Beam splitter cube for white light interferometry

Abstract

We describe a technique for constructing beam splitter cubes in which we monitor the optical path difference with a wavelength tunable laser. A white light interferometer should have path lengths equal to within λ/4. A larger path difference results in reduced fringe visibility and loss of signal-to-noise ratio. The beam splitter must be manufactured so that the two paths through glass are equal. We discuss the tolerance placed on the cube. One can assemble the cube from two right angle prisms in optical contact and in near perfect alignment. Alternatively, we adjust the OPD to zero by sliding the prisms into proper alignment on a layer of uncured cement. Using a dye laser, we have readily achieved acceptable path differences. This method of measuring the OPD with a tunable laser is shown to be useful for alignment of interferometers and measurement of aberrations in lens systems. A laser diode can be tuned by changing its operating current or temperature. We report results of optical path difference measurements using a laser diode.