Thermal stability and spectroscopic properties of Yb3+-doped zinc–tungsten–tellurite glasses

Thermal stability and spectroscopic properties of Yb3+-doped zinc–tungsten–tellurite glasses

Journal of Alloys and Compounds 373 (2004) 246–251 Thermal stability and spectroscopic properties of Yb3+-doped zinc–tungsten–tellurite glasses Guoni...

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Journal of Alloys and Compounds 373 (2004) 246–251

Thermal stability and spectroscopic properties of Yb3+-doped zinc–tungsten–tellurite glasses Guonian Wang∗ , Shiqing Xu, Shixun Dai, Junjie Zhang, Zhonghong Jiang Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai 201800, China Received 2 September 2003; received in revised form 22 October 2003; accepted 22 October 2003

Abstract A series of new glasses of 70TeO2 -(20-x) ZnO-xWO3 -5La2 O3 -2.5K2 O-2.5Na2 O (mol %) doped with Yb3+ is presented. Thermal stability and spectroscopic properties of Yb3+ ions have been measured. It found that 70TeO2 -15WO3 -5ZnO-5La2 O3 -2.5K2 O-2.5Na2 O composition glass had better thermal stability ((Tx − Tg ) > 160 ◦ C) than 75TeO2 -20ZnO-5Na2 O glass, high-stimulated emission cross-section of 1.32 pm2 for the 2 F5/2 → 2 F7/2 transition and existed measured fluorescence lifetime of 0.93 ms and the broad fluorescence effective linewidth of 74.5 nm. Evaluated from the good potential laser parameters, this system glass is promising for miniature solid fiber lasers or high-peak power and high-average power lasers, also for wave-guides or tunable lasers. © 2003 Published by Elsevier B.V. Keywords: Yb3+ ; Telluride glass; Spectroscopic property

1. Introduction Ytterbium-doped glass is a promising material with numerous attractive properties [1]: the broad fluorescence spectrum (compared with Nd3+ ) provides sufficient bandwidth to generate and amplify ultrashort laser pulses, the millisecond upper-state lifetime enables free-running lasers and laser diodes for pump sources. It is also particularly advantageous for Q-switched lasers and high-power ultrashort pulse amplification. The simple electronic energy level structure (2 F5/2 → 2 F7/2 ) results in a small quantum defect and the location of the absorption band (900–1000 nm) is suited for pumping with InGaAs laser diodes. Besides, glasses with a relatively high linear refractive index, doped with Yb3+ , usually also show a high nonlinear refractive index, which is a desirable effect for ultrashort pulse generation [2]. The Yb3+ ions are of interest also as a sensitizer of energy transfer for infrared to visible up conversion and infrared lasers [3]. Glasses are materials capable of high-peak power generation due to their high saturation fluencies, broad emission bandwidth and long upper-state lifetime. Laser glasses are ∗ Corresponding author. Tel.: +86-21-59914293; fax: +86-21-59914516. E-mail address: [email protected] (G. Wang).

0925-8388/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.jallcom.2003.10.031

usually evaluated by means of these spectroscopic properties. Emission cross-section and fluorescence lifetime are calculated by the three Judd–Ofelt parameters [4,5]. Since there is only the 2 F5/2 → 2 F7/2 transition for Yb3+ , it is impossible to calculate directly the Judd–Ofelt parameters for this ion. For this reason, the compositional dependence of the spectroscopic properties of Yb3+ -doped glasses is not well established. Up to now, there are only a few papers involving the effect of composition on the emission cross-section of Yb3+ in simple systems as borate, phosphate, silicate and fluoride phosphate glasses [6–9]. As laser or amplifier host materials, tellurite glasses combine the attributes of (1) a reasonably wide transmission region (0.35–5 ␮m), compared to only 0.2–3 ␮m for silicate glasses [10]; (2) good glass stabilities and corrosion resistances, which are better than those in fluoride glasses [11]; (3) a relative low phonon energy among oxide glass formers (largest phonon energy ∼800 cm−1 ) [12]; (4) high refractive index, n (typical e.g. 1.8–2.3) [13,14] and high non-linear indices, n2 (typical e.g. 2.5 × 10−19 m2 /W), which are small in both fluoride (typical n: ∼1.5; typical n2 : ∼10−21 m2 /W) and silica glasses (typical n: ∼1.46; typical n2 : ∼10−20 m2 /W) [15,16]. The non-linear refractive index and the phonon energy make the tellurite glasses uniquely suitable for non-linear and laser applications. Furthermore, they are resistant to atmospheric moisture and capable of

G. Wang et al. / Journal of Alloys and Compounds 373 (2004) 246–251

incorporating large concentrations of rare-earth ions into the matrix [17]. Until now, a few studies about tellurite-based glasses doped with Yb3+ were done only with relatively simple binary and ternary but without multi-component. Moreover, thermal stability and corrosion resistance were not desirable [7,18]. At present, TZN glass (75TeO2 -20ZnO-5Na2 O) is known as a new candidate for fiber devices because it has good chemical durability, thermal stability and excellent rare earth solubility, but the spectroscope properties of doped Yb3+ ion is not very perfect and needed to be improved [15]. The objective of this work is, on the basis of TZN glass composition and regulating the composition with 2.5K2 O and 2.5Na2 O instead of 5.0Na2 O for considering the effect of relative alkali content on absorption linewidth [19], adding 5.0 La2 O3 for similar solubility of La2 O3 and Yb2 O3 in oxide glasses and large quantities of Yb2 O3 can be incorporated into the host by substituting La2 O3 with Yb2 O3 , replacing of ZnO with WO3 gradually for improving the spectroscopic properties, to investigate the thermal stabilities and spectroscope properties of Yb3+ -doped multi-componential tellurite-based glasses and to survey the feasibility of these tellurite-based glasses as the candidate for a new type of Yb3+ -doped laser host material.

2. Experimental procedure and theoretical analysis 2.1. Sample preparation Reagent grade commercial oxides (>99.5% pure) were used as the starting materials. Each sample was doped with 1.0 mol % Yb2 O3 in the batch. Mixed batches were melted in platinum crucibles at 750–850 ◦ C for ∼30 min while being bubbled with dry oxygen gas; then the liquids were poured into stainless-steel molds preheated near their glass transition temperatures and annealed for 2 h above glass transition temperature and then were annealed to room temperature in 48 h. Samples for optical and spectroscopic properties measurements were cut and polished to the size of 20 mm × 10 mm × 1 mm. 2.2. Measurement of physical and spectroscopic properties Differential thermal analysis (DTA) (LCP-1 differential thermal analyzer (made in China)) was employed. The index of refraction was measured at 486.1, 589.3, and 656.3 nm, with a precision V-prism refractometer (made in China) using H2 and Na lamps as spectral source. The Cauchy’s equation, n (λ) = A + B/λ2 , [20] was used to determine the refractive index at the mean wavelength of Yb3+ : 2 F7/2 → 2F 5/2 transition. The absorption spectra were measured using a Perkin–Elmer 900 spectrophotometer in the range 870–1100 nm at room temperature. The emission spectra were obtained by exciting the samples with LD940 nm as pumping laser (excited at 940 nm). The light from the light

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source was chopped at 80 Hz and focused on the 20×10 mm2 face of the samples. The position of 1 mm from the edge was excited to minimize the re-absorption of emission. The emission from the sample was focused onto a monochrometer and detected by a Ge detector. The signal was intensified with a lock in amplifier and processed by a computer. The relative errors in these emission measurements are estimated to be <5%. The fluorescence lifetimes were measured by exciting the samples with the same LD940 nm as above and detected by a S-1 photomultiplier tube. 2.3. Sample evaluations 2.3.1. Spectroscopic analysis The potential performance of an Yb3+ -doped glass laser may be assessed from the emission and absorption properties. An efficient host for laser operation should show a combination of the following properties: large emission cross-section to provide high gain, long fluorescence lifetime to minimize pump losses incurred from spontaneous emission, large absorption cross-section at the pump wavelength and the possibility to incorporate a high concentration of Yb3+ ions. The integrated absorption cross-section ( abs ), the spontaneous emission probability (Arad ), fluorescence effective linewidth ( ␭eff ), the absorption and emission cross-section (σ abs and σ em ) for 2 F5/2 → 2 F7/2 transition of Yb3+ were calculated by the following eqations [21,22]: 2.303 log(I0 /I) σabs (λ) = (1) Nl   2.303 log(I0 /I) abs = σabs (λ)dλ = dλ (2) Nl Arad =

1 τrad

=

32πcn2 abs 3λ4p

λ4 Arad 8πc n2 λeff  I(λ)dλ = Imax

(3)

σem =

(4)

λeff

(5)

where σ abs is the absorption cross-section, λp is the peak wavelength of the absorption band, and n is the refractive index at the peak wavelength, l is the sample thickness, and N is the concentration of Yb3+ ions, log(I0 /I) is absorbance. From Eqs. (1)–(4), in order to get high emission cross-section glass with a large integrated absorption cross-section should be pursued. 2.3.2. Laser performace parameters For the two level laser with broad emission and absorption spectra, σ em is given by the following reciprocity equation [6]:   Zl Ezl − hcλ−1 σem (λ) = σabs (λ) exp (6) Zu kT

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G. Wang et al. / Journal of Alloys and Compounds 373 (2004) 246–251

Table 1 The variation of refraction indices (nd ), densities (ρ) and characteristic temperatures (±1 ◦ C): Tg (glass transition), Tx (onset of glass crystallization) and stability parameters (Tx − Tg ) with TWZ series glasses, which compared with TZN glass Series

x (mol %)

ρ (g/cm3 )

nd

Tg (◦ C)

Tx (◦ C)

Tx − Tg (◦ C)

TWZ1 TWZ2 TWZ3 TWZ4 TWZ5 TZN

0 5 10 15 20 –

5.387 5.443 5.516 5.573 5.624 –

1.989 2.026 2.053 2.067 2.089 –

335 345 350 362 368 299

– – – 525 445 417

8 8 8 163 77 118

hc λp τf σabs (λp )

Imin = βmin × Isat hc = λp σabs (λp )τf

(8)



 −1 Ezl − hcλ−1 Zl 0 1+ exp (9) Zu kT

where 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 (␭0 ). Isat is the pumping saturation intensity and can be obtained by the absorption cross-section σ abs and fluorescene lifetime τ f . Moreover, E = Ezl − hcλ−1 0 neither changes very much in the glass, nor does βmin . So from Eq. (9), Imin is mainly determined by σ em and τ f , the figure of merits of the Yb3+ laser materials is given by the Imin and σ em , and it turns out to be given by the emission cross-section σ em and fluorescence lifetime τ f ; The higher σ em and longer lifetime τ f give a better Yb3+ laser material.

2.12 Density Refractive index

5.60

2.10

5.55

2.08

5.50

2.06 2.04

5.45

2.02 5.40 2.00 5.35 -2

0

2

4

6

1.98 8 10 12 14 16 18 20 22

WO3 content (mol%)

Fig. 1. Refractive index and density of TWZ series glasses.

Refractive index

Isat =

(7)

Table 1 shows the measured densities, refractive indices and DTA data of some 70TeO2 -xWO3 -(20-x) ZnO-5La2 O3 -2.5K2 O-2.5Na2 O (x = 0, 5, 10, 15 and 20 mol %; Yb2 O3 1.0 mol %) (named as TWZ1, TWZ2, TWZ3, TWZ4, and TWZ5, respectively for short glass samples. Fig. 1 shows that densities and refractive indices of the glass samples increase slightly with the increasing WO3 content gradually. Such feature may be explained as follows: due to the molecular weight of W6+ is bigger than that of Zn2+ , with the gradual replacement of the Zn2+ ions by W6+ ions, the average molecular weight of unit volume enhances and the structure of glasses become compact, which results in densities increasing. Correspondingly, the refractive indices also increase. To the measured results of temperature parameter, with increasing of the WO3 and decreasing of the ZnO content, some general observations may be noted: there is an increase in Tg values; when the content of WO3 = 15 mol %, the difference (Tx − Tg ) between the glass transition temperature (Tg ) and the onset crystallization temperature (Tx ) of samples >160 ◦ C, it indicates that the samples are stable for fiber drawing; but when WO3 = 20 mol %, (Tx − Tg ) is only 77 ◦ C, and not suitable for fiber drawing. The absorption and emission cross-section of TWZ4 are shown in Fig. 2. The absorption peak wavelength corresponding to the energy separation of the lowest crystal field components of the ground and excited state is situated at

3

σabs (λ0 ) σabs (λ0 ) + σemi (λ0 )   −1 Ezl − hcλ−1 Zl 0 = 1+ exp Zu kT

βmin =

3. Results

Density (g/cm )

where Zl , Zu , k, h, T, c and Ezl represent the partition functions for the lower and upper levels of Yb3+ , Boltzman’s constant, Plank’s constant, temperature, the velocity of light and zero line energy which is defined to be the energy separation between the lowest components of the upper and lower states, respectively. Ezl is associated with the most intense peak in the absorption spectrum of a Yb3+ -doped glass and can be easily determined. Zl /Zu can be obtained from the Yb3+ energy structure [23]. Yb3+ ions are basically the two level laser, emission and absorption coexisting at a given wavelength. Therefore, the figure of merits of Yb3+ laser materials is given by Imin which is a measure of the ease of pumping the laser material, and σ em , which is a measure of extracting the stored energy. Imin is given by the following equation [6]:

G. Wang et al. / Journal of Alloys and Compounds 373 (2004) 246–251

1.8 Absorption Emission

2.0

1.7 2

Cross-Section(pm )

2

Cross-Section(pm )

2.5

249

1.5 1.0 0.5

Absorption Emission

1.6 1.5 1.4 1.3 1.2 1.1

0.0 900

950

1000

1050

1100

0

5

Wavelength(nm)

10

15

20

WO 3 content (mol%)

Fig. 2. Absorption and emission cross-section of TZW4 glass.

Fig. 3. The change of absorption and emission cross-section with the content of WO3 increasing.

977 nm. The wavelength of the fluorescent peak emission, related to the 2 F5/2 → 2 F7/2 transition, normally varies with the host and was measured at about 1006 nm. The maximum emission cross-section is 1.32 pm2 . Table 2 summarizes some important spectroscopic properties of Yb3+ in a series of TWZ samples calculated by the reciprocity method. It can be seen that, the trends of peak absorption cross-section (σ p ), integrated absorption cross-section ( abs ), fluorescence effective linewidth ( ␭eff ), spontaneous emission probability (AR ) and emission cross-section (σ em ) increase while fluorescence lifetime (τ f ) shortens with WO3 replace of ZnO from 0 to 15 mol % gradually; but the trend of σ em decreases when WO3 replace of ZnO entirely. Fig. 3 shows the change trends of absorption and emission with the content of WO3 from 0 up to 20 mol %.

4. Discussion The difference between Tx and Tg , T, has been frequently used as a rough estimate of the glass formation ability or glass stability. Since fiber drawing is a reheating process and any crystallization during the process will increase the scattering loss of the fiber and then degrade the optical properties. To achieve a large working range of temperature during our sample fiber drawing, it is desirable for a glass host to have T as large as possible [24]. From Table 1, the glass transition temperatures,Tg , registered for glasses obtained in our TWZ systems increase regularly with

WO3 take the place of ZnO gradually. This compositional dependence of Tg can reveal a transformation of the glass structure referred to the growth in the network connectivity. The presence of tungsten atoms leads to a densification of the TeO2 glass matrix [25]. While, the onset crystallization temperature (Tx ) decreases which indicates that the thermal stabilities get worse with more WO3 . Compared with TZN glass which had a glass transition temperature at 299 ◦ C and a crystallization onset temperature at 417 ◦ C, giving a working range (Tx − Tg ) = 118 ◦ C [15], the thermal stability of our TWZ4 glass, (Tx − Tg ) > 160 ◦ C, is better than TZN glass, so it has good thermal stability and is perfect for fiber drawing. According to the structural analysis discussed else where, it has been concluded that: the coordination states of tellurium atoms change from TeO4 (tbp) to TeO3+1 polyhedron and TeO3 (tp) with increasing of ZnO contents; both BO (Te–O–Te) and NBO (Te–O–Zn, Zn–O–Zn,etc.) atoms exist in these binary TeO2 –ZnO glasses [26,27]. To the binary TeO2 –WO3 glasses the main structural constituents are trigonal bipyramids TeO4 (tbp), when the WO3 content increases, some trigonal pyramids TeO3 (tp) are formed and some W atoms are incorporated into the glass network to form Te–O–W linkages [28]. To our TWZ glasses we can conclude that, with WO3 content increasing, the deformation of the glass former units leads to lower symmetry of the local structure in the vicinity of Yb3+ ions; meanwhile, the distortions about Yb3+ ions in TWZ glasses may also be increased by the difference between Te4+ (and/or Zn2+ ,

Table 2 The variation of spectroscopic properties of TWZ series glasses at room temperature Series

x (mol %)

λzl (nm)

σ p (pm2 )

abs (104 pm3 )

AR (s−1 )

λeff (nm)

λem (nm)

σ em (pm2 )

τ f (ms)

σ em τ f (pm2 ms)

TWZ1 TWZ2 TWZ3 TWZ4 TWZ5

0 5 10 15 20

977 977 977 977 977

1.49 1.68 1.73 1.80 1.81

5.24 6.06 6.22 6.38 6.33

2286 2742 2889 3004 3048

72.47 72.60 73.69 74.54 75.29

1006.5 1005.5 1005.5 1006 1006.5

1.06 1.19 1.25 1.32 1.22

0.98 0.96 0.96 0.93 0.95

1.04 1.14 1.20 1.23 1.16

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G. Wang et al. / Journal of Alloys and Compounds 373 (2004) 246–251

Table 3 Spectroscopic properties of some laser glasses doped with Yb3+ Glasses

σ em (pm2 )

τf (ms)

λem (nm)

σ em τ f (pm2 ms)

Imin (kW/cm2 )

QX ADY LY PN PNK FP Our glass

0.70 1.03 0.80 1.35 1.08 0.50 1.32

2.0 1.58 1.68 1.36 2.00 1.20 0.93

1018 1020 1028 1035 1016 1020 1006

1.40 1.63 1.35 1.83 2.16 0.60 1.23

1.80 1.12 1.95 0.59 1.29 0.80 0.92

W6+ ) ion sizes, for this difference makes the polarization of Yb3+ ions larger in TeO2 -based glasses containing ZnO and WO3 . In previous studies [6,21], it was demonstrated that integrated absorption cross-section and secondary peak emission cross-section of Yb3+ in oxide glasses are affected mainly by the asymmetry in the local structures surrounding Yb3+ ions more than the covalency of the bond between Yb3+ and oxygen ion. So, when WO3 content increase from 0 to 15 mol %, the σ abs and σ em of Yb3+ increase, but when WO3 content add up to 20 mol %, then ZnO content is zero, the symmetry of the vicinity structure of Yb3+ increases, therefore, the absorption and emission cross-section of Yb3+ ions decrease (shown as Fig. 3). In order to compare with our TWZ4 glass, Table 3 and Fig. 4 list the spectroscopic properties and minimum pump intensities(Imin ) between some laser glasses (QX, ADY, LY, PN, PNK, FP) reported in published papers [6–8,29–31]. It can be seen that the TWZ4 composition glass has excellent laser performance parameters. Fig. 4 shows the figure of our glass that is close to PN glass and has an advantage over other laser glasses. Therefore, we believe that our glass system is a promising laser glass host for high-peak power and high-average power lasers. At the same time, our TGL glass has large fluorescence effective linewidth (77 nm). It should be pointed out that such a large fluorescence effective

1.6 PN our glass

2

Emission cross-section(pm )

1.4 1.2

PNK ADY

1.0

LY QX

0.8 0.6

FP

0.4 0.2 0.0 0.0

0.5

1.0

1.5

2.0

2.5

2

Minimum pump intensity(kw/cm ) Fig. 4. Relationship between emission cross-section (σ em ) and minimum pump intensity (Imin ).

lingwidth, as well as its absorption cross-section (1.62 pm2 at 977 nm) is an important feature for short pulse generation in diode pumped lasers or for tunable lasers.

5. Conclusion By studying Yb-doped 70TeO2 -(20-x) ZnO-xWO3 -5La2 O3 -2.5Na2 O-2.5K2 O (x = 0, 5, 10, 15, and 20 mol %) glasses, the results indicate the glass of x = 15 mol % has better glass-forming ability than TZN glass, high absorption and emission cross-sections (1.80 and 1.32 pm2 ), broad fluorescence effective linewidth (74.54 nm), long fluorescence lifetime (0.93 ms), which indicates this system glass doped with Yb3+ is promising as a 1.02 ␮m laser source for miniature solid fiber lasers or high-peak power and high-average power lasers, also for wave-guides or tunable lasers.

Acknowledgements This research was financially supported by the Shanghai Science and Technology Foundation (grant 022261046) and by the Chinese National Natural Science Foundation (grant 60207006).

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