1487004 Controlling light; automatic control of direction CARL-ZEISS-STIFTUNG [trading as CARL-ZEISS] 20 Jan 1975 [25 Jan 1974] 2331/75 Headings G2F and G3R In a system wherein a tunable laser beam is mixed with a further laser beam, or frequency multiplied, in a non-linear crystal 3, the crystal is retroactively controlled so that the converted beam has the same optical path whatever the frequency of the tunable beam. In Fig. 1, alteration of the tunable beam leads to non-collinearity therewith of the converted beam, such displacement being sensed via a partial mirror 11 and photo-cells 12, 13 coupled to the differential inputs of an op-amp 14. The output controls a servomotor 5 for rotating the crystal until the outputs of cells 12, 13 are equal. To prevent lateral displacement of the beam due to rotation of the crystal, the crystal is immersed in a refractive index matched liquid 35, or followed by a second contra-rotated crystal, Fig. 2 (not shown). In a second embodiment, Fig. 3 (not shown), the temperature and hence the refractive indices of the crystal towards the various optical frequencies are controlled by the output of amplifier 14.
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