No dead-space optical time-domain reflectometer

An optical time-domain reflectometer operating at room temperature with the single-photon counting technique is described. It features subcentimeter resolution without data deconvolution, and the capability of observing Rayleigh scattering a few centimeters after a reflecting (Fresnel) discontinuity. The key component for the instrument is a silicon single-photon detector of a new design. Time resolution of these diodes is better than 50 ps full-width at half-maximum (FWHM), corresponding to the ability of spatially separate two Fresnel reflections placed 5 mm apart without requiring any mathematical treatment of the data. Sensitivity in the 10^-15 W range at 1-cm resolution is obtained. There is no dead space, since the digital detection process avoids problems related to the nonlinearity and saturation of analog amplifiers