Theoretical And Experimental Study On High-Peak-Power Short Pulse Solid-State Lasers At 2 ?M

The 2 ?m laser is safe for human eyes,which is not only located in the infrared atmospheric transmission window and water absorption peak,but also in the area of multiple gas molecular fingerprints.It is widely used in transparent plastic processing,environmental pollution gas detection,laser surgery,photoelectric countermeasures,etc.With the increasing demand for applications,2 ?m nanosecond short pulse laser with the high peak power and high repetition rate has become one of the research hotspots in the laser technology field in recent years.At present,the main means of achieving nanosecond short pulse laser output is Q-switching technology.Compared with passively Q-switching technology,actively Q-switching technology represented by acousto-optic(AO)and electro-optic(EO)modulators has the advantages of good stability,adjustable repetition frequency,high damage threshold,etc.Therefore,it has become an important technical means for achieving high-performance nanosecond short pulse laser.Tm3+ ion-doped crystals have the advantages of high quantum efficiency,large damage threshold,and many types of substrates,which can also be directly pumped by mature commercial LD,so they are currently one of the main gain media for obtaining high efficiency 2?m laser.In addition,Tm3+ions usually have a longer upper energy level lifetime(ms level),which is conducive to laser energy storage,thereby generating high energy pulse laser.However,the currently reports on output powers,especially the pulse peak power of Tm3+ion-doped 2 ?m nanosecond pulse solid-state lasers are still low.Therefore,it is difficult to meet increasing application demands.This paper focuses on how to increase the peak power of the nanosecond pulse laser at 2 ?m.AO and EO Q-switched laser characteristics of the LD pumped Tm3+ ions-doped crystals were studied from the theoretical and experimental aspects to achieve the laser output with high peak power and short pulse width at 2 ?m.In terms of theoretical research,firstly,based on the physical and spectral characteristics of Tm:YAP and Tm:YLF crystals,the thermal effects were analyzed by the thermal lens focal length formula and LASCAD software,which laid the foundation for the design of laser resonators.Secondly,the energy level transition characteristics of a Tm3+ ion-doped laser system were analyzed,and corresponding rate equation model under actively Q-switched operation was established.MATLAB software was used to theoretically simulate the output characteristics of LD pumped actively Q-switched Tm:YAP and Tm:YLF lasers,such as the density of inverse population,peak power,single pulse energy,and pulse width.The above simulation analysis results provide theoretical guidance for experimental research.In terms of experimental research,firstly,using the two-dimensional AO modulator with the high diffraction efficiency,the AO Q-switched laser characteristics of LD pumped 2 ?m Tm:YLF crystals were studied,achieving a pulse laser output with high power and high repetition frequency.Secondly,the special wavelength continuous wave(CW)and pulse laser output characteristics of LD pumped Tm:YAP crystals were studied using specially coated optical mirrors.Finally,utilizing Tm:YAP as the gain medium,based on LiNbO3(LN)and La3Ga5SiO14(LGS)EO modulators,the EO Q-switched laser characteristics at 2 ?m were studied,achieving a Q-switched short pulse laser output with high peak power.The main experimental research contents and results of this paper include:1.Based on the two-dimensional AO modulator,Tm:YLF pulse laser operation with the high power and the high repetition frequency was realized.At a repetition rate of 5 kHz,the maximum output power of 7.32 W was obtained with a shortest pulse width of 68 ns and a maximum single pulse energy of 1.4 mJ,corresponding to a peak power of 21.5 kW.This is the highest average output power for the LD pumped Q-switched Tm:YLF laser reported so far,mainly due to the strong optical path shutdown capability and the relatively small device volume of the two-dimensional AO modulator.2.Using special coated optical cavity mirrors,the special wavelength laser output characteristics of Tm:YAP crystals were studied.Under CW laser operation,the single-wavelength output at 1910 nm was realized,and the maximum output power was 10.79 W in the case of double-end output;Under the AO Q-switched operation,the average output power of 2.1 W was demonstrated with a shortest pulse width of 91 ns at a repetition frequency of 8 kHz,corresponding to the peak power of 2.72 kW.Through loss control and optimized cavity design,dual-wavelength output of 1906 nm and 1951 nm was further achieved with the maximum output power of 2.26 W.The high power laser output of the Tm:YAP crystal at special wavelengths was achieved without any wavelength tuning element inserted3.Based on the LN EO modulator,the low repetition frequency pulse laser operation of LD pumped Tm:YAP crystal was realized.The modulation frequency of the EO modulator varied from 200 Hz to 800 Hz.When the pulse repetition frequency was 200 Hz,the shortest pulse width of 28 ns was obtained with a single pulse energy of 2.9 mJ,and the corresponding peak power was 103.6 kW.The study found that LN EO switches operated more stably at low repetition frequencies due to the piezoelectric ringing effect.However,due to the low damage threshold of the LN crystal,a higher peak power output was not achieved in the experiment.4.Using the LGS EO modulator,the pulse laser operation of LD pumped Tm:YAP crystal with a high peak power was realized.The modulation frequency of the EO modulator was between 200 Hz and 1 kHz.At a repetition frequency of 200 Hz,an average output power of 0.63 W was obtained with a maximum single pulse energy of 3.15 mJ and a minimum pulse width of 17 ns,the peak power was 185.3 kW,which is currently the highest peak power for Tm:YAP crystals to the best of our knowledge.The research results show that to achieve a high peak power 2 ?m laser,on the one hand,an EO crystal with a high damage resistance threshold should be used,and on the other hand,the environmental humidity must be strictly controlled.