Research On The Characteristics Of Long-wave Emission Sideband Lasers Of Yb-doped Rare Earth Calcium Oxyborate Crystals

Yb-or Nd-ion solid-state lasers working in the 1-?m near-IR region usually emit in the 1020-1070 nm wavelength range.Under free-running conditions,the Yb-or Nd-ion laser media capable of generating laser operation in the long-wavelength region above1080 nm prove to be very rare.Yb-doped rare-earth calcium oxyborates such as Yb:YCa₄O（BO₃）₃（Yb:YCOB）have in their emission spectra,in addition to the main emission band,an emission sideband with its center wavelength being around 1084 nm,making it possible to produce long-wavelength high-power output power.It is therefore of practical importance to investigate the laser properties in the long-wavelength emission sideband of the Yb-doped rare-earth calcium oxyborates.In this work,we conducted a systematic investigation into the continuous-wave（cw）laser properties in the long-wavelength emission sideband of the ordered Yb:YCOB and the mixed Yb_0.14:Y_0.77Gd_0.09Ca₄O（BO₃）₃（Yb:YGdCOB）crystals,as well as into the effects of crystal temperature on the laser performance.We also realized passively Q-switched operation of an Yb:YCOB microchip laser,with two dimensional（2D）MoTe₂acting as saturable absorber.With high-gain oscillation in the main emission band being suppressed,highly efficient cw laser operation was realized in the long-wavelength emission sideband of the ordered Yb:YCOB and the mixed Yb:YGdCOB crystals cut along different orientations.In the polarized laser action（E//X）emitting at a central wavelength of 1084 nm achieved with Z-cut Yb:YCOB crystal,a maximum output power of 15.5 W was produced,with optical-to-optical and slope efficiencies being respectively 44.4%and 52%.While with Z-cut Yb:YGdCOB mixed crystal,the attainable output power could reach 16.2 W,the corresponding optical-to-optical efficiency was 50.3%,and the slope efficiency was 58%.In free-running mode,the highest output power produced with the Z-cut Yb:YGdCOB mixed crystal could increase to 18.1 W,with an optical-to-optical efficiency of 60.0%and a slope efficiency of 70.0%.Experimental investigations show that crystal temperature rising will lead to deterioration in laser performance;but the degree,to which the laser action is affected by the crystal temperature,proves to be much less for the long-wavelength sideband than for the main emission band.In the case of laser operation occurring around 1084 nm in the emission sideband of the Yb:YCOB crystal,the reduction rate of output power with crystal temperature rise was measured to be 0.006 W/?C;it amounted to 0.028 W/?C for1023 nm operation in the main emission band.A 2D MoTe₂ passively Q-switched Yb:YCOB microchip laser formed with a 4 mm long plane-parallel cavity was demonstrated,producing,with 5.0 W of incident pump power,a pulsed output power of 1.58 W at a repetition rate of 704 kHz with a slope efficiency of 36%,the resulting pulse energy and peak power were respectively 2.25μJ and 40.8 W,while the shortest pulse duration was 52 ns.These results reveal the potential of Yb:YCOB and other Yb-doped rare-earth calcium oxyborates in making passively Q-switched lasers with 2D saturable absorbers.