Abstract: When a sufficient reflectivity and a low absorption power are given to its dihedron of reflective surfaces, a Fizeau wedge can serve as a directional optical selector which supplies a narrow band of wavelengths in a first direction and the remainder of the incident wavelengths in another direction. A laser with colorant COL can then be defined by the Fizeau wedge CF and an output mirror M.sub.1, with two diaphragms D interposed. Tuning is variable by relocation of the Fizeau wedge in its plane and relates either to a narrow line or a narrow gap in a wide band. Several lines can also be tuned separately.