Tunable semiconductor lasers with fast tuning speed (~ns) are key components for performing all-optical switching. It would be ideal to use advanced modulation formats and coherent detections in these systems to maximize spectral efficiency (SE) and throughput. However, switching tunable lasers can result in large and time-varying frequency offsets (FO) between the transmitter laser and local oscillator (LO) used at the coherent receiver, increasing the waiting time considerably before the signal can be demodulated correctly. One modulation format that was suggested to overcome this issue is doubly differential quadrature phase shift keying (DDQPSK), where switching time is significantly reduced. To further increase the SE, higher-order modulation formats are being considered to replace QPSK.
Prof. Yonglin Yu’s group from Wuhan National Lab for Optoelectronics (WNLO), cooperating with Prof. L. P. Barry’s group from Dublin City University, applied the doubly differential (DD) technology to the 16-QAM modulation format. As the mostly used square-16-QAM constellation diagram is not suitable to perform DD coding, the doubly differential star 16-ary quadrature amplitude modulation (DD-star-16-QAM) that has the same modulation efficiency is introduced. Simulations and experiments are then performed to investigate the performance of the proposed scheme, which demonstrate the ability of the DD-star-16-QAM to overcome the FO transients issue and increase the network efficiency by reducing the time after a wavelength switch when the signal can be demodulated successfully.
This work, “Doubly Differential Star-16-QAM for Fast Wavelength Switching Coherent Optical Packet Transceiver” published in Optics Express (vol.26, No.7, Apr 2018), was supported by National Natural Science Foundation of China (NSFC) (61675073); China Scholarship Council (CSC); 111 Program (B07038); Fundamental Research Funds for the Central Universities (2016YXZD004); Science Foundation Ireland (SFI) (IPIC-12/RC/2276 and CONNECT-13/RC/2077).
Fig. 1. Results for the time-resolved BER after a wavelength switch.
