Optical interferometric gyro having reduced return light to the

Details Optical interferometric gyro having reduced return light to the Optical interferometric gyro having reduced return light to the

1992-04-24

1994-07-05

Turner, Samuel A.

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

385 12, G01C 1972

Patent

active

053272145

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to an optical interferometric gyro in which light from a light source is split into two light beams, the light beams are provided as clockwise and counterclockwise light beams to both ends of a looped optical transmission line, the light beams entering and leaving the looped optical transmission line are phase shifted by a modulation signal of a phase modulator at one end of the optical transmission line, the two light beams emitted from the looped optical transmission line at both ends thereof are combined to interfere with each other, the resulting interference signal is converted into an electrical signal, and the electrical signal is synchronously detected by the modulation signal of the phase modulator to thereby detect an angular velocity applied to the looped optical transmission line about its axis.


BACKGROUND ART

FIG. 1 shows a conventional optical interferometric gyro. Light emitted from a light source 11 is split by a beam splitter 12 into two, one of which is provided via a polarizer 13 to an optical coupler 14 and the other of which is terminated at a terminating element 20. The light beam led to the optical coupler 14 is split into clockwise and counterclockwise light beams. The clockwise light beam is phase modulated by a phase modulator 15 immediately after being emitted from the optical coupler 14, and is provided to one end of an optical fiber coil 16 used as a looped optical transmission line. The clockwise light beam reaches the optical coupler 14 again after propagating through the optical fiber coil 16 clockwise. On the other hand, the counterclockwise light beam is provided to the other end of the optical fiber coil 16 and, after propagating therethrough counterclockwise, it is phase modulated by the phase modulator 15, immediately thereafter reaching the optical coupler 14 again. In the optical coupler 14 the clockwise and counterclockwise light beams having propagated through the optical fiber coil 16 meet and interfere with each other. At this time, a periodic phase difference occurs between the clockwise and counterclockwise light beams because they have been subjected to periodic phase shifts by the phase modulator 15. Now, assume that the frequency f.sub.m of the modulation signal for driving the phase modulator 15 is, for example, 1/(2.tau.) (where .tau. is the time for light to propagate through the optical fiber coil 16). In this instance, when the phase shift of the clockwise light beam is sinusoidal as shown in FIG. 2A when it has just returned to the optical coupler 14 after being subjected to a phase shift .phi..sub.cw by the phase modulator and then having propagated through the optical fiber coil 16, a phase shift .phi..sub.ccw to which the counterclockwise light beam is subjected in the phase modulator 15 lags behind the modulation signal of FIG. 2A by the time .tau., and hence is 180.degree. out-of-phase with the phase shift .phi..sub.cw as depicted in FIG. 2B. Accordingly, the phase difference .phi..sub.cw -.phi..sub.ccw between the clockwise and counterclockwise light beams, which are combined by the optical coupler 14, varies with a 2.tau. period as indicated by the curve 17 in FIG. 2. In consequence, the two light beams, which are combined into the interference light, strengthen and weaken each other repeatedly with a period .tau., that is, the interference light varies its intensity with the period .tau.. The intensity of the interference light varies with the phase difference .phi..sub.cw -.phi..sub.ccw between the two light beams as indicated by the curve 18 in FIG. 2, and consequently, the intensity variation is repeated with the period .tau. as indicated by the curve 19.
In FIG. 1 the interference light from the optical coupler 14 is provided via the polarizer 13 to the beam splitter 12, wherein it is split into two beams, one of which is converted by a photodetector 21 into an electrical signal. This electrical signal becomes a signal which varies at a frequency twice higher than the phase modulation frequen

REFERENCES:
patent: 4712306 (1987-12-01), Cahill et al.
patent: 5018859 (1991-05-01), Chang et al.
patent: 5018860 (1991-05-01), Bielas et al.
"Source Statistics And The Kerr Effect In Fiber-Optic Gyroscopes" by R. A. Bergh et al, Optics Letters, vol. 7, No. 11, pp. 563-565 (Nov. 1982).
"Fiber-Optic Rotation Sensor Technology" by W. C. Goss et al, SPIE Milestone Series, vol. MS8, pp. 160-166 (1980).

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