Detection System for Coherent Light Beams

摘要

1,163,313. Photo-electric detection apparatus. NORTH AMERICAN AVIATION Inc. Sept. 1, 1966 [Oct. 1, 1965], No.39146/66. Heading G1A. [Also in Division H1| The information signal modulating a coherent carrier light beam is extracted by mixing with the carrier beam a reference light beam so as to produce an interference pattern of fringes the variations in intensity and/or movement of which are detected photo-electrically as representative of the modulations on the carrier beam. The light used is that from laser sources. The modulations impressed on the carrier may be of the amplitude of the light beam, in which case the fringes of the interference pattern vary in intensity, or of the frequency, phase or polarization, when they appear to move across the pattern. In detecting frequency modulations on a carrier beam 13 of varying frequency f 1 , Fig. 2, a reference beam 14 of a fixed frequency f 2 is mixed with the carrier beam and an optical system used to focus and locate the interference pattern formed on the surface of a detector means 7. This comprises two photo-diodes 11, 12 with interlocking fingers each of one (dark or light) fringe width. The outputs of detectors 11, 12 are two signals representing the movement of the fringes across detector, the speed of which movement is proportional to the frequencies f 1 and f 2 i.e. proportional to the information signal. The components of this signal in the output on lines 17 and 18 are 180 degrees out-of-phase, while noise signals generated by the incidence of stray light on the detector are in phase in the two outputs. Thus subtraction of the outputs by transformer 20 produces a signal 19 representing the information required, while noise signals cancel out. The sign of the frequency difference of f 1 and f 2 is given by the direction of fringe movement. In a modification of the detector construction, Fig. 4a (not shown), a block of rectangular photodiodes, each of width equal to half a fringe, is connected to a two-transformer circuit, to enable the sign of the frequency difference to be readily detected using an oscilloscope display. In another arrangement (not shown), a mirror provided with an array of slits passes alternate fringes to a detector behind it, and reflects the rest of the fringes towards a further detector. These detectors are connected to subtraction means as before. Alternatively, a double photo-multiplier is used, having a transparent photo-cathode coated on one side with a photo-sensitive layer of the configuration of diode 11, Fig. 2, and on the other, a layer of the configuration of diode 12, Fig. 2, activated by light transmitted through the electrode. These layers each have their own associated accelerating electrodes and anode, feeding a common subtraction circuit. Instead of a rectangular fringe pattern, a circular one may be used with appropriately designed detectors.