Dispersion compensation.

The G.652 fiber requires 10-30 km reserved in the WDM system. However, some G. 652 fibers require full or over dispersion compensation. Why?
Answer: The dispersion is related to the chirped return-to-zero (CRZ) or non-return-to-zero (NRZ) code instead of the advanced forward error correction (AFEC). For a 40-channel system using common code patterns, the G. 652 fiber requires 10-30 km reserved in the WDM system. For super CRZ, the relative dispersion compensation of each span should be in the range of 10 km. The G.655 and G.652 fibers share the same compensation principles.
The super WDM dispersion compensation follows the following principles:
The super WDM introduces a specific dispersion tolerance. The dispersion compensation should try to control the self-phase modulation (SPM) generated by lines and cancel light source chirp, receiving narrow spectrum in the receive end. In this way, the nonlinear effect will not cause a serious system penalty. The receive-end bit error rate (BER) performance is good. The sideband of the spectrum disappeared. A maximum dispersion tolerance of -300 ps/nm to +500 ps/nm can be obtained.
Dispersion compensation principles:
1. Select proper dispersion pre-compensation.
2. Use distributed compensation and control the accumulated dispersion values of optical signals at each incident location to control the SPM effect of optical fibers.
Different optical fibers have different nonlinear effect and dispersion. Therefore, the nonlinear effect and dispersion observe different compensation principles.
G.655 fiber dispersion compensation principles:
1. The G.655 fiber is over-compensated by 40 km, and under-compensated by 80 km. (An OSNR penalty no greater than 1 dB is introduced.)
2. The dispersion pre-compensation value depends on the number of spans. If the number of spans is greater than or equal to six and less than or equal to 11, the G.655 fiber is pre-compensated for 20 km. If the number of spans is greater than 11, the G.655 fiber is not pre-compensated.
3. The relative compensation dispersion value of each span is within 10 km. The relative dispersion compensation value should be offset if possible. That is, the incident dispersion of each optical line amplifier (OLA) station must be within the range of -10 km minus the pre-compensated dispersion to 10 km minus the pre-compensated dispersion. If 20 km is pre-compensated, the incident dispersion of each OLA station is in the range of -30 km to -10 km. If no pre-compensation is required, the incident dispersion of each OLA site is in the range of -10 km to +10 km.
4: If a span distance is long and out of the compensation range of a DCM module, the G.655 fiber can be under-compensated. (The maximum compensation value can be 160 km.) But the incident dispersion must be compensated in the next one or next two spans following the principle three.)
5. For a regeneration span whose span number is larger than 12, if the first span is long and the dispersion must be pre-compensated, 20 km can be pre-compensated. And the incident dispersion is compensated in the range of -10 km to +10 km in the next one or two spans.

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