GB1535507A - Light beam measurement - Google Patents

Abstract

1535507 Radiation beam measurements DEFENCE SECRETARY OF STATE FOR 24 Nov 1975 [25 Nov 1974] 51011/74 Heading G1A Radiation beam measurement apparatus comprises: means 1 for splitting the beam into first and second beams; a pair of movable plates 2a, 2b arranged in the first beam path to define a slit 5 of variable width; and means including first and second detectors 3, 4 for finding the ratio of the power of the portion of the first beam transmitted by the slit to the power of the second beam. A slit 5, Fig. 3, defined between micrometeradjustable plates 2a, 2b is first made narrow, and is moved across the path of a first portion of a laser beam S transmitted by a beam-splitter 1, until the position of maximum brightness is found. Thereafter the plates are separated symmetrically to vary the slit defined between their parallel edges, until a ratiometer R, comparing signals from photodetectors 3, 4, indicates that the power transmitted by the slit is 68À26% of the total power in the incident first portion of the beam. (A reference reading of the total power has previously been taken with the plates separated to be clear of the first portion of the beam.) The width of the slit which transmits 68À26% of the power is then taken as the value of the effective radius <SP>w</SP>(Z) of the beam in the plane of the slit. By the addition of a lens 6 Fig. 4 positioned in the main beam and separated from the slit 5 by its own focal length, a new reading of w is taken, from which the divergence # of the beam, Fig. 1 and hence the waist radius <SP>w</SP>O and the distance Z of the waist from the measurement plane, may be calculated.