The invention relates to a wavelength-switchable, semiconductor laser (10), for generating of laser light in the optical range between 1 μm and 100 μm, wherein the laser (10) comprises an external cavity (11) and at least the following components: a semiconductor optical amplifier (12), comprising an intra-cavity facet (121) facing towards; a first optical element (13) for collimating a laser beam exiting the intra-cavity facet (121) arranged at the intra-cavity facet (121) of the semiconductor optical amplifier (12); a transmissive, wavelength-adjustable interference filter (1), comprising at least i) a wavelength-adjustable Fabry-Perot filter (100) and ii) a Fabry-Perot etalon (110) comprising a first planar surface (111) and a second planar (112) surface, wherein the first and second surface (111, 112) of the etalon (110) are arranged opposite and plane-parallel to each other and delimit an etalon cavity (113); wherein the Fabry-Perot etalon (110) and the Fabry-Perot filter (100) are arranged in a row; wherein the interference filter (1) is arranged between the first optical element (13) and the beam splitter (14), and wherein the components are arranged such that laser light comprising a selected laser wavelength can stably resonate in the external cavity, wherein an on-axis, retro-reflecting beam splitter (14) is arranged at one end of the external cavity (11). Furthermore, the invention relates to a method for selecting a laser wavelength using the laser.