Extending PON reach or/and splitting ratio requires the system designer to amplify both up- and downlink signals. Semiconductor optical amplifiers (SOA) are often used
for this purpose as they amplify a relatively large spectrum (>40 nm), are compact (~1 mm) and potentially low-cost compared to EDFA. However, SOAs suffer from relatively
high noise figures (~10 dB) and from cross-gain modulation (XGM). XGM is caused by the fast gain dynamics of the SOA (~100 ps). This results in patterning effects
in single-channel systems and also in channel crosstalk in WDM systems limiting the system performance.
In this example (see ) the WDM technique (2x3 lambdas) is used to increase the number of users (3x64) of a classical TDM PON . A so-called
"extender", consisting of SOAs, is placed after the 80-km fiber trunk to amplify the up- and downlink signals (located in the 1500 and 1300 nm bands respectively). In such
configuration, optical packets coming from Optical Network Units (ONUs) operating at different wavelengths can interfere in the SOA, leading to XGM-induced crosstalks.
Directly modulated DFB lasers are used at the ONU side to generate the 2.5-Gb/s upstream signals. The 10-Gb/s downstream is generated using external EA modulators at the Optical
Line Termination (OLT).
For the low SOA gain, XGM is reduced, but signal performance is limited by noise (), while at the high gain, XGM affects the signal().
The injection current of the 1300-nm band SOA (upstream) is varied to find the best trade-off between noise and pattern effect ().
The different amplitude levels of the 'marks' observed in , result from crosstalk of neighboring channels emitting their own random bit streams. This pattern-dependent,
deterministic distortion shouldn't be treated as noise, but it should be treated properly using advanced BER estimation methods (as shown in ).
Keywords: SOA, PON, Burst, Transient, Optical Amplifiers, Dynamics, BER Estimation
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 F.T. An et al., "SUCCESS: A Next-Generation Hybrid WDM/TDM Optical Access Network Architecture", JLT, Vol. 22, Nov. 2004.