Laser Diode Pumped Solid State Lasers

Associated with the development of high-power laser diodes(semiconductor lasers)in the past two decades,research on the diode-pumped solid state lasers has also gained rapidly advancement.Due to the advantages of reliability,high efficiency, and compactness,diode-pumped solid state lasers have been widely used in various fields such as industry processing,medical treatment,national defense and scientific researches.However,it still remains a major task to develop various high-performance diode-pumped solid state lasers to meet different application requirements.Current research on the diode-pumped solid state lasers has been mainly focused on:1.New materials for solid state lasers,especially polycrystalline ceramics.2.Ultra-fast solid state lasers,in which high power,high repetition rate and ultra-short pulse duration are aimed.Following the trend of the diode-pumped solid state laser development,I have conducted the following research works for my PhD project:1.Development of a mode-locked femtosecond Nd:glass laser,and demonstration of multiplication of the pulse repetition rate by adding a coupled cavity.Compared with other neodymium-doped phosphate glass,the used N21-type phosphate glass has wider gain bandwidth and higher nonlinear refractive index. Therefore,it could potentially generate ultra-short mode-locked pulses.A diode-pumped mode-locked Nd:glass laser was developed,which generated mode-locked pulses with 85 fs pulse duration.The obtained 85-fs mode-locked pulses in the laser are the shortest one ever generated in the neodymium-doped phosphate glass lasers. Furthermore,by adding a coupled cavity,the repetition rate of the mode-locked pulses was increased by 1 or 2 times,and a theoretical analysis on the approach was also given in the dissertation.The developed Nd:glass laser provides a reliable femtosecond laser resource for the practical applications.In addition,the approach of multiplying pulse repetition rate could also be applied in other solid state lasers.2.Investigation on the characteristics of a CW Nd:YAG ceramic laser and a Q-switched GaAs-Nd:YAG ceramic laser.Demonstration of the mode-locking of a Nd:YAG ceramic laser by optical nonlinear interference modulation in GaAs wafer for the first time,and demonstration of the mode-locking of a Nd:YAG ceramic laser by use of SESAM. Compared with single crystals,the polycrystalline ceramics have the advantages of short fabrication period,potentially low cost,and high-doping concentration and large-size samples available.In this dissertation we analyzed the thermal,optical and mechanical properties of Nd:YAG ceramic,and studied its CW lasing performance. The results show that Nd:YAG ceramic has comparable lasing efficiency to Nd:YAG single crystal.Using GaAs wafer as the saturable absorber,the Q-switching performance of a Nd:YAG ceramic laser was investigated.We found in the experiment that there exist two absorption mechanisms in GaAs.They resulted in distinct Q-switching performances.When laser intensity in GaAs is low,a linear type of single-photon saturable absorption dominates in GaAs,and the Q-switched pulses generated have long pulse duration and low pulse energy;when laser intensity in GaAs is high, besides single-photon absorption,two-photon nonlinear absorption and free carrier absorption occur,and the formed Q-switched pulses have short pulse duration and high pulse energy.Finally,the mode-locking performance of a Nd:YAG ceramic laser was investigated.We firstly proposed and demonstrated the mode-locking of a Nd:YAG ceramic laser by the optical nonlinear interference modulation in GaAs wafer.In addition,we have also demonstrated the mode-locking of a Nd:YAG ceramic laser with a SESAM.It was found that shorter mode-locked pulses could be obtained by the former approach.This work is very important for applications of Nd:YAG ceramic lasers,and it also helps us to have a better understand on the complicated nonlinear effects in GaAs.3.Development of a high-power self-mode-locked Yb:Y₂O₃ ceramic laser.Through taking advantage of the thermal lensing aberration together with the Kerr self-focusing effect in the ceramic,a nonlinear loss modulation mechanism is formed, which results in the mode-locking of the laser.We have numerically simulated the thermal lensing aberration in a high-power Yb:Y₂O₃ ceramic laser,and calculated the resulted diffraction loss by the thermal lensing aberration in the laser.The numerical results indicated that thermal lensing aberration together with Kerr self-focusing effect could form large nonlinear loss modulation which can be explored for mode-locking of lasers.The mode-locking of a diode-pumped high-power Yb:Y₂O₃ ceramic laser using the technique was firstly demonstrated.Since no additional active or passive element is required,the laser setup is very simple and compact.The mode-locking approach could also be extended to other high-power diode-pumped solid state lasers,thus it is a good supplement for the SESAM mode locking.The research was highlighted by the Optics & Laser Europe magazine.4.Investigation on the optical and lasing characteristics of the Yb:NaY(WO₄)₂ single crystal,including the absorption and emission spectra,lasing efficiency, wavelength tuning and self-pulsing phenomenon.Yb:NaY(WO₄)₂ is a novel single crystal laser material.Due to the random distribution of the Na and Y/Yb ions at the same lattice sites,the absorption and emission spectra are broadened in the Yb:NaY(WO₄)₂ crystal,which is beneficial for the wavelength tuning and mode-locking of the laser.In the dissertation we have investigated its spectral characteristics,lasing efficiency,wavelength tuning and self-pulsing phenomenon,and analyzed the origin of self-pulsing in the laser.This work is useful for applications of Yb:NaY(WO₄)₂ laser in the future.5.Theoretical study on the compression of picosecond pulses generated in mode-locked lasers by cascaded quadratic process.In general,the generated pulses by neodymium-doped mode-locked lasers have a pulse-duration of 10-100 ps.Pulse compression is demanded to achieve femtosecond pulses.The compression from picosecond to femtosecond level requires large nonlinear phase shift generated in cascaded quadratic process.Since nonlinear phase shift generated in cascaded quadratic process tends to saturate at a large value,it is difficult to obtain high compression rate by single-stage compression.In the dissertation we proposed a multi-stage compression scheme.Numerical results indicated that a compression rate of～130 could be achieved by 2-stage compression and it could attain～1000 for 3-stage compression.The numerical results showed that multi-stage compression could be an efficient approach to achieve high compression rate.This work is useful as a reference for the following experimental research.