In a tunable optical source, a diffraction device 115 is used to provide wavelength selective feedback to a laser diode 100. The diffraction device 115 is at least partially fabricated in a material whose refractive index is controllable so as to control the diffraction performance of the device. This allows the diffraction device 115 to be used in tuning the optical source, potentially without using moving parts. For instance, the refractive index can be controlled using a temperature change across material of the device 115. The diffraction device 115 can be mounted in free space in an external cavity with respect to the laser diode 100. Novel forms of diffraction device 115 are described. Tunable optical sources of this type can be used for instance in communications, particularly wavelength division multiplexing.