Gaussian beam interaction with an air-gap Fizeau interferential wedge.

We describe a plane-wave-expansion approach for calculation of the fringe pattern in transmission and reflection for a Gaussian monochromatic beam. Both positive and negative incidence, at which the incident light beam undergoes multiple reflections within the wedge in direction of increasing or decreasing wedge thickness respectively, are analyzed. It is shown that the two opposite incidences of the light beam are described by the same mathematical expressions; i.e., the transmitted/reflected fringe pattern at positive incidence is a continuation of the pattern at negative incidence at some distance from the wedge. Numerical simulations are made for a high-reflectivity-coating air-gap Fizeau interferential wedge with apex angle of 5-100 microrad and thickness of 5-500 microm as a useful optical element in laser resonator design. Experimental verification is also provided.