An autocollimator with a mid-infrared laser for angular measurement of rough surfaces

In this study, we proposed an autocollimator for the angular measurement of rough surfaces. A mid-infrared laser having a wavelength of 2.8 μm was employed as the light source because it possesses high specular reflectivity on rough surfaces. Although a conventional autocollimator using a visible laser has a high resolution of angular measurement, it can only be used to measure mirror surfaces because the intensity distribution of the reflected light from a rough surface changes significantly due to the influence of diffused reflection. The suitability of a mid-infrared laser in an autocollimator for the angular measurement of rough surfaces (sub-micrometer level) was verified through simulations using reflection models. Therefore, a continuous-wave mode fluoride glass fiber based mid-infrared laser was developed for the autocollimator. Furthermore, an autocollimation unit was designed and constructed for detecting the angular displacement of mid-infrared laser light reflected from a target surface. The capability of the developed autocollimator for the angular measurement of rough surfaces was demonstrated through experiments.