| 摘要 |
1,259,175. Lasers. WESTERN ELECTRIC CO. Inc. 8 May, 1969 [13 May, 1968], No. 23532/69. Heading H1C. Transverse mode locking and beam-scanning is achieved in a laser by phase modulating the laser beam with a travelling wave of change of index of refraction propagating in the laser cavity 12, 14 transversely at a frequency equal to the transverse mode separation frequency. The phase modulator 15, Fig. 2A, comprises an electro-optical KDP crystal 16 provided with a plurality of electrodes on opposite faces connected in pairs at periodic points across a transmission line 18 driven by signal source 20 whereby a travelling electric field normal to the direction of propagation 17 of the laser beam is produced across the crystal 16. The travelling electric wave produces a travelling wave of change of index of refraction which propagates in the crystal and thereby phase modulates the light beam passing therethrough. By making the frequency of source 20 equal to the transverse mode separation frequency phase locking is achieved characterized by an elongated region of energy extending between the cavity reflector mirrors 12, 14 forming a spot of coherent light on the reflectors. Since the elongated region oscillates transversely in the resonator in step with the transversely varying index of refraction the spot scans the reflectors. In an alternative arrangement a standing wave is created in the crystal 16 by employing an additional signal source 24 which also produces a field at the transverse modeseparation frequency. The modulation may be acoustic by converting the applied electric field to an acoustic wave, Fig. 2C (not shown), by a transducer coupled to one end of the crystal 16. Both the longitudinal and transverse modes may be phase locked simultaneously by designing the cavity such that the longitudinal mode separation is an integral multiple of the transverse mode separation, the energy distribution in this case being a pulse of energy which bounces back and forth between the reflectors in a zig-zag path in both a longitudinal and a transverse direction simultaneously, Fig. 1D (not shown). Two dimensional scanning may be provided by using a pair of mutually orthogonal phase modulators. |
