Study On Mid-infared Continuous Wave Optical Parametric Oscillator Technique Based On MgO:PPLN Crystal

3-5μm mid-infrared coherent light source is the most transparent one among the three infrared atmosphere-windows(1-3μm, 3-5μm and 8-12μm) , which includes absorption peaks of many atoms and molecules. It is more and more important in many fields such as photoelectrical countermeasure, laser spectroscopy, atmospheric sensing, laser medicine, optical wireless networks, space communications, and so on. As broadly tunable laser source, mid-infrared optical parametric oscillator (OPO) have become indispensable laser source for many research fields by virtue of its many merits which other solid-state lasers have not. Especially, continuous-wave(CW) mid-infrared OPO is more suitable for high-resolution spectroscopy analysis. In this thesis, continuous-wave(CW) mid-infrared OPO based on MgO-doped periodically poled LiNbO3 (MgO:PPLN) is studied on the basis of theoretical analysis and experiments.Theoretically, the threshold of a singly resonant OPO and the relation between the threshold and conversion efficiency are discussed according to the basic principles of OPO. All kinds of the elements influencing line width of the parametric light such as bump light width and the divergence angle of pumping beam are analyzed. And the theory of quasi-phase matching(QPM) based on periodically poled crystals is introduced. Acceptance parameters based on QPM technique are discussed. And output wavelength tuned characteristic of CW OPO based on MgO:PPLN crystal is calculated theoretically. The temperature distribution of the composite Nd:YVO₄ crystal in the experiment and the thermal lensing caused by the thermal gradient is analyzed at certain pump power. The stability of OPO resonator with symmetrical four-mirror cavity configuration is discussed. The influences of the arm length, the curvature radius of cavity mirrors, incident angle, and the thermal lensing effect of the nonlinear crystal on the stability of OPO cavity are evaluated.Experimentally, laser diode(LD) end-pumped CW single-frequency Nd:YVO₄ laser is investigated as a pump source of CW OPO. And a four-mirror ring cavity with small incident angle is designed. Spatial hole burning effect is eliminated by inserting a Faraday rotator and a half wave plate to make the laser unidirectional operation. The ring laser produced single-frequency output of 13W, which is suitable for bumping CW PPLN OPO.An intra-cavity CW MgO:PPLN OPO is studied in the thesis. A CW Nd:YVO₄ ring laser is used as a pump source. Symmetrical four-mirror ring cavity configuration is adopted for the pump laser in the experiment. The nonlinear crystal of the OPO is MgO:PPLN crystal. Symmetrical four-mirror linear cavity structure is adopted for the OPO, which was resonant for the signal frequency. The OPO is able to yield 3-4μm mid-infrared laser by changing the nonlinear crystal grating periods and the crystal temperature. Because the laser crystal is a composite Nd:YVO₄ crystal with low doping concentration, the thermal effect of the crystal is weaken effectively. The pump laser output is linear polarization, which avoides the losses from inserting polarizer and 1/2 wave plate. Thus the OPO threshold is reduced greatly and the parametric light conversion efficiency is improved. The solid-state instrument is compact and easy to miniaturize.An external-cavity CW optical parametric oscillator based on MgO:PPLN crystal is studied. On the basis of theoretical analysis, MgO:PPLN OPO with symmetrical four-mirror cavity pumped by Yb fiber laser is studied. In the experiments, the OPO cavity is resonant for the signal frequency. How to yield optimal focusing parameter for pump beam and how to utilize the largest nonlinear coefficient of the nonlinear crystal are taken into account to improve conversion efficiency of parametric light. A continuous-wave mid-infrared laser source is obtained by changing nonlinear crystal grating periods from 28.5μm to 31.5μm and the crystal temperature from 30°C to 160°C. The OPO was able to generate light with idler wavelengths ranging from 2.9μm to 4.1μm. For a pump power of 40 W,the idler output power is 10.2W at 3.004μm. And by using the signal light output coupling mirror to substitute for a cavity mirror, signal and idler are obtained at the same time. For the pump power of 25W, parametric light output reached a total power of 10.7W, and parametric light conversion efficiency reached 42.8%. 1.53μm signal and 3.47μm idler output power are 7.2W and 3.5W, respectively. To obtain high power of about 3.8μm, a symmetrical four-mirror ring cavity is optimally designed for OPO output 3.87μm idler, the maximum output power of 3.87μm idler is 4.9W for pump power 44W from Yb fiber.Finally, a continuous-wave single-frequency mid-infrared optical parametric oscillator is studied experimentally. The pump source is a continuous-wave single-frequency Nd:YVO₄ laser emitting 1064nm. A symmetric four-mirror ring cavity is designed for the OPO, which is resonant for the signal frequency. An etalon is inserted into the cavity in order to compress the width of the oscillation signal. Thus, a continuous-wave single-frequency mid-infrared laser output is obtained, which is suitable for high-precision spectroscopy.