| The single crystal fiber is a gain medium between the traditional bulk laser crystal and the quartz fiber.The raw material for preparing the single crystal fiber is usually a mature laser crystal,such as Yb:YAG,Nd:YAG,Ho:YAG,etc.On the one hand,the single crystal fiber inherits the advantages of the large length-to-diameter ratio of the traditional fiber.Compared with the traditional bulk crystal,it has a waveguide effect on the pump light and improves the pumping efficiency;on the other hand,compared with the quartz fiber,it has higher thermal conductivity,smaller non-linear effect coefficient(SBS),and theoretically the single crystal fiber's ultimate output power is higher.In this paper,0.2%and 0.5%doped Nd:YAG single crystal fiber is selected as the research object.The size of the crystal is?1×40mm.Starting from the rate equation of the four-level system,the key parameters such as the light output threshold,working efficiency,and output power in the Nd:YAG laser are theoretically derived,and the basic characteristics of the Nd:YAG laser are preliminarily analyzed.Then,Tracepro was used to simulate the pump light distribution in the single crystal fiber,and the effects of the crystal size,pump beam waist size,and pump light numerical aperture on the pump light distribution in the single crystal fiber were studied.Follow-up experiments provide guidance.In order to ensure the stable operation of Nd:YAG single crystal fiber laser at high power,this paper redesigned the heat dissipation scheme on the basis of the predecessors,and figured out the crystal heat dissipation packaging process and the crystal end face high-precision polishing process.First of all,this paper conducted a continuous output experiment of Nd:YAG single crystal fiber.In the experiment,two pump sources of 808nm and 885nm were used.The highest power,slope efficiency,and beam quality of the output laser were compared and analyzed at 0.2%.In the 0.5%doped crystals,the central wavelength of the pump source,the pump coupling ratio,the effect of the parallel planar cavity and the flat concave cavity on the Nd:YAG single crystal fiber laser.At 0.5%doping,885nm pumping,and a flat concave cavity output,a continuous output of 26.1W is obtained,corresponding to a slope efficiency of 62.3%,which is currently the highest slope efficiency among Nd:YAG single crystal fibers.In the continuous output,we found that the Nd:YAG single crystal fiber has a very high slope efficiency,so it has a very high gain for the output laser,so we conducted a ps amplification experiment using the Nd:YAG single crystal fiber.We chose a domestically produced 8ps seed source.In order to obtain higher amplification output,this article divides the amplification experiment into"single pass","double pass"and"four times pass"according to the number of times the signal light passes through the crystal during the amplification process three phases.In the"single pass",this paper studies the influence of the signal light and the pump beam waist on the amplification performance;in the"double pass",the method of returning the"single pass"output light to the original path is analyzed in detail,and the proposed A combination of concave lens,convex lens,and high mirror is used to return the"single pass"output method;in the"four times pass",it is found that due to the higher gain of Nd:YAG single crystal fiber,significant spontaneous radiation amplification(ASE)is generated.Due to the limitation of the experimental conditions,this problem cannot be solved well.Among them,in the"double pass",the amplified output with the highest average power of 11.8W is obtained,corresponding to the extraction efficiency of 21.1%.And when the repetition frequency is 100k Hz,a single pulse energy output of 116?J is obtained,the pulse width is 9.2ps,the peak power is 12.6MW,and the beam quality changes fromM_X~2=2.8,M_Y ~2=2.2 before amplification toM_X~2=2.4,M_Y ~2=2.1.This is also the highest peak power currently obtained with Nd:YAG single crystal fiber amplifiers. |
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