中红外宽带发射氟化铅激光晶体性能研究

Properties of Mid-Infrared Broadband Emission Lead Fluoride Laser Crystals

中红外激光在光电对抗、激光雷达、遥感探测、激光手术、光纤通信、科学研究等领域中有着非常重要的应用。中红外激光增益材料作为中红外激光器的核心工作介质,具有举足轻重的地位。其中,设计和生长既满足当前泵浦条件又具有高效中红外荧光发射性能的激光晶体成为了研究热点。从优化中红外发光离子光谱性能角度出发,回顾了本课题组近年来关于新型宽带发射稀土离子掺杂氟化铅中红外激光晶体的研究结果,以期为后续研究提供一定的参考。

Objective Mid-infrared (MIR)lasers,specifically broadband tunable MIR lasers operating at~3 mm have attracted much attention for numerous applications in scientific research and practical technologies such as laser microsurgery,remote sensing,atmospheric lidar measurement,wavelength division multiplexing (WDM)systems,and ultrashort pulsed lasers.It is well known that trivalent erbium (Er^(3+)),dysprosium (Dy^(3+)),and holmium (Ho^(3+)) ions are natural candidates for 2.7-3mm lasers owing to the Er^(3+):^(4)I_(11/2)→^(4)I_(13/2),Dy^(3+):^(6)H_(13/2)→^(6)H_(15/2),and Ho^(3+):^(5)I_(6)→^(5)I_(7) transitions.However,the~3mm laser operations of Er^(3+),Dy^(3+),or Ho^(3+) ions cannot be obtained efficiently because of the fluorescence lifetime of the self-termination effect(i.e.,fluorescence lifetimes at the upper levels are considerably shorter than fluorescence lifetimes at the lower levels,which results in selfsaturation of population inversion)and the lack of an appropriate commercialized laser diodes(LDs).Therefore,in order to solve the bottleneck problem of Er^(3+),Ho^(3+),and Dy^(3+)ions activated mid-infrared laser crystals and obtain broadband fluorescence emission in the~3μm band,which is beneficial to realize the broadband tunable output,we need a proper sensitizer ion with large absorption cross sections for Ho^(3+)and Dy^(3+)ions,appropriate deactivated ions with efficient depopulation of Er^(3+)and Ho^(3+)ions for population inversion,and a proper host with low phonon energy.Methods The Er^(3+)/Ho^(3+)/Pr^(3+):PbF_(2),Yb^(3+)/Er^(3+)/Dy^(3+):PbF_(2),and Yb^(3+)/Ho^(3+)/Dy^(3+):PbF_(2)crystals are grown by the Bridgman technique.The concentrations of Yb^(3+),Dy^(3+),Er^(3+),Ho^(3+)and Pr^(3+)ions in the as-grown crystals are measured by the inductively coupled plasma atomic emission spectrometry analysis.Crystal structure identification is undertaken with an X-ray diffraction(XRD)method using Cu Kαradiation.The absorption spectra are measured by a spectrometer.Spectrophotometers are used to measure the fluorescence spectra and the fluorescence decay curves.All the measurements are taken at room temperature and other conditions are kept as same as possible.Results and Discussions For the Er/Ho/Pr:PbF_(2)crystal,an enhanced broadband~3mm emission with a large effective bandwidth(330nm)pumped by a 980nm LD is obtained(Fig.3).Compared with the Er:PbF_(2)and Ho:PbF_(2)crystals,the Er/Ho/Pr:PbF_(2)crystal presents a strong fluorescence emission intensity.It is also demonstrated that the Pr^(3+)ion is a suitable deactivated ion for Er^(3+)and Ho^(3+)ions with an efficient depopulation effect on the lower laser levels of Er^(3+):^(5)I_(13/2)and Ho^(3+):^(5)I_(7)for enhancing the~3mm fluorescence emission.For the Yb/Er/Dy:PbF_(2)crystal,broadened and enhanced emission with a full width at half maximum of 265nm is obtained at~3μm due to the fact that Er^(3+)and Dy^(3+)ions are used as the emission center at the same time(Fig.5).For the Yb/Ho/Dy:PbF_(2)crystal,broadband emission extending from 2600nm to 3400nm with a full width at half maximum of 283nm is obtained(Fig.7).Conclusions The mid-infrared tunable lasers have a wide range of applications in military,medical,biological imaging,and scientific research fields.As the core working medium of mid-infrared tunable lasers,the gain medium plays an important role.In order to solve the bottleneck problem of Er^(3+),Ho^(3+),and Dy^(3+)ion activated mid-infrared laser crystals and obtain broadband fluorescence emission in the~3μm band,which is beneficial to realize a broadband tunable output,this work proposes a relevant optical performance optimization control scheme.Based on the existence of coupling interaction between rare earth ion levels,three coupling schemes of Er^(3+)/Ho^(3+)/Pr^(3+):PbF_(2),Yb^(3+)/Er^(3+)/Dy^(3+):PbF_(2)and Yb^(3+)/Ho^(3+)/Dy^(3+):PbF_(2)are proposed,and the optical properties are studied systematically.At the same time,an enhanced and widened 3μm fluorescence emission is obtained,indicating that these kinds of crystals are expected to be gain materials for broadband tunable lasers.