.e. the four beams, passing through the mirror are directly heterodyned, and the resulting signals may be applied to three circuits: a laser cavity length control circuit; a rotation rate detection circuit; and a rotation direction determination circuit, which are described below.

In such a practical embodiment and as described further below, the direction of rotation may be determined through the use of a power supply having an AC component for oscillating, also called dithering, the laser plasma current and using the AC component from the plasma power supply as a phase reference in determining the direction of rotation of the ring laser gyroscope.

For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made, by way of example, to the accompanyin drawings, in which: Figure 1 is a schematic block circuit diagram of an arrangement according to the invention, Figure 2 is a diagram illustrating the four frequencies of the four modes associated with the arrangements of Figure 1, Figure 3 is a graph showing the beat frequencies plotted against rotation characteristics associated with the arrangement of Figure 1, and Figure 4 is a diagram illustrating an intermodulated beat signal generated by part of the arrangement of Figure 1.

With reference to Figure 1, the four-mode ring laser gyroscope 12 is provided with four mirrors at its corners, including the two simple mirrors 14 and 16, and the mirror 18 which is secured to a piezoelectric transducer 20 for controlling the cavity length of the ring laser gyroscope. The fourth mirror 22 is only partially reflecting, thus permitting the transmission of four beams associated with the four modes through the mirror 22.

The four-mode ring laser gyroscope is also provided with a suitable crystal 21 and Faraday effect device 23, as is well known in the art, for causing the generation of four beams, associated with four modes, involving four distinct frequencies, as shown in FIGURE 2.

The crystal 21 may be of quartz.

As is well known in the art, there are two modes or different types of polarized beams which are incident on the partially reflective mirror 22. These are the so-called left and right circularly polarized beams. Moreover, only the "P" polarized components of the four beams pass through the output mirror readily, while the "S" polarized components of the beams do not, where the "P" polarized components have their electric vector parallel to the plane of the laser beam path and the "S" polarized components are othogonal to the plane of the laser beam path. While the "S" waves are not totally blocked, the ratio of transmission - to - rejection for "P" and "S" components is typically about 100 to 1.

Passing through the mirror 22 are the two "P" components of those laser modes which are associated with frequencies F1 and F4, see FIGURE 2, and which impinge on mirror 24 from one direction of propagation around the ring laser beam path, and the two oppositely directed "P" components of those laser modes which are associated with frequencies F2 and F3 (FIGURE 2) and which impinge on mirror 26. These two sets of beams are combined by the partially reflecting "beam splitter", in the present instance used as the beam combiner mirror 28, and all four beams simultaneously impinge on the photo diode 30.As indicated in FIGURE 2, the two oppositely propagating beams with frequen cie Fl and may be thought of as making up a first gyroscope, designated GYRO 1, and the other two oppositely propagating beams with frequencies F3 and F4 which, under non-rotation conditions are spaced apart by approximately the same frequency, make up a second gyroscope, designated GYRO 2.

To give an idea of the frequency ranges and the frequencies which may be involved, a normal helium-neon laser gyroscope which exhibits the familiar bright and light red color has a frequency of approximately 5 x 1014 hertz, i.e. cycles per second.

Depending on the type of crystal and Faraday effect device, i.e. Faraday cell, which are employed in the four-mode ring laser gyroscope of FIGURE 1, the differential frequency of separation between GYRO 1 and GYRO 2, as shown in FIGURE 2, may be of the order of approximately 10 to 500 megahertz, i.e. millions of cycles per second.

Also, it may be noted that, depending on the Faraday effect device which is employed, the difference in frequency between the two frequencies of counter-rotating, i.e. oppositely propagating, beams making up each of the gyroscopes, designated GYRO 1 and GYRO 2 may be approximately 10 to 500 kilocycles per second, i.e. kilohertz.

The photodiode 30 is a square-law detector and serves as a non-linear mixing or intermodulation element. At the input to the square-law detector are frequencies F1, F2, F3 and F4, as shown in FIGURE 2. At the output of photodiode 30 are the many sum and difference frequencies obtained by beating the various frequencies F1, F2, F3 and F4 together in the photodiode 30 which has a non-linear characteristic. The low level signals at the output of photodiode 30 are amp lified by a broad band amplifier 32 and applied to a low-pass filter 34.The low-pass filter 34 is set to pass only the beat frequency between F1 and F2; referred to as frequency Fl2; the beat frequency between frequencies F3 and F4, referred to as frequency Pea4; and the difference frequency between the difference frequencies F12 and F84, which is called herein frequency F12-F34. Therefore the filter 34 may have a cut-off frequency of about two megahertz. It may be noted that, if the ring laser is not subject to any rotation, frequency F12 will be equal to frequency F34 so that there will be no beat frequency F12 minus F24, because then F12-Fa4 equals zero.

FIGURE 3 illustrates the interrelationship between the various beat frequencies diagrammatically. In FIGURE 3, the horizontal axis represents rotation of the ring laser about its central axis. The vertical axis in FIGURE 3 represents frequency, ranging from about 10 kilohertz to 500 kilohertz.

GYRO 1 as represented by frequency Fl2 has a linear response characteristic and is represented by line 36 extending from the upper left-hand side of FIGURE 3 to the lower right-hand side. Similarly, GYRO 2 is r e- resented by line 38 which extends from te upper right-hand side to the lower left-hand side in FIGURE 3. At zero rotation, the two gyroscopes operate at the same frequency and, therefore, their characteristics intersect at the vertical center line 40 of FIGURE 3 which represents zero rotation. As the ring laser is rotated in either one direction or the other, the operating frequency of one of the two gyroscopes increase, while that of the other gyroscope decreases.The beat frequency F12 minus F34 is then developed and this is indicated by the distance between lines 36 and 38 along a line parallel to the vertical center line. For example, with rotation in the positive sense, as indicated by the dashed line 42, the intermodulation beat frequency between frequencies F12 and F34, i.e. frequency F12-F24, is represented by the length of arrow 44.

Further reference will be made to FIG URE 3 in discussing other aspects of the present system.

Referring back to FIGURE 1, the rotation and the rate of rotation of the ring laser gyroscope are determined by circuits 52 and 54.

Circuit 52 detects the beat frequency F12 minus F24, and the beat counter 54 counts the number of beats generated by the difference between the two differential frequencies Fl2 and F24. The number of beats detected by beat counter 54 indicates the rotation, and the number of beats per second indicates the rate of rotation of the ring laser gyroscope.

By determining the output from beat counter 54, the rate of rotation of the ring laser becomes known. However, assuming that the rate of rotation is known and corresponds to either the dashed line 62 or the dashed line 64, as shown in FIGURE 3 to the left- and right-hand sides of the zero rotation line, it is still not known whether the detected beat between the two differential frequencies is that indicated by the arrow 66, representing rotation in one direction, or that indicated by arrow 68, corresponding to rotation in the opposite direction. With reference to FIGURE 1, this ambiguity is resolved by circuitry including the rectifier 72, the low pass filter 74, the capacitor 76, the synchronous demodulator 78, and the plasma power supply 80.Incidentally, the power supply 80 which energizes the gas plasma within the ring laser includes both a DC power supply and also a source for a differential AC dithering voltage which is superimposed on the DC power which energizes the plasma. This dithering voltage, which increases and decreases the plasma current differentially or oppositely in the two opposite gain sections of the laser, has much the same effect as rotation of the laser gyroscope. A phase reference voltage synchronized with the AC dither of the plasma is applied to the synchronous demodulator 78 over lead 82. A slight change in the plasma current will cause both of the arrows 66 and 68 to shift in one direction, for example to the right-hand side, thus reducing the beat frequency represented by arrow 66 or increasing the beat frequency represented by arrow 68 as the plasma current is shifted.Through the use of the phase reference signal on line 82 which is synchronized with the dithering of the plasma current, it is possible to determine the direction of rotation of the ring laser by determining whether the detected signal 84 or 86 is in phase with the phase reference signal or out of phase with It. This is determined by the synchronous demodulator 78 and is indicated by the rotation sign indicator also called the direction indicator 88.

For proper operation of the ring laser, it is important that the amplitude of the signals making up GYRO 1 (see FIGURE 2) and the amplitude of the signals making up GYRO 2 be substantially equal. When these conditions prevail, the beats between the frequencies Fl2 and F34 are as indicated m Fl(i- URE 4 of the drawings, and periodically drop to zero, as indicated at point 89 in FIGURE 4. Thus, FIGURE 4 indicates the characteristic intermodulation of the two differential frequencies F12 and F34 and is the classical form of amplitude modulation with maxima occurring when the sums of the instantaneous components are in phase, and the minima occurring when these components are 180C out of phase.If one of the signals F12 or F34 is significantly greater than the other, however, the beat signal will not drop to zero. Accordinglv bv detecting the minimum points of the beat signal and then determining whether it is actually zero, and finally adjusting or servo-ing the length of the cavity to achieve the desired equality of the two signals, proper laser gyroscope action may be achieved. In the present circuit, the DC voltage which controls the cavity length through the use of the piezoelectric transducer 20, see FIGURE 1, is supplied by voltage supply 92. Oscillator 94 provides a superposed alternating current for varying the cavity length in a manner somewhat similar to the dithering of the plasma by the power supply 80.Other circuits included in the cavity control servo-loop are the voltage level detector 96 and the synchronous demodulator and analyzer circuit 98. If zero voltage points are not obtained, see FIG URE 4, a feedback signal is applied from circuit 98 to the voltage supply 92 on lead 100.

With these control arrangements, the propervoltage is applied to the piezoelectric transducer 20 to maintain it at the proper position.

In closing, it is to be understood that the present invention may be implemented by other known equivalent laser or electronic components performing the function as set forth herein.

WHAT WE CLAIM IS: 1. An arrangement for determining the rate and the direction of rotation of a ring laser operating with four beams at four mutually distinct frequencies, two of which beams propagate in a first direction through the laser cavity and are circularly polarised in opposite senses, and the other two of which beams propagate in a second, opposite, direction and are circularly polarised in opposite senses, the arrangement comprising: servo means for regulating the length of the laser cavity so as to substantially equalize the intensity of each of the four beams; a photodetector arranged to receive light energy simultaneously from said laser at said four mutually distinct frequencies and to produce an electrical output signal; means for superimposing onto the laser plasma current a phase reference signal of periodic changes of the dithering type; and processing means coupled to said photodetector for determining from said electrical output signal and from said phase reference signal the rate, and the direction of rotation, of the laser.

2. An arrangement according to claim 1 wherein the photodetector is a photodiode having a characteristic such that said electrical output signal contains sum and difference frequencies of said four frequencies, there being means, responsive to the electrical output signal, operable to produce first and second gyroscope signals, such that said first gyroscope signal increases and said second gyroscope signal decreases in substantially linear relationship to the rate of rotation of the laser.

3. An arrangement according to claim 2 wherein said gyroscope signal producing means comprises a low pass filter to pass the difference frequencies while rejecting the sum signals.

4. An arrangement according to claim 2 or claim 3 wherein said processing means is arranged to receive the two gyroscope signals and to produce a signal proportional to the frequency of the beat between the two gyroscope signals, which signal is indicative of the rate of rotation of the laser.

5. An arrangement according to any one of the preceding claims comprising means for determining the direction of rotation of the laser by comparing the phase of the laser plasma current with the phase of a low frequency component of the photodetector output signal.

6. An arrangement according to claim 2, 3 or 4 wherein said regulating means is operable to regulate the length of the cavity so that the difference between the frequencies of the two gyroscope signals can be servoed to a minimum.

7. An arrangement for determining the rate and the direction of rotation of a ring laser substantially as hereinbefore described with reference to the accompanying drawings.

8. A ring laser gyroscope comprising a ring laser operable to produce four beams at four mutually distinct frequencies, and an arrangement according to any one of the pre

Claims (1)

1. ceding claims.