A laser source (10, 30, 40) comprising a laser medium (10) having a back facet (10B) and a front surface (10A), whereby the laser medium (10) is adapted to emit a laser beam through the front surface into an external cavity defined in length by a cavity end mirror (40) reflecting the laser beam back towards the laser medium (10). A wavelength tunable filter (30) is arranged between the laser medium (10) and the cavity end mirror (40) adapted for tuning the wavelength of the laser beam. The laser medium (10), the wavelength tunable filter (30), and the cavity end mirror (40) are arranged in a spatially linear cavity structure substantially in a line without angular redirection of the laser beam in the cavity between the laser medium (10) and the cavity end mirror (40). At least one portion of the laser beam within the cavity after passing the wavelength tunable filter (30) and before again passing the laser medium (10) is coupled out as an output beam (50) of the laser source (10, 30, 40), and the cavity end mirror (40) is provided to be partly transparent for coupling out a first output beam (50).