High resolution distance measurement of laser-induced diffraction signals by digital signal processing

A diameter measurement method is presented offering an extremely good resolution, high signal-to-noise ratio and a high measuring rate. The measuring principle is based on the evaluation of the diffraction of a laser beam at fibers or thin wires for the determination of the diameter. The high accuracy of up to 0.1 μm resolution and the robustness are obtained by using digital signal processing and with the aid of a signal processor. Signal processing methods are discussed for a laser-induced diffraction signal by using the fast Fourier transform. The measuring method was implemented on a signal processor board placed in a PC. The diffraction signal is generated by a measuring head, consisting of a semiconductor laser collimator and charged coupled device line scan camera. The measuring head and evaluation unit are applicable as a complete measuring system for quality assurance in the production of fibers and thin wires. Experimental results are presented