Calibration of Optical Fiber Time Domain Reflectometers in Accordance with IEC 61749-1:2009

Abstract

Optical time domain reflectometers (OTDR) are widely used in testing, installation and maintenance of optical communication networks. An OTDR launches a series of high speed pulses into a fibre network and measures the amplitude and the delay time of reflected signals to locate events or faults along a fibre link. Precise measurement capabilities of an OTDR on distance, loss/attenuation and reflectance are required to locate and evaluate the severity of faults accurately. The Standards and Calibration Laboratory (SCL) has developed a calibration system for calibrating single mode OTDRs fitted with FC connectors at wavelength 1310 nm and 1550 nm in accordance with the international standards IEC 61746-1:2009. The characteristic parameters calibrated include the distance deviation, attenuation deviation and reflectance deviation. The principle of the calibration is to apply a set of reference standards, namely distance calibration artifact, attenuation calibration artifact and reflectance calibration artifacts, to the OTDR, and to compare against the displayed values for a given OTDR setting. The distance calibration artifact consists of a recirculating delay line and of a lead-in fiber which generates a series of reflection peaks separated by calculable distances. The linear regression of the measured data is processed in Microsoft Excel to estimate the distance scale deviation and zero location offset. The attenuation artefact consists of a G.652 fibre spool with known spectral attenuation and attenuation uniform. The reflectance artifact is composed of a set of known reflectances ranging from -10 dB to -50 dB. The best measurement uncertainties for the distance, attenuation and reflectance deviation calibration are 2m, 0.06 dB and 1.7 dB respectively.