Patent GB-1356035-A (GB)
Title: Methods of filtering light and light apparatus
IPC: G02F1/11
Filed: 1971-08-02
Abstract: 1356035 Modulating light HEWLETTPACKARD CO 2 Aug 1971 [7 Aug 1970] 36186/71 Heading H4F [Also in Division H1] Optical filter apparatus comprises a piezoelectric birefringent crystal plate 2, e.g. of quartz or LiNbO 3 , provided with electrodes 3, 4 and a circuit 5-8 for exciting .acoustic waves therein, so that a narrow pass band of light from a plane polarized polychromate source 9 propagating collinearly with the acoustic waves is cumulatively diffracted into the orthogonal polarization plane and separated from the non-diffracted beam by an analyser 13, the frequency f 0  of the optical pass band being determined by the acoustic frequency according to a k-vector matching requirement where c is the speed of light in vacuo, V and f a   are the acoustic velocity in the crystal and the acoustic frequency, and #n is the birefringence of the crystal. The plate 2 is preferably cut for a thickness shear or extensional mode of resonance to increase the power efficiency of the device, and preferably an overtone resonant mode is employed. Several such overtone modes may be used sequentially or simultaneously to vary the colour and ratio of the components of the transmitted beam 14 and examples of quartz plates for transmitting combinations of red, blue and yellow, or of red, blue and green, are given. The thickness of the plate also determines the bandwidth of the beam 14, in an inverse sense. The acoustic frequency may be determined solely by generator 7, or alternatively the plate 2 may itself form part of an oscillatory circuit. Tuned electric filters may be used to select the required overtone, and may be paralleled in lines 5 and 6 with suitable gating circuits for selecting different overtones. In a modification of the filter apparatus, Fig. 4 (not shown), analyser 13 is a Rochon prism disposed to deflect the diffracted light and to transmit the notch filtered main beam. In Fig. 3 (not shown) the electrode 4 is reflective and a Rochon prism is disposed between the source 9 and electrode 3 to act as polarizer and analyser, selectively deflecting the diffracted beam. An array of filter elements may be formed on a single crystal plate 2, Fig. 5 (not shown). By arranging for the thickness of the plate to decrease across each row, and from row to row, it is possible to address any desired element by controlling the frequency of source 7. Cross-talk is avoided either by using a "mesa" construction wherein the electrodes 3 and 4 are deposited in a dot pattern on either side of plate 2 and wherein the plate thickness between the electrodes is greater than in the surrounding regions, or by using a loading configuration in which electrodes 3 and 4 are thick enough to physically load the plate and reduce the resonant frequency in the electroded regions. Each resonator need be separated from its nearest neighbour by only 10<SP>-5</SP> of the resonant frequency. The frequency of generator 7 may thus be programmed to provide a desired colour display operating with ambient light, or a light source 9, and in the transmissive or reflective modes. In addition the display provides a short term memory due to the finite acoustic Q. A laser cavity may include one or more such filters. Figs. 8 and 10 (not shown) illustrate the use of an array of the type described above disposed between plane or complementary spherical mirrors of the laser cavity for determining the position of the laser output beam. In Fig. 12 (not shown) a single filter of the type shown in Fig. 1 within the cavity is energizable at different overtone frequencies for tuning the laser by varying the gain of the laser at different wavelengths. Similarly a ring laser of triangular configuration may contain a single filter in each of two sides for tuning purposes, Fig. 11 (not shown). The band pass frequencies of such filters must be chosen to coincide with a laser frequency.