Monitoring equipment for detecting and locating bends, material degradations or breaks in optical fibre networks is traditionally set up on dark fibre as this provides good balance between material cost, system provisioning effort and fault detection success rate.
Most monitoring systems used for physical fault detection and positioning employ OTDRs (optical time domain reflectometers); a dark-fibre monitoring approach typically uses a 1550 nm window test unit since this window produces lower fibre attenuation, maximising the measurement range.
Skipping over terminals, amplifiers and OADMs (optical add-drop multiplexers) only requires optically connecting the appropriate ports and the next segment becomes visible for the OTDR, with almost no additional loss. It is also a well-known fact that monitoring only a few fibres within a cable geometry and structure (such at its outer part, one per tube or one per ribbon) is enough to provide information about the optical link quality.
So why will live-fibre testing and monitoring continue to generate such interest?
* There may just not be enough dark fibre available to provide good fault detection sensitivity.
* In many situations, degradations can be local, such as a connection removed voluntarily or not, a worn-out splice, aerial cable damage due to bullets, rodents or sabotage where only a few outer fibres were cut. So, depending on the regional problems that are experienced or foreseen, the dark-fibre logic may not apply. In these cases, live-fibre monitoring or testing becomes the only solution that can provide complete surveillance.
* More importantly, optically testing live segments is a straightforward method to correlate transmission and physical path errors. In a long-haul system, more than 100 km can separate two network elements, a considerable distance to deal with if the only information is a B1 byte status change, signalling a specific section fault in the SONET/SDH section overhead coding. In metro, core or access networks, the trend is to remove expensive fast-detecting and processing electronics by optically switching paths, which directly remove points of surveillance between destinations. Unless these intelligent optical switches can embed an optical surveillance function, this movement drives the need for live-fibre and optical signal monitoring.
Deploying OTDR-based live-fibre testing and monitoring requires solid understanding of the stimulated Raman scattering (SRS) cross-effects between the OTDR's high-power pulses emitted in wavelength windows such as 1625 nm or 1650 nm and the traffic, generally emitting in the C and/or L band. To some degree, Raman scattering can affect both the OTDR dynamic range and the live signals. Minimising stimulated and spontaneous Raman scattering is therefore a must in order to achieve seamless integration of OTDR-over-traffic signals.
For detailed experimental procedures and results, and a few design rules to adequately address the Raman scattering problem, download Exfo's comprehensive white paper at www.dataweek.co.za/+dw3501. The purposes of this article are to present some background to the issues involved, and to summarise the important effects.
Spontaneous Raman scattering
Raman scattering can be spontaneous and eventually stimulated. The phenomenon starts occurring when signals in the medium get to an intensity level at which a small fraction of the incident wave starts generating new frequency-shifted waves propagating in both directions. This spontaneous emission occurs due to the fibre medium's vibrational modes, widely spread around the Raman gain peak, whose frequency is about 13 THz from the pump's wave frequency.
Spontaneous Raman scattering, which is usually a negligible feature for transmission systems, becomes an important limitation for live-fibre monitoring. This is due to the fact that a non-negligible fraction of the traffic signal is scattered in the OTDR wavelength band and adds to the Rayleigh backscattering measured by the OTDR.
Stimulated Raman scattering is a well-known limitation for ultra-high-power transmission. As its threshold is in the 1 to 2 W range, it is usually negligible for low- to medium-traffic power levels. In the context of live-fibre monitoring with long wavelength OTDR, the WDM traffic acts as a pump and the OTDR pulse acts as a signal that is being amplified. Since the OTDR power provides a seed for the amplification, Raman gain can be experienced at a power well below the usual threshold for stimulated Raman scattering. In the case of an OTDR pulse propagating at a wavelength where Raman gain exists, the OTDR pulse will acquire energy from the traffic and experience some gain. This energy transfer depletes the traffic bits' amplitude.
The parameters that can control the amount of SRS are the following (somewhat in order of importance):
* The transmission, or pump, signal intensity and distribution along the fibre.
* The OTDR pulses' peak power level and propagating direction (co- or counter-propagating relative to pump).
* Difference in frequency between the pump and the seed (here, the OTDR pulse).
* The fibre type used for transmission, from which a Raman gain coefficient is derived.
Effects summary
Coupling an OTDR to a live fibre for testing purposes requires adding WDMs and filters to protect transmit/receive equipment from the backscattered or transmitted OTDR signals. Component insertion loss, isolation and cross-talk specifications are the first issues to consider in the implementation of live-fibre OTDR testing. Once the link loss budget is established with these new components, system engineers should evaluate the impact of SRS on both the OTDR performance and, eventually, on traffic depletion.
The principal limitation to live-fibre monitoring at 1625 nm (and any wavelength within a large range about 100 nm beyond the transmitting signals), will come from the spontaneous Raman scattering noise that reaches the OTDR port. Since the OTDR is a very sensitive device, it takes a rather small CW signal at its input to raise the noise floor. This typically limits the maximum amount of traffic that can be injected in a live system.
Counter-propagation live-fibre OTDR testing, due to the non-synchronised nature of OTDR pulses and traffic signals, provides definite advantages, especially in case of rather long links, since it not only reduces the spontaneous noise reaching the OTDR, but also blurs the stimulated Raman scattering. In case of bidirectional transmission, OTDR power and transmission power levels may require adjustments so that the effect remains negligible.
The future of live-fibre testing and monitoring
Live-fibre testing and monitoring is defined as a non-intrusive test on a given fibre, whether it is one section or multiple sections carrying live data, voice or video signals, for assessing the quality of an optical fibre link. The use of an OTDR for detecting and locating degradations is more valuable on live fibres than on dark ones because one can measure, pinpoint and eventually correlate problems raised by other systems dedicated to measuring signal quality such as BER, Q-factor or other more sophisticated parameters.
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