Optical fibre sensor system where variations in temperature of optical source is minimized or equal

An optical fibre sensor system comprising a transmitter, a receiver and a sensor interconnected by an optical link, wherein the transmitter comprises an optical source arranged to transmit to the sensor via the optical link two optical signals at different wavelengths, the sensor is arranged to transmit to the receiver via the optical link measurement and reference signals, the measurement signal being derived by modulating the intensity of one of the two optical signals in dependence upon a parameter monitored by the sensor and the reference signal having an intensity which has a predetermined relationship with the intensity of the other optical signal, and the receiver is arranged to compare the intensities of the measurement and reference signals to derive a measure of the monitored parameter, characterised in that the two wavelengths are selected such that changes in the intensities of the optical signals resulting from variations in the temperature of the optical source are minimised or are of substantially equal magnitude for both the optical signals.