Itu-t rec. G. 957 (06/99) Optical interfaces for equipments and systems relating to the synchronous digital hierarchy
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- 6.2.2 Spectral characteristics
| 6.2 Transmitter
6.2.1 Nominal source type Depending on attenuation/dispersion characteristics and hierarchical level of each application in Table 1, feasible transmitter devices include Light-Emitting Diodes (LEDs), Multi-Longitudinal Mode (MLM) lasers and Single-Longitudinal Mode (SLM) lasers. For each of the applications, this Recommendation indicates a nominal source type. It is understood that the indication of a nominal source type in this Recommendation is not a requirement and that SLM devices can be substituted for any application showing LED or MLM as the nominal source type and MLM devices can be substituted for any application showing LED as the nominal source type without any degradation in system performance. 6.2.2 Spectral characteristics For LEDs and MLM lasers, spectral width is specified by the maximum Root-Mean-Square (RMS) width under standard operating conditions. The RMS width or value is understood to mean the standard deviation ( σ ) of the spectral distribution. The measurement method for RMS widths should take into account all modes which are not more than 20 dB down from the peak mode. For SLM lasers, the maximum spectral width is specified by the maximum full width of the central wavelength peak, measured 20 dB down from the maximum amplitude of the central wavelength under standard operating conditions. Additionally, for control of mode partition noise in SLM systems, a minimum value for the laser side-mode suppression ratio is specified. Recommendation G.957 (06/99) 10 There is currently no agreed reliable method for estimating the dispersion penalties arising from laser chirp and finite side-mode suppression ratio for SLM lasers. Because of this, SLM laser linewidths and maximum fibre dispersion values for the L-4.2, S-16.1, S-16.2, L-16.1, L-16.2 and L-16.3 applications are under study. Until this study is complete, transverse compatibility for these systems cannot be guaranteed. Present indications are that spectral width definitions based on time-averaged spectral measurements can provide necessary, but not sufficient criteria for SLM devices. However, in combination with additional tests such as outlined below, such criteria could prove adequate. A possible need to specify dynamic laser characteristics more accurately is being recognized, particularly for long-haul systems. This includes associated measurement methods. One possible method is a fibre transmission test. Its configuration consists of a transmitter under test, test fibres with maximum dispersion specified for the maximum system length, and a reference receiver. The dynamic characteristics of the transmitter can then be evaluated using a bit error rate measurement. The above method could also be adapted for the purposes of laser acceptance testing. Thus, the laser would be evaluated by incorporation into the transmitter of an emulated transmission system. Lasers having acceptable spectral characteristics would be identified on the basis of satisfactory error performance of the emulated system. Any such arrangement, intended for use by laser manufacturers, would need to be made available in a form capable of periodic calibration and, if necessary, repair by a supplier of test equipment. These and alternative methods for characterizing laser dynamic performance are for further study. For SDH networks utilizing optical amplifiers, a transmitter with appropriate spectral characteristics is necessary to achieve target distances exceeding those defined for long-haul applications. tải về 235.99 Kb. Chia sẻ với bạn bè của bạn:
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