TECHNICAL FIELD

This invention relates to inertial instruments, and more particularly, to laser gyroscopes.

BACKGROUND ART

Commonly assigned U.S. Patent No. 4,326,803, issued April 27, 1982 describes thin film laser gyroscopes, also known as micro-optic gyroscopes (MOGs) . Therein, a Sagnac effect is said to define a linear relationship between the rate of rotation of a circuital waveguide, or loop, and the difference in frequency in oppositely directed electromagnetic wave disturbances travelling through that waveguide at resonance.

In U.S. Patent No. 4,326,803 issued April 27, 1982, entitled "Thin Film Laser Gyro", to Lawrence, there is described a thin film, passive waveguide that provides a substantially closed circular propagation path for optical signals. A laser and associated beam splitter are adapted to generate two coherent optical signals. A first directional coupler introduces the two optical signals to the waveguide in a manner establishing oppositely directed coherent optical signals in the waveguide. In one embodiment, both optical signals are frequency-controlled so that these signals resonate within the waveguide. Frequency control for the optical signals may be achieved by the use of acousto-optic modulators, such as Bragg cells, which shift the frequency of applied optical signals as a function of a frequency of a radio frequency (RF) signal applied to the modulator. With this configuration, the optical signals from a laser and a beam splitter pass along separate paths, through the frequency shifters, directional couplers, and into the waveguide. In addition, optical detectors and a second directional coupler

are adapted to detect the intensity of the oppositely-directed optical signals in the waveguide. Servo networks responsive to the detectors generate the RF signals for controlling the frequencies of the optical signals (by way of the modulators) that are injected into the waveguide in their respective directions.

In U.S. Patent No. 4,514,088, issued April 30, 1985, entitled "Single-Coupler Guided-Wave Passive Resonant-Ring Optical-Gyro Instrument" to Coccoli, there is described a single-coupler guided-wave passive resonant-ring optical gyro system that is responsive to a coherent light source and includes an optical-fiber resonant ring, as opposed to a thin film planar waveguide, and a single directional fiber coupler. The directional coupler serves as an input for exciting clockwise and counterclockwise traveling wave resonances in the resonant fiber ring and serves as an output for extracting the output signals representative of those resonant traveling waves.

In U.S. Patent No. 4,456,377 issued June 26, 1984, entitled, "Multimode Fiber Optic Rotation Sensor", to Shaw et al. , there is described a fiber optic rotation sensor that utilizes a multimode optical fiber to improve power coupling and reduce back scattering.

In U.S. Patent No. 4,480,915 issued November 6, 1984, entitled "Ring Interferometer Device And Its Application To The Detection Of Non-Reciprocal Effects", to Arditty et al., there is described a ring interferometer device that causes two fractions of a coherent radiation to travel in opposite directions in a closed loop. The device detects the interference of the two radiations after traveling through the loop, and provides a filter for selecting a particular mode among all the modes likely to be propagated in the loop and to arrive at the detection device.

In U.S. Patent No. 4,747,111 issued May 24, 1988, entitled "Quasi-Planar Monolithic Unidirectional Ring Laser", to Trutna, there is described a quasi-planar monolithic

unidirectional ring laser that includes an Nd:YAG crystal that is used for the ring. The crystal is geometrically shaped so that the desired ring configuration is imposed by internal reflections. In a crystal in which the front face serves as the output coupling port, a slanted rear face can serve as the locus of the center reflection defining the plane characterized by the out-of-plane angle.

In U.S. Patent No. 4,445,780 issued May 1, 1984, entitled "Fiber Optic Rotation Sensing Gyroscope With (3x2) Coupler", to Burns, there is described a Sagnac gyroscope having an optical coupler with three input waveguides, wherein a middle input waveguide is disposed between two outer input waveguides, and with two output waveguides forming branching ends of a central coupling waveguide structure. The waveguides are disposed in a common plane about an axis of symmetry in the common plane. The middle input waveguide is adapted to transmit a light beam into the coupler and the output waveguides are optically coupled to the ends of a fiber-optic loop. The intensities of the light beams exiting the coupler via the outer input waveguides are measured and compared to determine the rate of rotation.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved passive thin film (i.e. integrated optic) ring resonator laser gyroscope that utilizes a single directional coupler.

It is another object of the invention to provide a thin film laser gyroscope that is less complex to manufacture, thereby increasing the device yield and decreasing manufacturing costs.

The laser gyro of the present invention is an improvement upon the laser gyro described in commonly assigned U.S. Patent No. 4,326,803 issued April 27, 1982, entitled "Thin Film Laser

Gyro" to Lawrence. As was previously described, the laser

gyro described in that patent utilizes two directional couplers. One of the directional couplers introduces the two optical signals to the waveguide in a manner establishing oppositely directed coherent optical signals in the waveguide. The second directional coupler receives a portion of each oppositely directed coherent optical signal in the waveguide in order for the two separate detectors to detect the intensity of each signal.

The laser gyroscope of the present invention utilizes a single directional coupler and a bi-cell detector to perform the same functions as the two directional couplers and the two separate detectors, respectively, in U.S. Patent No. 4,326,803.

The single directional coupler technique of the present invention maximizes the optical signal-to-noise ratio at both coupler outputs, increases the Q-factor of the resonant cavity by reducing the total coupler loss, and reduces distortion introduced to the laser via resonant optical feedback. The reduction in the number of couplers also results in less stringent tolerances in fabricating the directional coupler, thereby reducing manufacturing costs.

The bi-cell detector concept of the present invention allows detection of both oppositely directed coherent optical signals in the waveguide at just one location on the substrate, on which the waveguide is disposed, thereby minimizing temperature gradients across the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention will be made more apparent in the ensuing detailed description of the invention when read in conjunction with the attached drawing, wherein:

FIG. 1 shows the basic configuration of the laser gyroscope of the prior art utilizing two directional couplers

and two separate detectors;

FIG. 2 shows an alternate embodiment of the laser gyroscope of FIG. 1;

FIG. 3 shows the basic configuration of the laser gyroscope of the present invention, which is an improvement upon the configuration of FIG. 1, utilizing one directional coupler and one bi-cell detector; and,

FIG. 4 shows an alternate configuration of the laser gyroscope of the present invention which is an improvement upon the configuration of FIG. 2.

MODES OF CARRYING OUT THE INVENTION

Fig. 1 shows the laser gyro 10 of U.S. Patent No. 4,326,803. In the configuration of Fig. 1, gyro 10 includes a thin film, dielectric waveguide 12 defining a circular propagation path for optical signals. The waveguide 12 is disposed on a planar substrate 14. A controllable frequency laser 20 is also disposed on substrate 14. The laser 20 includes two output ports leading to thin film optical waveguides 22 and 24 for transmitting coherent optical signals from the laser. Waveguide 22 is coupled by way of a frequency shifter 28 to a directional coupler 32. Similarly, waveguide 24 is coupled by way of a second frequency shifter 30 to the directional coupler 32. The frequency shifters 28 and 30 may be conventional acousto-optic modulators such as Bragg cells.

The waveguides 22 and 24 and frequency shifters 28 and 30 are configured with respect to coupler 32 and the waveguide 12 so that optical signals travelling from the laser 20 by way of waveguide 22 are coupled to the waveguide 12 in a first direction (clockwise as shown in Fig. 1) and optical signals travelling from the laser 20 by way of waveguide 24 are coupled to the waveguide 12 in the opposite direction (counterclockwise as shown in Fig. 1) . A second directional coupler 34 is disposed on the opposite side of the waveguide 12 from coupler 32. Thin film waveguides 38 and 40 extend

from coupler 34 to detectors 44 and 48, respectively. The coupler 34, waveguide 38 and detector 48 are configured so that detector 48 receives a portion of counterclockwise travelling optical signal in waveguide 12. Detector 48 in turn is responsive to that received signal to generate a signal on line 48a representative of the intensity of that counterclockwise optical signal in waveguide 12. Similarly, the coupler 34, waveguide 38 and detector 44 are configured so that detector 44 receives a portion of the clockwise- travelling optical signal in waveguide 12. Detector 44 is responsive to that received signal to generate a signal on line 44a representative of the intensity of the clockwise optical signal in waveguide 12.

The use of just one coupler allows the unique possibility (i.e. not possible with multi-coupler resonators) of achieving complete destructive interference on resonance at the output detector. The coupling is set to equal the round trip loss in the resonator. Under these conditions, the intensity contrast (off versus on resonance) is maximized and the signal-to-noise ratio is optimized.

This technique also has the advantage of eliminating resonant feedback from the circuital waveguide onto the laser.

It has been found that the utilization of one directional coupler, as shown in Fig. 3, further maximizes gyroscope sensitivity because the total coupling loss to the ring waveguide resonator 20 can be reduced, thereby increasing the Q-factor of the resonant cavity. A high Q-factor resonator requires weak total coupling, i.e., the more "taps" on the cavity, the weaker each tap or coupling must be. As it is very difficult to fabricate an ultra-weak coupler, the fabrication of only one coupler, at a relatively higher coupling level, is clearly preferred.

The preferred embodiment of the invention utilizes a directional coupler having a double-concave geometry that is formed by two curved waveguides, though of different radius of curvature. In contrast to a coupler formed, for example, via

a straight waveguide and a curved waveguide, the double- concave coupler achieves the desired coupling over a relatively long interaction length and as such has proved to be more tolerant of fabrication errors. It has also been found that utilization of a bi-cell detector, in place of separate detectors, minimizes temperature gradients across the substrate, because detection of both ring waveguide resonator output signals occur at one location on the substrate. Fig. 3 shows a laser gyroscope 170 and is a presently preferred embodiment of the invention. Elements corresponding to elements in the embodiment of the configuration in Fig. 1 are identified with the same reference designations. Directional coupler 100 and bi-cell detector 150 are utilized in place of couplers 32 and 34, and detectors 44 and 48, respectively, of Fig. 1. Coupler 100 performs the same function as couplers 32 and 34 of Fig. 1. Coupler 100 is optically coupled to waveguide 20 and has two inputs and two outputs. One of the inputs is coupled to the output of frequency shifter 28 while the other input is coupled to the output of frequency shifter 30. Waveguides 22 and 24, and frequency shifters 28 and 30, are configured with respect to coupler 100 and waveguide 12 so that optical signals emanating from laser 20 and travelling through Y-branch beam splitter 26 and waveguide 22 are coupled to waveguide 12 in a first direction (clockwise), as shown in Fig. 3, and optical signals travelling through waveguide 24 are coupled to waveguide 12 in the opposite direction (counterclockwise), as shown in Fig. 3.

Coupler 100 has one output coupled to a first input of bi-cell detector 150 while the other output of coupler 100 is coupled to a second input of bi-cell detector 150. Bi-cell detector 150 has two detector cells 150a and 150b which perform the same functions as detectors 48 and 44, respectively. Cell 150b of bi-cell detector 150 receives a portion of the first coherent optical signal travelling in the first

direction (clockwise) in waveguide 12, while cell 150a of bi- cell detector 150 receives a portion of the second coherent optical signal travelling in the second direction (counterclockwise) in waveguide 12. Cell 150b outputs a signal that represents the intensity of the first coherent optical signal in waveguide 12. Cell 150a outputs a signal that represents the intensity of the second coherent optical signal.

A control network 56 is responsive to the signals from bi-cell detector 150 to provide control signals on lines 56a and 56b for adjusting the frequency shift provided by shifters 28 and 30, respectively. In addition, the control network 56 provides a center frequency control signal on line 56c which adjustably controls the center frequency, f , of laser 20. The control network 56 includes a demodulator 62 and an f _Q control network 64. The demodulator 62 is responsive to the intensity signal on line 44a to provide an output signal on line 62a proportional to the amplitude of the clockwise optical signal in the waveguide 12. The f control network 64 generates the laser f_ control signal on line 56c, in response to the signal applied from line 62a. This occurs in a closed- loop manner, which maximizes the intensity of the clockwise signal in waveguide 12, thereby causing the optical signal along that path to achieve resonance. The control network 56 also includes a difference amplifier 68, a frequency tracking loop 70, and a first RF driver 72 and a second RF driver 74. The frequency tracking loop 70 generates a frequency standard control signal which is applied by way of line 70a to RF driver 72, which in turn provides a fixed RF signal at frequency f. on line 56a to the frequency shifter 28. With this configuration, the clockwise wave in waveguide 12 is thereby controlled to be f_+f .

In addition, the frequency tracking loop 70 is responsive to the output from the difference amplifier 68 (which detects the difference between the intensities of the counter propagating optical signals in waveguide 12) to provide a

servo control signal on line 70b which minimizes this difference. The servo control signal for the tracking loop is applied by way of line 70b to variable frequency RF driver 74, which in turn provides an output RF signal on line 56b to frequency shifter 30 so that the counterclockwise optical signal has a frequency equal to a nominal frequency f +f , plus or minus an additional component Δf which is necessary to shift the frequency of the counterclockwise optical signal in waveguide 12 to be resonant. The control network 56 also provides an output signal on line 70c which is applied to a frequency comparator 80. Frequency comparator 80 in turn provides an output signal on line 80a proportional to the difference in frequency between the RF signal control signals applied to the frequency shifters 28 and 30. This signal is proportional to the rotation of the waveguide 12. When the waveguide 12 is at rest in inertial space, Δf equals 0; when waveguide 12 rotates about an axis normal to the plane of the waveguide, Δf is proportional to the rate of turn, in accordance with the Sagnac effect.

In operation, the laser frequency is modulated by the f control 64. The amplitude of the frequency scan is maintained so that it scans across the resonator peak, from one inflection point to its opposite. The f _Q control 64 then examines the demodulated signal from detector 44 and determines whether the laser line and the resonator peak center are coincident. If an error signal is detected, indicating lack of coincidence, f _Q control 64 changes the laser frequency until centering is achieved. The frequency response of this loop extends from DC to a relatively high frequency in order to maintain servo lock in the presence of disturbances, such as mechanical and acoustic vibrations, and frequency jitter in the laser itself.

When the servo loop is locked on the resonator peak for the clockwise path, the difference signal is examined for rate information. The differential output rate signal (ΔV)

provided by network 68 drives a frequency tracking loop 70 that introduces a frequency shift (Δf) between the drive to the acousto-optic shifters operating on the CW and CCW signals. The direction and sign of Δf are adjusted by the frequency tracking loop until the two peaks are brought back to a complete overlap condition. The waveguide, in effect, provides two interferometers which differ in path length but which, because of the differing wavelengths, both contain an integral number of wavelengths. When the ΔV is nulled, Δf relates linearly to rate. The Δf, read out as the rate signal, is the difference frequency between the signals to the acousto-optic frequency shifters for the CW and CCW paths. A center frequency of 300 MHz for the shifters requires that the two oscillators maintain a stable frequency difference of as low as 1 Hz and, for high rate situations, as high as a few hundred KHz.

Fig. 2 shows laser gyroscope 1 which is an alternate embodiment of the laser gyroscope of Fig. 1 where elements corresponding to elements in the embodiment of Fig. 1 are identified with the same reference designations. Fig. 4 shows an alternate embodiment of the present invention which is an improvement over the laser gyroscope configuration in Fig. 2. In Fig. 4, coupler 100 and bi-cell detector 150 replace couplers 32 and 34, and detectors 44 and 48, respectively. The output from laser 20 is split by Y-branch beam splitter 26 and is coupled by way of frequency shifters 28 and 30 and coupler 32 to waveguide 12. Portions of the counter rotating optical signals in waveguide 12 are applied by way of coupler 34 and waveguides 38 and 40 to bi-cell detector 150. Bi-cell detector 150 is comprised of detector cells 150a and 150b.

The frequency of laser 20 is controlled by a clock signal and modulator 92. A first feedback network (including detector cell 150a, demodulator 94, and voltage-to-frequency convertor 104) controls the frequency (f +f ) of the counterclockwise signal in waveguide 12 by way of frequency shifter 34b. A second feed back network (including detector cell 150b,

demodulator 102, and voltage-to-frequency convertor 96) controls the frequency (f.+f.) of the clockwise signal in waveguide 12 by way of frequency shifter 34a. In that f. and f_ are derived from low jitter RF oscillators, the frequency jitter in fø+f*, and f ₀ +f ₂ ^are substantially identical. The rate information may be determined by measuring the difference between the two frequencies f. and f«. The measurement of the waveguide path length difference is accomplished by servoing f ₀ +f. for the counterclockwise optical signal in waveguide 12 to its resonant frequency in the waveguide (by means of the first electronic network which detects the intensity of that optical signal and then servos the frequency of the frequency shifter to maximize that intensity) , and similarly, servoing f_+f ₂ for the clockwise optical signal to its resonant frequency. In this way, the difference Δf between f. and f. is directly proportional to the inertial rotation rate of the waveguide about an axis normal to the plane of the waveguide. When the gyro is at rest in inertial space, Δf = 0 and both optical paths have the same resonant frequency. A component of the rotation normal to the plane of the waveguide will cause Δf to be non-zero.

In accordance with the invention, one servo loop controls one VCO of the two VCO\'s, and a second servo loop controls the laser. Based on the foregoing teaching, those having skill in the art may derive a number of modifications to the embodiments of the invention disclosed above. Thus, the invention is not to be construed to be limited only to these disclosed embodiments, but it is instead intended to be limited only as defined by the appended claims.