Measurement of the apex angle of a small prism by an oblique-incidence mode-locked femtosecond laser autocollimator

A major modification has been made to the conventional mode-locked femtosecond laser autocollimator so that the apex angle of a small prism, which is a transparent object difficult to be measured by the conventional optical methods, can be measured. In the oblique-incidence optical setup, a collimated mode-locked femtosecond laser is at first made incident to a target prism, and the laser beam reflected from the prism surface is then projected onto a reflective-type grating for angle measurement based on the principle of the femtosecond laser autocollimator. With the robust signal processing technique in the femtosecond laser autocollimator where the spectrum of the first-order diffracted beams emanated from the grating is captured by the fiber detector, high-precision angle measurement can be realized even on a surface having a small area for measurement and/or on a surface having low reflectivity. An oblique-incidence femtosecond laser autocollimator with a fiber-based femtosecond laser source is developed, and its basic characteristics are then evaluated in experiments. An experimental setup composed of the oblique-incidence femtosecond laser autocollimator and a high-precision rotary table is also constructed for prism angle measurement. To avoid the influence of the internally reflected beam in a prism, a method employing the characteristic of Brewster's angle is newly proposed, and its feasibility is verified through experiments. The uncertainty analysis on the measurement of the prism apex angle by the oblique-incidence mode-locked femtosecond laser is also carried out based on GUM.