Research On Quantum Cascade Laser For Its Application In Trace Gas Detection

The fundamental absorption bands of many important gas species in trace gassensing applications are located in mid-infrared wavelength region. Quantum CascadeLasers(QCLs)，which can cover2.6-24μm range, are ideal light sources for trace gassensing applications. Though many individual indexes of the output characteristics ofQCLs have been improved dramatically, the requirements of many trace gas sensingapplications such as air pollution, medical diagnostics are comprehensive, single-mode,broad tuning range, high output power, low threshold, good beam quality devices are indemand. In order to fulfill the requirements, four aspects including one-dimensionalgrating, grating external cavity, sampling grating, tapered waveguide are used toregulate the performance of device. The primary results and achievements aresummarized as follows:(1) Quantum cascade lasers are used to trace gas detection. Two computing method,equivalent RCL oscillating circuit and finite element modeling, are applied in thesimulation of our own-developed gas detection setup based on photoacoustic effect. Themethane detection limit of our system is189nmol mol1using light source of7.6μmdistributed feedback(DFB) QCL.4.6m DFB-QCL is used to trace CO gas detection,and the detection limit is70nmol mol1.（2）Distributed feedback quantum cascade lasers operating at target wavelengthregion are designed. By the fabrication of first-order grating based on reaching thestrong CO absorption lines, single-mode lasers operating at2161.97cm-1operating inpulsed mode at room temperature are achieved. The QCLs can operate in continuouswave(CW) mode with temperature up to60°C. Single mode emission with a side modesuppression ratio (SMSR) above25dB is observed.（3）Tunable grating external cavity (EC) quantum cascade lasers emitting at λ~4.7μm,7.5m,8.3m respectively are realized. Alow threshold current density of0.901kA/cm2for4.7μm EC-QCL operated in CW mode is realized. Single-mode CWoperation with a side-mode suppression ratio of30dB is achieved. Wide tuning range of131cm1in pulsed mode and116cm-1in CW mode are observed.(4) We use the buried sampling grating to manufacture broad wavelength range of QCLs. The multi-wavelength lasers are fabricated simultaneously on a single wafer bydifferent sampled gratings. Single-mode lasing at different wavelengths with a meanside mode suppression ratio (SMSR) above20dB is obtained. A wide wavelength rangeof238nm (~106cm-1) between4.87μm and4.63μm is achieved in pulsed mode at roomtemperature with different sampling periods.（5）Tapered sampling grating QCLs are fabricated. The devices can operate insingle longitudinal mode and transverse mode simultaneously. The single-facet outputpower is1134mW in pulsed mode, the single-lobe far-field divergence angle is13.6o,and beam quality factor is1.27. And we realize the single longitudinal mode taperedQCLs operate in CW mode. The emitting wavelength varies from4.89μm to4.65μm.When the current is800mA, the CW output power is204mW.