Fibre optic wavelength division multiple (WDM) telecommunications networks require tuneable sources which can cover all the spectrum allowed. Such sources exist and offer tuneable (by way of example DBR -Distributed Bragg Reflector- or DFB—Distributed Feed Back- laser) or highly tuneable (by way of example SG-DBR -Sampled Grating- or SSG-DBR -Super Structure Grating-) output, however WDM applications require sources having good stability in wavelength. In current practice, this is provided by temperature control since the main cause of wavelength drift is variation in laser temperature as tuneable laser output wavelength is temperature dependent (typical coefficient: 0.1 nm/degree). The temperature is usually controlled by means of a Peltier-effect thermoelectric cooler. Such devices consume up to 10 W of electrical power compared to an order of 0.2 W consumed by the laser. In large WDM systems with 50-200 channels the resultant prime power and cooling requirements are onerous. Furthermore, as laser diode operating temperature ranges are increased to from 0 to 85 degrees C., coolerless operation becomes very attractive for reduced power consumption and cost and increased reliability.