Traceable measurements for beam propagation ratio M2

Abstract

This paper reports an investigation into measurement techniques and infrastructure suitable for underpinning traceable calibration of devices for beam propagation parameter measurement. The accurate knowledge of the M2 parameter is required for a diverse range of applications including semiconductor manufacture, process control, laser machining, thin-film transistors (TFT), fundamental research into frequency doubling, and the production and characterisation of optical components. Characterisation of the size and position of the beam waist is essential for the correct hazard classification of extended source laser systems using the IEC 60825 series of standards. A system for measuring beam propagation factor measurement has been developed at NPL based on the methodology outlined in the ISO 11146 series of standards. This methodology uses spatial intensity profile measurements of the beam at defined points along the direction of propagation of the beam through a beam waist produced by a convex lens. The beam widths obtained using a CCD and a converging second moment method, are fitted to a hyperbolic propagation envelope and the beam propagation coefficients are obtained from this shape. The traceability of this method has been provided by using graticules calibrated against length standards and carefully measured CCD camera parameters.