JPS5832793B2 - ring laser gyroscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a four-mode ring laser gyroscope.

It is well known that two counter-rotating laser beams, i.e. beam forces propagating in opposite directions, can be shaped in a laser beam.

This type of device is described in detail, for example, in "Laser Gyros," edited by Monte Ross, "Laser Applications," Academic Press, Inc., New York, 1971, pages 134-200.

When the ring laser is rotated about an axis perpendicular to its plane, the frequency of oscillation is shifted, the frequency of the beam propagating in the direction of rotation decreases, and the frequency of the beam propagating in the opposite direction of rotation decreases. frequency increases.

The magnitude and direction of rotation is therefore determined by detecting the beat m wavenumber between counterpropagating beams.

By using a set of three laser gyroscopes, the phenomenon is used to determine rotational data in an inertial navigation system, thereby providing directional information to aircraft, etc.

Four-mode laser gyroscopes are well known to those skilled in the art.

This gyroscope uses an optical crystal and a Faraday effect device (
It uses a Faraday cell (also called a Faraday cell).

However, the biasing and detection methods proposed to date are extremely complex and have high noise levels.

Therefore, the primary objective of the present invention is to simplify and reduce noise in a four-mode multiple oscillator ring laser gyroscope.

According to the ring laser gyroscope of the present invention,
It operates with four different circularly polarized beams of the same wavenumber, two of which are oppositely circularly polarized and propagate in one direction, while the other two oppositely circularly polarized beams are propagated in the opposite direction. A portion of the optical energy in each beam is converted into electrical energy, and the frequencies present in the electrical output signal thus generated determine the speed and direction of rotation of the ring laser. The optical energy of all four frequencies is applied simultaneously to a single photodiode with non-linear characteristics and is thereby used to determine the speed and direction of rotation of the ring laser. A combined electrical signal is generated that includes the sum and difference frequencies of all four frequencies.

One embodiment of the invention is characterized by receiving the signal generated by the photodiode and transmitting only a difference frequency signal component that is applied to a circuit that determines the rate and direction of rotation of the ring laser gyroscope. It has a low pass filter.

A circuit is provided for introducing a dithered periodically varying phase reference signal into the laser plasma current, said phase reference signal also being combined with the rectified low frequency component of the photodiode output signal, determines the direction of laser rotation.

Additionally, one of the two low frequency difference signals from the photodiode is 10 by the other of the two low frequency difference signals.
A circuit is used to maintain the laser cavity length at a length such that 0φ modulation occurs.

An important feature of embodiments of the invention is the inclusion of a simple apparatus for detecting four optically identical wavenumbers passing through one of the mirrors of a multi-oscillator ring laser gyroscope.

The four optically identical wavenumbers, or beams, passed through the mirrors are directly heterodyned, and the resulting signals are applied to three circuits: a laser cavity length control circuit, a rotation speed detection circuit, and a rotation method determination circuit.

As described below, by using a power source having an AC component that oscillates (or dithers) the laser plasma current and using the AC component from the plasma power source as a phase reference, a ring, laser gyroscope, etc. The direction of rotation of can be determined.

The transducer controlling the cavity length is oscillated with appropriate synchronization to vary the gyroscope's 17-the mode frequency and is fixed at the transducer control voltage to
By applying beats n wavenumbers of equal amplitude to two two-mode laser gyros forming a mode ring laser gyroscope, the heterodyne output of the four signals is
Generates a 00% modulation signal.

The invention will be better understood by describing an embodiment thereof with reference to the drawings.

Referring to FIG. 1, a four-mode ring laser gyroscope 12 is provided with four mirrors at its four corners.

Two of them 14 and 16 are just mirrors, mirror 18
is fixed to a piezoelectric transducer 20 to control the cavity length of the ring laser gyroscope.

The fourth mirror 22 reflects only a portion of the light, so the four beams associated with the four modes can pass 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 to those skilled in the art, and includes four different equal wave numbers as shown in FIG. 4 modes and related 4
Generates two beams.

The crystal 21 may be a quartz crystal. As is well known to those skilled in the art, there are two modes, or different types of polarization, of the beam incident on the partially reflecting mirror 22.

These are the so-called left-polarized beam and right-polarized beam.

Furthermore, only the "P" polarized component of the four beams easily passes through the mirror, whereas the S" polarized component of the beam does not.

Here, the "P" polarization component has an electric vector parallel to the plane of the laser beam path, and the "S" polarization component is perpendicular to the plane of the laser beam path.

ゝS“ waves do not pass through the mirror at all, but ゝP ′′
The ratio of pass valves to check valves for the 18/' and 18/' sections is typically about 100 to 1.

In conjunction with frequencies P1 and F4 shown in FIG.
The two "P" components of the aforementioned laser mode incident on the mirror 24 from the 10,000 propagation directions of the laser beam path pass through the mirror 22 and are associated with frequencies F2 and F3 in FIG. The two % p // components propagating in the opposite direction of the laser mode incident on the mirror 22 also pass through the mirror 22 .

These two silk beams are the beam combination mirror 2 in this example.
8, all four beams are simultaneously incident on the photodiode 30.

As shown in Figure 2, the two oppositely propagating beams with frequencies F1 and F2 can be considered to form a first gyroscope, named GYROI, which has almost the same wavenumber in the absence of rotation. Two other oppositely propagating beams with wavenumbers F3 and F4 separated by a distance form a second gyroscope designated GYRO2.

To give an example of the extent of the same wavenumber region we are considering here, a normal helium neon laser gyroscope that exhibits a normal bright red color is approximately 5 x 10"
It has the same wave number of 4 hertz.

The crystal type and Faraday effect device used in the four-mode ring laser gyroscope shown in Figure 1
In other words, depending on the type of Faraday cell, the same wave number interval between GYROI and GYRO2 is approximately 10 to 5, as shown in Figure 2.
00 MHz, or on the order of 1 million cycles/second.

Also, depending on the Faraday effect device used,
The difference in frequency between the two oppositely rotating or counterpropagating beams forming each gyroscope, designated GYROI and GYRO2, is approximately 10 to 500 kilohertz.

The photodiode 30 is a square side detector and operates as a nonlinear mixing element, that is, an intermodulation element.

The photodiode used here may be of any type.

A photodiode typically generates an output proportional to the power of the irradiated light greater than 1, that is, an output that is nonlinear with respect to human illuminance.

The input of this square side detector has the same wave number F as shown in Figure 2.
1. F2. F3 and F4 are added.

The output of the photodiode 30 has a frequency F1. F2. A number of sum and difference frequencies are generated resulting from forming the beats of F3 and F4.

The low level signal at the output of photodiode 30 is amplified by broadband amplifier 32 and applied to low pass filter 34 .

The low pass filter 34 is Fl. The beat n wave number between and F2 (
This is called F1□), the beat n wave number between F3 and F4 (this is called F34), and the difference frequency between F12 and F34 (this is called Fl2 F34) are passed.

Therefore, filter 34 will have a cutoff frequency of approximately 2 MHz.

1 if the ring laser is not rotating, the frequency F12 is equal to the frequency F34, so F1□ and F3
No beat frequency occurs between 4 and 4.

FIG. 3 diagrammatically shows the interrelationship between the various beat frequencies.

In FIG. 3, the horizontal axis represents the rigid rotation of the central axis of the ring laser.

The vertical axis of FIG. 3 represents the same wave number from about 10 kHz to 500 kHz.

The GYROI represented by the same wave number F12 has a linear response characteristic, and is represented by a straight line 36 extending from the upper left to the lower right in FIG.

Similarly, GYRO2 is represented in FIG. 3 by a straight line 38 extending from the upper right to the lower left.

When the rotation is O, the two gyroscopes operate at the same frequency and their characteristic curves therefore follow the vertical center line 4 in Figure 3.
It intersects at 0, which means the rotation is O.

As the ring laser is rotated 10,000 or in other directions, the operating frequency of one of the two gyroscopes increases and the operating frequency of the gyroscope decreases.

The beat frequency F1□F34 is represented by the distance between straight lines 36 and 38 along a straight line parallel to the vertical center line.

For example, when rotated in the positive direction as shown by dotted line 42,
The intermodulation beat frequency between frequencies F1□ and F34 (ie, F12 F34 ) is represented by the length of arrow 44 .

Further reference will be made to FIG. 3 in discussing other features of the system.

Referring again to FIG. 1, the rotational speed of the ring laser gyroscope is determined by circuits 52 and 54.

The circuit 52 detects the beat frequency F1□F34, and the beat counter 54 counts the number of beats generated by the difference between the two difference frequencies F1□ and F34.

The number of beats detected by beat counter 54 represents the rotation, and the number of beats per second represents the rotation rate of the ring laser gyroscope.

By determining the output from the beat counter 54, the rotational speed of the ring laser can be determined.

t, = b - t, . Even if it is found that the rotational speed corresponds to the dotted line 62 or 64 shown on the left and right side of the straight line of rotation O in FIG. It is not known whether the beat indicated by arrow 66 represents rotation in one direction or the beat indicated by arrow 68, which corresponds to rotation in the opposite direction.

Referring to FIG. 1, this ambiguity is resolved by a circuit consisting of a rectifier 72, a frequency demodulator and low pass filter 74, a capacitor 76, a synchronous demodulator 78, and a plasma power supply 80.

A power supply 80 for exciting the gas plasma in the ring laser.
includes both a DC power source and a differential AC dither voltage source superimposed on the DC power source to excite the plasma.

This dither voltage, which differentially increases or decreases the plasma current in two opposing gain sections of the laser, has the same effect as the rotation of a laser gyroscope.

A voltage between the cathode and anode causes gas flow or movement.

The dither voltage causes the flow to oscillate back and forth.

The movement of the gas generates a signal as if the laser were mechanically rotated.

Mechanical rotation or gas flow in the ring laser creates a beat signal between gases propagating in opposite directions within the ring laser.

A phase reference voltage synchronized with the AC dither of the plasma is connected to the conductor 8.
2 to the synchronous demodulator 78.

Arrows 66 and 68 as the plasma current changes slightly
are both shifted in one direction, for example to the right, thereby decreasing the beat frequency represented by arrow 66 and increasing the beat frequency represented by arrow 68.

By using a phase reference signal on conductor 82 that is synchronized with the dither of the plasma current, the direction of rotation of the ring laser is determined by determining whether the detected signal 84 or 86 is in phase or out of phase with the phase reference signal. It is possible to do so.

Beat frequency 68 in phase with plasma current dither increase/decrease
When the beat frequency 66 increases or decreases, the increase or decrease in the beat frequency 66 is in reverse phase with the increase or decrease in the dither of the plasma current.

An increase or decrease in the beat frequencies 66 and 68 means an increase or decrease in frequency, that is, frequency modulation.

The beat frequency F1□-F34 signal output from the filter 34 is frequency-modulated in phase with or in phase with the dither signal depending on the rotation direction of the ring laser, so the beat frequency F1□-F34 after rectification by the rectifier 72 From the frequency demodulator and low frequency filter 74, the beat frequency F12 F3
A modulated signal 84 of 4 is obtained.

An AC signal component 86 is obtained by the capacitor 76 and the synchronous demodulator 7
8, the phase of the dither signal and the phase reference signal 82 synchronized with it is checked.

If signal 86 is in phase with signal 82, the beat frequency is 6
8, the ring laser was rotating counterclockwise.

This determination is made by synchronous demodulator 78 and is indicated by an indicator called rotation sign indicator or direction indicator 88.

For the ring laser to work properly, the GYROI
It is important that the amplitudes of the signals forming GYRO2 and GYRO2 are substantially equal.

When this condition is extended, the beat between frequencies F□2 and F34 becomes as shown in FIG. 4, and becomes O periodically as shown at point 89 in FIG.

In this way, FIG. 4 shows the two difference frequencies F12 and F
34 intermodulation and takes the form of amplitude modulation, which has been used for a long time.

This maximum value occurs when the sum of the instantaneous components is in phase, and the minimum value occurs when they are 180° out of phase.

If one of the signals F12 or F34 is significantly larger than the other, the beat signal will not go to O.

Therefore, by finding the minimum point of the beat signal, then determining whether it is actually 0, and adjusting the length of the cavity so that the two signals are equal, proper laser gyroscope operation can be achieved. Ru.

In the circuit described herein, a power supply 92 provides a DC voltage that controls the cavity length using a piezoelectric transducer 20, as shown in FIG.

Oscillator 94 provides a superimposed alternating current to vary the cavity length in a manner somewhat similar to dithering of a plasma by power supply 80.

Other circuits included in the cavity control servo loop are a voltage level detector 96 and a synchronous demodulator and analyzer circuit 98.

If the zero voltage point (see FIG. 4) is not obtained, a feedback signal is sent from circuit 98 via conductor 100 to power supply 9.
Added to 2.

These controls apply the proper voltages to the piezoelectric transducer 20 to hold it in the proper position.

Finally, it should be understood that the invention may be implemented with other well-known equivalent lasers or electronic components that perform the functions described herein.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a multi-oscillator ring laser gyroscope in accordance with one embodiment of the present invention; FIG. 2 is a diagram illustrating the four frequencies associated with the four modes detected in accordance with the system of the present invention; Figure 3 shows the ring in Figure 1.
A curve illustrating the beat frequency versus rotation characteristic of a laser gyroscope, FIG. 4, is a diagram illustrating the intermodulated beat signal produced by the detector of FIG. 1 when the resonant laser cavity has an optimal length. [Explanation of symbols of main parts] Claims All four frequency symbols 7" t t F2 t F32 F4 Photodiode 0 Low-pass filter 2 Difference frequency signal component

Claims (1)

[Claims] A ring using a multioscillation ring laser having a device for generating four laser oscillation modes such that two of the four laser oscillation modes at fourteen different frequencies propagate in each direction. - A photodetector that is a laser gyroscope and receives all of the four laser oscillation modes;
and a device coupled to the photodetector for generating signals measuring rotational speed and direction of the ring laser; wherein the four lasing modes have substantially equal intensities; A ring laser gyroscope comprising: a servo device coupled to the photodetector for adjusting the cavity length of the ring laser to achieve 2. In the ring laser gyroscope according to claim 1; the photodetector receives and mixes all of the four laser oscillation modes by a single photodiode and adjusts the rotational speed. a ring laser gyroscope generating first and second signals having substantially linear frequency characteristics but opposite slopes; 3. A ring laser gyroscope according to claim 2, wherein a device coupled to the photodetector is adapted to detect the rotation of the two gyros for generating a signal representative of the magnitude of the rotational speed of the gyroscopes. A ring laser gyroscope comprising a velocity determining device coupled to receive a scope signal and a direction determining device coupled to the photodetector for generating a signal indicative of the direction of rotation of the ring laser. 4. A ring laser gyroscope according to claim 3, wherein the direction determining device changes the plasma current of the ring laser and changes the phase and change of the beat signal obtained from the four lasing modes. The ring laser gyroscope generates a signal indicative of the direction of rotation of the ring laser gyroscope by comparing the phase of the plasma current with the phase of the generated plasma current. 5. In the ring laser gyroscope according to claim 1; the servo device is configured such that the beat signal of one of the four laser oscillation modes is 100% controlled by the beat signal of another of the four laser oscillation modes. A ring laser gyroscope in which the cavity length of the ring laser is adjusted to a length that provides modulation.