The Investigation Of Mid-Infrared Difference Frequency Laser Generation Based On Ppln Crystal

Mid-IR light source generated by Difference-frequency generation (DFG) based on a periodically poled lithium niobate Crystal (PPLN) in quasi-phase matched (QPM) technique has been demonstrated in recently year. Unlike in the birefringent phase matched bulk configuration, confinement of light in the PPLN crystal has no trade-off issue of beam size and interaction length. As a result, a long interaction length and thus high conversion efficiency can be achieved in a PPLN. Since the effective refractive index n is different for each wavelength, there are multiple sets of wavelengths that satisfy both the energy-conservation equation, and the momentum conservation equation, in a single PPLN with a given QPM period. As a result, a relatively broad tuning range can be obtained with a single given QPM period PPLN.In the mid-IR region, difference-frequency generation based on QPM PPLN has Impressive advantages such as a wide wavelength conversion bandwidth and a narrow line-width. A laser based sensor operating in the mid-IR spectral region is favored in the application of trace gas detection, since that region contains a large number of strong fundamental absorption bands of most atmosphere trace gasses. Mid-IR based gas sensor has wide applications include air-quality control in large buildings, hospitals, and aircraft, monitoring of emissions in mining and drilling, and combustion diagnostics.Due to the excellent properties of Mid-IR DFG sourse described above, in this thesis a broadband cw difference-frequency radiation based on PPLN has been investigated. First the principle of difference-frequency generation based on QPM PPLN, its advantages and characters are described in detail. Assuming pumped by a fixed-wavelength Nd-doped solid-state laser and mixed with a tunable near infrared laser in a nine QPM periods PPLN, numerical simulation indicate that a wide tuning range of IR light can obtained using the combination of varying the crystal temperature and QPM period. The energy conversion efficiency is investigated numerically for both plan wave and Gaussian beam. According to these calculations, experimental parameters for DFG are obtained and used for real experiments.