Hi,
As we know studies have been carried out on NG-PON2 enabling technologies, such as 40 G TDM-PON,wavelength division multiplexed PON (WDM-PON), time and wavelength division multiplexed PON (TWDM-PON) and orthogonal frequency division multiplexing PON(OFDM-PON).
And among all these technologies, TWDM-PON has beenselected as the best candidate for NG-PON2 because itsupports backward compatibility, flexibility and static sharing.

At the OLT side, a set of laser diodes such as distributedfeedback (DFB) laser diodes operating at different wave-lengths serve as downstream laser sources, followed by aWDM for multiplexing.
Therefore, ONU must contain tunable trans-mitters and receivers devices. A tunable filter at thereceiver is used to select or tune to any of the fourdownstream wavelengths. In case of upstream wave-lengths, the tunable laser is used to provide colorlessONU (free operation wavelength) to enable easier networklaying and maintenance.
The TWDM-PON ONU tunable transmitter is used totune its wavelength to any of the upstream wavelengths.
any techniques have been developed as candidates ofupstream laser source, including spectral slicing of broad-band sources, remote reflective modulators, and injectionlocking. The most common lasers for upstream lasersource are the directly modulated Fabry-Perot laser diode(FP-LD) and the reflection semiconductor optical amplifier(RSOA.
n addition to RSOA and FP-LD laser sources, there areother lasers such as DFB laser with temperature control(TC), DFB laser with partial TC multi-section distributedBragg reflector laser (electrical control) without cooling,external cavity laser (ECL) with mechanical control with-out cooling, and ECL with thermo/electro/piezo/magneto-optic control without cooling.

Thanks,