Research On Generation And Amplification For Sub-nanosecond 1.06 μm Laser Operating At Kilohertz Level

Generally speaking,the sub-nanosecond laser means the pulsed laser with the pulse width of several hundred picoseconds.Because of the narrow pulse width and high peak power,the sub-nanosecond 1.06-μm laser is widely used in many fields such as industry,military,and medicine,which is one of the hotspots in the field of solid-state laser.However,the development of sub-nanosecond lasers at the repetition rate of kilohertz level is still relatively slow,and the output energy is difficult to meet the application requirements in the field of lidar,laser ignition,and laser processing.Therefore,in this thesis,we mainly carry out research on the generation methods and energy enhancement of sub-nanosecond1.06-μm lasers operating at kilohertz level.A thorough introduction and analysis of the development of sub-nanosecond laser technology were conducted firstly.A technical solution that combines the Q-switching technique with the master oscillator power amplifier structure was determined.Based on the rate equations,the pulse width in Q-switched lasers was studied.The impacts of the pump parameters,the gain medium,and the seed pulse energy on the amplification gain were discussed.In order to obtain sub-nanosecond pulse width and high amplification gain,the gain medium needs to operate at high pump density in both the oscillator and the amplifier.Therefore,a theoretical model for the thermal effects of the gain medium was constructed.Based on the photoelastic theory,how the thermally induced birefringence of the Nd:YAG crystal under high pump power affected the output performance of the laser system was analyzed.A gradient-concentration Nd:YVO₄ crystal was proposed to replace the single-concentration Nd:YVO₄ crystal as the gain medium,and a comparative study was conducted based on the finite element analysis method.Under the same pump condition,the use of gradient-concentration crystal could reduce the temperature gradient within the gain medium from 115.14 K to58.60 K,reducing by 49.1%,which could avoid the damage to the gain medium under high pump power effectively.These theoretical studies provided guiding functions for constructing high-performance sub-nanosecond laser systems.Based on the theoretical research,an Nd:YAG/Cr⁴⁺:YAG passively Q-switched microchip laser was designed.An undoped YAG crystal was used to alleviate the thermal effects of the gain medium,which made the laser operate at kilohertz repetition rate safely.A（110）cut Cr⁴⁺:YAG crystal was used as the passive Q switch to achieve outputs with linear polarization.Laser pulses with a repetition rate of 1 kHz,a pulse energy of 140μJ,and a pulse width of 490 ps were obtained.Based on the single-passed laser diode side-pumped Nd:YAG amplifier structure,how the thermally induced birefringence of the Nd:YAG crystal affected the performance of the laser amplifier was studied,and the pulse energy was amplified to 1.7 mJ with the double-passed amplifier structure.In order to obtain sub-nanosecond seed pulses with higher pulse energy,the research on electro-optically Q-switched sub-nanosecond laser technique was carried out.Based on the Rb Ti OPO₄ electro-optic Q-switched technique,laser pulses with the pulse energy of 140μJ and 110μJ were obtained at the repetition rate of 1 kHz and 5 kHz,respectively.The pulse width was about 730 ps.It was found that the parasitic oscillation could be easily formed between the high reflectivity mirror and the end face of the electro-optic crystal,which limited the improvement of the pulse energy.To avoid the parasitic oscillation,the oblique-incidence electro-optically KD₂PO₄ Q-switched technique was proposed in this thesis,which was verified theoretically and experimentally.In addition,by using the gradient-concentration Nd:YVO₄ crystal as the gain medium,the damage of the gain medium under high pump power was avoided effectively.Laser pulses with a pulse energy of 520μJ and a pulse width of 900 ps were obtained at 5 kHz.Pointing at the low efficiency and serious thermal effects of the sub-nanosecond laser amplification stage,a 914 nm laser diode in-band pumped amplifier with the gradient-concentration Nd:YVO₄ crystal as the gain medium was proposed,which effectively improved the Stokes efficiency and alleviated the thermal effects in laser amplifiers.Based on the polarized absorption characteristics of the Nd:YVO₄ crystal around 914 nm,a polarized pump structure was designed,which effectively improved the absorption efficiency of the gain medium.Under the repetition rate of 5 kHz and the absorbed pump energy of11.0 mJ,the pulse energy was amplified from 520μJ to 4.2 mJ,with the optical-optical conversion efficiency of the amplifier reaching 33.5%.The laser pulses were very stable,with a coefficient of variation of 1.94%.The peak power is as high as 4.7 MW,which showed the application potential in the field of lidar,laser processing and laser ignition.