Method and arrangement for the measurement of rotations

A method for the measurement of rotation rates by the Sagnac effect in a light path which encircles an area, said light path being defined along at least a portion thereof by an optical fiber which is capable of being rotated, said method employing two coherent light beams which are coupled with fixed states of polarization respectively into opposite ends of said fiber and which are recombined after having traversed the fiber in opposite directions, said rotation rate .OMEGA. being evaluated from the intensity of one of the output light fluxes which results after recombination of said two coherent light beams, and wherein said intensity varies as a function of the phase shift which results between said two coherent beams if the fiber defined along said light path is rotated, said method including the steps of:

• phase modulating said two coherent light beams by a phase modulation signal having an amplitude of .PSI. sin 2.pi.f.sub.o t, where .PSI. is the maximum phase excursion and f.sub.o is the modulation frequency;
• detecting the intensity of said output light flux to produce a detector output signal;
• passing said detector output signal through a synchronous rectifier with reference frequency f.sub.o ;
• and
• controlling a current through a solenoid which is wound around said fiber along at least a part of its length, said control being accomplished by an automatic control loop which uses as its error signal the filtered output signal of said synchronous retifier and which maintains the condition
• .DELTA.= .pi. n
• where n is an integer and 2.DELTA. denotes the phase difference which said two coherent light beams have when they emerge from said light path, said phase difference 2.DELTA. being the algebraic sum 2.phi.+2.phi..sub.F of a first phase shift 2.phi. which results from the rotation of said light path and of a second phase shift 2.phi..sub.F which results from the Faraday effect which is caused by the magnetic field of said solenoid is said part of the fiber, said method further comprising the steps of:
• a. storing the momentary value of said current in a first state of vanishing error signal;
• b. changing said current by a fixed amount in a time interval that is shorter than the response time of said control loop, said amount being chosen so that the resulting change of said phase difference 2.DELTA. corresponds approximately to .pi., and simultaneously the phase of the reference signal of said synchronous rectifier is changed by .pi.;
• c. allowing said control loop to settle in a second state of vanishing error signal, said current assuming a second value which corresponds to a change of exactly 2.pi. in said phase difference;
• d. comparing said second current value with said stored first value, the difference between said two current values representing a current which corresponds to a phase change of 2.pi., said current difference being used as a scale factor in the evaluation of said phase shift 2.phi. from said second current value;
• and
• e. repeatedly performing steps a. through d. to provide a continuous updating of said scale factor.

An apparatus for the measurement of rotations by the Sagnac effect in a light path which encircles an area and which is rotating at a rate to be measured, comprising:

• a beamsplitter including means for splitting an input light flux into two coherent light beams and respectively coupling said beams into both ends of said light path to traverse said light path in opposite directions, to thereby produce a phase difference of 2.DELTA. between said beams and which further includes means for recombining said light beams after they have traversed said light path to form first and second output light fluxes, and means for directing said first output light flux back towards the source of said input light flux;
• a photoelectric detector coupled to receive said first output flux to measure the intensity thereof;
• a phase modulator which modulates the phase of said two coherent light beams;
• a phase-sensitive rectifier coupled to receive an output signal of said detector for producing an electrical error signal whose magnitude and sign are proportional to sin (2.DELTA.);
• an automatic control loop coupled to receive said error signal from said rectifier, said loop maintaining with high accuracy said phase difference 2.DELTA. equal to an integer multiple of 2.pi.;
• and
• a combination of a monomode waveguide and a polarizer provided between said beamsplitter and a branching device which separates said first output light flux going to said detector from the path of the input light flux, said combination being traversed in opposite directions by said input light flux and by said first output light flux which is used for the detection.