| High power all-solid-state lasers are widely used in laser processing,remote sensing,and high-energy physics research.The slabs are cracked and the cooling area is large.In the direction of the large crystal face in zig-zag slab amplifiers,the temperature gradient is one-dimensional;the signal light is totally reflected or zigzag transmission on the pump surface inside the crystal,which can compensate the quality degeneration of the beam caused by the thermal gradient in the pump direction.In recent years,it has been one of the main directions of high-power solid-state lasers.Amplified Spontaneous Emission(ASE)is also known as super fluorescence.Fluorescence from spontaneous emission is significantly amplified in high gain media or there is strong optical surface positive feedback in the amplifier.When the fluorescence in the laser medium has a closed path and the gain is larger than the loss,the parasitic oscillation(PO)is established,which becomes quite serious as the size of the laser gain medium and the pumping power density increase.It greatly consumes the population inversion and affects the output power.The large aperture and high gain of the amplifier can cause severe ASE effects,decrease the energy storage density and efficiency of the laser medium,redistribution of energy storage in the medium occurs,and the spatial gain uniformity of the beam is destroyed.It has become one of the severe factor limiting the development of slab laser systems.In this issue,the ASE effect in large-caliber high power Nd:YAG slab amplifiers is taken as the research object.The purpose is to explore the law between the ASE intensity and the slab structure as well as the suppression methods to improve the amplification performance with the consideration of the gain medium and structural characteristics.The experimental verification of the related issues has been carried out.The main contents include:1.This paper introduces the basic configuration and related research progress of slab amplifiers,and carried out detailed investigation and introduction on the study of ASE and parasitic oscillation effects.The specific content is classified according to whether the gain medium has circular symmetry,mainly including the progress of numerical calculation and the development of suppression technology;2.The paper summarizes the theoretical basis of spontaneous emission fluorescence and the theory of slab gain amplification.The four-level structure of Nd:YAG as well as the slab amplification mechanism are also discussed.The quasi-classical theory,quantum theory,and rate equation theory of amplified spontaneous emission are introduced respectively,and the rate equation theory is finally determined as the main method of ASE analysis;3.A numerical calculation model of ASE effect in slab amplifiers based on geometric optical ray tracing has been established.The entire three-dimensional asymmetrical slab space is meshed,and the propagation and reflection characteristics of fluorescence are calculated.The effects of spontaneous emission fluorescence propagation on the inverted population density and small signal gain coefficients are analyzed.The related parameters are corrected for accuracy.The model analyzes the ratio on each surface of the slab amplifier,the small signal gain coefficients under different slab size structures are calculated to analyze the effect of the ASE on the amplification characteristics of slab with different calibers.The calculated results provide guidance for experimental inquiry;4.The relevant experiments of the ASE and parasitic oscillation characteristics in high-power slab amplifiers have been completed.The unit experiment includes:1)We used the high-speed photoelectric probes to perform fluorescence characteristic detection experiments,intuitively understand the ASE effect and parasitic oscillations;The generation process of parasitic oscillations and the possible paths of parasitic oscillations are also discussed;2)We used the slab gain module to perform the amplification experiment,we have demonstrated a Nd:YAG slab amplifier(150.2mm*30mm*2.5mm)with the repetition frequency and pulse width of 1 kHz and 48 ns.The output energy is 236 mJ with a high beam quality of 1.57 and 1.86 in the horizontal and vertical directions;The small signal gain coefficients of different size slabs(150.2mm*30mm*2.5mm and 150.2mm*40mm*2.5mm)are measured respectively,and the results have been compared them with the results of the ASE calculation model to verify the accuracy of the model calculations;3)According to the calculation results of the model,the ASE suppression methods of increasing the fluorescence absorption on the large surfaces and deriving the fluorescence on the side surfaces are respectively proposed.The metal Ti and Cr are selected as the absorbing materials.The absorption performance has been tested from the theoretical calculation and experimental verification.A composite film has been designed and its fluorescence absorption effect is calculated.The metal-dielectric composite absorption film system designed using Cr and SiO2 achieved 92%absorption theoretically at 1064nm in the range of incident angle of 0-60°,confirming the positive effect of the absorption film layer in suppressing the internal ASE of the slab;4)The quartz material is selected as the side light guide material and three different sizes material(130mm*2mm*5mm,130mm*2mm*8mm,130mm*2mm*10mm,respectively)are processed.The light guide material is combined on the rough side surfaces with pure water.The influence of the light guide bar's size and the fixed material on the effect of fluorescence extraction has been analyzed.The average gain improvement ratio under 21600W is 60.25%.The experiment confirm that the fluorescence derivation measures effectively reduce the ASE intensity by reducing the fluorescence reflection on the side surface,the measures improve the gain and they ensure the output performance of the slab in high-power environment.This article has completed innovative work in the following areas:1.Based on the geometric optical ray tracing technique,a three-dimensional ASE calculation model is established.The model calculates the ASE spatial distribution,and the ASE contribution ratio of the fluorescence reflection on each surfaces is analyzed.2.The structure of the large-surface metal fluorescence absorption film is proposed,and the absorption of the single-layer metal film has been verified both theoretically and experimentally.A kind of metal-dielectric composite absorption film is designed,and its absorption performance is calculated theoretically;3.A lateral fluorescence derivation structure is proposed.The relevant experiments verify the positive effects of lateral fluorescence derivation measures to suppress fluorescence reflections and improve the system gain. |
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