焦耳级大能量中红外固体Fe∶ZnSe激光器

Joule-Level High Energy Mid-Infrared Solid Fe∶ZnSe Laser

为实现焦耳级大能量中红外4~5μm波段激光输出,基于波长为2.94μm的Er∶YAG激光泵浦源和液氮恒温控制器,搭建了固体Fe∶ZnSe激光器实验装置,研究了温度和晶体参数对Fe∶ZnSe激光器输出时域波形及能量的影响。闪光灯泵浦的Er∶YAG泵浦激光时域波形展现出固体激光典型的弛豫振荡现象,Fe∶ZnSe激光脉冲波形与泵浦激光波形保持强相关性,且单个尖峰脉冲宽度随温度的升高而变小。在低温79 K条件下,当Er∶YAG泵浦能量为2.75 J时,实现了Fe∶ZnSe激光1.04 J大能量输出,对应的斜率效率和光光转换效率分别为36.4%和37.8%,激光中心波长为4.1μm。在热电致冷可达的240 K温度条件下,当泵浦能量为500 mJ时,Fe∶ZnSe激光的能量输出为50 mJ,激光中心波长为4.4μm。研究结果为焦耳级大能量中红外固体激光器的研制提供了参考。

Objective Mid-infrared(4-5μm)radiation lies in the atmospheric transmission window and has broad application prospects in fields such as atmospheric remote sensing,environmental monitoring,and space communications.Compared with chemical lasers,nonlinear frequency conversion lasers,and other means of obtaining mid-infrared lasers,solid Fe:ZnSe lasers have the advantages of compact volume and high energy,offering a new way to achieve high-energy mid-infrared laser output.This study presents a high energy mid-infrared solid Fe∶ZnSe laser.We use the Er∶YAG laser as the pump laser and design an Fe∶ZnSe laser system whose crystal temperature can be controlled.The work performance of the Fe∶ZnSe laser is studied at different temperatures.In addition,Fe∶ZnSe laser spectra are obtained at different temperatures.Methods The Fe2+∶ZnSe crystal is sensitive to temperature,which causes a temperature quenching effect at higher temperatures and affects the laser efficiency.When the temperature is above 100 K,the lifetime of the laser upper level decreases rapidly with an increase in temperature,from 60μs at 77 K to 360 ns at 294 K.To improve the lifetime of the laser upper-level,we use a liquid nitrogen Dewar temperature controller.The Fe∶ZnSe crystals are placed in a low-temperature vacuum chamber.A 2.94-μm Er∶YAG laser with axial pumping is incident on the crystal surface.The maximum output energy of the Er∶YAG laser is 3 J,and its pulse width is 50-300μs,which comprises multiple spike pulses with a duration of several hundred nanoseconds.The resonant cavity is formed by a flat input mirror M1 and flat output coupler M2 with a cavity length of 50 mm.The input mirror M1 exhibits>98%transmittance for the pump laser and>99.5%reflectivity for the Fe^(2+)∶ZnSe laser,whereas the output coupler M2 exhibits>99.9% reflectivity for the pump laser and 70% reflectivity for the Fe∶ZnSe laser.The energy density of the pump light incident on the cavity can be adjusted by changing the optical interval of the telescope.An iris is used to adjust the size of the pump spot incident on the Fe∶ZnSe crystal;in addition,it is used to suppress transverse parasitic oscillations.Previous research has shown that smaller pump spots can suppress transverse parasitic oscillations and improve laser efficiency.In this study,three Fe∶ZnSe crystals are grown via the vertical Bridgman method and simultaneously doped during growth with a higher doping uniformity.The crystal size of crystal#1 is 20 mm×20 mm×4 mm,with a doping concentration of 5×1018 cm^(-3).Crystals #2 and #3 have the same size,and their doping concentrations are 0.9×1018 cm^(-3) and 4.5×1018 cm^(-3),respectively.Results and Discussions At 79 K,the maximum output energy of the Fe∶ZnSe laser is 1.04 J with the slope efficiency of 36.4% and optical-to-optical conversion efficiency of 37.8% at a pump energy of 2.75 J[Figs.2(a)and(b)].Figure 2(a)shows the output energy and slope efficiency of different Fe∶ZnSe crystals.Because of the difference in the doping concentration and gain length,the absorption and slope efficiencies of the crystals are different.The total absorptivities of crystals #1 and #3 are similar;therefore,the slope efficiency of the Fe∶ZnSe laser is also similar.Owing to the low doping concentration of crystal #2,the total absorption of the pump light is only 69%;therefore,the laser slope efficiency is lower than those of the other two crystals.The temporal profiles of the Fe∶ZnSe laser are shown in Figs.2(c)-(f)and Figs.3(a)and(b);it can be observed that the Fe∶ZnSe laser waveform remains strongly correlated with the pump laser waveform,and the width of a single spike pulse shortens with the increase in temperature.The output spectrum of the Fe∶ZnSe laser is shown in Fig.3(d).The output spectrum redshifts with increasing temperature,and the tunable range widens.Conclusions In this study,a low-temperature Fe∶ZnSe laser is fabricated using an Er∶YAG laser as the pump energy source.The Fe∶ZnSe laser has the potential to produce large amounts of energy in the mid-infrared region.At 79 K,the output energy of the Fe∶ZnSe laser is 1.04 J with a slope efficiency of 36.4% and an optical-to-optical conversion efficiency of 37.8% at a pump energy of 2.75 J;the wavelength of the Fe∶ZnSe laser is 4.1μm.At the thermoelectric cooling temperature of 240 K,the energy of the Fe∶ZnSe laser is 50 mJ with a wavelength of 4.4μm and pump energy of 500 mJ.The Fe∶ZnSe laser introduced in this study has many potential applications in mid-infrared fields,such as environmental monitoring and laser communication,which provides a basis for further miniaturization and fabrication of wavelength-tunable Fe∶ZnSe lasers.