Extending OTDR Distance Span by External Front-End Optical Preamplifier

Abstract

Optical time-domain reflectometer (OTDR) is used to characterize fiber optic links by identifying and localizing various refractive and reflective events such as breaks, splices, and connectors, and measuring insertion/return loss and fiber length. Essentially, OTDR inserts a pulsed signal into the fiber, from which a small portion that is commonly referred to as Rayleigh backscatter, is continuously reflected back with appropriate delays of the reflections expressed as the power loss versus distance, by conveniently scaling the time axis. Specifically, for long-distance events visibility and measurement accuracy, the crucial OTDR attribute is dynamic range, which determines how far downstream the fiber can the strongest transmitted optical pulse reach. As many older-generation but still operable OTDR units have insufficient dynamic range to test the far-end of longer fibers, we propose a simple and cost-effective solution to reactivate such an OTDR by inserting a low-noise high-gain optical preamplifier in front of it to lower the noise figure and thereby the noise floor. Accordingly, we developed an appropriate dynamic range and distance span extension model which provided the exemplar prediction values of 30 dB and 75 km, respectively, for the fiber under test at 1550 nm. These values were found to closely match the dynamic range and distance span extensions obtained for the same values of the relevant parameters of interest by the preliminary practical OTDR measurements conducted with the front-end EDFA optical amplifier, relative to the measurements with the OTDR alone. This preliminary verifies that the proposed concept enables a significantly longer distance span than the OTDR alone. We believe that the preliminary results reported here could serve as a hint and a framework for a more comprehensive test strategy in terms of both test diversification and repeating rate, which can be implemented in a network operator environment or professional lab.

Optical OFDM Error Floor Estimation by Means of OTDR Enhanced by Front-End Optical Preamplifier

“…So, to obtain the impulse response h(τ), we need to de-convolve it from the OTDR transmitted pulse P T 1/2 (τ), i.e., solve (5) per h(τ), which leads to [12]:…”

“…This implies that the normalized power-delay profile of the fiber can be approximated by the normalized (to unity area) relative OTDR trace as [12,13]:…”

“…In addition to pulse width, receiver bandwidth is another crucial parameter, which determines the actual OTDR resolution, as for short transmitted pulses, the OTDR receiver (photo-detector) bandwidth should be large enough, at least 40 % wider than the reciprocal pulse width [12,13].…”

“…Let us express NEP eff of a receiver cascade of blocks by the more common noise figure F relating the (shot) noise power spectral density P no at the receiver output to the input noise power spectral density P ni multiplied by the receiver gain G [12]:…”

“…the above classic relation shows that F 2 (unless it is very large), does not contribute significantly to F 1,2 characterizing the whole cascade [12]. However, in case of a noise-rich (older-generation) OTDR receiver's APD [1], the passive front-end ODC with insertion loss A DC (equal to its own NF), deteriorates the noise figure (13) of the cascade even further to [12]:…”