Patent GB-1141512-A (GB)
Title: Improvements in or relating to optical devices employing nonlinear birefringent crystals
IPC: G02F1/39
Filed: 1966-05-19
Abstract: 1,141,512. Light modulators. WESTERN ELECTRIC CO. Inc. 19 May, 1966 [27 May, 1965; 23 Dec., 1965], No. 22241/66. Heading H4F. [Also in Division G2] The invention relates to a parametric device as described in Specification 1,070,570 in which a voltage is applied across a non-linear birefringent crystal e.g. LiNbO 3  (lithium metaniobate) or KDP, to produce a control over the output frequency of a coherent light beam through the linear electro-optic effect, this voltage being in addition to a primary frequency control by means of temperature control. In a first embodiment, Fig. 1, a laser 11 directs a coherent light beam, preferably plane polarized, into a LiNbO 3  crystal 12 perpendicularly to its optic axis. Crystal 12 is maintained at a fixed temperature which affords optimum phase matching for second harmonic generation such that the output beam contains not only the fundamental frequency  wo  from laser 11 but also second harmonic components at frequency W p , where W p  is twice W o . Frequency W o  is filtered out and the beam at W p  is directed into LiNbO 3  crystal 16 perpendicularly to its optic axis. Crystal 16 has dielectric coatings 17 and 18 or mirrors are spaced from the crystal to produce an optical cavity resonator. The non-linearity and birefringence of the crystal permits parametric interaction and thus an extraordinary ray pump wave w p  can be phase-matched to ordinary ray signal w s  and idler w i  waves within the crystal. The phase-matching determines the frequencies w s  and w i  which may be varied over a wide range by means of variations in the crystal temperature. Crystal 16 is thus maintained at a fixed temperature by suitable means 19 to produce the desired signal w s  and idler w i  output frequencies. Pump frequency W p  and where desired idler frequency w i  may be eliminated by one or more filters 21. To stabilize the output frequencies during temperature variations a portion of the output beam is directed (partially reflecting mirror or prism 22) into a Fabry-Perot interferometer 23 the output of which is focused on to a plate 26 and through apertures 27 and 28 to a pair of photomultipliers 29 and 31. A change in frequency produces a shift in the interferometer rings and thus a difference in the outputs of the photomultipliers 29 and 31 which are fed to a difference amplifier 32. Amplifier 32 compares the outputs, the apparatus being adjusted so that they are usually equal, and produces a voltage output proportional to the difference in outputs of the photomultipliers which is applied across crystal 16 by means of contacts 33 and 34. This voltage causes a change in the matching conditions and hence a shift in the output frequencies in a direction to correct the original frequency drift i.e. automatic frequency stabilization of the output of crystal 16 is achieved. Lyot filters may be used to detect frequency changes. In a second embodiment, Fig. 2 (not shown), frequency modulation of the output of the crystal is obtained by applying a modulating signal across the crystal which produces a change #w in both the signal and idler frequencies. A frequency pulsed output may be produced, Fig. 3 (not shown). Large frequency changes may be achieved by means of controlled temperature changes. In a third embodiment, Fig. 4 (not shown), amplitude modulation of the light beam may be produced by applying pump and signal waves into the crystal, their phases being matched for optimum parametric amplification of the signal, and by applying an electric field across the crystal to vary the phase match and hence the degree of amplification of the signal. In a fourth embodiment, Fig. 7 (not shown), the device is arranged to operate close to a desired frequency by applying a net voltage across the crystal in a direction which is transverse to the light beam and at an angle to the Y and Z crystal axes.