Continuous Wave And Passively Q-switched Pulsed Laser Properties Of Yb:YLuGdCOB Mixed Crystals In A Microchip Cavity

Yb-doped rare-earth calcium oxyborates,Yb:ReCOB（Re=Y,Gd,La）,which can be readily grown by Czochralski method,and possess high chemical stability as well as strong resistance to optical damage,are a class of Yb-ion laser crystals of important potential for applications.The ground-state of Yb ions in these crystals has a large Stark splitting,which proves to be desirable for realizing low-threshold and high-efficiency continuous-wave（CW）laser operation at room temperature.Due to the long fluorescence lifetime of upper state of Yb ions in these crystals,high-energy pulsed laser operation can also be achieved through Q-switching.In addition to the ordered crystals,Yb:ReCOB mixed crystals consisting of Lu are also worth exploring and studying,since theoretically,the substitution of Yb for Lu would result in the minimum lattice distortion,making it easier to grow laser crystals with high optical quality.This thesis involves a systematic and deep experimental investigation into the CW and passive Q-switching laser properties of a mixed Yb:ReCOB crystal consisting of three optically inactive rare-earth ions（Y,Lu,Gd）,whose chemical formula is expressed as Yb_0.19Y_0.34Lu_0.12Gd_0.35Ca₄O（BO₃）₃（Yb:YLuGdCOB）.The main results and primary conclusions obtained from the thesis work are summarized as follows.Both the crystal sample length and its orientation are of important influences on CW as well as pulsed laser output characteristics.A thin crystal sample is prone to suffering from absorption saturation,so the output power attainable will be limited largely;whereas a long crystal sample is difficult to achieve high lasing efficiency,because of its great resonant absorption losses.Additionally,the laser polarization state and lasing spectrum are also affected significantly by the crystal length.Of the crystal samples cut along the three principal optic axes（X,Y,Z）,the Y-cut sample is the optimal in generating high output power and lasing efficiency;while the X-cut one is more suitable for Q-switching pulsed laser operation.High-power/high-efficiency CW laser operation was realized with Y-cut crystals in a microchip resonator.The output power produced with a 0.6 mm thin crystal plate reached3.15 W,the optical-to-optical efficiency being 65.5%;while with a 1.0 mm thick crystal,the maximum optical-to-optical efficiency achieved amounted to 91.9%,very close to the upper limit imposed by the quantum defect in the laser emission process.The output power produced from the laser operation with a 3.2 mm long crystal could reach 14.30 W,the optical-to-optical and slope efficiencies being,respectively,70.0%and 81%.Passively Q-switched laser operation was realized with an X-cut crystal in microchip resonator,with a 2D Mo Te₂acting as the saturable absorber.A pulsed output power of1.56 W at 1033 nm was produced at a repetition rate of 481 k Hz;while the maximum pulse energy,minimum pulse width,and the highest peak power,which were measured in the passively Q-switched operation,were 3.87mJ,26.2 ns,and 147.7 W,respectively.