A sensor unit for detecting Rayleigh scattering of laser light, guided in a fiber along a light path, is configured with a holder. The holder has a U-shaped recess centered along an axis which extends transversely to the light path. The curved bottom of the recess has a matte reflective surface and is configured with such a radius of curvature that Rayleigh scattered light indicated on the bottom is collected in a focal plane of a light input window of a meter coaxial with the recess. The reflected amount of the Rayleigh scattering on the light input window is sufficient to provide a measurement data which deviates from a reference value at no more than about +/-3%.