Patent GB-1388418-A (GB)
Title: System for and a method of detecting rotation of a structure
IPC: G01P3/36
Filed: 1972-02-28
Abstract: 1388418 Lasers RAYTHEON CO 28 Feb 1972 [3 March 1971] 9131/72 Heading H1C A ring laser uses two pairs of beams, each pair comprising a right and left hand circularly polarized ray, and all four beams having different frequencies. The beam frequencies are shown in Fig. 2 at 21, 22, 23, 24, where beams 21 and 24 form one pair and rotate clockwise around the ring while beams 22 and 23 form the other pair and rotate anticlockwise. Suppose that the whole'of the ring laser is rotated clockwise; the frequencies of beams 21 and 24 increase, and the frequencies of beams 22 and 23 decrease. Hence the difference in frequency of beam 21 and 22 decreases, while the difference between beams 23 and 24 increases. The algebraic sum of the changes in these differences, i.e. (24-23)- (22-21) is directly proportional to the rate at which the laser is rotated, while the direction in which the laser rotates is given by which of the two differences is the larger. This arrangement avoids locking of the beams at low rotation rates by using the frequency differences of the four beams, while the effects of power supply variation, vibration and thermal variations are equal on all four beams and are hence removed by the algebraic summation. Apparatus.-Fig. 1 shows a laser medium 10 in a ring resonator formed of mirrors 12, 13, 14, 15. A pair of polarization rotators 17, 18 are included in the ring and serve to introduce a delay between right and left hand circularly polarized light. The delay is equivalent to a change in the resonator path length, so that each frequency mode which the laser will support splits into two frequencies, one right hand polarized and the other left hand. This is the origin of the four beams 21, 22, 23, 24 in Fig. 2, where a pair of modes have been split. The laser gain is adjusted to eliminate other modes such as 92, Fig. 2; alternatively the effects of these other modes can be removed by electronic filters following the photodetectors 46, 48, Fig. 1. The counter-rotating beams emerge through mirror 14 and pass via quarter wave plates, reflectors 33, 34, analyzers 35 and reflectors 45, 47 to the photodetectors. The analyzers separate the left and right hand polarizations, so that the paths 42 and 43 actually represent the frequencies 22 and 23 of Fig. 2. The outputs of the photodiodes are filtered by stray capacitances to contain only the beat frequencies (34-23) and (22-21) which are then passed via counters to an inverting adder 55 (effectively a subtractor) whose output is the rotation rate. The diode signals are also rectified and filtered to control a piezoelectric mount for mirror 12 so that the four frequencies are maintained symmetrically around the centre of the laser gain curve in conventional manner. In the preferred embodiment, Fig. 3, a gas laser is used with a central anode 76 to compensate for flow of the gas along the laser tube caused by the pumping discharge. A crystal of quartz, 85, replaces the pair of rotators shown in Fig. 1. One or more of the resonator mirrors may be concave or may be incorporated into the laser windows. Extra laser tubes may be used. It is stated that the invention may be applied to semiconductor microwave devices.