View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Shanghai Institute of Optics and Fine Mechanics,Chinese... 第 23 卷 第 4 期 物 理 学 进 展 Vol. 23, No. 4 2003 年 12 月 PROGRESS IN PHYSICS Dec. , 2003 Article ID:1000O0542( 2003) 04O0473O10 Yb3+ _doped fluorophosphate glasses: a good candidate for high_energy, ultra_short_pulse, tunable fiber lasers ZHANG Li_yan, HU Li_li, JIANG Zhong_hong ( Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, P. R. China) Abstract: In co mparison w ith other rare_earth ( RE ) ions, Yb3+ has the simplest energ y lev els, w hich bring it some unique properties such as eliminating excited state absorption, avoiding upconversion and concentration quenching, thus make it a good do pant for laser media to realize hig h po wer laser output. F luorophosphate ( FP) glass can bring tog ether both t he advantages of fluoride glass and phosphate glass, like decreasing the phonon energy and improving the easy moistening property of phosphate glasses, enhancing the physics and chemical stability of fluoride g lasses. T his makes it a pro mising RE_doped fiber laser media w ith wide tunable ranges. Lots of researches show ed Y b3+ doped FP glasses hav e a good future as laser media. T his paper summarizes t he characteristics, properties, structure and problems of Yb3+ doped fluorophosphate glass as an operat ion material for lasers. Key words: F luoro phosphate g lass; Y b_do ped; fiber laser; phonon energy CLC number: TQ171 Document code: A 1 Introduction RE_doped glasses are of great interest in optical communication technology, applications in the field of infrared _to_visible converters and laser sources[ 1] . Ytterbium lasers have the advantage of a w ide absorption and fluorescence bandw idth and low thermal loading. T hese properties make them suitable for diode_pumping, ultrashort pulse g eneration, and high average pow er operation. Yb3+ : glasses arose particular interest because of their smooth fluorescence spectrum and low re_absorption at the laser w avelength. Yb3+ laser is reg arded R eceived date: 2003O08O15 * Corresponding author: Zhang L iyan Phone: + 86O21O59910994 Fax : + 86O21O59910393 Email address: jndxzly@ hotmail. com 474 物 理 学 进 展 23 卷 as the most promising candidate for high energ y, ultra_short_pulse laser needed in inertially confined fusion ex periments because of its simple quantum structure[ 2~ 4] . But now almost all the practical g lass matrix for Yb3+ doped fiber laser are silica glass or phosphate glass. They all have the relatively hig h phonon energy, and don. t have w ide tunable ranges. H igh phonon energy increases nonradiatively relaxation rates thus reducing the luminescence quantum efficiency and the excited state lifetime of doping ions[ 5] . Table 1 shows the phonon energy of various glasses. Fluoride glasses have low phonon energy compared with phosphate and silica glasses. Therefore, FP glasses can have phonon energies betw een fluoride g lasses and phosphate glasses ( lower the intensity of the 1300cm- 1 phosphate peak g reatly) . And also the low numerical apertures of silica fibers ( < 0. 3) make the pump absorption efficiency low , and the mid_infrared absorption of OH - at about 3000nm caused by the easy moistening nature of phosphate glasses deteriorate its spectrum properties. A little fluoride content can g reatly enhance the OH- resistant ability, thus makes FP glass a preferable choice to combine both the optical properties and thermal stability of phosphate glasses and the low phonon energy property of fluoride glasses. Table 1 Phonon energy of some kinds of glasses G lass Phono n energ y ( cm- 1) Glass Phonon energ y( cm- 1) Bo rate 1400 F luoride 350~ 600 Phosphate 1300 Chlo ride 285 Silicate 1100 Bromide 190 T ellurite 780 Iodide 160 T able 2 shows the spectroscopic properties of three kinds of Yb: g lasses[ 4] . QX/ Yb and QO 246/ Yb are successful commercial Yb3+ doped laser glasses. In comparison with phosphate and silicate glasses, FP glass exhibits some good properties. Yb3+ doped FP glass has the potentiality for cheap diode pumped tunable room temperature cw_lasers acting at w aveleng ths around 1040 nm[ 6, 7] . Yb_doped fiber laser has achieved great development. University of Southampton reported in 1999 a sing le_clad Yb_doped fiber laser having an output over 5W, but now they improve the output to 20W at 1085 nm, see Fig. 1, and their Q_switch multimode Yb_doped fiber laser has the 10W average pow er, and they also obtained a Yb_doped jacketed air_clad fiber laser w hich has a w ide flat tunable rang es of 100 nm, shown in Fig. 2 and Fig. 3, and over 800m J output power. T heir diode pum ping source is show n in Fig. 4. Till now the w idest tunable range is 104 nm ( 1502~ 1606 nm ) in Er3+ : Yb3+ _doped FP g lass w ith an average output of 9. 5 mw reported by J. F. Philipps et al [ 8] . 4 期 张丽艳等: 高能、超短脉冲、可调谐激光器用掺镱氟磷酸盐玻璃研究进展 475 Table 2 Spectroscopic parameters of three kinds of Yb:glasses L aser G lass Phosphate( P) Silicate( S) Fluoride Q X/ Y b QO246/ Y b Phosphate( FP) Emission Bandw idt h vK/ nm 62 77 81 Emission cross section @ 0. 975 Lm/ @ 10- 20 cm2 0. 67 0. 71 1. 2 ( L) - 20 2* Emission cross section Rem / @ 10 cm 0. 05 0. 095 0. 16 ( L) - 20 2* A bsorption cross section Rabs / @ 10 cm 0. 001 0. 002 0. 002 2 Gain saturation fluence F sat, L/ J/ cm 370 190 120 Fluorescence lifetime S/ ms 1. 3 1. 1 1. 3 ( P) - 20 2 A bsorption cross section Rabs @ 0. 97 Lm/ @ 10 cm 0. 25 0. 19 0. 4 ( P) - 20 2 Emission cross section Rem / @ 10 cm 0. 07 0. 11 0. 2 2 Pump saturation intensity I sat, P@ 0. 97 Lm/ kw / cm 38 62 26 ( L) 2 T ransparency intensity IP, trans/ kw/ cm 1. 3 2. 1 0. 5 3+ 21 - 3 Yb doping density N tot/ @ 10 cm 2 1. 7 1. 7 * at 1. 06 Lm waveleng th F ig. 1 Yb_doped fiber laser with 83% slope efficiency F ig. 2 Yb_doped jacket ed air_clad fiber ( U niversity of Southampton) ( U niversity of Southampton) F ig. 3 T unable range of the jacketed fiber F ig. 4 Diodes source ( U niv ersity of Southampton) ( U niv ersity of Southampton) 476 物 理 学 进 展 23 卷 J. Limpert et al [ 9] reported a cladding_pumped, Yb_doped large_core_area fiber amplifier that is capable of g enerating 51. 2 W of average power at 1064 nm, w hith a repetition rate of 80OMHz and 10_ps pulse duration. U . Griebner* reported recently a high energy ultrafast Yb_doped fiber chirp pulse amplify ing system w ith average power of 22 W at 1060 nm, and a femtosecond Yb: glass laser w ith pulse duration of 75 fs, average output power of 25 mW, and repetition rate of 95 MH z at 1050 nm. Bell Lab* * researchers have demonstrated ultra_hig h_pow er sing le_mode fiber laser from 1065 to 1472 nm, using Yb_doped cladding pumped and cascaded Raman laser, and the highest power level is over 40 W. 2 Characteristics of Yb3+ ions Because of its special quantum structure, Yb3+ exhibits some excellent properties for laser application: 3+ 2 2 1) Yb has only tw o energ y levels: F7/ 2 and F5/ 2. Compared with other RE_ions, its simple energy level scheme avo ids the detrimental complications, such as no excited state absorption, no upconversion, no concentration quenching ; minimizes thermal load ( because no nonradiative relax ation) , so enables efficient lasing operation. 2) Yb3+ has w ide absorption, em ission and tunable bands: its absorption band is between 800 ~ 1100 nm, and absorption peak is located at 970 nm , which is in the w aveleng th range of InGaAs diode laser ( 0. 9~ 1. 1 um ) ; and also its short pump absorption lengths decreases the demands on the beam quality of diode laser 3) The lack of intermediate levels and the large separation ( > 104cm- 1 ) betw een the low er and upper laser state m anifolds eliminate multi_phonon relaxation. Its long fluorescence lifetime is beneficial for energy restore, thus enhances the pumping efficiency[ 10~ 13] T he sim ple energ y level of Yb3+ also makes it a good sensitizer to other RE_ions, like Nd3+ , Tb3+ , Pr3+ and Er3+ , especially for Er3+ , as shown in Fig. 5. 4I11/ 2 of Er3+ and 2 3+ 3+ F 5/ 2 of Yb have the sim ilar high energy position, Er emission transition occurs betw een 4 4 3+ I13/ 2 y I15/ 2, however, Er absorption itself is too week to allow direct pumping, but the spectroscopic behavior of Er3+ doped materials can be enhanced by codoping w ith Yb3+ , 2 w hich can be pum ped by a diode laser around 980 nm in the F5/ 2 state that can subsequently 4 3+ 4 4 transfer the excited energy to I11/ 2 state of Er , g iving rise to I13/ 2 y I15/ 2 sensitized emission[ 14~ 18] . This energy transfer property can greatly enhance the average pow er of Er3+ _doped lasers and amplifiers used for w avelength_division _multiplexing ( WDM ) system . Because of Yb3+ ion operating as a quasi_four_level in normal tem perature, the higher stark levels of the g round state m anifolds are used as low er laser levels, the g round state manifolds are in 200 ~ 600 cm- 1, and it. s not much different from the thermal energ y * w w w .