Study On Tunable Semiconductor Lasers And Photonic Integrated Devices

Optical fiber communication is one of the main transmission methods in modern communication networks.Since network services such as cloud computing,cloud storage,"internet+" continue to rise in recent years,the demand for transmission speed is increasing.The main target of optical fiber communication is high speed,large capacity,and low cost.Systems with separate devices can no longer meet the demand due to the cost and speed bottleneck,and then photonic integration has become an inevitable trend in the development of optical fiber communication devices.Especially in the 21st century,the development of large-scale photonic integrated chip is coming into a booming period.Integrated tunable laser is the main light source in optical coherent communication systems and next-generation passive optical networks(PONs).It has huge market potential.However,the tunable laser chips used in China nowadays are mainly imported from US or Japan due to various reasons,that is,the chip volume and price are still controlled by others.Therefore,the research and development of tunable laser chips have economic and social significance.In the thesis,based on the demand of optical fiber communication systems,tunable lasers are studied both theoretically and experimentally.Monolithic integration on indium phosphide(InP)and hybrid integration on silicon are two main technologies for integrated chips.The thesis focuses on tunable distributed feedback(DFB)semiconductor laser,and both of the integration platforms are studied.The work relates to the design,fabrication,measurement and analysis of the laser chip.The experiences and results obtained from the research work will benefit the follow-up study.The main results of the thesis are:1.A cascaded tunable DFB laser on InP is developed.The proposed laser combines the cascaded configuration and the reconstruction equivalent chirp(REC)technology.Tunable laser with eight laser array sections is realized.It covers 32 wavelength division multiplexing(WDM)channels with 100GHz spacing,that is,the wavelength tuning range is 25.6nm.REC technology is used to precisely control the laser wavelength,in which the phase-shifted gratings can be realized by holographic exposure combining with conventional lithography.The cascaded configuration does not need a multiplexer,which means the active and passive integration technology can be avoided,further reducing the manufacturing difficulty and cost.The thesis discusses the basic principles of REC technology;the design details of the tunable laser,such as the methods to reduce the crosstalk between adjacent sections and the methods for signal modulation.In addition,the thesis develops the fabrication process of the tunable laser,which will benefit the future study.2.A phase-shifted DFB laser with linearly chirped grating is developed.The bandwidth of chirped grating is large,so it is promising to design and fabricate tunable lasers.The thesis uses equivalent chirp structure to simplify the fabrication process,trying to achieve tunable properties.Although the expected performance is not observed,the phased-shifted DFB laser with linearly chirped grating is studied in depth by simulation and experiment.For example,the photon distribution in the proposed laser is more flat than conventional lasers,and the photon distributions for long wavelength and short wavelength are different.It determines the different spectral shape from the two facets of the laser,which is verified in the experiment.In addition,the chirp factor changes with the injection current,and it will cause the spectral broadening or mode hoping.3.A hybrid integrated tunable DFB laser on silicon is developed.A bonding process based on benzocyclobutene(DVS-BCB)is used to combine the silicon on insulator(SOI)chip and InP epitaxy together.The gratings,waveguides,multiplexers,and other passive components are fabricated on SOI chip,and the light from InP material can be coupled into the silicon waveguide by a taper structure.Comparing with conventional hybrid integrated lasers,the tunable laser has a pnp configuration for electrical tuning,and two gratings with different sampling periods to achieve wide tuning range.The thesis discusses the confinement factor and loss of the mode;design of the grating structure and taper structure;the series resistance and threshold current estimation;the thermal problem and the methods to improve it,etc.The thesis also develops the fabrication process of the tunable laser,including the waveguide and quantum well etching,device planarization,electrode deposition and other issues.Optically pumped lasing and lasing under pulsed current source are observed.The innovations of the thesis are:1.It is the first time to combine the REC technology and the cascaded configuration to achieve tunable DFB lasers.REC technology has the advantage to precisely control the laser wavelength,while the cascaded approach can avoid the use of a multiplexer.Both of them will simplify the fabrication process and reduce the cost.2.It is the first time to combine the tunable twin guide structure and the bonding technology to realize an electrically tunable laser.The tuning speed is faster than thermally tunable lasers,so it can be used in fast optical switching systems.