Itu-t rec. G. 957 (06/99) Optical interfaces for equipments and systems relating to the synchronous digital hierarchy
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| 6.3.3 Reflections
Reflections are caused by refractive index discontinuities along the optical path. If not controlled, they can degrade system performance through their disturbing effect on the operation of the laser or through multiple reflections which lead to interferometric noise at the receiver. In this Recommendation, reflections from the optical path are controlled by specifying the: – minimum Optical Return Loss (ORL) of the cable plant at point S, including any connectors; and – maximum discrete reflectance between points S and R. The possible effects of reflections on single fibre operation using directional couplers have not been considered in this Recommendation and are for further study. Measurement methods for reflections are described in Appendix I. For the purpose of reflectance and return loss measurements, points S and R are assumed to coincide with the endface of each connector plug (see Figure 1). It is recognized that this does not include the actual reflection performance of the respective connectors in the operational system. These reflections are assumed to have the nominal value of reflection for the specific type of connectors used. The maximum number of connectors or other discrete reflection points which may be included in the optical path (e.g. for distribution frames, or WDM components) must be such as to allow the specified overall optical return loss to be achieved. If this cannot be done using connectors meeting the maximum discrete reflections cited in Tables 2 to 4, then connectors having better reflection Recommendation G.957 (06/99) 13 performance must be employed. Alternatively, the number of connectors must be reduced. It also may be necessary to limit the number of connectors or to use connectors having improved reflectance performance in order to avoid unacceptable impairments due to multiple reflections. Such effects may be particularly significant in STM-16 and STM-4 long-haul systems. In Tables 2 to 4, the value –27 dB maximum discrete reflectance between points S and R is intended to minimize the effects of multiple reflections (e.g. interferometric noise). In Tables 3 and 4, the value − 27 dB for maximum receiver reflectance will ensure acceptable penalties due to multiple reflections for all likely system configurations involving multiple connectors, etc. Systems employing fewer or higher performance connectors produce fewer multiple reflections and consequently are able to tolerate receivers exhibiting higher reflectance. As an extreme example, if only two connectors exist in the system, a 14 dB receiver return loss is acceptable. For systems in which reflection effects are not considered to limit system performance, no values are specified for the associated reflection parameters and this is indicated in Tables 2 to 4 by the entry NA (not applicable). However, when using this Recommendation for a particular application, it should be noted that if upgradability to other applications having more stringent requirements is contemplated, then these more stringent requirements should be used. The possible need to develop a specification for transmitter signal-to-noise ratio under conditions of worst-case optical return loss for the applications in Tables 2 to 4 is for further study. tải về 235.99 Kb. Chia sẻ với bạn bè của bạn:
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