Accuracy of optical component reflectance measurements using an OTDR

Discrete reflections are produced by physical discontinuities at connectors, mechanical splices, or at fiber-to-component connection points in fiber-optic systems. Even a small amount of reflected light arriving at the source laser can affect its modulation performance and increase the relative intensity noise. Consequently, the link signal-to-noise ratio deteriorates, causing a power penalty in both analog and digital systems. Multiple reflections aggravate the situation. The accepted solution is to restrict the component reflectances to below a certain maximum. This paper reports our findings on the accuracy of optical continuous wave reflectometry (OCWR) and optical time domain reflectometry (OTDR) techniques down to minus 70 dB reflectance. If the reflected power is estimated by integrating under the reflection peak, the OTDR is more accurate at low reflectances; it enables measurements down to minus 62 dB with an accuracy of plus or minus 1.0 dB or down to minus 65 dB with an accuracy of plus or minus 2.0 dB. The sensitivity of the measurements to the pulsewidth and to the characteristics of the optical lead-in fiber were also evaluated.