In this paper we investigate the variation of free spectral range (FSR) for the Fabry-Perot interferometer (FPI) consisting of mirrors with phase shift dispersion. The reflection phase shift on a mirror has been calculated employing the Transfer-Matrix Method and the values of FSR have been calculated under the condition of normal incidence of light beam. Fabry-Perot (FP) cavities have been fabricated employing bulk micromachining technology, and silicon wafers coated with multilayer dielectric films were used as mirrors. FSR of these FP cavities have been experimentally measured. The experimental data match the calculated results very well. The conclusion is that FSR shortening effect must be taken into account for the FPIs with a small plate gap, as the finesse and the tunable range of tunable FPI can be affected by the shortening effect greatly.
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