Common path interferometer based on second harmonic generation

A common path interferometer utilizing noncritically phase-matched frequency doubling crystals is presented. The interference pattern is directly sensitive to the wave front shape (as the Mach-Zehnder interferometer) and is in the second harmonic of the laser source. This interferometer has twice the sensitivity of a two-beam interferometer. All the optics are 100% transmitting—no beam splitters, the contrast is adjustable, and the laser intensity pattern is not affected by the interferometer. This interferometer is similar in some ways to the point diffraction interferometer, and the two are compared in this paper. A theory of operation is given, although the reference wave of the interferometer is flatter than predicted. Several possible reasons for this are examined, but no conclusion was found.