Differential wavelength meter for laser tuning

A simple interferometer for matching the wavelengths of tunable lasers is described. Our interferometer uses the angular dispersion of a diffraction grating at the Littrow angle to produce a tilted wavefront with respect to a reference mirror in an opposing arm of a Michelson interferometer. As a first step, the resulting interference pattern is adjusted to produce a null fringe using a reference laser as a wavelength standard. When a tunable laser, such as a laser diode, is used to illuminate the system, the laser is simply tuned to reproduce the null fringe pattern established using the reference laser. When so tuned, the wavelength of the tunable laser is matched to that of the reference laser within about 3 GHz, close enough for optical heterodyning.