Patent GB-1222551-A (GB)
Title: Arrangement for controlling a supply voltage to an electro-optical polarisation switch
IPC: G02F1/03
Filed: 1968-02-28
Abstract: 1,222,551. Light deflectors. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 28 Feb., 1968, No. 9627/68. Heading H4F. In a digital light deflecting system utilizing an electro-optical polarization switch an auxiliary beam is passed through the switch and the polarization condition of the emergent beam utilized to control the supply voltage applied to the switch. By controlling the supply voltage in this manner the propagation of light in unwanted directions due to variations in the supply voltage and to temperature effects is prevented. Thus, as shown in Fig. 1, an auxiliary beam 8 plane polarized at 10 is passed through the polarization switch 3 (which comprises a Kerr cell) at right angles to the beam 2 which is to be modulated and the emergent beam is directed via a birefringent plate 11 and plane polarizer 12 to a photodetector 13 the plate 11 being adjusted so that the beam intensity at the photodetector is zero. Fluctuations in the voltage supplied to electrode 4 of the switch from generator 6 then result in an output from 13 and this is utilized via connection 14 to adjust the generator output to its desired voltage i.e. that which determines the first switching condition. By employing a second auxiliary beam 8<SP>1</SP> and adjusting the birefringent plate 11<SP>1</SP> such that the beam intensity at photo-detector 13<SP>1</SP> is other than zero when the applied voltage has the desired value, the sign of the fluctuation may be deduced and this, it is stated, is desirable when only a short time is available for effecting the correction adjustment. A similar arrangement of two auxiliary beams may be employed for stabilizing the output of the generator at the voltage necessary to set the switch in its other switching condition. Fig. 2, shows an embodiment in which an auxiliary plane polarized beam 22 is passed through the (Kerr cell) switch 23 such that the optical path length in the cell is twice the effective length of the cell (or an integral multiple of this length) at least one reflector 24 being employed, and the emergent beam directed to a photo detector 25 via a birefringent plate 27 and polarizer 26 which are adjusted such that with the supply voltage required to set the switch in one switching condition applied to the switch electrodes the beam has zero energy. When the supply voltage is changed to set the switch in the other switching condition the plane of polarization is rotated by 180 degrees (i.e. 90 degrees before and 90 degrees after reflection) and the beam energy at the detector 25 is again zero (or minimum). Departure from these conditions are then utilized, as described above to stabilize the two voltages necessary for switching between the two conditions. In a modification of this arrangement two auxiliary beam following the same path in the switch are employed the emergent beams being split and directed to respective photo-detectors via respective polarizers at right angles to one another (Figs. 3, not shown). By using a single plane polarized beam and plane polarizing the emergent beam before it impinges on the beam splitter the last arrangement provides a linear relationship between the potential difference across the switch electrodes and the intensity of the auxiliary beam at the photodetectors (Fig. 4, not shown). By comparing the outputs of the photodetectors in a bridge circuit the magnitude and the absolute sense of the difference between the switch electrode voltages may be determined. A plurality of the switches may be used in a ditital light deflecting system.