Passive ring resonator angular rate sensor.

This invention relates to a passive ring resonator angular rate sensor according to the preamble of claim 1.

Passive cavity laser angular rate sensors are well known and illustrated, for example, in US-A 4,135,822. Briefly, in such sensors, an electromagnetic wave is coupled to a passive ring resonator so as to cause counter-propagating waves therein. Since rotation affects the optical cavity closed-loop path length in proportion to rotation, the resonant frequency of the waves is affected in relation to the rotation rate. If the frequency is fixed, the amplitude or intensity of the counter-propagating waves is affected in relation to the rotation rate. In the above cited US-A, the magnitude and direction of an inertial rotation is determined by adjusting the frequency of each counter-propagating electromagnetic wave to the corresponding cavity resonant frequency in each direction. The amount of adjustment may then be used as a measurement of rotation rate. There are, of course, other schemes for determining the rotation rate in such sensors.

The advancement of the art of passive laser angular rate sensors now include optical fibers, semiconductor waveguides, and the like, as the means for providing the passive cavity.

It is the object of the present invention to reduce errors due to light back scattering at the mirrors. This object is achieved according to the characterizing features of claim 1. Further advantageous embodiments of the invention may be taken from the sub-claims.

With respect to the figures of the attached drawings the invention shall be further described, where <ul id="ul0001" list-style="none"><li>Figure 1 is a schematic block diagram of a passive cavity laser angular rate sensor in accordance with the present invention.</li><li>Figure 2 shows another schematic block diagram of another embodiment of a passive cavity angular rate sensor in accordance with the present invention.</li></ul>

Shown in Figure 1 is substantially the schematic diagram of the passive ring resonator rotation sensor shown and described in a publication .entitled, Passive Ring Resonator Method for Sensitive Inertial Rotation Measurements in Geophysics and Relativity, by G.A. Sanders, M.G. Prentiss, and S. Ezekial, Optic Letters, Nov. 1981, Vol. 6, No. 11. This article is incorporated herein by reference thereto. Shown in Figure 1 is a square ring resonator 10 including two spherical maximum-reflectivity mirrors 11 and 12, and two 99%-reflectivity flat mirrors, 13 and 14, used for coupling into and out of the cavity 10. Source 20 provides an electromagnetic wave f_o such as a laser beam. Beam 21 is split into two beams 21a and 21b by way of beam splitter 30. Beam 21a is shifted in frequency by an acousto-optic modulator 32 to provide beam 40', and beam 21b is shfited in frequency by acousto-optic modulator 31 to produce beam 41'. Beams 40' and 41' are coupled into resonator 10.' Beam 40' passes through mirror 13 to produce beam 40 in a clockwise direction of cavity 10, and beam 41' also passes through mirror 13 to produce beam 41 in the counter-clockwise direction of cavity 10. Beam 40 propagating in the clockwise direction in cavity 10 has frequency fo shifted by a fixed frequency f₁ +Δf, and the counter-clockwise beam 41 has frequency fo shifted by f₂. The shift frequencies f₁ and f₂ are provided by a voltage controlled oscillator (VCO) 33 connected to acousto-optic modulator 32, and VCO 34 connected to acousto-optic modulator 31, respectively. Shift frequency Δf is produced by sinusoid drive source 35 through signal summing means 38.

The frequency shift Δf produced by modulator 33 will be described in further detail. The frequency shift A f is utilized for reducing scattering errors as will be subsequently described. However, in the following discussions, the resonant frequencies of cavity 10 will be described in terms of frequencies f₁ and f₂ substantially produced by the acousto-optic modulators 32 and 31, respectively.

Flat mirror 14 provides a portion of beam 40 to impinge on photodetector 42 and a portion of beam 41 to impinge on photodetector 43. The position of mirror 12 is mounted on a PZT crystal for cavity-length adjustment. The PZT device is capable of controlling the cavity length of resonator 10. The PZT is controlled by a feedback control circuit 50 including a phase sensitive demodulator 51, integrator 52, summing amplifier 53, and source 54 providing a modulation frequency f_m. Feedback control circuit 50 locks the clockwise resonance frequency to the laser frequency fo + f₁ via the PZT crystal.

In operation, the output of detector 42 is demodulated by demodulator 51 at frequency f_m (36 kHz) and then passed through integrator 52 followed by a voltage summing amplifer which drives the PZT coupled to mirror 12. The system adjusts the cavity so that frequency fo + f₁ is resonant frequency. This is accomplished by the intensity peak detecting control circuit 50.

A second feedback control circuit is shown including a phase sensitive demodulator 55, integrator 56, and voltage controlled oscillator 34. The second feedback control circuit adjusts frequency f₂ by means of acousto-optic modulator 31 so that fo + f₂ is locked to the center of the counter-clockwise resonance of the cavity.

In operation, demodulator 55 demodulates the output of photodtectors 43 at the modulation frequency f_m of source 54. The output is integrated by integrator 56 to drive the voltage controlled oscillator 34. The output of VCO 34 is applied to modulator 31 to adjust f₂ as required. If there is no inertial rotation (or any.other noise or bias source) the clockwise and counter-clockwise resonant frequencies of the cavity are identical and, therefore, f_l- fz. In the presence of inertial rotation, few and fccw will be separated by a frequency, f₁- fz. which is an indication of the rotation rate. Furthermore, the measurement of f₁ - f₂ is also an indication of the difference frequency fcw - fccw which is related to the rotation rate by a scale factor. In ideal circumstances, with no inertial rotation input, the frequency of beams 40 and 41 are identical. Accordingly, frequencies f₁ and f₂ are also identical.

As noted in the paper by Sanders, et al cited above, the passive cavity sensor includes a source of error due primarily, among others, to back scattering from the cavity mirrors and scattering in all the acousto-optic transducers. This results in an oscillatory non-reciprocal frequency shift at the difference frequencies f₂ - f₁. This error source, as taught by Sanders, et aI, was reduced by the addition of the acousto-optic modulator 33 which inserted a modulating frequency difference or bias signal A f. This resulted in the frequency difference varying by a modulating value of Δ f. The modulating frequency shift Δf is intended to be small. This small shift can be produced by the acousto-optic modulator 33 or a phase modulator as illustrated in the Sanders, et al paper. The modulator is driven by a periodic modulating signal such as sinusoidal drive voltage signal.

In the present invention, as illustrated in Figure 1, the acousto-optic modulator 33 is driven by the sum of a sinsusoidal drive signal 35 and a random noise generator 37 produced by summing circuit 38. The combination of a sinsusoidal signal and the random noise generator 37 produces a frequency difference bias signal between the frequency difference f₁ and f₂ which not only modulates the frequency difference between the beams 40 and 41, but also randomizes the modulation. The combination of the modulation and the randomization of that modulation substantially reduces the sources of rotation error caused by the back scatter of the mirrors and optical error terms caused by the acousto-optic modulators.

Random noise generator 37 may be provided by a wide variety of sources including a white noise generator, a diode noise generator or a pseudo-random noise generator, or the like.

It should be noted by those skilled in the art, that the passive cavity resonator of Figure 1 is of the type where the resonator is fined-tuned to one of the frequencies, namely f₁, and the frequency f_a is controlled to obtain the resonant frequency due to rotation. The frequency difference between frequecies f₂ and f₁ being that indicative of the rotation rate. The circuit of Figure 2 illustrates another form of the invention in which the passive resonator includes a fiber optic loop. This variation of the passive cavity resonator system is particularly described in Applied Optics, Vol. 23, Nov. 1, 1984, in an article entitled Effect of Rayleigh Back Scattering in an Optical Passive Ring-Resonator Gyro, by Iwatsuki, Hotate, and Higashiguchi. This article is also incorporated herein by reference thereto. Figure 2 particularly shows a pair of beams 40' and 41' injected into a fiber optic closed-loop resonator 200 including a reflection means 210 for coupling beams 40' and 41' into the loop 200. Optical means 210 also forms in part the closed-loop optical cavity by allowing beams 40 and 41 to circulate through the closed-loop of the optical fiber 200. Like the Sanders publication, Iwatsuki also includes a phase modulator 220 driven by a sinsusoidal source 222. In the present invention, like Figure 1, a random noise generator 230 is added to the circuit of Iwatsuki in order to substantially eliminate the affects of back scattering. The combination, as indicated earlier, of both the sinusoidal modulation of the frequency difference of beams 40' and 41' and the means for randomizing the modulation provides substantial reduction of the back scattering and error source in the passive cavity sensor in both the rectangular closed-loop path and the fiber optic closed-loop path.

It should be recognized by those skilled in the art, that the resonators or cavities may include optical fibers, semiconductors, or other light reflection systems and the like. While only certain particular embodiments have been described in detail herein, it will be apparent to those familiar with the art that certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

It should be specifically recognized that there exists a wide variation of control circuit schemes to obtain the intended function as those shown herein. Specifically, circuit 100 is shown responsive to the inputs to the VCO's 31 and 32, namely frequencies f₁ and f₂. A more precise beat frequency of those beams impinging on photodetectors 42 and 43. Furthermore, many of the techniques shown with reference to Figures 1 and 2 are shown in analog form, nevertheless, they may be provided by well-known digital techniques. These digital techniques including microprocessors and the like.