Investigation Into The Phase Noise Of Tunable Semiconductor Lasers And Its Effect On Coherent Communication Systems

Due to the rapid development of bandwidth-hungry applications,e.g.cloud computing,Internet of things,big data,there is a constantly increasing demand for higher capacity in networks.One key technology for the next-generation optical network is wavelength tunable lasers,which can change wavelength flexibly.To further increase the spectral efficiency of the optical communication systems,advanced modulation formats and coherent detections are preferred.However,the linewidth requirement is much more stringent than before when the coherent systems are employed,especially putting forward a greater challenge for tunable lasers.Therefore,it is meaningful to investigate the phase noise of distributed Bragg reflector?DBR?based wavelength tunable lasers and its effect on the coherent communication systems,making the tunable lasers more suitable for high-speed coherent optical networks.In this thesis,this issue is investigated deeply by laser simulation model and coherent transmission experiments.The limiting factors for the tunable lasers used in static and switching coherent communication systems are analyzed,the digital signal processing?DSP?and modulation formats are then modified based on the analysis.The research works can be summarized as follow:?1?We have investigated the effect of nonlinear gain on the phase noise of tunable semiconductor lasers.The phase noise is first analyzed by experiments,and some interesting phenomena are observed.In order to explain these,multi-mode rate equations including Langevin noise are employed to study the effect of nonlinear gain on the phase noise.The simulated results show that the fluctuation of the side-mode can result in excess phase noise,which is more obvious when the side mode is on the long wavelength side of the main mode.In addition,a bi-stable state can be introduced by the nonlinear gain,leading to the phase noise hopping.This research is useful to understand the complicate phase noise characteristics of semiconductor lasers,and the experimental results have been explained properly.?2?The phase noise of Y-branch lasers is investigated by experiments and simulations,mainly focusing on the influence of the additive Vernier effect.A comprehensive laser model is developed by considering spontaneous emission noise in the active section,carrier noise in the passive sections,1/f noise of the injection currents and the de-tuning effect induced by the DBR.The difference between the multiplicative and additive Vernier effect is studied by comparing the simulated results of the Sampled Grating-DBR?SG-DBR?laser and the Y-branch laser.It indicates that different supermode selection methods can result in different reflection characteristics,then leading to different phase noise,so larger linewidth of the Y-branch laser is introduced by the additive Vernier effect.Finally,the phase noise is measured by experiments and the results are in agreement with the simulations,proving the reliability of the simulation model.?3?A carrier phase estimation method based on Kalman filter is proposed to overcome the excess phase noise of fast tunable lasers.The phase noise of an SG-DBR laser is first analyzed,and the state parameter of the Kalman filter is modified.Then data transmission simulations are performed with the excess phase noise taken into account,which shows that the proposed scheme is able to mitigate the excess phase noise and it is suitable for parallel processing.These results are further investigated by experiments,which shows the Kalman filter can obtain a lower BER than the 2^nd order phase lock loop?PLL?due to a better linewidth tolerance.At last,the transmission performance is tested in the whole tuning range?30 nm?of the SG-DBR laser,which further proves the feasibility of the proposed scheme.?4?In order to increase the efficiency of the reconfigurable networks employing tunable lasers,a doubly differential star 16-ary Quadrature Amplitude Modulation?DD-star-16-QAM?format is proposed.The coding and decoding processes for this modulation format are initially presented and the transmission performance is investigated in static and dynamic scenarios.The static results demonstrate that this modulation format can cancel out the influence of the frequency offset?FO?.In the switching case,where the laser has time-resolved linewidth and frequency,DD-star-16-QAM can overcome the effect of time-resolved frequency and the waiting time of the receiver after the wavelength switching is determined by the time it takes the dynamic linewidth to reach the requisite level.Therefore,the proposed modulation format can achieve a short switching time than that of the standard 16-QAM,which is limited by the time-varying FO.