Study On Multi-channel Interference Widely Tunable Semiconductor Laser

Monolithic widely tunable semiconductor lasers are key devices in photonic integrated circuits and fiber-optic communication systems.In dense wavelength division multiplexing(DWDM)systems,tunable semiconductor lasers can be used to replace fixed single-wavelength lasers so as to provide effective inventory management and sparing with tremendous reduction in cost.In the next generation reconfigurable optical networks,they can be utilized to provide automatic wavelength configuration,wavelength conversion and wavelength routing.At present,commercially available monolithic widely tunable semiconductor lasers are almost based on gratings to realize mode selection,such as distributed feedback laser array and distributed Bragg reflector based tunable lasers.However,fabrication of grating needs high-resolution lithography technologies,like electron beam lithography.Buried gratings need good quality of regrowth.Due to high costs,tunable semiconductor lasers are mainly used in the bone networks and gradually applied to the metropolitan area networks.Therefore,tunable semiconductor lasers with low cost,ease of fabrication and robust tuning mechanism are always pursued by researchers.In the last several decades,a lot of novel monolithic widely tunable semiconductor lasers are proposed and demonstrated,but performance of them still can't be comparable to the grating based tunable semiconductor lasers.In this article,a new monolithic widely tunable semiconductor laser,multi-channel interference laser,is proposed and demonstrated for the first time.Research works are focused on theoretical design,wavelength characterization,experimental demonstration and performance improvement of the MCI laser.Chapter two introduces the working principle of the MCI laser in the first place.The MCI laser is based on constructive interference of several arms with unequal length difference,so it can be fabricated with conventional photolithography so as to reduce the fabrication costs.There is an arm phase section on each arm so that the phase of each arm can be independently tuned,so the MCI laser is insensitive to the initial phase of each arm and consequently has large fabrication tolerance.Influence of the arm length difference,number of the arms and the shape of gain spectrum on the mode selection of the reflection section is discussed in detail.Overall considering the single-mode and tuning performance and complexity of wavelength control,eight arms are chose for the MCI laser.Then,realization of 1×8 splitter based on multi-mode interferometer(MMI)is discussed,including 1×8 MMI and 1×8 MMI based on cascaded 1×2 MMI.Self-imaging,fabrication tolerance and optical bandwidth of MMI are introduced and analyzed.An on-chip broad-band reflector is needed to integrate at the end of each arm so as to reflect the lights back.Multi-mode interference reflector(MIR)is a novel and easy-to-fabricate on-chip broad-band reflector.Working principle of MIR is simply introduced and one-port MIR is simulated by a hybrid method.After deciding the basic structure of the MCI laser,steady multi-mode rate equations are utilized to simulate the lasing performance of the MCI laser.Threshold currents,output optical power,side mode suppression ratio and tuning range are acquired.The MCI laser is theoretically predicted to have a tuning range of more than 40 nm and SMSRs above 40 dB across the tuning range.Chapter three introduces a new characterization scheme based on optimization algorithm for tunable lasers.Wavelength tuning of conventional tunable lasers usually need to adjust no more than three control currents,so they are normally characterized by sweeping control currents and monitoring the output optical spectra,output optical power or the junction voltage of the active section so as to establish the relationship of the output wavelength and control currents.Wavelength tuning of the MCI laser needs to adjust the phase of eight phase shifters simultaneously,so conventional characterization schemes are infeasible for the MCI laser.In order to solve the problem of wavelength control of the MCI laser,an optimized algorithm based characterization scheme is proposed,which utilizes the ratio of the power of the desired wavelength and the power except the desired wavelength to approximatively represent the SMSR so that proper control current settings can be found and mode hopping can be avoided.The proposed scheme is based on power measurement and takes advantage of optimization algorithm to improve efficiency,so it can be very fast.Characterization schemes based on spectral measurement,power measurement and junction voltage measurement are introduced firstly.After that,the optimization algorithm based characterization scheme is introduced in detail and the feasibility of the proposed scheme is demonstrated with two kinds of commercially available tunable semiconductor lasers.Chapter four introduces the fabrication and characterization results of the MCI laser.In the first fabrication spin,to demonstrate the feasibility of the MCI laser,both the active and passive waveguides used deep ridge structure.Through characterization,the MCI laser was demonstrated to have a tuning range of more than 53.6 nm and good single-mode performance with SMSRs higher than 40 dB across the tuning range,which is better than the simulation results.Because the active section was deeply etched,which increases the loss and Auger recombination of the active section,threshold currents of the MCI laser was relative high and thus the output power is small.Besides,semiconductor optical amplifier was successfully integrated with the MCI laser with a two-port MIR.The two-port MIR can be fabricated with the deeply etched passive waveguides of the MCI laser at the same time,so no extra fabrication processes are needed,which is advantageous for reduce the cost and fabrication complexity of the integrated device.Chapter five introduces the fabrication and characterization results of the improved MCI laser.In the second fabrication spin,the active section and the arm phase sections used surface ridge waveguide structure while the other passive waveguided were deeply etched.In order to reduce the losses arising from the mode mismatch of the surface ridge and deep ridge waveguides,a compact and efficient tapered transition structure was used to connect the surface ridge and deep ridge waveguides.After characterization,threshold currents of the MCI laser and MCI laser with integrated SOA were reduced to 17 mA and 17 mA respectively.Besides,wavelength control of the MCI laser was studied and the relationship of the output wavelength and the eight control currents was established for the first time.