GB1496515A - Laser output measuring instruments - Google Patents

Abstract

1496515 Measurement of laser output GENERAL ELECTRIC CO Ltd 5 Sept 1975 [12 Sept 1974] 39833/74 Heading G1A [Also in Division H1] An instrument for measuring the energy or power of a laser comprises a photo-detector associated with an optical system for diffusing the light from the laser, a circuit for integrating and amplifying the photo-detector output and visual display means, the photo-detector being provided with heating means to maintain its temperature constant during operation. As shown in Figs. 1 and 2, the detector is a double photodiode whose face exposed to the radiation comprises an inner first anode and an outer concentric second anode, the common cathode being located on the opposite face of the detector crystal. A D.C. voltage of 100-140 is applied to the cathode. The oven 4 for the detector is electrically heated, to maintain 70‹C. The optical system comprises opal glass scatter screens 5, 6, 7, 8 and a perforated metal plate 9, a final opal plate 11 forming a closure for the oven 4. The detector has a fast output from its second anode 24 to terminal 26, whereby to provide a direct display of the detector input variations on a digital voltmeter, an oscilloscope (e.g. for indicating laser pulse length or shape) or an analog meter. The first anode (25) signal is used to supply the integrator circuit C 1 , R 2 . When laser pulses are being analyzed, each pulse charges up capacitor C 1 and after the pulse has ended the voltage stored thereby, adjusted in accordance with the photo-detector dark current, is amplified by operational amplifier A 1 . The amplifier output is used to trigger analog-todigital converter 28, via switch S, capacitor C 3 and amplifier A 2 , so that the converter is triggered to receive the laser output representative signal only at the end of each pulse. The converter is provided with a numerical four-digital electroluminescent visual display 12 and an output to a voltmeter at 35 via digital-to-analog converter 34 and amplifier A 3 . In the other position of switch S, the instrument can be zeroed. Alternative arrangements. The instrument may be modified to deal with high pulse repetition rates or continuous beams. The detector may be a silicon photodiode which is particularly sensitive in the range 0À8-0À9Á, for use with a neodymium doped laser. Alternative detectors are of germanium mercury cadmium telluride or lead sulphide. A pyroelectric device may be used. The optical system may incorporate beam-splitter; diffusing/reflecting devices and absorption vanes. The apparatus may be all in one housing or divided into two housings as indicated in Fig. 1 by the dashed line.