Low-cost coherent UDWDM-PON by hybrid DBPSK/ASK-2 modulation

The coherent ultra-dense wavelength division multiplexed passive optical network (UDWDM-PON) has been widely studied recently, in which the envelop detection based coherent reception of on-off keying (OOK) signal has been shown to possess both high receiver sensitivity and tolerance against laser linewidth/chirp. In order to increase the spectral efficiency (SE) to 2 bit/s/Hz, researchers formulated a hybrid DBPSK/ASK-2 modulation using the silicon Mach-Zehnder modulator (MZM), where both amplitude and phase modulation are employed. The experimental result shows that the proposed DBPSK/ASK-2 modulation is of better performance than PAM-4 at the same SE of 2 bit/s/Hz. When the low-cost silicon MZM and DFB laser of 4-MHz linewidth are employed, the receiver sensitivity of DBPSK/ASK-2 exceeds that of the PAM-4 by about 5.7 dB. This work can achieve about −46 dBm receiver sensitivity at 2.5 GBaud after transmission over 80-km standard single mode fiber (SSMF). The proposed DBPSK/ASK-2 modulation using low-cost silicon MZM and DFB laser is considered as a practical and reliable method for the future UDWDM-PON at the SE of 2 bit/s/Hz.