Investigation Of Efficient Nonlinear Optical Frequency Conversion

Nonlinear frequency conversion technology has established itself as a reliable and efficient way to obtain diverse and versatile laser sources with novel and valuable wavelengths.It has been one important aspect and one of the focus topics in optics.Optical parametric oscillation (OPO)and stimulated Raman scattering(SRS)are two kinds of efficient nonlinear optical processes.High output powers can be obtained from both technologies.Based on the second order nonlinear effect of crystals,OPO is operated under phase-matching conditions and two new laser frequencies,i.e.signal and idler waves,can be obtained.The most attractive merit of OPOs is that frequency-tunable output can be achieved through angular or temperature tuning.SRS is a third order nonlinear effect and is the base of a Raman laser.In a Raman laser,the frequency shift will be fixed once the Raman medium is selected.The advantage of Raman lasers over OPOs is no need for phase matching.All solid-state optical parametric oscillators and Raman lasers are compact,highly efficient and stable.Hence they can find important applications in many fields such as information,traffic,measurement,medicine, national defence,industry and agriculture.In this dissertation,by using actively Q-switched Nd:YAG lasers as the fundament sources, intracavity OPOs based on typeâ…¡non-critically phase-matched KTiOPO₄(KTP),KTiOAsO₄ (KTA)crystals are studied,respectively.Rate-equation theoretical model is set up to analyse the performances of the diode-end-pumped actively Q-switched intracavity OPOs.During the experiments on KTA OPOs,efficient SRS of the KTA crystals are found.Then,the Raman characteristics of KTA are researched and the first-Stokes and second-Stokes Raman lasers based on KTA crystals are carried out,respectively.In order to improve the conversion efficiencies of the nonlinear optical processes,electro-optically Q-switching technology of the fundamental laser soures is investigated.The Q-switching principles of La₃Ga₅SiO₁₄ (LGS),a new electro-optic Q-switch,are investigated,which supplies as the theoretical basis for designing LGS EO Q-switches.The main contents of this dissertation are as follows:1.Based on the analysis on the properties of LGS,we innovate in the research method of the LGS electro-optic(EO)Q-switch by introducing Jones matrix.This makes it possible to study the polarization state influenced simultaneously by the optical activity and EO effect of LGS.For the first time,the working principle of the LGS Q-switch is expained and its "on","off" states are modeled accurately,which supplies as the theoretical basis for designing LGS EO Q-switches.With the help of Jones-matrix method,the EO coefficient,r₁₁,is measured correctly and the result is 2.2 pm/V. 2.By using Nd:YAG crystal as the laser gain medium(1064.2 nm),the diode-end-pumped acousto-optically(AO)Q-switched intracavity OPO(IOPO)is studied based on a 20-mm-long x-cut KTP crystal.The signal wavelength in this case is determined to be 1572 nm.The output signal powers and pulse widths are measured with different incident diode powers and different pulse repetition rates(PRRs).When the diode power is 6.93 W and the PRR is 15 kHz,an output signal power of 0.72 W is obtained,corresponding to an optical-to-optical(diode power to signal power)conversion efficiency of 10.4%.The peak power is 7.7 kW.Two different kinds of pulse-series phenomena are observerd during the experiments.3.Theoretical analyses are performed to study the output characteristics of the diode-end-pumped AO Q-switched Nd:YAG/KTP IOPO.By taking into account the Gaussian distributions of the initial population inversion density(IPID)and the intracavity photon densities,the rate-equation model is deduced.Thermal lens effect in the Nd:YAG crystal is considered.The IPID is determined by a relatively accurate method.The theoretical results are in good agreement with the experimental data,which comfirms the applicability of the theoretical model.4.A diode-side-pumped AO Q-switched intracavity Nd:YAG/KTA eye-safe OPO is studied. The x-cut KTA is 22 mm long.The signal wavelength in this case is determined to be 1535 nm.The output signal powers and pulse widths are measured with different incident diode powers and different pulse repetition rates(PRRs).When the diode power is 96.3 W and the PRR is 10 kHz,output signal power of 2.54 W is obtained,corresponding to an optical-to-optical conversion efficiency of 2.6%.The pulse width is 5.01 ns and the peak power is 7.7 kW.5.The diode-end-pumped AO Q-switched intracavity Nd:YAG/KTA OPO is reported for the first time.The output signal powers and pulse widths are measured with different incident diode powers and different pulse repetition rates(PRRs).When the diode power is 7.43 W and the PRR is 15 kHz,output signal power of 0.93 W is obtained,corresponding to an optical-to-optical conversion efficiency of 12.5%.6.During the investigations of the diode-end-pumped AO Q-switched intracavity Nd:YAG/KTA OPO,the parametric conversion and SRS are simultaneously realized in a 30-mm-long x-cut KTA crystal.With a diode power of 7.43 W and a PRR of 20 kHz,an output signal power of 0.92 W is obtained,corresponding to an optical-to-optical conversion efficiency of 12.4%.Simultaneously,a first-Stokes(1091.4 nm)power of 0.17 W is obtained.7.The spontaneous Raman scattering characteristics of KTA are investigated.The backscattering spectra are measured with the excitation laser propagating along the X-axis of the KTA crystal.Two prominent Raman scattering peaks of KTA are found:the strongest one at 234 cm^-1with the linewidth of 5.8 cm^-1and the second strongest one at 670 cm^-1with the linewidth of 19 cm^-1.By comparing the spontaneous Raman scattering spectra of the KTA and YVO₄ crystals with the same thickness,the relative Raman cross sections of KTA are obtained.8.The first-Stokes Raman laser based on KTA is realized efficiently.The Raman laser utilizes intracavity configuration with diode-end-pumped AO Q-switched Nd:YAG laser as the pumping source.The x-cut KTA crystal is 30 mm long.A first-Stokes power as up to 1.38 W is obtained with a diode power of 8.11 W and a PRR of 25 kHz,corresponding to an optical-to-optical conversion efficiency of 17%.The pulse width in this case is 6.5 ns and the peak power is 8.5 kW.9.The second-Stokes Raman laser based on KTA is realized efficiently.The laser configuration is the same with that in item 8.The second-Stokes power of 0.63 W is obtained with a diode power of 6.7 W and a PRR of 20 kHz,corresponding to an optical-to-optical conversion efficiency of 9.4%.The pulse width in this case is 23.5 ns and the peak power is 1.3 kW.The main innovations of this dissertation are as follows:1.We innovate in the research method of the LGS electro-optic(EO)Q-switch by introducing Jones matrix.This makes it possible to study the polarization state influenced simultaneously by the optical activity and EO effect of LGS.For the first time,the "on", "off" states of the LGS Q-switch are modeled accurately,which supplies as the theoretical basis for designing the LGS EO Q-switches.With the help of Jones-matrix method,the EO coefficient,r₁₁,is measured correctly.2.Diode-end-pumped AO Q-switched intracavity Nd:YAG/KTP OPO at 1572 nm is reported for the first time.The optical-to-optical conversion efficiency is 10.4%.3.For diode-end-pumped actively Q-switched intracavity OPOs,the rate equations are deduced by taking into consideration the Gaussian distributions of the IPID and intracavity photon densities.Theoretical results are in good agreement with the experimental data.4.Diode-side-pumped AO Q-switched intracavity Nd:YAG/KTA OPO emitting at 1535 nm is realized.With the diode power of 96.3 W and PRR of 10 kHz,output signal power of 2.54 W is obtained,corresponding to an optical-to-optical conversion efficiency of 2.6%. This is the highest efficiency reported for the diode-side-pumped intracavity KTA OPOs. 5.Diode-end-pumped AO Q-switched intracavity Nd:YAG/KTA OPO at 1535 nm is reported for the first time.With the diode power of 7.43 W and PRR of 15 kHz,output signal power of 0.93 W is obtained,corresponding to an optical-to-optical conversion efficiency of 12.5%.This is the highest efficiency reported for the intracavity KTA OPOs.6.The relative Raman scattering cross sections of KTA are measured.For the first time, KTA Raman lasers are realized.The highest powers for the first- and second-Stokes lines are 1.38 W and 0.63 W,respectively.The optical-to-optical conversion efficiencies are 17%and 9.4%,respectively.7.For the first time,the parametric conversion and SRS of the fundamental wave are observed in the same crystal.