Study On The Continuous Wave Optical Parametric Oscillator And Coexistent Stimulated Raman Scattering Phenomenon

While lasers have been in use for more than50years, it is still difficult todevelop laser output wavelength that can cover the regions of the optical spectrum,from the ultraviolet and visible to the near-infrared and mid-infrared wavelengthrange. As the broadly tunable coherent laser source, continuous wave (CW) opticalparametric oscillators (OPO) have become an important part of the nonlinear opticalfrequency conversion and laser tuning technology fields by virtue of its many merits.Especially, CW OPO will have more important applications in many fields such ashigh resolution spectroscopy analysis, military, laser medicine, and so on.In this thesis, CW OPO based on quasi-phase-matched (QPM) periodicallypoled MgO-doped lithium niobate crystal (PPMgLN) is studied on the basis oftheoretical analysis and experiments. Using the PPMgLN crystal as the nonlinearconverter, we have accomplished low threshold, high power and highly efficient CWOPO, driven by high beam quality, efficient and compact diode-end-pumped CWNd:YVO4laser.Theoretically, the coupled wave equations of three-wave interaction are deducedfrom the basic principles of nonlinear optical. The fundamental theory of QPMtechnique based on periodically poled crystal and properties of PPMgLN crystal areintroduced. The gain of single resonant continuous wave OPO (SRO) is analyzed, and the threshold and conversion efficiency in the plane wave approximation andGaussian beam approximation are discussed according to the fundamental theory onOPO system. The tuning characters of four tuning modes of thequasi-phase-matching OPO, namely, pump wavelength tuning, grating period tuning,temperature tuning and angle tuning, are numerical calculated and analyzed based onPPMgLN crystal, and a thorough analysis on period tuning and temperature tuning isgiven according to the experiments. A contrastive analysis of the thermal lens effectbetween conventional Nd:YVO4crystal and bonding Nd:YVO4crystal is made atcertain pump power. The threshold and coupled wave equations of the coexistentstimulated Raman scattering (SRS) are derivated.Experimentally, a CW Nd:YVO4laser end-pumped by laser diode array (LDA)is investigated as the pump source of CW OPO. A two-mirror linear cavity isdesigned, and by choosing a single-ended bonding Nd:YVO4crystal to reduce thethermal lens effect caused by end deformation, a CW1064nm laser with highconversion efficiency, good quality of light beam and maximum output power of11.79W is realized, which is suitable for pumping CW PPMgLN OPO.The output characteristics of high power CW OPO is studied in the dissertation.An experiment using CW1064nm laser and two-mirror linear cavity is carried outbased on the PPMgLN crystal with0.5μm cycle interval. In the experiments, theOPO cavity is resonant for the signal frequency. Optimized pattern matching forpump beam is taken into account to improve the output power and conversionefficiency of CW OPO. Experimental results indicate that the OPO’s threshold is justonly0.3W, and the OPO is able to yield laser spectrum from1.43μm to1.67μmand mid-infrared spectrum from2.93μm to4.12μm. The maximum total signal andidler output power at30.5μm poling period is4.29W by pumping power11.79W,which corresponds to an optical-optical efficiency of36.4%.1.55μm signal and3.40μm idler output powers are3.14W and1.15W, respectively.The coexistent stimulated Raman scattering (SRS) in CW OPO is researched.The study finds that there is an important impact on the generation of idler. The higher order SRS can be inhibited by enhancing the threshold of SRS with adjustingthe output mirror. With the loss of SRS increasing,1.98W of the maximum idleroutput power at3.40μm is obtained, corresponding to an optical-optical efficiencyof16.8%. Due to the impact of factors such as low photon energy and non-resonant,SRS phenomenon is not found in the idler wavelength.Finally, the output characteristics of a multi-wavelength tunable CW OPO areinvestigated experimentally. The pump source is a CW Nd:YVO4laser emitting1064nm. The PPMgLN crystal with0.2μm cycle interval is adopted for the OPO. Awidely and continuously without separation tunable near-infrared spectrum from1.49μm to1.68μm and mid-infrared spectrum from2.88μm to3.68μm areobtained by changing crystal grating periods from29.8μm to31.4μm and thecrystal temperature from30℃to100℃. By using nonlinear crystal with smallperiodic intervals, the needed temperature range can be greatly reduced, and thespeed and efficiency of the combined tuning are improved effectively. The mostimportant features of the CW OPO are widely tunable range, high speed, and widelyspectrum range without separation.