| High peak-power pulsed lasers with simultaneous multi-wavelength oscillation possess significant practical applications in laser detection,laser interferometers,spectroscopic analysis,holographic measurement,biomedical science,differential absorption laser lidar(DIAL)and nonlinear optical frequency conversion.The stimulated Raman scattering(SRS)effect of Raman gain medium provides an efficient and flexible approach to generate multi-wavelength oscillation.The advantages such as free of phase-matching condition,pulse compression effect and clean-up effect of SRS make Raman lasers become an international hotspot in laser and optical field.Combining SRS conversion process with a typical "sandwich"structured laser-diode(LD)pumped passively Q-switched microchip laser to form a passively Q-switched Raman microchip laser(PQSRML)not only can broaden the output wavelength range and flexibility in laser wavelength selection of traditional microchip lasers,but also provides a reasonable solution for developing miniature Raman lasers with high peak power,sub-nanosecond pulse duration.Compared with Nd3+ doped laser crystals,ytterbium(Yb)ions doped laser gain media are more suitable for developing passively Q-switched microchip lasers with high peak power because of their excellent properties such as free of concentration quenching effect,easy of high doping concentration,and low thermal loading.Yb:KGW,Yb:KYW,Yb:KLuW and Yb:YVO4 crystals have been used to generate dual-wavelength oscillation with fundamental laser and Raman laser in passively Q-switched self-Raman lasers.Unfortunately,the performance of self-Raman lasers is restricted by the severe thermal loading induced by the simultaneously fundamental laser and Stokes laser oscillating in one crystal.Moreover,the tradeoff between the fundament laser and Raman laser oscillation in self-Raman gain medium makes the self-Raman laser difficult to be optimized and the output laser wavelengths difficult to control.Therefore,it is extremely necessary for developing PQSRMLs with separated laser medium and Raman medium to achieve excellent multi-wavelength oscillation.In this dissertation,excellent performance of simultaneous multiple wavelength Raman lines under continuous-wave(CW)and passive Q-switching conditions have been generated from a LD end pumped PQSRML with a Yb:YAG crystal as gain medium and a YVO4 crystal as Raman medium.Firstly,pump power dependent multi-wavelength laser oscillation with adjustable frequency separation around 1.05 μm,1.08 μm,and 1.11 μm have been demonstrated in laser diode pumped Yb:YAG/Nd:YVO4 CW Raman microchip laser.The maximum Raman laser output power of 260 mW is achieved.The fundamental laser and Raman laser exhibit stable elliptical and linearly polarized state,respectively.Multi-wavelength laser oscillation with adjustable frequency separation around 1.05μm,1.08 μm,and 1.11 μm in LD pumped Yb:YAG/Nd:YVO4 CW microchip Raman laser opens a new window for designing a compact laser source for terahertz generation.The Raman laser at 1.08 μm with high optical efficiency has been obtained by use of the heavy doped Yb:YAG crystal and un-doped YVO4 crystals with the perfect lattice structure.The maximum output power is 1.16 W,and the optical-to-optical efficiency is 18.4%.Moreover,optical vortex beams of the doughnut-shaped LG0,1 mode,two-vortex array,and four-vortex array have been obtained depending on the applied pump power.The LG0,1 mode,two-vortex array,and four-vortex array possess 1,2,and 4 phase singularities,respectively.The comb-like Raman laser emitting spectrum covers from 1072.49 nm to 1080.13 nm with a spectral bandwidth of 7.64 nm and 30 equidistant longitudinal modes.This work paves a road to generate optical vortices with an optical frequency comb in miniature solid-state lasers.Secondly,on the basis of the CW Raman microchip laser,four-wavelength laser emitting at 1030 nm,1123 nm,1134 nm,1260 nm and six-wavelength laser emitting at 1030 nm,1050 nm,1079 nm,1134 nm,1156 nm,1260 nm have been obtained at different pump power levels by combing Yb:YAG,YVO4 and Cr4+:YAG crystal as a PQSRML with a typical "sandwich" structure.The maximum peak power of 9.2 kW and narrowest pulse duration of 440 ps are achieved in the Raman laser.Compared with those reported self-Raman lasers based on Yb:KGW,Yb:KYW,Yb:KLuW and Yb:YVO4 crystal,the Yb:YAG/Cr4+:YAG/YVO4 PQSRML with separated laser medium and Raman medium configuration offer a higher peak power and potential laser source for flexible applications,which is benefiting from the good thermal management.The peak power of Raman laser obtained in this work is five times higher than the latest reported Yb:KLuW self-Raman microchip laser(2015,119 mW,3 μJ,1.5 kW,2 ns).The pulse width of Raman laser obtained in this work only about a quarter of the Yb:KLuW self-Raman microchip laser.And then,multi-wavelength,sub-nanosecond near-infrared Raman laser and green laser have been generated by combing SHG(second harmonic generation)as well as SFG(sum-frequency generation)and employing the commercial plane mirrors originally designed for 1030 nm laser as output couplers.The maximum average output power of Raman laser is 111.4 mW.Meanwhile,the maximum pulse energy of 7 μJ and peak power of 8.9 kW for Raman laser are achieved.The tunable relative intensity of dual-wavelength green laser at 530 nm and 549 nm is achieved by manually by tuning the angel of KTP crystal and the incident laser.In order to optimize the pulsed laser performance of Yb:YAG/Cr4+:YAG/YVO4 PQSRMLs,the effects of the transmission(TOC)of the output couplers(OCs)and the initial transmission(T0)of Cr4+:YAG crystals have been investigated theoretically by solving the coupled rate equations numerically.The results show that one can obtain a better laser performance of Raman laser,in terms of a high pulse energy,a high peak power and a narrow pulse width of Raman laser,by employing the optimum TOC for different T0s of Cr4+:YAG crystals.The maximum average output power of Raman laser is 143 mW with the combination of TOC=11%OC and T0=90%Cr4+:YAG Enhanced Raman laser pulse performance of 24.1 μJ pulse energy and 45.1 kW peak power can be achieved by using a combination of TOC=16%OC and T0=85%Cr4+:YAG.The shortest pulse width of 505 ps can be achieved.The pulse trains of Raman laser are very stable.The peak power of Raman laser obtained in this work is the maximum value to our best knowledge under CW pumping condition in PQSRMLs.In order to reduce the thermal effect of Raman lasers,a quasi-continuous-wave(QCW)940 nm LD is employed as the pumping source.The maximum peak power of 95 kW with the corresponding pulse width of 466 ps for Raman laser has been obtained by adopting plane-concave cavity based on Yb:YAG-Cr4+:YAG composite crystal and YVO4 Raman crystal.The maximum repetition rate is 87.8 kHz.The plane-concave cavity configuration has advantages of easily adjusting cavity length,waist position and size.The results have the guiding significance to the design of multi-wavelength,sub-nanosecond Raman lasers at visible wavelengths by inserting second-order nonlinear optical crystals such as KTP,LBO.The results in this dissertation provide the theory basis and practice guideline for developing PQSRMLs with controllable wavelength separation,multi-wavelength laser radiation,high pulse energy and high peak power. |
PDF Full Text Request