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.
Highlights
Let us make an exemplar prediction of dynamic range extension and distance span extension by means of Equations (23) and (24), as well as Equation (25), respectively, adopting common values for the relevant parameters of interest: By conducting measurement, we found the optical time-domain reflectometer (OTDR) noise floor NEPOTDR value to be close to −50 dBm, which corresponds to NEPOTDR = 10 nW on the linear scale
As our distance span extension model is centered around the dynamic range, according to the considerations in Section 2.2. that longer pulses reach farther along the fiber, we selected 5 μs as quite sufficient for the fiber that we tested
The exemplar preliminary test traces of the OTDR alone, and of the OTDR with the front-end optical amplifier (OA), are presented in Figure 8a,b, respectively. As it can be seen, the significant difference is evident between the noise floor values in these cases, with the dynamic range enhancement of about 30 dB, which conforms to the predicted value obtained from Equation (24), and verifies the distance span extension of 75 km estimated by Equation (25) as well
Summary
Concerning the long-distance measurements, especially the far-end events visibility and measurement accuracy, the crucial OTDR attribute is dynamic range, which is the difference between the backscatter level at the front end and the noise floor at the far end of the fiber. The dynamic range determines how far downstream the fiber can the strongest (i.e., of longest duration) transmitted optical pulse reach, taking into account OTDR span reduction introduced by connectors, splices, and splitters. On the other hand, looking from the bottom up, the lower boundary determining the OTDR dynamic range is the noise floor value, which is (mostly) where the signal-tonoise ratio (SNR) equals unity, and is observed against the longest pulse width, after the three-minute averaging time [9] The dynamic range should be about 5 to 8 dB larger than the maximal loss that the OTDR can measure [1,4,5,6,7,8].
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