Study On Mid-infrared Difference Frequency Generation Laser Source Based On MgO:PPLN Crystal

Trace gas detection and analysis can be achieved by using the interaction characteristics between the mid-infrared (mid-IR) light waves and the materials, since large amounts of gas molecules have intensive base absorption lines in 3-5μm waveband. Currently, mid-IR laser sources based on the quasi-phase matching difference frequency generation (QPM-DFG) schemes have been the preferred choice for spectrum detection application due to the advantages of simple structure, convenient tuning, room temperature running and no threshold limit, ect. In this thesis, a mid-IR DFG laser system based on MgO:PPLN is built and the DFG output properties of widely tuning and multi-wavelength operations are studied in detail. The main research work are as follows:Based on the three-wave coupling equations and the QPM theory, the physical process of DFG is firstly discussed. The frame scheme of a mid-IR DFG/QPM laser source is then designed. Through optimal design the parameters of signal and pump light sources, MgO:PPLN nonlinear crystal, optical collimating-focusing and DFG unit, the DFG/QPM laser source is constructed, and mid-IR DFG laser radiations are achieved. Then the output properties of the DFG laser system are systematically investigated. The experimental results show that, the QPM acceptance bandwidth for the crystal temperature and the pump wavelength are 4.8℃ and 5.2nm, respectively. The DFG tuning range can reach 390nm with the power fluctuation< 1% within 2 hours.In addition, to meet the multi-component gas synchronous detection applications, a novel DFG scheme for simultaneously achieving multi-wavelength mid-IR emission is proposed. Multiple idler QPM peaks can be obtained by using the segmented temperature controlling techniques on a MgO:PPLN chip. Moreover, the multiple idler QPM peaks can be widely tuned by adjusting the segmented crystal temperatures and thus the tunability for the generated multiple mid-IR wavelengths is realized.Based on the results described above, the application of trace gas detection based on the mid-IR laser system is preliminary explored. The extraction and processing methods of weak signals in the trace gas absorption spectrum are studied. Moreover, a preliminary gas detection scheme based on the DFG laser source is proposed, and a data acquisition system by using LabVIEW software is designed, which lay the foundation for the practical applications.