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Yb Doped Fluorophosphate Glasses: a Good Candidate for High Energy, Ultra Short Pulse, Tunable Fiber Lasers

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Yb Doped Fluorophosphate Glasses: a Good Candidate for High Energy, Ultra Short Pulse, Tunable Fiber Lasers 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 .
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