Distance scale calibration of optical fiber OTDR

In this paper we present the distance scale calibration of optical fiber Optical Time Domain Reflectometers (OTDR) using the optical time-delay recirculating line approach. Particularly we demonstrate the optical modulation - phase-shift technique to measure the length of the standard fiber which formed the loop of the delay line. Two stages of modulations of 10 KHz and 1 MHz were used to achieve, step-by-step, the final optical length uncertainty of less than 0.02m+1.20X10(-6)[L]. The length measurement results were self-checked and they are well within the quoted uncertainty. With the calibrated delay recirculating-line as a transfer standard the distance scale of the OTDRs is verified to the uncertainty of about 2 a 10(-5), depending on the OTDRs' performance.