Fabrication of cube beamsplitters for white light interferometry

Abstract

Interferometric applications utilizing short coherence length sources, such as white light interferometry, require precise matching of optical path lengths for the two arms of the interferometer. If a cube beamsplitter element is utilized, the added dispersive material in the optical path could introduce undesirable optical path difference (OPD) effects and a consequent degradation in fringe visibility. For this reason a cube beamsplitter must be well matched for equal geometric path length in glass during the fabrication process. Two degrees of freedom must be controlled; the lateral position and relative rotation of the two prisms that comprise the beamsplitter. A method is described for efficiently assembling cubed beamsplitters utilizing a kinematic mount to adjust the relative position of the beamsplitter prisms with sub-micron precision for both degrees of freedom. The OPD is monitored simultaneously at three separate wavelengths during assembly by exploiting the color separation capabilities of a spatially congruent 3-CCD color camera. Once positioned to minimize the OPD, the prisms are bonded together with UV curing adhesive and any residual aberrations are quantified. The technique was proven by aligning prisms to 0.1μm accuracy and measuring the OPD error to 0.05μm accuracy.