Transmission spectra of Fabry-Perot etalon filter for diverged input beams

The transmittance spectrum of an Fabry-Perot (FP) etalon for an arbitrary laser mode size was analyzed theoretically and experimentally. An asymmetry and peak wavelength shift in a transmittance spectrum of the FP etalon can be explained by the decomposition of an input beam into plane waves. It was found that a wavelength locker can be realized without collimation lens by using a spot-size integrated laser diode.