Interband Cascade Lasers Based On V-coupled Cavity And Gas Sensing

Mid-infrared(IR)semiconductor lasers have shown great advantages in many applications such as gas sensing,environmental monitoring,medical diagnostics,and industrial process control.As the gas absorption lines are very narrow,single-mode lasers are required for sensitive detection of many gases.In addition,single-mode lasers that can be tuned over a wide wavelength range are desirable for identification of multiple species.Usually,distributed feedback(DFB)gratings are incorporated into semiconductor mid-IR lasers to achieve single-mode operation.However,the tuning range of a DFB laser is typically limited to a few nanometers and the fabrication of the DFB gratings is complicated and expensive.Alternatively,a semiconductor laser can be placed in an external cavity to achieve a single-mode output with a wide tuning range,but the extra mechanical parts outside the laser chip make the system bulky and less reliable.V-coupled-cavity lasers(VCCL)have been successfully implemented at telecommunication wavelengths with the advantage of no grating,compactness,and easy wavelength control.In this paper,we initially introduce the concept of V-coupled cavity to interband cascade lasers to develop monolithically integrated mid-IR widely tunable single-mode lasersInterband cascade lasers(ICLs)have emerged as efficient mid-IR semiconductor lasers and achieved room-temperature operation at the wavelength of 3-6μm.However,the device fabrication of GaSb-based ICLs much less mature as compared to that of the InP-based material systems for near-IR applications.Additionally,the ICL is based on a ridge waveguide that is deeply etched(through all the active cascade stages)for uniformly confining injected current over every cascade stage.In this dissertation,we will describe and discuss the design,fabrication and characterization of V-coupled cavity lasers based on a type-Ⅱ GaSb-based interband cascade structureAt the beginning,we introduce the ICL structure used in this work and design FP lasers.After the research on the structure and basic operation principle,we derive the threshold condition of the VCCL and obtain the main design factor.The simulation of the structure of the bend waveguide and half-wave coupler is achieved to optimize the parameter in the VCCL.Next,we elaborate the fabrication processes of GaSb-based lasers.We developed the wet etching and dry etching processes and the procedure of the passivation and via window for deep-etching ridges and facets with perpendicular sidewalls.Furthermore,as the contact layer is thin and easily destroyed during the procedure of via window,we add the processes of the isolation gap and a 100-nm-thick Ti/Au layer before ridge etching.Finally,GaSb-based VCCLs were fabricated as designed.After fabrication,we established a measurement system using liquid nitrogen to characterize the ICLs.We compare the measurement results of FP lasers with different structure parameters to calculate the internal loss and group index.Also,we determine the temperature and injection current dependent wavelength tunability.Then,we elaborate the LIV and spectral characteristics of two V-coupled cavity lasers.By changing the injection current at a fixed heat-sink temperature,a tuning range over 35 nm can be achieved with a side-mode-suppression-ratio(SMSR)up to 28 dB.The tuning range can be extended to 60 nm when combined with the adjustments of the heat-sink temperature.The laser output power from one facet of the long cavity and can be higher than 1 mW.we have demonstrated,to the best of our knowledge,the first monolithically integrated widely tunable single-mode mid-IR ICLs based on the half-wave V-coupled cavity configuration.Finally,as the mid-IR interband cascade VCCLs do not work at the room temperature at this time,we introduce a TDLAS gas sensing system based on a near-IR VCCL for multiple species detection.The VCCL was calibrated using a reference gas chamber.With the technology of direct absorption spectroscopy(DAS),CO2 with the concentration higher than 10%and CO with the concentration lower than 1%are detected and the linearity between the detection signal and the gas concentration is calculated to be 0.994 and 0.976.The latter one is improved to be 0.997 with the method of wavelength modulation spectroscopy(WMS),which is proved to be higher signal-to-noise ratio and detection sensitivity.This gas sensing system demonstrates the advantage of VCCLs in the application of optical gas detection and provides experimental foundation for the trace gas detection based on mid-IR VCCLs.