Patent GB-1488691-A (GB)
Title: Opto-electronic positioning-system
IPC: H03G3/34
Filed: 1974-09-27
Abstract: 1488691 Adjustable focusing PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 27 Sept 1974 [1 Oct 1973 25 Feb 1974] 42172/74 Heading G1A In an opto-electronic system for determining a deviation along an optical axis of an optical imaging system 11, 12, L, between the actual and desired position of a substantially flat surface 1 (a record carried) normal to said optical axis, a radiation source and at least two detectors are employed. The detectors are offset so that under "in focus" conditions, real or virtual sources S 2 , S 3  formed from source S 1  are brought to a focus just before and just after the focal plane of the lens L that contains the image of the record carrier surface, so the detectors D2, D 3  are equally illuminated. The detector outputs are used to control means adjusting the position of the lens L. Fig. 3 shows a system wherein a source S 1  on the optical axis is focused, under "in focus conditions" by the lens L on to the recording surface. The reflected light is brought via a semi-reflector to focus on a detector D 1  whose output is processed to get the information stored on the carrier 1. Alternatively, the stored information can be obtained by processing the high frequency output of the detectors D2, D3 Fig. 10 (not shown). Instead of the sources Si, S 2 , two virtual sources may be provided by an assymetrically positioned Fresnel zone plate 12 as shown, or, provided by a diffraction grating in combination with a stepped glass plate Figs. 7 and 8 (not shown), or provided by a Wollaston prism in combination with a polarization responsive lens (Fig. 10). Alternatively, the Fresnel lens may be positioned in front of the detector Fig. 4 (not shown), to give the effect of two virtual sources. The signal controlling the lens positioning means may be derived by investigating the magnitude of D.C. signals provided by the detectors, or by investigating the modulation depth of the high or low frequency components of the detector outputs, or by utilizing the line synchronizing pulses provided by the record carrier surface. Fig. 6 shows a system for writing information on a record carrier 50, wherein radiation from a laser 51 is directed via an electro-optic modulator 54 that is connected to electronic control device 55 supplied with, for example, a television programme signal. The modulated light beam is then directed on to the record carrier which may comprise a radiation reflective surface below a photo resist layer. Detectors D 2 , D 3  receive radiation reflected off the record carrier surface from a separated auxiliary beam (not shown) or by means of a semi-reflector 11 from the modulated "write" beam itself. A Fresnel zone plate 12 can provide the offset virtual source feature referred to above and the detector outputs are processed as above to control the energization of a magnetic coil 60 that moves the lens 59. Fig. 11 shows a further feature for dealing with larger errors in focus. Behind the set of detectors 111 is placed a lens 110 shaped as shown, together with a further set of detectors 112, 113, 114, 115. The set of detectors 111 function as before, but if the beam 116 is brought to a focus a comparatively larger distance before or after the detectors (111) Figs. 12a and 12b (not shown), the lens functions to direct the beam to the pair of detectors 113, 114 or the pair of detectors 112, 115 respectively. The detector's outputs may then be used to adjust the focus of the lens. Other lens shapes or lens combinations, Figs. 14 and 15 (not shown), corresponding to the lens 11 are disclosed. The row of detectors (111) Fig. 13 (not shown), may also include two extra detectors that are offset relative to the central row and that are used to centre the beam.