Narrow linewidth tunable ECDL using wide bandwidth filter

We show how an external cavity diode laser with interference filter for wavelength selection can operate on a single external cavity mode, even when the filter bandwidth is hundreds of times greater than the external cavity mode spacing, and many times greater than the intrinsic laser diode mode spacing. Single mode operation is demonstrated for a laser using a 780 nm filter with 3 nm bandwidth and an external cavity formed by a cat-eye reflector. The cat-eye reflector is intrinsically stable with respect to intra-cavity optical misalignment, and allows a mechanically rigid design free of significant mechanical resonances, illustrated by comparison of the frequency noise spectrum with that of a common external cavity diode laser design which uses a diffraction grating and kinematic mount. The laser can be tuned discontinuously over a wavelength range of 9 nm, without alteration of the output beam direction. The linewidth of the laser was found to be 40 kHz using the delayed self-heterodyne technique.