A method for increasing the fringe shift resolution of a two‐dimensional optical interferogram by more than an order of magnitude without loss of spatial resolution is described. A Fabry‐Perot cavity is used as an interferometer to increase the number of passes. The sharpened fringes are multiplied in number by using a multimode laser as a light source. The fringe pattern (observed without a lens) is equivalent to that of a Mach‐Zehnder interferometer except that a shift of one fringe corresponds to an optical path length difference of &lgr;/2&agr; (instead of &lgr;) when the cavity lengths are in the ratioL/L0=1/&agr;, and &agr; is the number of excited laser modes.