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Overview

Impact of Burst Mode in Extended WDM-TDM GEPON

Description

Extending PON reach or/and splitting ratio requires 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 (~10dB) 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 system and also in channel cross-talk in WDM systems limiting the system performance.

Typical Results

In this example (see Figure 1) the WDM technique (2x3 lambdas) is used to increase the number of users (3x64) of a classical TDM-PON [1]. A so-called "extender", consisting of SOAs, is placed after the 80km fiber trunk to amplify the up and downlink signals (located in the 1500 and 1300nm 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 cross-talks.
Directly modulated DFB lasers are used at the ONU side to generate the 2.5Gb/s upstream signals. The 10Gb/s downstream is generated using external EA modulators at the Optical Line Termination (OLT).
For low SOA gain, XGM is reduced but signal performance is limited by noise (Figure 2), while at high gain XGM affects the signal (Figure 3). The injection current of the 1300nm band SOA (upstream) is varied to find the best trade-off between noise and pattern effect (Figure 4).
The different amplitude levels of the 'marks' observed in Figure 3, result from cross-talk of neighboring channels emitting their own random bit streams. This pattern dependent, deterministic distortion shouldn't be treated as noise, but treated properly using advanced BER estimation methods (see Figure 4).

Keywords

SOA, PON, Burst, Transient, Optical Amplifiers, Dynamics, BER Estimation