engleski

Practical Enhancement of Fiber Installation and Maintenance Test Tools - Example of Extending OTDR Distance Range by Optical Preamplifier

Lightwave test equipment has a difficult task to keep up with fast evolving fiber optic communication systems that require ever more sophisticated measurement methods. In this presentation, first a survey of measurement techniques and test tools that are needed to design, install and maintain state-of–the-art fiber optic transmission systems, are reviewed. Then we focus the optical time-domain reflectometer (OTDR) as the troubleshooting and fault locating equipment of choice for fiber characterization in terms of identification and localization of its various refractive and reflexive events such as e.g. breaks, as well as measuring attenuation, splice and connector insertion/return losses, and fiber length. Essentially, OTDRs insert a pulsed signal onto the fiber, from which a small portion that is commonly referred to as Rayleigh backscatter, is continuously reflected back by irregularities in the optical fiber structure, with the appropriate delays of the reflections expressed as the power loss versus distance, by conveniently scaling the time axis. Moreover, in contrast to legacy OTDRs designed to test dark fibers only, the new generation is equipped with filtered ports, enabling in-service measurements on fibers carrying live traffic, in addition to some other features such as macrobend location and automatic trace acquisition. Specifically, with regard to 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. Consequently, the dynamic range determines how far downstream the fiber can the strongest (i.e. longest- duration) transmitted optical pulse reach, also taking into account OTDR span reduction introduced by connectors, splices and splitters. On the other hand, the lower boundary determining the OTDR dynamic range is the (root-mean-square) noise floor value, mostly related to the longest pulse width with the three-minute averaging time, where the signal-to-noise ratio equals unity. With this regard, still many older OTDR units have insufficient dynamic range to test the far-end of long fibers. For example, out of a typical dynamic range of, say, 35 dB, just 30 dB is effective, whereas the fiber attenuation takes 0.20 dB/km, and the splices between the 2 km-long hops take 0.1 dB each. This implies that only distances up to 120 km can be reliably tested with these OTDR units, making them lay down scattered in test tools inventories of fiber optic network operators. However, we introduce a simple and cost effective solution to activate these OTDRs and make them capable of reaching significantly farther away, by extending their dynamic range. As maximal-power impulse is in use anyway, lowering the noise floor is the only option for widening the dynamic range. We did it by inserting a low- noise-figure high-gain optical preamplifier in front of the OTDR, so making the noise figure of the cascade dominantly determined by the (lower) noise figure of the optical amplifier, which, according to our preliminary tests, enabled the OTDR under test to reach almost 190 km towards the end of the same fiber.

fiber test ; otdr ; distance range ; noise floor

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