Linear phase ramp type fiber optic gyro having ramp reset and reset connection signals

A linear phase ramp type fiber optic gyro comprising:

• a light source;
• a beam splitter for splitting a light beam from said light source into two light beams;
• a looped optical transmission line into which said two light beams from said beam splitter are introduced as clockwise and counterclockwise light beams;
• interference means for causing said clockwise and counterclockwise light beams emitted from said optical transmission line to interfere with each other;
• photoelectric transducer means responsive to interference light from said interference means for outputting an electric signal corresponding to the intensity of said interference light;
• first and second optical phase modulators interposed respectively between opposite ends of said optical transmission line and said beam splitter, said first and second optical phase modulators being electrically controlled to shift the phase of light;
• a biasing signal generator for generating a biasing signal in the form of a rectangular waveform having a period that is two times as long as the light propagation time .tau. through said optical transmission line and a 50% duty ratio, said biasing signal being applied to said first optical phase modulator for alternately producing phase differences of +.pi./2 rad and -.pi./2 rad at intervals of the time .tau. between said clockwise and counterclockwise light beams in said interference means;
• an AC amplifier for amplifying the AC component of the output of said photoelectric transducer means;
• synchronous detector means for synchronously detecting the output of said AC amplifier in accordance with said biasing signal;
• phase cancel signal generating means for generating, based on the output of said synchronous detector means, a sawtooth phase cancel ramp signal having a slope which cancels a Sagnac phase differences between said clockwise and counterclockwise light beams produced by an angular velocity inputted to said optical transmission line, said phase cancel ramp signal being applied to said second optical phase modulator;
• reset signal generating means for generating a reset signal which resets said phase cancel ramp signal at the time when said phase cancel ramp signal has reached a predetermined positive or negative threshold voltage, said reset signal generating means also generating a reset sign signal that represents whether said phase cancel ramp signal has reached said positive threshold voltage or said negative threshold voltage at that time;
• means for detecting the polarity of a pulse which is output from said synchronous detector means at the time when said phase cancel ramp signal is reset;
• up-down control means for outputting a count-up or count-down command depending upon the detected polarity of said pulse and said reset sign signal;
• an up-down counter responsive to said command for counting said reset signal up or down;
• a D/A converter for converting the count value of said up-down counter to an analog signal and for outputting said analog signal as a correcting value;
• and
• correcting means for correcting said positive and negative threshold voltages based on said correcting value so that the absolute values thereof two increase or decrease at the same time.