Laser beam quality factor (M$^{2}$) measured by distorted fresnel zone plates

Abstract

In this paper we present a novel method, based on diffractive optics, for measuring M$^{2}$. This novel optical arrangement consists of a lens along with a set of 1 or 2 distorted diffraction gratings and a single array detector. As a result, this system produces simultaneous images of a number of different object planes centred over the laser beam propagation axis symmetrically to the laser beam waist. This image is gathered around the image plane using a CCD camera producing, in a single image and after some image processing, an instantaneous measurement for the laser beam quality. This technique allows for the possibility of monitoring the beam quality in real time, in a continuous or pulse-to-pulse mode operation. Nowadays, high power laser machining is a standard technique in manufacturing material processing that is widely used in industry. Although these lasers are not always stable for long periods of time, which is required in this kind of process, it usually takes considerable time to diagnose them and restore them to working order. The M$^{2}$ parameter is a numerical expression for laser beam quality that can be measured in real time over a process line. However, to measure the M$^{2}$ parameter, several measurements in the neighbourhood of the laser beam waist are required.