| Recently, near infrared lasers with emission wavelength range between 1.6μm and 2.1μm are attractive for using as light sources of trace-gas sensing because of the strong absorption lines of various gases, such as:CH4, HCl, H2O COand NO. High-strained InGaAs/InGaAsP multiple quantum well (MQW) distributed feedback (DFB) laser with the grating fabricated in the upper SCH layer can obtain a stable single-mode operation with the line width of several million hertz which is much narrower than the absorption lines of gas molecules (hundred million hertz). And the laser wavelength can also be tuned in the gas absorption lines accurately by adjusting the laser current and temperature, so it is a suitable light source for gas sensor system. The purpose of this dissertation is to design and fabricate the high-strained MQW DFB lasers with long wavelengths for gas sensing system, and the following achievements have been obtained:1. optimized the processing technique of 1.65-μm distributed feedback lasers with InGaAs/InGaAsP multiple-quantum wells for methane gas sensing system. Presently, we had basically realized small batch production of 1.65μm wafer for the detection of methane. The threshold current of less than 25 mA, a continuous wave (CW) output power of more than 10 mw, the external differential quantum efficiency of higher than 20%, and the stable single-mode operation with a line width of less than ten million hertz and a side-mode suppression ratio of more than 40 dB were achieved for the devices. All the chips passed the aging procedure, so we predicted that the life of the laser is about 11 years. After packaged in a compact module, the output power of the device was more than 8mW at Ith+80mA, which meets the requirements of methane gas sensing completely.2. High-strained InGaAs/InGaAsP MQW DFB lasers fabricated using metal organic chemical vapor deposition (MOCVD) were presented with an emission wavelength of 1.7945μm for Nitric Oxide gas sensing system. Samples were grown with ten seconds interruption in the InP buffer layers to improve the quality of epitaxial layers and carrier blocking layer was adopted to enhance the temperature stability of the device. Threshold current of 19.7 mA and a continuous wave (CW) output power of 9.9 mw were achieved under an operation current of 100 mA at 20℃for the uncoated 300-um-long devices. From -20℃to 50℃, the threshold currents were between 10-35 mA. The device kept single-mode operation at a large current range, and wavelength was tuned continuously by current at a rate of 0.012 nm/mA. The characteristic temperature, T0=58K, was comparable to that of conventional 1.55μm DFB laser.3. High-strained InGaAs/InGaAsP MQW DFB lasers fabricated using MOCVD are presented with an emission wavelengths of 1.82μm,1.84μm,1.86μm, and 1.9μm for H2O sensing system. The growth interruption of ten seconds in the InP buffer layers and a strained buffer layer in the SCH layer were employed to improve the quality of epitaxial layers. And carrier blocking layer was adopted to enhance the temperature stability of the device. The side-mode suppression ratio of 1.82-μm DFB lasers were more than 30 dB. The current-tuning rate and temperature-tuning rate of the DFB wavelength were almost linear at 0.013nm/mA and 0.12nm/℃, respectively, and the characteristic temperature To is 58K. The threshold current of less than 25 mA and a continuous wave (CW) output power of more than 8 mw were achieved under an operation current of 100 mA at room temperature for the uncoated 300-μm-long devices. All the chips passed the aging procedure, so we predicted that the life of the laser is about 11 years. After packaged in a compact module, the output power of the device was more than 3mW at 60mA, which meets the requirements of H2O gas sensing. Stable single-mode operation of 1.84-μm DFB lasers with a side-mode suppression ratio of more than 30 dB was obtained, and the emission wavelength of the laser was finely tuned by the injection current at 0.011 nm/mA. The current-tuning rate of 1.86-μm DFB lasers with a side mode suppression ratio of 38.9 dB was 0.014nm/mA. The threshold current and output power was approximately 40 mA and more than 7 mw for the uncoated 600-μm-long devices. The sidemode suppression ratio of 1.9-μm DFB lasers was more than 25 dB, and the threshold current was about 30 mA for the uncoated 600-μm-long devices.Based on the application in light sources of CH4, NO, H2O gas monitoring, we fabricated six types of DFB lasers with emission wavelengths range between 1.6μm and 1.9μm in this work. |
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