A cavity ring-down sensor having an optical path of mirrors and an interface proximate to a mirror with a normal incidence optical fiber coupling. The interface may be a block of transmissive materials having different indices of refraction for receiving a light beam of normal incidence from an optical fiber and directing the beam into the optical path. The sensor may be used applications where the optical fiber needs to be straight into the sensor without unusual bending because of space limitations. One application of this sensor may be for ground water testing via a narrow well casing.