Research And Design Of High Speed Directly Modulated Semiconductor Lasers

With the rapid development of multimedia service,data center and cloud computing,people's demand for network bandwidth is increasing.Directly modulated semiconductor lasers have important applications in access and data communication systems.The requirements of the access network and data communication system are high-speed and low cost.In this thesis,we makes an in-depth study on how to improve the modulation bandwidth of semiconductor lasers.In addition,this thesis also proposed some solutions for cuting the cost for the photoelectric device.To improve the modulation bandwidth of the directly modulated laser(DML),we use following methods.Firstly,a full physics based model is used to systematically study the modulation bandwidth of the directly modulated strained-layer multiple quantum well(SL-MQW)laser in this thesis.The active region of the directly modulated laser(DML)is optimized in terms of the number of QWs,barrier height and doping concentration,as well as the graded-index separate confinement hetero-structure(GRIN-SCH).To compromise the device dynamic performance at different operating temperatures,we present an overall optimized design for a 25Gbps DML under an ambient temperature ranging from 25 to 85 ?.To further enhance the modulation bandwidth,we've also proposed a mixed QWs design that increases the 3dB bandwidth by almost 44%compared to the one without undergoing optimization.The experimental results show that the 3dB bandwidth of the optimized DML can reach 19 GHz.A clear eye diagram with a bit rate of 25Gbps was observed at 25 ?.Then,we propose a DFB laser with a horn ridge waveguide(HRW)to suppress the longitudinal spatial hole burning effect in the lasers cavity,thus to reduce the roll-off at low-frequency,which greatly improves the modulation bandwidth of the DFB laser with large kL.Finally,this thesis studies the push-pull modulated DFB lasers.By optimizing the structure of the DFB laser,to make the photon-photon resonant(PPR)frequency of the push-pull modulated semiconductor laser is as close as possible to the electron-photon resonance(CPR)frequency,as thus,the photon-photon resonant peak can be used to compensate rolling down of the laser frequency response at high frequency.This can achieve greatly improve the purpose of laser modulation bandwidth.The theoretical simulation results show that the modulation bandwidth of DFB laser with push-pull modulation DFB laser with second order grating can be up to 50GHz.To cut the cost for the photoelectric device,we propose a novel approach to enhance the single longitudinal mode(SLM)yield for uniform grating distributed feedback(DFB)laser diode by breaking the dual-mode degeneracy.By introducing a horn ridged waveguide(HRW)in replace of the conventional straight ridge waveguide(RW),we create an effectively chirped grating as well as a longitudinally varying modal gain.As such,the field distribution along the laser cavity is different for the two possibly lasing longitudinal modes on each side of the Bragg stopband,and the field having a larger overlap with the gain becomes the only dominant lasing mode.We have optimized the ridge width of HRW-DFB lasers with different normalized coupling coefficients(kL)in terms of the SLM yield.Our experimental results show that the SLM yield of the HRW-DFB laser with kL of 2.9 can reach 65%as opposed to 19%obtained by the conventional RW-DFB lasers fabricated on the same wafer.In addition,in order to improve DFB lasers' immunity to external optical feedback to save the cost of the system by removing the optical isolators in the system,we propose an external optical feedback resistant distributed feedback(DFB)laser diode(LD)by exploiting parity-time symmetric complex coupling in this thesis.With its complex refractive index followed a parity-time symmetry,the grating shows a strongly asymmetric reflection to the contra-propagating light inside the DFB cavity,which effectively rejects the returning light from one end.Consequently,the DFB LD is much less sensitive to external optical feedback.On the contrary,the transmissivity of such grating is still symmetric so that the output light of the DFB LD is not affected.Numerical simulation result shows that the lasing wavelength drift can be less than 0.2 nm with a SMSR exceeding 45 dB under a coherent external optical feedback as high as-10 dB.In summary,the influence of the parameters of the quantum well active region on the modulation bandwidth of the laser is investigated,which provides theoretical guidance for designing high-speed direct modulation semiconductor lasers.At the same time,we proposed and manufactured a horn ridge waveguide DFB laser to improve the single longitudinal mode yield of DFB lasers in the thesis.We also proposes a PT symmetric complex coupled DFB laser to improve the DFB lasers' immunity to external optical feedback,which can be used isolator-free systems to save the cost for optical isolator.