Spectroscopic properties of Yb3+-doped silicate glasses

Journal of Alloys and Compounds 363 (2004) 1–5

Spectroscopic properties of Yb3+-doped silicate glasses NengLi Dai∗ , Lili Hu, Jianhu Yang, Shixun Dai, Aoxiang Lin Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, PR China Received 6 January 2003; received in revised form 5 March 2003; accepted 5 March 2003

Abstract The absorption, emission and potential laser properties of silicate glasses doped with Yb3+ ions are investigated in detail. The emission cross-sections (σ emi ) are calculated on the basis of reciprocity method according to the measured absorption spectra. The potential laser properties for Yb3+ -doped glasses are evaluated by the minimum fraction of Yb3+ (βmin ), the pump saturation intensity (Isat ), the minimum pump intensity (Imin ), and the product of emission cross-section (σ emi ) and fluorescence lifetime (τ f ). The σ emi near 1020 nm and τ f of Yb3+ in 61SiO2 –25PbO–6Na2 O–8K2 O glass are 0.49 pm2 and 2.00 ms, respectively, which are comparable to those in phosphate glasses. Systematical factor of laser properties (SFL) is introduced to evaluate the potential laser properties of Yb3+ -doped glasses, the results indicate that Yb3+ -doped silicate glasses are potential materials for double cladding ytterbium fiber laser application. © 2003 Elsevier B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Crystal structure; Crystal binding

1. Introduction Double-cladding fiber lasers (DCFL) have some unique advantages over traditional fiber lasers including [1–3] excellent beam quality, high optics-optics transfer efficiency, small volume and weight and robust laser cavity. From 1988 [4,5], high average power DCFL have attracted increasing attention because of its wide application in the fields such as automobile industry, communication, medical instrument, and printing technology. Recently, IPG company announced that 2000 W Yb3+ -doped double clad fiber laser had been successfully made. Since 1995, the research [6] on Yb3+ -doped various glasses have developed rapidly due to their higher Yb3+ doping concentration and low cost of the laser diode pumping sources. Compared with those glasses such as phosphate, borate, and gallium glasses which possess high emission cross-section [7–9], silicate glasses have their own benefits such as stable physical and chemical properties, low cost, and possible fused coupling with silica fiber. However, up to now, there are few systematic investigations on the optical properties of Yb3+ -doped silicate glasses. In this work, the spectroscopic properties of Yb3+ -doped silicate glasses and their potential laser properties are in∗

Corresponding author. E-mail address: [email protected] (N. Dai).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-8388(03)00379-7

vestigated systematically. Factors affecting the stimulated emission section and potential laser properties are discussed from the viewpoint of local environments around Yb3+ sites in glass. The potential laser properties of Yb3+ in different glass hosts are compared, which suggest that Yb3+ -doped silicate glasses are beneficial to be used for double cladding fiber lasers.

2. Experimental procedure and theoretical analysis 2.1. Sample preparation and measurement Reagent grade commercial oxides (> 99.5% pure) were used as the starting materials. 1.0 mol% Yb2 O3 (> 99.9% pure) was introduced to the batch shown in Table 1 for measurements of spectroscopic properties. Mixed batch was melted in alumina crucible at 1300–1450 ◦ C for about 60 min. Then the glasses melt was poured on the steel molds. The samples were annealed for 2 h near the glass transition temperature. Samples for optical and spectroscopic properties were cut and polished to the size of 20 × 10 × 1 mm3 . The absorption spectra were measured by Perkin-Elmer Lambda 900 in the range of 870–1100 nm at room temperature. The fluorescence lifetime were measured by exciting the sample with a 970-nm, 0.5-W laser diode. Fluorescence decay curves were recorded by the HP546800B 100-MHz

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N. Dai et al. / Journal of Alloys and Compounds 363 (2004) 1–5

Table 1 Glass compositions (mol%)

Isat =

Glass Glass compositions in mol% samples SBB SPb SB SAl

60SiO2 – xBi2 O3 – (30 − x)B2 O3 – 7Na2 O – 3K2 O – 1Yb2 O3 (x = 0,5,10,15,20,25,30) 61SiO2 – 25PbO – 6NaO – 8K2 O – 1Yb2 O3 (55 + x)SiO2 – (27 − x)B2 O3 – 18Na2 O – 1Yb2 O3 (x = 0,3,6,9,12) 72.2SiO2 – 3.2Al2 O3 – 2.1BaO – 22.6Na2 O – 1Yb2 O3

hc λp σabs (λp )τf

(5)

Imin = βmin · Isat =

  −1 Zl hc Ezl − hcλ−1 r 1+ exp λp σabs (λp )τf Zu kT

(6)

oscilloscope. The relative errors in these emission measurements were estimated to be < 8%.

where λl , λp are laser wavelength and the pump wavelength, Ezl represents the zero line energy, which is defined to be the energy separation between the lower components of the upper and lower energy level, respectively.

2.2. Evaluation of spectroscopic and laser properties of Yb3+ -doped glasses

3. Experimental results and discussion

2.2.1. Spectroscopic properties The absorption cross-section (σ abs ), integrated absorption cross-section (Σ abs ) and the stimulated emission cross-section (σ emi ) were calculated by the following equations [6,10] 2.303 log(I0 /I) N0 l
= σabs (λ)d(λ)

σabs =

(1)

Σabs

(2)

   Zl hc 1 1 exp σemi (λ) = σabs (λ) − Zu kT λp λ

(3)

where I0 and I are the primary optical intensity and the optical intensity throughout the sample, N0 is Yb3+ ion concentration (ions/cm3 ), l is the sample thickness, Zl , Zu represent the partition functions of the lower and the upper states, respectively, according to the energy-level diagram of Yb3+ in silicate glasses at room temperature, the value of Zl /Zu is very close to the denominator ration of lower and upper energy (4/3) [11,12], λp is the wavelength of peak absorption, h, c, k, T represent the Plank’s constant, velocity of light, Bolzman’s constant, room temperature, respectively. 2.2.2. Laser performance parameters The laser performance parameters of Yb3+ -doped glasses are affected strongly by the following parameters including βmin , Isat and Imin , respectively. βmin is defined as the minimum fraction of Yb3+ ions that must be excited to balance the gain exactly with the ground state absorption at laser wavelength λl ; Isat is defined as the pump saturation intensity; Imin is a parameter which evaluates the minimum absorbed pump intensity. They are given by the following equations, respectively [13] σabs (λl ) σabs (λl ) + σemi (λl )    −1 Ezl − hcλ−1 Zl l = 1+ exp Zu kT

βmin =

(4)

3.1. Spectroscopy of Yb3+ -doped silicate glasses It is well known that the stimulated emission cross-section (σ emi ) has strong relation with the local environment of the Yb3+ site characterized by both the Yb3+ site symmetry and its bonding characteristics [6,7,14]. At given oxide glasses, Yb3+ is in a network modifier position surrounded by the six oxygen ions which associated network formers. Therefore, the site symmetry and the bonding characteristics of Yb3+ ions are strongly affected by the bonding strength, polarizability and structure between the network former and oxygen ions. The more site asymmetry of [YbO6 ] octahedron the higher stimulated emission cross-section. The typical absorption and emission cross-section of Yb3+ -doped silicate glasses calculated by Eqs. (1) and (3) are shown in Fig. 1. As shown in Fig. 1, corresponding to the 2 F7/2 –2 F5/2 transition, Yb3+ -doped silicate glasses have broad absorption in the range of 910–980 nm and broad emission in the range of 975–1080 nm. SBB15 glass exhibits the highest peak absorption cross-section (σ abs ) of 1.98 pm2 at 976 nm, which is double of that in SAl sample. The σ emi near 1020 nm is more important than that of near 980 nm due to the self-absorption effect of Yb3+ ions for double-cladding fiber laser. It can be seen in Fig. 1 that the SBB15 glass has larger σ emi of 0.97 pm2 near 1020 nm, while the σ emi of SAl glass is 0.34 pm2 . The local structures of Yb3+ ions in silicate, borate, phosphate glasses were similar to those of Er3+ ions [14]. Yb3+ ions sit around the terminal region of the network or the region between the network in these glasses. In SBB glass, [YbO6 ] coordination sphere is surrounded by the network formers polyhedrons such as [SiO4 ], [BiO6 ] and [BO4 ] or (and) [BO3 ]. Therefore the [YbO6 ] in SBB glass has much more asymmetry than that of SAl in which the [YbO6 ] just is surrounded by sole [SiO4 ]. Finally SBB15 glass shows higher emission cross-section than SAl glass. Fig. 2 shows the relationship between stimulated emission cross-section σ emi (near 1020 nm) and integrated absorption cross-section (Σ abs ) calculated by Eq. (2) in Yb3+ -doped silicate glasses. The highest stimulated emission cross-section is achieved in SBB15 glass. It can be seen that it is almost a

N. Dai et al. / Journal of Alloys and Compounds 363 (2004) 1–5

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Fig. 1. The absorption and emission cross-section of Yb3+ -doped silicate glasses.

linear relationship between σ emi and Σ abs . So it is deduced that the σ emi is nearly determined by the Σ abs . This result agrees with that of Ref. [7] which indicates the stimulated emission cross-section σ emi increases with increase of Σ abs . 3.2. Laser properties parameters The laser parameters βmin , Isat , and Imin of Yb3+ -doped glasses which have been calculated by Eqs. (5), (6) and (7) are listed in Table 2. λ0 , and τ f indicate the sub-peak wave-

Fig. 2. The emission cross-section near 1020 nm of Yb3+ -doped silicate glasses versus the integrated absorption cross-section.

length of stimulated emission cross-section, and the measured fluorescence lifetime, respectively. SFL, the systematical factor, is defined as the general evaluation of laser properties in Yb3+ glasses. It is described as follows σemi · τf SFL = (7) Imin The larger the SFL, the better the general laser properties. In order to compare with other glasses, laser parameters of LSY glass (SiO2 –Al2 O3 –Li2 O–Na2 O–SrO) [6], QX Phosphate glass from Kigre [15] and PNK glass (P2 O5 –Nb2 O5 –K2 O) [6] are listed together with our Yb3+ -doped silicate glasses in Table 2. As shown in Table 2, the minimum fraction of Yb3+ (βmin ), the pump saturation (Isat ) and the minimum pump intensities (Imin ) occur in range of 0.082–0.165, 16.85–27.22 (kw/cm2 ) and 1.56–3.76 (kw/cm2 ) for Yb3+ -doped silicate glasses. 61SiO2 –25PbO–6NaO–8K2 O–1Yb2 O3 glass has better laser parameter than that of other glasses system. The value of βmin , Isat and Imin for SPb are 0.093, 16.85 and 1.53, respectively, which are better than those of LSY glass and near to QX glass and PNK glass. For laser glasses σ emi × τ f is generally desirable for to be as large as possible to provide high gain, for Imin to be as small as possible to minimize the pump losses. Therefore, the systematical factor of laser properties (SFL) is very useful to evaluate both the pump losses and the product of σ emi and τ f . Laser oscillation is easy to gain if the SFL is large enough. Fig. 3 shows a plot of the SFL value versus the Imin

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N. Dai et al. / Journal of Alloys and Compounds 363 (2004) 1–5

Table 2 The optical properties of Yb3+ in various glasses Sample

nd

λ0 (nm)

σ emi (pm2 )

τf (ms)

βmin

Isat (kw/cm2 )

Imin (kw/cm2 )

σ emi × τ f (pm2 ·ms)

SFL

SBB0 SBB5 SBB10 SBB15 SBB20 SBB25 SBB30 SPb SB0 SB3 SB6 SB9 SB12 SAl LSY [6] QX [13] PNK [6]

1.508 1.593 1.620 1.659 1.720 1.830 1.923 1.625 1.524 1.525 1.525 1.525 1.525 1.510 1.574 1.535 1.677

1021 1018 1020 1018 1020 1019 1018 1019 1023 1021 1020 1022 1022 1015 1022 1018 1016

0.76 0.83 0.79 0.90 0.71 0.56 0.54 0.49 0.82 0.74 0.61 0.55 0.46 0.32 0.46 0.70 0.83

0.39 0.45 0.65 0.78 0.84 0.95 1.04 2.00 0.72 0.85 0.94 1.05 1.15 1.42 1.04 2.00 1.360

0.098 0.126 0.111 0.109 0.134 0.132 0.110 0.093 0.082 0.094 0.102 0.122 0.135 0.165 0.0785 0.171 0.0812

27.22 26.30 21.72 17.24 19.34 24.16 22.54 16.85 18.65 19.75 21.23 23.75 23.56 22.78 22.23 10.79 11.03

2.68 3.32 2.42 1.88 2.59 3.18 2.47 1.56 1.53 1.86 2.16 2.89 3.18 3.76 1.75 1.82 0.90

0.30 0.37 0.51 0.70 0.59 0.53 0.56 0.98 0.59 0.63 0.57 0.57 0.53 0.45 0.37 1.40 1.12

0.112 0.111 0.211 0.372 0.223 0.167 0.227 0.628 0.386 0.339 0.264 0.197 0.167 0.120 0.211 0.769 1.244

is found in Yb3+ -doped silicate glasses. Due to the higher asymmetry of Yb3+ site environment resulting from the existence of three network formers in SBB15 (60SiO2 –15Bi2 O3 –15B2 O3 –7Na2 O–3K2 O–1Yb2 O3 ) glass, it has higher σ emi (σ emi = 0.9 pm2 ) but relative short fluorescence lifetime (0.9 ms) for the emission of the 2 F7/2 –2 F5/2 transition. SPb (61SiO2 –25PbO–6Na2 O–8K2 O–1Yb2 O3 ) glass has the longest fluorescence lifetime (2 ms), moderate σ emi , higher Imin and the largest SFL value in Yb3+ -doped silicate glasses. Its SFL value is close to that of QX glass. It is suggested that SPb glass is a good candidate for double-cladding glasses fiber application.

Acknowledgements Fig. 3. A plot of SFL versus minimum pump intensity of Yb3+ -doped glasses.

from several selected Yb3+ -doped glasses. Silicate glasses have the SFL value in the range of 0.120–0.628, PNK glass has the largest SFL value of 1.24. SPb glass has the SFL value close to that of QX glass. This result indicates that SPb glass is a good candidate for double-cladding glass fiber laser application.

The authors appreciate Professor Guosong Huang, Mr. Shunguang Li and Mrs. Meiying Huang for their assistance in measuring optical properties of glasses, and Dr. Zhongming Yang and Mr. Shiqing Xu for their helpful discussions. This work was supported by the Project of Optical Science and Technology of Shanghai (No. 022261046) and the National Nature Science Foundation of China (No. 60207006).

References 4. Conclusions The stimulated emission cross-section of Yb3+ -doped silicate glasses is calculated from the measured absorption spectra using the method of reciprocity. The potential laser parameters are evaluated by the βmin , Isat , Imin , and SFL. Linear relationship between stimulated emission cross-section and the integrated absorption cross-section

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