Research On Directly Modulated DBR Lasers And Applications For Access Network

In information era,the explosively growing network data has been drving new demands on the transmission capacity,flexibility and dynamics of optical communication networks.Tunable semiconductor lasers,as a kind of optical source devices capable of fast wavelength switching,have very important applications in the future dynamic optical network.As a key component in WDM system and all-optical switching networks,tunable semiconductor lasers are attracting more and more attention.With the development and application of passive optical networks based on wavelength-division multiplexing(WDM)and time-and wavelength-division multiplexing(TWDM),low-cost and high-speed directly modulated tunable semiconductor lasers gradually become a research hotspot.In this thesis,based on the basic theory of tunable semiconductor lasers,an in-depth analysis and theoretical research on direct modulation characteristics of three-section distributed Bragg reflector(3s-DBR)lasers is presented first.Then,directly modulated 3s-DBR lasers are studied for applications in 5G mobile fronthaul network(MFH).Furthermore,a novel tunable high-speed chirp-managed laser is proposed.The main content of this thesis is as following:(1)Based on the small-signal model and the chirp model,the direct modulation characteristics of the 3s-DBR laserare studied theoretically and experimentally,including modulation bandwidth and frequency chirp characteristics.Furthermore,a new method based on the impulse response is proposed for calculation of the the frequency response of the 3s-DBR laser.The frequency response obtained by fast Fourier transform of the impulse response of the laser,can be used to characterizes the small signal modulation response of the laser.Moreover,influences of the structure and material parameters of the 3s-DBR laser on its modulation bandwidth are discussed,which provides some guidance for the device design and optimization.(2)The mobile fronthaul network based on RoF-WDM-PON is investigated by experiments,and the feasibility of directly modulated 3s-DBR lasers in the system is demonstrated.The WDM-PON network architecture can effectively meet requirements of the high bandwidth and ultra-low latency data transmission in the future 5G mobile fronthaul network,while the use of radio-over-fiber(RoF)technology in the link can significantly improve the signal transmission capacity and spectrum efficiency.In the experiments,a directly modulated tunable 3s-DBR laser is used to generate RoF signal,and multi-band filtered-OFDM signal transmission over different lengths of fiber with different wavelength channels are studied.Experimental results verify the ability of the 3s-DBR laser for generating the multi-carrier analog format signal,and its wavelength tunability is inherently compatible with the dynamic and high spectral efficiency fronthaul links,this will be critical to support the development of future optical mobile communications.(3)Effect of the laser chirp and the fiber dispersion on the transmission of RoF signal is studied theoretically.The RF power fading and composite second order(CSO)distortion caused by interplay between the laser chirp and the fiber dispersion are analyzed through theoretical calculation.For the application of direct modulation 3s-DBR laser in MFH based on RoF-WDM-PON,by using the chirp compensation scheme of DBR type tunable laser,i.e.applying reversed modulation current of the active section to the phase section,the laser chirp can be reduced from about 30 GHz to below 8GHz.Results of transmission simulation show that the laser chirp is significantly reduced after using the chirp compensation,and the RF power fading and CSO distortion of RoF signal after optical fiber transmission are effectively supressed,verifying the effectiveness of the compensation scheme.(4)A TPSG-DBR laser based on cascaded tilted ? phase shift grating and uniform DBR grating is proposed for high-speed direct modulation.Static and dynamic output characteristics of the laser are simulated by using TLM model and TDTW model.Simulation results show that the detuning effect and optical spectrum reshape(OSR)effect in the TPSG-DBR laser can be enhanced by utilizing the cascaded grating,leading modulation speed up to 25Gb/s and extinction ratio(ER)up to 6.2dB.Also,the frequency chirp can be reduced.Fiber transmission simulations show that,compared with the conventional 3s-DBR laser,the dispersion power penalty induced by the fiber dispersion and the frequency chirp of the TPSG-DBR laser has been significantly reduced.