Abstract: Optical sensing apparatus (10) for measuring the temperature and flow-rate of a fluid comprises a DFB fibre-laser (20) and an AO modulator (16) for modulating the power of pump radiation delivered thereto. Modulated pumping results in cyclical heating of a grating structure in the fibre-laser and hence wavelength variations in laser radiation output therefrom. The temperature and flow-rate of a fluid surrounding the fibre-laser may be inferred from the wavelength excursion and the minimum value of the wavelength respectively. The invention provides simple optical sensing apparatus capable of providing measurements of both the temperature and the flow-rate of a fluid.