Abstract: A fluid detector and a method of detecting a fluid are described. The fluid detector includes a broadband light source (10) for providing a light beam (12) including light of at least a predetermined bandwidth, and an optical power detector (40) arranged to provide an output signal indicative of the total power of incident light. An optical path extends from the light source to the optical power detector. The optical path includes a tunable optical filter (20) and a fluid sampling region (30). The tunable optical filter is a tunable optical band-rejection filter, and has a rejection band narrower than the bandwidth of the light beam. The rejection band is swept across the wavelengths of the predetermined bandwidth of the light beam. The tunable optical filter may comprise a fiber Bragg grating.