Mid-infrared Ppln Optical Parametric Oscillator Technology Research

Mid-infrared lasers (3-5μm) have many applications. Mid-IR optical parametric oscillator (OPO) can offer a unique combination of wide wavelength tunability, compact configuration, high power and high energy output. Because of its large nonlinear coefficient, wide transparent range, multiple choices of wavelength tuning, periodically poled lithium niobate (PPLN) becomes the most universally used nonlinear crystal for research in and abroad. At present, the average output power and single pulsed energy of mid-IR lasers from PPLN-OPO are over 10W and 100mJ, which are the highest in the mid-IR OPO field. This dissertation is a general research on mid-IR PPLN-OPO.The heat expansion effect of PPLN crystal towards the output wavelength of OPOs is investigated. The theoretical wavelength tuning curves of PPLN-OPO are calculated. The broadening of the line width in OPO is fully studied. The continuous, precise and wide tunability of PPLN-OPO is achieved based on the temperature adjusting. When the temperature of a 31.2μm PPLN is changed from 25℃to 200℃, the wavelength tuning ranges of the idler and signal lasers are 2.65-3.12μm and 1.61-1.78μm, respectively. The average adjusting precision is about 0.3nm/0.1℃. The heat expansion effect of PPLN would be ignored at low temperature, but the difference of the central wavelength caused by the heat expansion is about several ten nms at higher temperature. Besides the tunability of PPLN-OPO, the high power output characteristic is also studied.The highest power ever reported of 2.7μm and 3.8μm lasers based on optical parametric technique is achieved when PPLN-OPO is pumped by an elliptical beam. The threshold, conversion efficiency, acceptable angle and line width of a singly resonated OPO are calculated. The 1.064μm laser from an acousto-optical Q-switched Nd:YAG laser pumps the singly resonated PPLN-OPO after the coupled system which adjusts the beam size and shape into a proper elliptical beam to match the end face of the crystal. When the PPLN crystal with a grating period of 31.9μm is operated at room temperature, the central wavelengths of the signal and idler laser are 1.75μm and 2.71μm, respectively. When the pump power is 175W at 7kHz, the total output power of PPLN-OPO is 95.6W, conversion efficiency is 54.6%. The power of the 2.71μm laser is 35.2W,the corresponding conversion efficiency is 20.1%, the pulsed width is 72ns, beam quality is about Mx2=3.59, My2=6.26, respectively. Through the similar method, average power of 22.6W at 3.8μm is obtained when pumped by a 145W 1.064μm laser at 7kHz. The beam quality is about Mx2=3.68, My2=7.24, respectively. The mid-IR OPO pumped by a 1μm laser generates a near-IR signal laser around 1.5μm besides the mid-IR idler laser around 3-5μm. If the mid-IR idler laser is the desired output, the greater power of the near-IR signal laser of a common OPO will be wasted. The final output power of the mid-IR laser is restricted by the quantum conversion efficiency and photon energies. If the signal laser is used as a pump laser to generate a secondary OPO, new wavelength mid-IR lasers will be obtained, this process is called tandem OPO (TOPO). The conversion efficiency from the pump laser to mid-IR lasers is obviously enhanced by the use of the signal laser. When the twice OPOs generate the same wavelength mid-IR laser, optical parametric amplifier (OPA) from the signal laser to the idler laser is achieved, and the output power of the idler laser increased by the twice optical parametric generations.TOPO is achieved based on a single-grating-period PPLN. The wavelength tunability of PPLN-TOPO is studied by the temperature adjusting. Multiple wavelength mid-IR lasers and wider tuning range are achieved. The theoretical wavelength tuning curves of PPLN-TOPO are calculated. The temperature that the dual OPOs generate the same wavelength mid-IR lasers is obtained. The conditions that a pump photon converts to three mid-IR photon of the same wavelength are calculated. PPLN-TOPO is pumped by 1.064μm laser at high intensity. When the temperature of a 31.4μm PPLN is changing from 25℃to 200℃, three (or two) mid-IR lasers are obtained. The whole tuning range is 1.6-4.4μm, which is much wider than a common OPO. Beside the wavelength tunability of PPLN-TOPO, the high efficiency output characteristic is also studied.The state that the dual OPOs generate the same wavelength mid-IR lasers from a TOPO is achieved experimentally based on a single-grating-period PPLN crystal. The quantum efficiency of the mixing lasers and condition of complete pump depletion are obtained from the coupling waves equations. The threshold, conversion efficiency, acceptable angle and line width are analyzed theoretically by separating the dual OPOs individually. When a 31.2μm PPLN is operated at 151.3℃, the TOPO processes are:1.064μmâ†'2.83μm+1.70μm, 1.70μmâ†'2.83μm+4.30μm. The twice optical parametric processes generate the same mid-IR laser of 2.83μm, the idler laser of 2.83μm is amplified by the signal laser of 1.70μm through different frequency process. At the highest pump power of 25W, an average power of 8.25W at 2.83μm is obtained, corresponding conversion efficiency reaches 33%, which is beyond the theoretical maximum of the mid-IR conversion efficiency from a singly resonated OPO based on the gauss model. The output power of 4.30μm laser is about 0.8W.