| Laser is a kind of electromagnetic wave generated by stimulated radiation.Since the first ruby laser is invented in the years of 1960,with its characteristics of high power density,monochromaticity,directionality and coherence,laser has been applied in various fields which relevant to the national economy.It can be said that the emergence of laser has not only promoted the great transformation of optical technology and application,but also promoted the progress and development of physics and other science technology.Especially after the years of 2000,Nobel Prize will be awarded every 4 or 5 years for laser research,which further stimulates the development and application of laser materials and laser technology.With the widespread application of the laser,requirements for laser gradually improve,various types and operation modes lasers also gradually appear.Laser with high peak power can be achieved by the Q-switching and the mode-locked teachnology,and the pulsed laser can satisfy the demand of information society on the strong electric field and magnetic field.Therefore,the pulsed laser has always been at the forefront of laser technology research.Generally speaking,Q-switched laser has the characteristics of large pulse energy and peak power,and it has important demands and applications in materials processing,laser ranging,laser remote sensing,information storage and other fields of scientific research and industrial production.At present,with the continuous upgrading and deepening of"made in China",laser processing technology has become an important tool integrating advanced technology,intelligent manufacturing and intelligent equipment.However,the Q-switched laser is one of the basic light sources of laser processing technology,among which Q-switch is the core component of Q-switched laser.According to the driving mode,the Q-switching can be divided into active Q-switching and passive Q-switching.With the advantages of stable repetition frequency,large and stable pulse energy,high peak power,strong controllability and small time jitter,the active Q-switching is the first choice for laser processing,laser ranging and other systems.For example,in laser machining,the processing speed and ability depend on the performance of Q-switching,especially the repetition rates.Acousto-optic Q-switching and electro-optic Q-switching are two main types of active Q-switching.The acousto-optic Q-switching can obtain pulsed laser with strong controllability,but its switching ability is limited by its diffraction efficiency,so it is difficult to achieve high switching ratio and limit the laser energy output.The electro-optic Q-switching mainly depends on the electro-optic performances of the crystal to achieve polarization modulation of the laser,which can achieve high switching ratio and obtain pulse laser with large energy,short pulse and peak power.At present,all the practical electro-optic crystals are based on the linear electro-optic effect(Pockels effect),and their structures have no symmetrical center,which also determines that the linear electro-optic crystals have the piezoelectric effect,and the piezoelectric ringing effect caused by the operation of high repetition frequency will limit the output of the laser with high repetition frequency.The most widely used electro-optic crystals comprise KD2PO4(KD*P),LiNb03(LN),β-BaB2O4(BBO),Rb6TiOPO4(RTP)and La3Ga5SiO14(LGS).KD*P crystal has a higher light damage threshold and better optical uniformity,but it is water-soluble crystal and easy deliquescence,so it must be sealed when used.At the same time,its piezoelectric effect is strong,and piezoelectric ringing effect appears when the repetition rate exceeds 10 kHz.LN has a stable physical and chiemical properties,and a wide light transmission ranges,but the laser damage threshold is relatively low(100 MW/cm2),which is only suitable for medium and low power laser applications.RTP crystal has a small piezoelectric ringing effect,which can be applied to the operation of high repetition rates,but this kind of crystal belongs to the low symmetry orthogonal crystal,in order to overcome its birefringence,need to be used in pairs,so that this kind of electro-optical switch is more sensitive to temperature,vibration and other external environment.As an important electro-optic crystal,BBO has always attracted people’s attention.However,it is difficult for such crystal to grow in the Z direction of the device,so a breakthrough in growth technology is urgently needed.LGS has the advantages of high light damage threshold(950 MW/cm2),wide light transmission range(0.5-4.5 μm),large size and high optical quality crystals,etc.It is a potential electro-optical crystal,especially its small piezoelectric coefficient(6×10-12 C/N),which may be suitable for high repetition frequency operation,However,it is necessary to explore the interaction between electro-optic and optical rotation,and develop high repetition rates electro-optical devices.This paper is oriented to the development status of Q-switched laser.Aiming at the development of Q-switching with high repetition rates and broadbands,it grasps the key factors determining the piezoelectric ringing effect and selects LGS crystal as the research object.Combined with the advantages of LGS crystals with large size and high optical quality that can be grown by our research group,the interaction between electro-optic and optical rotation of the LGS crystal was analyzed,we explored the electro-optic Q-switched performances of LGS crystal with high repetition rates and designed laser devices,and realized the high repetition rates Q-switched laser with the wavelength ranges from 1.0661μm to 1.988 μm.And the main work as follows:1.The interaction between optical rotation and electro-optical properties of LGS crystal and the design of "odd transit time" LGS Q-switchingLGS crystal is an electro-optic crystal with excellent comprehensive performance,but its optical rotation performance limits its electro-optic application.Based on the propagation equation of light,the effects of optical rotation impacted on the phase and electro-optical properties are discussed in theory,it is found that the optical rotation of crystal only changes the polarization direction of light and does not produce additional phase difference and corresponding electro-optical switching ability.On the basis of theoretical research,combined with the "reversible" characteristics of optical rotation,a new "odd transit time" LGS electro-optic Q switched design scheme is proposed,to eliminate the single pass through the electro-optic crystal produced by the effect of optical rotation to cope with electro-optical effects by rotating a quarter of the wave plate.This scheme simplifies the resonant cavity design,is more conducive to the compact structure of the laser design and short pulse width laser output,laid the theoretical foundation for the follow-up experimental exploration,and provides a new way for the optical rotation electro-optic crystal to achieve electro-optic Q-switched application.2.1.066 μm high repetition rates LGS electro-optic Q-switchingBased on the theoretical research,the "odd transit time" LGS electro-optic Q-switching is designed.Using a cut 0.4at%Nd:LuVO4 crystal as the laser gain medium,1.066 μm LGS electro-optic Q-switched pulsed laser was achieved.The plused laser can be obtain the highest repetition rates of 200 kHz,the narrowest pulse width of 5.16 ns,the maximum output power of 4.4 W with slope efficiency of 29.1%.Relevant studies have become the highest repetition rate operation of electro-optic Q-switching for such crystals,and no piezoelectric ringing effect has been found,proving that LGS crystals have an important application prospect in electro-optic Q-switching laser with high repetition rates,and there is room for improvement.3.1.342 high repetition rates LGS electro-optic Q-switchingNd:YVO4 crystal was selected as the laser gain medium to study the influence of different doping concentration(0.27a%,0.5at%)on 1.342 μm LGS electro-optic Q-switched experiment and choice optical crystral concentration.Under the existing conditions,the pulsed laser with the highest repetition rate of 100 kHz can be achieved.Using doping concentration of 0.27at%,the laser can be achieved the shortest pulse width of 3.1 ns and the highest average output power of 2.42 W with the slope efficiency of 16.8%.When changing to 0.5at%crystal,the laser can be obtain the narrowest pulse width of 2.4 ns and the highest output power of 2.21 W,slope efficiency of 17.7%,all the results did not appear piezoelectric ring effect.The theoretical value of energy output under different repetition rates was calculated,and the calculated values of the pulse energy were almost consistent with the experimental values.4.1.988μm high repetition rates LGS electro-optic Q-switchingUsing its wide transmission spectrum,the 1.988 μm electro-optic Q-switched performane of LGS crystal was investigated.Aiming at the problem that the driving voltage required by the electro-optic switch is proportional to the laser wavelength of the working band.The electro-optic switching is designed by combining the gain size of the laser crystal.A low driving voltage and high repetition frequency LGS electro-optic crystal switch is designed by balancing the gain ald loss in the cavity.Tm3+ doped laser crystal is utilized and optimized,guided by theoretical calculation of gain and loss.It is found that under the pump power of 12 W,the driven voltage of the switching can be reduced to 3.9 kV which is 45%lower than the previous driven voltage.We demonstrated the LGS electro-optic Q-switched 1.988 μm laser with a repetition rate of 200 kHz and pulse width of 5.52 ns.The related research can be used for reference and guidance for the development of low driving voltage,high repetition rates electro-optic Q-switching and Q-switched laser,especially the mid-wave infrared laser. |
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