高功率连续光和波长可调谐Er∶CaF_(2)激光器

High-Power Continuous-Wave and Tunable Er∶CaF_(2) Lasers

本文报道了一种高功率半导体激光二极管双端泵浦Er∶CaF_(2)连续激光器和宽波长调谐范围的中红外激光器。在单端泵浦的谐振腔系统中,当泵浦功率为19.7 W时,输出功率达到了3.54 W,斜率效率为19.6%;在双端泵浦的谐振腔系统中,当泵浦功率为32.5 W时,连续光输出功率达到了5.04 W,斜率效率为16.5%,中心波长为2799.27 nm。据笔者所知,这是目前所有报道的激光二极管泵浦掺铒氟化物晶体激光器实现的最高连续光功率。采用厚度为1、2、4 mm的MgF_(2)双折射滤光片,实现了波长可调谐范围分别为168.89、148.87、141.17 nm的Er∶CaF_(2)激光器,这是目前所有报道的掺铒氟化物晶体激光器中实现的最宽的波长可调谐范围。

Objective In the mid-infrared region,erbium-doped lasers at 2.8µm have attracted significant attention owing to their wide applications in the medical treatment,detection,and military fields.The phonon energy of erbium-doped crystalline oxides(e.g.,Er∶YAG and Er∶YSGG)is high(860 cm-1 and 728 cm-1),which causes the self-termination phenomenon.The phonon energy of erbium-doped sesquioxides,such as Er∶Lu2O3 and Er∶Y2O3,is low(618 cm-1 and 597 cm-1)and the thermal conductivity of these materials is high.However,the fabrication of these active laser materials is complicated and expensive mainly due to their high melting temperature.Erbium-doped fluorides,such as Er∶YLF,Er∶CaF_(2),and Er∶SrF_(2),show lower phonon energy(560 cm-1,322 cm-1,and 280 cm-1)than sesquioxides,which can effectively suppress the non-radiative transition.Especially,the doped Er3+ions prefer to form clusters in CaF_(2)and SrF_(2) crystals.These spontaneous clusters can achieve strong energy transfer between Er3+ions with very low doping concentration(approximately 1%),accordingly obtaining a high-efficiency and high-power continuous-wave(CW)laser at 2.8µm.The emission spectrum of Er∶CaF_(2)and Er∶SrF_(2) crystals is wide,approximately 250 nm(from 2600 nm to 2850 nm),around 2.8µm.Therefore,spectral tuning of the Er∶CaF_(2)laser output wavelength is viable.Methods To obtain high-power CW laser,a dual-end pumped Er∶CaF_(2)laser is demonstrated[Fig.1(a)].The two pumping sources are wavelength-stabilized 976 nm fiber coupled laser diodes(LD)with a fiber core diameter of 105µm and a numerical aperture of 0.22.The pumping radiations are focused into the laser crystal by the optical coupling systems with a coupling ratio of 30∶60(L1∶L2)and 30∶75(L4∶L3),respectively.A plane-concave resonator with a cavity length of 23 mm is formed by a concave dichroic mirror(DM1)and a plane output coupler(OC).The DM1,with a radius of curvature of 50 mm,is AR coated(T=95%)for 960-980 nm and HR coated(R=99.8%)for 2.65-2.85µm.An OC with T=3%at 2.65-2.85µm is used.The DM2 is placed between the OC and the L3 lens to separate the laser from the pumping radiations.With a dimension of 2 mm×2 mm×20 mm,an uncoated 2%-doped Er∶CaF_(2)crystal is wrapped with indium foil and placed in a water-cool copper block with a temperature of 18℃for heat dissipation.In addition,a tunable Er∶CaF_(2)laser is also demonstrated[Fig.1(b)].Uncoated MgF_(2)birefringent filters(BRF),with a diameter of 15 mm and three different thicknesses,1,2,and 4 mm,are mounted on a rotating frame and inserted between the Er∶CaF_(2)crystal and OC at a Brewster angle(θB=53.7°),respectively.The cavity length is approximately 67 mm.The spectral tuning is achieved by rotating the angle between the BRF s optical axis and the incident light.Results and Discussions In the single-end pump scheme,the laser exhibits a saturation trend when the pump power reaches 19.7 W,and a CW output power of 3.54 W with a slope efficiency of 19.6%is obtained(Fig.2).In the dual-end pump scheme,there is still no saturation trend when the pump power reaches 32.5 W.However,to protect the crystal,the pump power is not further increased.The maximum achieved CW output power is 5.04 W with a slope efficiency of 16.5%,a central wavelength of 2799.27 nm,and a beam quality factor of M^(2)_(x)=5.16 and M^(2)_(y)=5.69[Figs.2(a)and(b)].To the best of our knowledge,this is the highest CW output power among all reported LD-end-pumped erbium-doped fluoride crystal lasers at 2.8µm.The dual-end pump scheme alleviates the saturation trend of the laser,which is an effective scheme to realize high-power CW laser.There is a slight red-shift of the central wavelength with increasing output power(Fig.3):from 2749.85 nm at a CW output power of 0.5 W to 2799.27 nm at a CW output power of 5.0 W.In addition,the emission spectrum of the 2%-doped Er∶CaF_(2)crystal is wide,covering a range of approximately 250 nm,from 2600 nm to 2850 nm(Fig.3).These observations lead us to believe that spectral tuning of the laser output wavelength is feasible.Based on MgF_(2)BRFs with thicknesses of 1,2,and 4 mm,Er∶CaF_(2)lasers with tuning ranges of 168.89 nm(from 2682.55 nm to 2851.44 nm),148.87 nm(from 2696.96 nm to 2845.83 nm),and 141.17 nm(from 2703.60 nm to 2844.77 nm)are achieved,respectively(Fig.5).Owing to the strong absorption of the 2.8µm laser by water molecules and other substances in the air,the tuning power curves are not smooth and the emission spectrum is modulated.A wider spectral tuning range can be obtained by using BRFs with a smaller thickness,but the processing difficulty and price cost will also be greatly increased.Conclusions In this work,high-power CW and tunable Er∶CaF_(2)lasers are demonstrated.In the single-end pump scheme,the output power reaches 3.54 W when pump power is 19.7 W.In the dual-end pump scheme,the CW output power reaches 5.04 W with a slope efficiency of 16.5%and a central wavelength of 2799.27 nm when pump power is 32.5 W.To the best of our knowledge,this is the highest CW output power among all reported LD-end-pumped erbium-doped fluoride crystal lasers at 2.8µm.Er∶CaF_(2)lasers with tuning ranges of 168.89 nm,148.87 nm,and 141.17 nm are achieved by using MgF_(2)birefringent filters with thicknesses of 1 mm,2 mm,and 4 mm,respectively.To the best of our knowledge,this is the widest spectral tuning range among all reported erbium-doped fluoride crystal lasers.