The use of reflective SOA (RSOA) to implement the Optical Network Unit (ONU)
in WDM PONs has been established as a promising device attributable to its simultaneous
amplification and modulation capabilities.
In RSOA-based ONUs, the seed light is generated at the central office and injected towards the ONUs.
It can be either a continuous wave or a modulated signal. At each of the ONUs, the seed light is
modulated with the corresponding data and sent back in upstream direction to the central office.
As light is exposed twice to link loss, amplification is required at each ONU, ideally coming
directly from the RSOA . In this application example, we show a full-duplex remodulating scheme.
The schematic is represented in
The downlink signal modulated in amplitude (ASK) is divided by an optical coupler into two parts at
the ONU side. The upper branch is used for downstream data detection. At the bottom branch, the signal
enters into the RSOA, where it is modulated with the upstream data, and is reflected back in upstream
direction. The optical amplifier operates in saturation and thus the two levels of the incoming
signal can be equalized. However, the impact of the downstream signal cannot be fully cancelled out
and induces amplitude ripple on the upstream signal, which leads to a system penalty.
The eye diagram of the optical signal generated by the RSOA is displayed in
where the residual modulation of the downstream data channel can be observed. In order to see the
influence of the extinction ratio of the downstream signal on the remodulated data, the amplitude
of the driver of the Mach-Zehnder modulator at the central office has been varied. As we can see
the bit error rate of the downstream signal is degraded when decreasing the driver amplitude (as
a consequence of the lower extinction ratio). The opposite trend is observed in
for the upstream signal (reduced impact of the downstream signal).
Keywords: Amplitude Shift Keying (ASK), remodulation, Reflective Semiconductor Optical Amplifier (RSOA), WDM PON
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.
 C. Arellano, C. Bock, J. Prat, K.-D. Langer, OFC-NOFC 2006, paper OTuC1, 2006.