A tunable laser system

The invention relates to the production of laser light, particularly the production of laser light at variable and selected wavelengths. The tunable laser system comprises a sealed chamber (201), including an optical window, a laser diode located within the chamber and a diffraction grating located outside the chamber (203), wherein the laser diode, optical window and diffraction grating (203) are arranged such that light emitted from the laser diode can pass through the optical window to the diffraction grating (203), and wherein a thermal control means is provided to control the temperature of the laser diode, and wherein an atmosphere control means is provided for controlling the moisture content of the atmosphere within the chamber (201).