Research On 2 μm Passive Q-switched Solid-state Laser Based On Bn Crystal

Solid-state lasers in the mid-infrared 2 μm band have very common applicability in many fields today,and it is an important light source for industrial,military,medical and scientific research fields.Pulsed lasers have advantages over continuous lasers in terms of high peak power,high repetition frequency and narrow pulse width,and Q-switched technology is one of the main technical paths to obtain high performance nanosecond pulsed lasers,including both actively Q-switched and passively Q-switched.Passively Q-switched has the advantages of simple construction,low process cost and excellent output characteristics,so it has attracted much attention.In recent years,saturable absorbers prepared with the help of simple and broadband absorbing two-dimensional materials have become a new research hotspot in solid-state laser technology.In this thesis,the optical absorption characteristics of boron nitride(BN)crystals of different thicknesses and graphene/BN heterojunctions as saturable absorbers in the 2 μm band are investigated respectively.By analyzing the thermal effect of the gain medium and optimizing the resonant cavity design,a 2 μm Ho:YLF passively Q-switched laser with BN and graphene/BN heterojunction as saturable absorbers was successfully built to achieve a stable pulsed laser output.The optical modulation performance of BN as saturable absorber in the 2 μm band was experimentally verified,extending the application range of two-dimensional materials as optical modulation devices.The main research contents of the paper are as follows:(1)Various solutions such as ethanol,water and acetone were selected as solvents for BN materials with 99 wt.% purity,and BN saturated absorbers were fabricated and characterized using various methods such as acoustic degradation method and liquid stripping method,which were measured to have a small absorption intensity in the range of 1000 nm to 2400 nm.In terms of theoretical analysis,we established a three-dimensional heat conduction and thermal stress model of the bulk crystal,performed thermal analysis simulations using Ho:YLF crystal as the gain medium,and optimized the design of the resonant cavity to study the thermal stress as well as the effects of the injected pump power and the size of the spot on the thermal distribution of the crystal.(2)A flat-cavity Ho:YLF passively Q-switched laser was built,and BNs with varying thicknesses were inserted into the resonant cavity to achieve optical modulation.The laser output characteristics were then investigated under different transmittance output mirrors.The shortest pulse of 2.24 μs was obtained with the monolayer BN passively Q-switched laser using 1.5%transmittance output mirror,and the highest single pulse energy and peak power of 588.62 n J and262.77 m W were obtained at the output center wavelength of 2066.86 nm,respectively,and the highest repetition frequency of 45.87 k Hz was obtained.The pulse width at the output mirror with1.5% transmittance was smaller than that at the output mirror with 2.5% transmittance in the BN passive Q tuning experiments at the same thickness.The single pulse energy increases with the increase of BN thickness.(3)A graphene and BN heterojunction as a saturable absorber passively Q-switched laser was built.Passively Q-switched laser output was achieved at 2.5% and 1.5% transmittance output mirrors,respectively,and concluded by experimental comparison that compared to both single-layer graphene and single-layer BN passively Q-switched lasers at the same transmittance output mirrors,the graphene/BN heterojunction passively Q-switched laser obtained shorter pulses.The highest single pulse energy and peak power of 730.36 n J and 466.96 m W were obtained at 2066.8 nm with a maximum repetition frequency of 52 k Hz using a 1.5% transmittance output mirror.The graphene/BN heterojunction exhibits superior saturable absorption properties than graphene and BN alone.