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Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate laser glass
JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011, P. 614
Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate laser glass XU Yongchun (ᕤ∌), LI Shunguang (ᴢ乎)ܝ, HU Lili (㚵ББ), CHEN Wei (䰜ӳ) (Shanghai Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China) Received 10 November 2010; revised 27 December 2010
Abstract: N31-type phosphate laser glasses doped with different concentrations of Cu were prepared. Their optical loss coefficient at 1053 nm wavelength and nonradiative transition rate from the Nd3+ 4F3/2 state were determined and analyzed in detail. The optical loss coefficient per unit of Cu2+ (cm–1/ppmw) and the fluorescence decay rate (Hz/ppmw) caused by Cu2+ and Nd3+ interaction were 0.0024 and 7.9, respectively. Cu impurity affected both optical loss at 1053 nm and fluorescent emission of Nd3+ 4F3/2 state seriously in N31 laser glass. Keywords: Nd-doped phosphate laser glass; optical loss; nonradiative energy loss; Cu impurity; rare earths
The technical parameters for Nd3+ doped phosphate glass are becoming more and more strict with the investigation and development of inertial confinement fusion (ICF)[1]. N31-type phosphate glass has been used in the Shenguang II facility and this glass will be used in Shenguang III facility in China. The high quality of the Nd3+ doped phosphate glass may improve the property of laser performance in high-energy and high-peak-power laser systems. Therefore, one must deeply understand the static optical loss and Nd3+ nonradiative energy loss of the Nd3+ doped phosphate glass in amplifier material. The influence of impurities on the static optical loss and Nd3+nonradiative energy loss in Nd3+ doped phosphate glass is one of the important factors. Although many workers have investigated these problems in early development period of Nd3+ doped glasses, neither the investigated scope nor the objects can satisfy the development of ICF. In recent years some famous laboratories in the world such as Lawrence Livermore National Laboratory (LLNL) have studied this issue in LHG-8 and LH-770 glasses[1–5]. In this work we investigated the influence of Cu impurity on the static optical loss and Nd3+ nonradiative energy loss in our N31-type phosphate glass prepared from Shanghai Institute of Optics and Fine Mechanics. Because Cu impurity can deeply degrade the laser performance of the glass by increasing both static optical loss at the 1053 nm laser transition and Nd3+ nonradiative energy loss, even at contaminant concentrations in the low ppmw range. Cu impurity is present in most raw materials which are difficult to completely separate. For these reasons, Cu impurity was the main focus of this study.
1 Experimental
high purity raw materials with the following weight (wt.%) composition: (56–60)P2O5-(8–12)Al2O3-(13–17)K2O-(10–15) BaO-3Nd2O3–xCuO, x=0–500 ppmw. In brief, the glass samples were melted in quartz crucible and homogenized by stirring. The melts were bubbled with both O2 and Cl2 to remove OH groups from the raw materials. The glasses were first melted at 1100 °C and then refined for 5 h at 1350 °C to remove bubbles. The melt was then cooled to 900 °C and cast into a mold. It was annealed to reduce residual stress at about 500 °C. Samples were cut and polished to the size of 25 mm× 10 mm×5 mm for optical absorption, IR spectra and fluorescence lifetime measurements. The concentration of Nd and Cu in each sample was determined by analysis of fully dissolved glass solutions with inductively coupled plasma (ICP) emission spectrometry (Thermo Iris Intrepid) after acid cleaning and acid dissolution. The measured Nd concentration is in close agreement with the mean composition (5 wt.%). The measured Cu concentration can be quantified in 3 ppmw range. Absorbance measurements were carried out with a spectrophotometer (Perkin-Elmer 900UV/VIS/NIR). The Nd emission spectrum for the 4F3/2 to 4I11/2 transition was measured from 1000 to 1100 nm using a TRIAX550 fluorescence spectrometer. The Nd3+ fluorescence decay time was measured by exciting the samples with the laser diode (800 nm). The fluorescence decay curves were recorded and averaged with computer-controlled transient digitizer. The relative errors in these measurements were estimated to be ±5 us. Static optical loss measurements were carried out by a self- built device using polished round stick samples sized ij8 mm× 150 mm. The samples without striaes and bubbles were used for measurement. The measurement error is ±1×10–4 cm–1.
The N31-type laser glass samples were prepared from Corresponding author: HU Lili (E-mail: [email protected]; Tel.: +86-21-59910854) DOI: 10.1016/S1002-0721(10)60508-X
XU Yongchun et al., Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate…
2 Results and discussion 2.1 Static optical loss by Cu impurity Fig. 1 shows the measured transmission spectra for N31-type phosphate glass doped with various Cu impurity concentrations but without Nd3+. These spectra show the major absorption bands due to Cu impurity without the influence of Nd3+ absorption bands. Because of the lack of interference by the Nd3+ absorption bands, these spectra show the major absorption bands due to Cu impurity between about 300–1150 nm. It is found that the absorption from 600 nm to the near-infrared increases rapidly with Cu concentrations increasing. In this paper, the attention is focused on the absorption near 1053 nm that corresponds to 4F3/2–4I11/2 Nd3+ laser transition. The measured absorption spectra between 500–1100 nm for 5 typical samples containing various Cu concentrations are shown in Fig. 2. It shows that 5 characteristic Nd3+ absorption bands at about 870, 800, 750, 580, 530 nm are due to the Nd3+ transition from the 4I9/2 ground state to states of 4 F3/2; 4F5/2, 2H9/2; 4F7/2, 4S3/2; 2G7/2, 4G5/2 and 2K13/2, 4G7/2, 2G9/2, respectively[2,6–11]. It is found that from 600 nm to the near-infrared, the optical absorption cross-section of Nd3+ is influenced deeply by Cu concentrations. The existence of Cu impurity in the glass can reduce the Nd3+ absorption of the pump efficiency, thereby affecting the laser performance of
Fig. 1 Absorption spectra of N31 samples with various Cu2+ concentrations and using La rather than Nd as the rare-earth dopant
Fig. 2 Absorption spectra of N31 samples with various Cu2+ concentrations
615
glass. Moreover, the absorption near 1053 nm corresponding to the 4F3/2–4I11/2 transition is also influenced deeply by Cu. It is found that the absorption near 1053 nm increases rapidly with the increase of Cu concentration. The contribution of Cu impurity on absorption at 1053 nm is much larger than that of Fe impurity[10]. Here we assume Cu is present only as 100% Cu2+ in fully equilibrated melts prepared in air and containing less than 50% CuO[2]. All the melts in our study were prepared using O2 and thus satisfied this criteria. The net gain of a glass laser amplifier is a function not only of the gain coefficient but also of the combined passive transmission losses in the glass due to impurity absorption. The total static optical loss in laser glass at 1053 nm is a sum of contributions Į=ĮNd+ĮScatter+ĮCu+
Įimpurities
(1) –1
Where ĮNd is the absorption loss coefficient (cm ) due to Nd3+ ion, ĮScatter is the static optical loss coefficient (cm–1) due to scattering by defects and inclusions (such as bubbles and Pt particles), ĮCu is the absorption loss coefficient (cm–1) due to Cu impurity, and
Įimpurities is the absorption loss
coefficient (cm–1) due to other transition metal ions (TM) and rare earth ions (RE) impurities. Scatter losses can be negligible because of highly polished surfaces and lack of inclusions or bubbles in the laser glass samples. Generally the scatter loss is <10–5 cm–1 [2,3,9]. Thus, the total static optical loss is mainly due to Nd3+ and impurities. Nd3+ absorption loss coefficient for 4I11/2 to 4F3/2 transition at room temperature in phosphate glasses can be described by the empirical formula[1–3] ĮNd(T)=1.03×10–20[Nd3+]exp(–2576/T) (2) Where ĮNd(T) is the temperature-dependent absorption coefficient (cm–1) and [Nd3+] is the Nd ion concentration (ion/cm3). The Nd3+ concentration used in this work is 3.06×1020 ion/cm3. Therefore, Nd3+ ion absorption loss remains constant. Because of all samples in this study prepared from the same high purity raw materials, we assume that the absorption loss from other transition metal (TM) and rare earth (RE) impurities remains constant. Thus, the static optical loss coefficient of Cu at 1053 nm can be determined for samples with various Cu concentrations, because the absorption loss from Nd3+ and other TM and RE remains a constant for these samples. The optical loss at 1053 nm can be quantified by using static optical loss measurement. The results from static optical loss data of the samples with various Cu2+ concentrations are summarized in Table 1. In this work, we assume that the optical loss of the sample C1 without Cu2+ doping is only caused by Nd3+ ions and impurities such as other transition metal ions and rare earth ions, and the optical loss of Cu2+ in sample C1 is nearly zero. Therefore, the optical loss caused by various concentrations of Cu2+ doping in the samples can be modified, taking sample C1 as a standard. To more fully analyze our data from Table 1, we chose to fit the measured Cu2+ optical loss at 1053 nm to a simple equation of the form
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Table 1 Optical loss at 1053 nm corresponding to various Cu2+ concentrations Į/(10–2 cm–1)
ĮCu(/10–2 cm–1)
N31 samples
[Cu]/ppmw
C1 (undoped)
0
0.12
0
C2
15.5
4.7
4.6
C3
20.5
5.9
5.8
C4
51.6
13.2
13.1
C5
67.8
16.3
16.2
ĮCu=0.0024(cm–1/ppmw)×[Cu] (3) 2+ Where Įcu is the optical loss coefficient of Cu , [Cu] is the Cu2+ concentration (ppmw). It is found that the correlation between the optical loss of Cu2+ and Cu2+ concentration is considered to be nearly liner (see Fig. 3). Fig. 3 shows the modified optical loss of Cu2+ in samples as a function of Cu2+ doping concentrations. The solid line is calculated results from Eq. (3). It is found that the optical loss of Cu2+ at 1053 nm increases rapidly from 0 to 70 ppmw of Cu2+. The slope of Fig. 3 giving an optical loss coefficient per unit of Cu2+ (cm–1/ppmw) is the constant. It is about 0.0024 cm–1/ ppmw in N31-type phosphate glass. The magnitude of optical loss coefficient per unit of Cu2+ is similar to that obtained in both LHG-8 and LG-770. Campbell et al.[1,2] reported a value of 0.0027 cm–1/ppmw in these glasses prepared in O2 melting atmosphere. This can provide a quantitative estimation of optical loss contributed by Cu2+ in our N31 laser glass. The Cu impurity can degrade the laser performance of Nd-doped phosphate glass seriously. Nowadays, N31-type phosphate glasses used in high-energy solid-state laser systems require the optical loss <1.5×10–3 cm–1. From Eq. (3), it is deduced that not more than 0.5 ppmw of Cu2+ contamination in N31-type phosphate glass is permitted. Therefore, Cu2+ concentration in raw materials and Cu2+ contamination in melting process must be strictly controlled. 2.2 Non-radiative decay by Cu impurity Non-radiative energy loss from the Nd3+ metastable 4F3/2 state can reduce the stored energy and thereby affect the laser gain and overall system performance[4–7]. The rate of energy transfer between Nd3+ and an impurity is generally de-
Fig. 3 Optical loss caused by Cu2+ at 1053 nm as a function of Cu concentrations
scribed on the basis of a dipole-dipole interaction model. In the model, Nd3+ is the ‘donor’ (D) and the impurity such as Cu impurity is the ‘acceptor’ (A), the energy transfer rate is given as [1] G (v ) K (v ) 6 K DA K R DA ³ D v 4 A dv (4) Where Ș is a constant for a given base glass composition and RDA is the inter-atomic distance between the donor and acceptor. The integral describes the spectral overlap between the donor and emission, GD(Ȟ) and the acceptor absorption K A (v) , where Ȟ is in wave numbers. Thus acceptors that are the strongest absorbers at the emission wavelength tend to produce the obvious increase in the non-radiative decay rate. Cu impurity is known to have great influence on the 1053 nm emission[1–4]. We measured Nd3+ fluorescence intensities of N31-type phosphate glasses with different Cu2+ concentrations. Fig. 4 shows the fluorescence spectra of N31-type phosphate glasses with various Cu2+ concentrations. It indicates that with the increase of Cu2+ concentrations the fluorescence intensities decrease rapidly. Cu impurity in glass is effective quencher of the radiative transition of Nd3+ ion[2,8]. For 1053 nm emission, the upper energy level for Nd3+ is 4 F3/2 state and the rate of energy transfer from this state is given by the inverse of the fluorescence lifetime(W 1 )[1–3]. Cu impurity reduces the lifetime and thereby reduces the stored energy in the 4F3/2 state, resulting in a corresponding degradation in the laser output energy. To understand the effect of Cu impurity on fluorescence decay rate of Nd3+ (1053 nm), we measured the fluorescence lifetimes (IJ) of different Cu2+ concentrations doped N31-type phosphate glasses. The results are shown in Table 2. It can be seen that the measured decay rate, W 1 , increases nearly linearly with various Cu2+ concentrations as shown in Fig. 5. Considering the rate of energy transfer to Cu impurity, the total decay, W 1 , of Nd3+ (4F3/2–4I11/2) IR emission can be given by 1 =K +K (5) rad Cu–Nd+Kother W Where Krad is the sum of all radiative decay of 4F3/2 (Nd3+) level, KCu–Nd is the rate of energy transfer to Cu impurity and Kother is the decay rate caused by other factors such as
Fig. 4 IR fluorescence spectra of N31-type phosphate glasses with various Cu2+ concentrations
XU Yongchun et al., Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate… Table 2 Fluorescence lifetime (IJ) as a function of various Cu2+ concentrations for N31-type phosphate glasses N31 samples
[Cu]/ppmw
Fluorescence
Fluorescence decay
lifetime IJ/ȝs
rate IJ–1 /KHz
C1(undopped)
0
330
3.13
C2
15.5
305
3.28
C3
20.5
300
3.33
C4
51.6
280
3.60
C5
67.8
270
3.70
C6
88.6
260
3.80
C7
179.3
232
4.30
C8
255.2
192
5.20
C9
454.5
148
6.76
617
According to Eq. (11) and Fig. 5, we calculated KCu constant of Nd3+ in N31-type phosphate glass. KCu=ȕ=7.9 Hz/ppmw. This large KCu vaule indicates that there is strong interaction between Nd3+ ions and Cu impurity.
3 Conclusions Cu impurity had great influence on the optical loss at 1053 nm and the nonradiative transition of Nd3+ 4F3/2 state in the N31-type phosphate glasses. The optical loss from Cu2+ increased with Cu2+ concentration, ranging up to 70 ppmw. The quantitative relations between Cu2+ doping concentrations and the optical loss at 1053 nm were determined and analyzed in detail. The fitting constant of 0.0024 cm–1/ppmw was similar to these of LHG-8 and LG-770 glasses. Cu impurity also had great influence on the 1053 nm emission of Nd3+ ion. The fluorescence decay rate, W 1 , increased linearly with Cu impurity content. The constant KCu, which represented the interaction between Nd3+ and Cu impurity, was calculated to be 7.9 Hz/ppmw.
References:
Fig. 5 Decay rate, W 1 , as a function of various Cu concentrations for N31-type phosphate glasses
OH-groups, cross-relaxation, multiphonon relaxation, etc. The energy transfer rate to Cu impurity, which is proportional to the acceptor (Cu2+) and donor concentration (Nd3+), can be expressed by[2] KCu–Nd=Į[Nd][Cu] (6) Where Į is a constant. [Nd] is the Nd3+ concentration (donor concentration), [Cu] is the measure of the Cu2+ content (acceptor concentration). The Nd3+ concentration in this study remains constant in all glass samples, allowing one to rewrite Eq. (6) as KCu–Nd=ȕ[Cu] (7) Because of the linear correlation between the total decay rate 1/IJ and [Cu] as shown in Fig. 5, the decay rate can be described as follows: 1 =KCu[Cu]+ 1 (8) W W
Where KCu is the slope of the fitting curve and 1/IJ0 is the decay rate in the absence of Cu impurity. The comparison of Eq. (5) with Eq. (8) leads to 1
W
1
W
=Krad+Kother
(9)
=ȕ[Cu]+ 1 W
(10)
From Eqs. (8) and (10), we can also get KCu–Nd=KCu[Cu]= ȕ[Cu]
(11)
[1] Campbell J H, Suratwala T I. Nd-doped phosphate glasses for high-energy/high-peak-power lasers. J. Non-Cryst. Solids, 2000, 263&264: 318. [2] Ehrmann P R, Campbell J H, Suratwala T I, Hayden J S, Krashkevich D, Takeuchi K. Optical loss and Nd3+ non-radiative relaxation by Cu, Fe and several rare earth impurities in phosphate laser glasses. J. Non-Cryst. Solids, 2000, 263&264: 251. [3] Ehrmann Paul R, Campbell John H. Nonradiative energy losses and radiation trapping in neodymium-doped phosphate laser glasses. J. Am. Ceram. Soc., 2002, 85: 1061. [4] Li Shunguang, Huang Guosong. The influence of OH groups on laser performance in phosphate glasses. Chinese Optics Letters, 2005, 3: 222. [5] Doris Ehrt, Wolfgang Seeber. Glass for high performance optics and laser technology. J. Non-Cryst. Solids, 1991, 129: 19. [6] Zhang Long, Hu Hefang. The effect of OH–1 on IR emission of Nd3+, Yb3+ and Er3+ doped tetraphosphate glasses. J. Chem. Phys., 2002, 63: 575. [7] Campbell J H, Suratwala T I, Thorsness C B, Hayden J S, Thorne A J, Cimino J M, Marker A J, Takeuchi K, Smolley M, Ficini-Dorn G F. Continuous melting of phosphate laser glasses. J. Non-Cryst. Solids, 2000, 263&264: 342. [8] Stokowski S E, Krashkevich D. Transition-mental ions in Nddoped glass: Spectra and effects on Nd fluorescence. Mater. Res. Soc. Symp. Proc., 1986, 61: 273. [9] Toratani H, Meissner H E, Izumitani T, Stokowski S E. Phosphate laser glass at absorption loss of 10–4 cm–1. J. Non-Cryst. Solids, 1987, 95&96: 701. [10] Xu Yongchun, Li Shunguang, Hu Lili, Chen Wei. The effect of Fe impurity on the optical loss of Nd-doped phosphate laser glass. Chinese Optical Letters, 2005, 12: 701. [11] Ebendorff-Heidepreim H, Seeber W, Ehrt D. Spectroscopic properties of Nd3+ ion in phosphate glasses. J. Non-Cryst. Solids, 1995, 183: 191.
Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate laser glass XU Yongchun (ᕤ∌), LI Shunguang (ᴢ乎)ܝ, HU Lili (㚵ББ), CHEN Wei (䰜ӳ) (Shanghai Institute of Optics & Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China) Received 10 November 2010; revised 27 December 2010
Abstract: N31-type phosphate laser glasses doped with different concentrations of Cu were prepared. Their optical loss coefficient at 1053 nm wavelength and nonradiative transition rate from the Nd3+ 4F3/2 state were determined and analyzed in detail. The optical loss coefficient per unit of Cu2+ (cm–1/ppmw) and the fluorescence decay rate (Hz/ppmw) caused by Cu2+ and Nd3+ interaction were 0.0024 and 7.9, respectively. Cu impurity affected both optical loss at 1053 nm and fluorescent emission of Nd3+ 4F3/2 state seriously in N31 laser glass. Keywords: Nd-doped phosphate laser glass; optical loss; nonradiative energy loss; Cu impurity; rare earths
The technical parameters for Nd3+ doped phosphate glass are becoming more and more strict with the investigation and development of inertial confinement fusion (ICF)[1]. N31-type phosphate glass has been used in the Shenguang II facility and this glass will be used in Shenguang III facility in China. The high quality of the Nd3+ doped phosphate glass may improve the property of laser performance in high-energy and high-peak-power laser systems. Therefore, one must deeply understand the static optical loss and Nd3+ nonradiative energy loss of the Nd3+ doped phosphate glass in amplifier material. The influence of impurities on the static optical loss and Nd3+nonradiative energy loss in Nd3+ doped phosphate glass is one of the important factors. Although many workers have investigated these problems in early development period of Nd3+ doped glasses, neither the investigated scope nor the objects can satisfy the development of ICF. In recent years some famous laboratories in the world such as Lawrence Livermore National Laboratory (LLNL) have studied this issue in LHG-8 and LH-770 glasses[1–5]. In this work we investigated the influence of Cu impurity on the static optical loss and Nd3+ nonradiative energy loss in our N31-type phosphate glass prepared from Shanghai Institute of Optics and Fine Mechanics. Because Cu impurity can deeply degrade the laser performance of the glass by increasing both static optical loss at the 1053 nm laser transition and Nd3+ nonradiative energy loss, even at contaminant concentrations in the low ppmw range. Cu impurity is present in most raw materials which are difficult to completely separate. For these reasons, Cu impurity was the main focus of this study.
1 Experimental
high purity raw materials with the following weight (wt.%) composition: (56–60)P2O5-(8–12)Al2O3-(13–17)K2O-(10–15) BaO-3Nd2O3–xCuO, x=0–500 ppmw. In brief, the glass samples were melted in quartz crucible and homogenized by stirring. The melts were bubbled with both O2 and Cl2 to remove OH groups from the raw materials. The glasses were first melted at 1100 °C and then refined for 5 h at 1350 °C to remove bubbles. The melt was then cooled to 900 °C and cast into a mold. It was annealed to reduce residual stress at about 500 °C. Samples were cut and polished to the size of 25 mm× 10 mm×5 mm for optical absorption, IR spectra and fluorescence lifetime measurements. The concentration of Nd and Cu in each sample was determined by analysis of fully dissolved glass solutions with inductively coupled plasma (ICP) emission spectrometry (Thermo Iris Intrepid) after acid cleaning and acid dissolution. The measured Nd concentration is in close agreement with the mean composition (5 wt.%). The measured Cu concentration can be quantified in 3 ppmw range. Absorbance measurements were carried out with a spectrophotometer (Perkin-Elmer 900UV/VIS/NIR). The Nd emission spectrum for the 4F3/2 to 4I11/2 transition was measured from 1000 to 1100 nm using a TRIAX550 fluorescence spectrometer. The Nd3+ fluorescence decay time was measured by exciting the samples with the laser diode (800 nm). The fluorescence decay curves were recorded and averaged with computer-controlled transient digitizer. The relative errors in these measurements were estimated to be ±5 us. Static optical loss measurements were carried out by a self- built device using polished round stick samples sized ij8 mm× 150 mm. The samples without striaes and bubbles were used for measurement. The measurement error is ±1×10–4 cm–1.
The N31-type laser glass samples were prepared from Corresponding author: HU Lili (E-mail: [email protected]; Tel.: +86-21-59910854) DOI: 10.1016/S1002-0721(10)60508-X
XU Yongchun et al., Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate…
2 Results and discussion 2.1 Static optical loss by Cu impurity Fig. 1 shows the measured transmission spectra for N31-type phosphate glass doped with various Cu impurity concentrations but without Nd3+. These spectra show the major absorption bands due to Cu impurity without the influence of Nd3+ absorption bands. Because of the lack of interference by the Nd3+ absorption bands, these spectra show the major absorption bands due to Cu impurity between about 300–1150 nm. It is found that the absorption from 600 nm to the near-infrared increases rapidly with Cu concentrations increasing. In this paper, the attention is focused on the absorption near 1053 nm that corresponds to 4F3/2–4I11/2 Nd3+ laser transition. The measured absorption spectra between 500–1100 nm for 5 typical samples containing various Cu concentrations are shown in Fig. 2. It shows that 5 characteristic Nd3+ absorption bands at about 870, 800, 750, 580, 530 nm are due to the Nd3+ transition from the 4I9/2 ground state to states of 4 F3/2; 4F5/2, 2H9/2; 4F7/2, 4S3/2; 2G7/2, 4G5/2 and 2K13/2, 4G7/2, 2G9/2, respectively[2,6–11]. It is found that from 600 nm to the near-infrared, the optical absorption cross-section of Nd3+ is influenced deeply by Cu concentrations. The existence of Cu impurity in the glass can reduce the Nd3+ absorption of the pump efficiency, thereby affecting the laser performance of
Fig. 1 Absorption spectra of N31 samples with various Cu2+ concentrations and using La rather than Nd as the rare-earth dopant
Fig. 2 Absorption spectra of N31 samples with various Cu2+ concentrations
615
glass. Moreover, the absorption near 1053 nm corresponding to the 4F3/2–4I11/2 transition is also influenced deeply by Cu. It is found that the absorption near 1053 nm increases rapidly with the increase of Cu concentration. The contribution of Cu impurity on absorption at 1053 nm is much larger than that of Fe impurity[10]. Here we assume Cu is present only as 100% Cu2+ in fully equilibrated melts prepared in air and containing less than 50% CuO[2]. All the melts in our study were prepared using O2 and thus satisfied this criteria. The net gain of a glass laser amplifier is a function not only of the gain coefficient but also of the combined passive transmission losses in the glass due to impurity absorption. The total static optical loss in laser glass at 1053 nm is a sum of contributions Į=ĮNd+ĮScatter+ĮCu+
Įimpurities
(1) –1
Where ĮNd is the absorption loss coefficient (cm ) due to Nd3+ ion, ĮScatter is the static optical loss coefficient (cm–1) due to scattering by defects and inclusions (such as bubbles and Pt particles), ĮCu is the absorption loss coefficient (cm–1) due to Cu impurity, and
Įimpurities is the absorption loss
coefficient (cm–1) due to other transition metal ions (TM) and rare earth ions (RE) impurities. Scatter losses can be negligible because of highly polished surfaces and lack of inclusions or bubbles in the laser glass samples. Generally the scatter loss is <10–5 cm–1 [2,3,9]. Thus, the total static optical loss is mainly due to Nd3+ and impurities. Nd3+ absorption loss coefficient for 4I11/2 to 4F3/2 transition at room temperature in phosphate glasses can be described by the empirical formula[1–3] ĮNd(T)=1.03×10–20[Nd3+]exp(–2576/T) (2) Where ĮNd(T) is the temperature-dependent absorption coefficient (cm–1) and [Nd3+] is the Nd ion concentration (ion/cm3). The Nd3+ concentration used in this work is 3.06×1020 ion/cm3. Therefore, Nd3+ ion absorption loss remains constant. Because of all samples in this study prepared from the same high purity raw materials, we assume that the absorption loss from other transition metal (TM) and rare earth (RE) impurities remains constant. Thus, the static optical loss coefficient of Cu at 1053 nm can be determined for samples with various Cu concentrations, because the absorption loss from Nd3+ and other TM and RE remains a constant for these samples. The optical loss at 1053 nm can be quantified by using static optical loss measurement. The results from static optical loss data of the samples with various Cu2+ concentrations are summarized in Table 1. In this work, we assume that the optical loss of the sample C1 without Cu2+ doping is only caused by Nd3+ ions and impurities such as other transition metal ions and rare earth ions, and the optical loss of Cu2+ in sample C1 is nearly zero. Therefore, the optical loss caused by various concentrations of Cu2+ doping in the samples can be modified, taking sample C1 as a standard. To more fully analyze our data from Table 1, we chose to fit the measured Cu2+ optical loss at 1053 nm to a simple equation of the form
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JOURNAL OF RARE EARTHS, Vol. 29, No. 6, Jun. 2011
Table 1 Optical loss at 1053 nm corresponding to various Cu2+ concentrations Į/(10–2 cm–1)
ĮCu(/10–2 cm–1)
N31 samples
[Cu]/ppmw
C1 (undoped)
0
0.12
0
C2
15.5
4.7
4.6
C3
20.5
5.9
5.8
C4
51.6
13.2
13.1
C5
67.8
16.3
16.2
ĮCu=0.0024(cm–1/ppmw)×[Cu] (3) 2+ Where Įcu is the optical loss coefficient of Cu , [Cu] is the Cu2+ concentration (ppmw). It is found that the correlation between the optical loss of Cu2+ and Cu2+ concentration is considered to be nearly liner (see Fig. 3). Fig. 3 shows the modified optical loss of Cu2+ in samples as a function of Cu2+ doping concentrations. The solid line is calculated results from Eq. (3). It is found that the optical loss of Cu2+ at 1053 nm increases rapidly from 0 to 70 ppmw of Cu2+. The slope of Fig. 3 giving an optical loss coefficient per unit of Cu2+ (cm–1/ppmw) is the constant. It is about 0.0024 cm–1/ ppmw in N31-type phosphate glass. The magnitude of optical loss coefficient per unit of Cu2+ is similar to that obtained in both LHG-8 and LG-770. Campbell et al.[1,2] reported a value of 0.0027 cm–1/ppmw in these glasses prepared in O2 melting atmosphere. This can provide a quantitative estimation of optical loss contributed by Cu2+ in our N31 laser glass. The Cu impurity can degrade the laser performance of Nd-doped phosphate glass seriously. Nowadays, N31-type phosphate glasses used in high-energy solid-state laser systems require the optical loss <1.5×10–3 cm–1. From Eq. (3), it is deduced that not more than 0.5 ppmw of Cu2+ contamination in N31-type phosphate glass is permitted. Therefore, Cu2+ concentration in raw materials and Cu2+ contamination in melting process must be strictly controlled. 2.2 Non-radiative decay by Cu impurity Non-radiative energy loss from the Nd3+ metastable 4F3/2 state can reduce the stored energy and thereby affect the laser gain and overall system performance[4–7]. The rate of energy transfer between Nd3+ and an impurity is generally de-
Fig. 3 Optical loss caused by Cu2+ at 1053 nm as a function of Cu concentrations
scribed on the basis of a dipole-dipole interaction model. In the model, Nd3+ is the ‘donor’ (D) and the impurity such as Cu impurity is the ‘acceptor’ (A), the energy transfer rate is given as [1] G (v ) K (v ) 6 K DA K R DA ³ D v 4 A dv (4) Where Ș is a constant for a given base glass composition and RDA is the inter-atomic distance between the donor and acceptor. The integral describes the spectral overlap between the donor and emission, GD(Ȟ) and the acceptor absorption K A (v) , where Ȟ is in wave numbers. Thus acceptors that are the strongest absorbers at the emission wavelength tend to produce the obvious increase in the non-radiative decay rate. Cu impurity is known to have great influence on the 1053 nm emission[1–4]. We measured Nd3+ fluorescence intensities of N31-type phosphate glasses with different Cu2+ concentrations. Fig. 4 shows the fluorescence spectra of N31-type phosphate glasses with various Cu2+ concentrations. It indicates that with the increase of Cu2+ concentrations the fluorescence intensities decrease rapidly. Cu impurity in glass is effective quencher of the radiative transition of Nd3+ ion[2,8]. For 1053 nm emission, the upper energy level for Nd3+ is 4 F3/2 state and the rate of energy transfer from this state is given by the inverse of the fluorescence lifetime(W 1 )[1–3]. Cu impurity reduces the lifetime and thereby reduces the stored energy in the 4F3/2 state, resulting in a corresponding degradation in the laser output energy. To understand the effect of Cu impurity on fluorescence decay rate of Nd3+ (1053 nm), we measured the fluorescence lifetimes (IJ) of different Cu2+ concentrations doped N31-type phosphate glasses. The results are shown in Table 2. It can be seen that the measured decay rate, W 1 , increases nearly linearly with various Cu2+ concentrations as shown in Fig. 5. Considering the rate of energy transfer to Cu impurity, the total decay, W 1 , of Nd3+ (4F3/2–4I11/2) IR emission can be given by 1 =K +K (5) rad Cu–Nd+Kother W Where Krad is the sum of all radiative decay of 4F3/2 (Nd3+) level, KCu–Nd is the rate of energy transfer to Cu impurity and Kother is the decay rate caused by other factors such as
Fig. 4 IR fluorescence spectra of N31-type phosphate glasses with various Cu2+ concentrations
XU Yongchun et al., Effect of copper impurity on the optical loss and Nd3+ nonradiative energy loss of Nd-doped phosphate… Table 2 Fluorescence lifetime (IJ) as a function of various Cu2+ concentrations for N31-type phosphate glasses N31 samples
[Cu]/ppmw
Fluorescence
Fluorescence decay
lifetime IJ/ȝs
rate IJ–1 /KHz
C1(undopped)
0
330
3.13
C2
15.5
305
3.28
C3
20.5
300
3.33
C4
51.6
280
3.60
C5
67.8
270
3.70
C6
88.6
260
3.80
C7
179.3
232
4.30
C8
255.2
192
5.20
C9
454.5
148
6.76
617
According to Eq. (11) and Fig. 5, we calculated KCu constant of Nd3+ in N31-type phosphate glass. KCu=ȕ=7.9 Hz/ppmw. This large KCu vaule indicates that there is strong interaction between Nd3+ ions and Cu impurity.
3 Conclusions Cu impurity had great influence on the optical loss at 1053 nm and the nonradiative transition of Nd3+ 4F3/2 state in the N31-type phosphate glasses. The optical loss from Cu2+ increased with Cu2+ concentration, ranging up to 70 ppmw. The quantitative relations between Cu2+ doping concentrations and the optical loss at 1053 nm were determined and analyzed in detail. The fitting constant of 0.0024 cm–1/ppmw was similar to these of LHG-8 and LG-770 glasses. Cu impurity also had great influence on the 1053 nm emission of Nd3+ ion. The fluorescence decay rate, W 1 , increased linearly with Cu impurity content. The constant KCu, which represented the interaction between Nd3+ and Cu impurity, was calculated to be 7.9 Hz/ppmw.
References:
Fig. 5 Decay rate, W 1 , as a function of various Cu concentrations for N31-type phosphate glasses
OH-groups, cross-relaxation, multiphonon relaxation, etc. The energy transfer rate to Cu impurity, which is proportional to the acceptor (Cu2+) and donor concentration (Nd3+), can be expressed by[2] KCu–Nd=Į[Nd][Cu] (6) Where Į is a constant. [Nd] is the Nd3+ concentration (donor concentration), [Cu] is the measure of the Cu2+ content (acceptor concentration). The Nd3+ concentration in this study remains constant in all glass samples, allowing one to rewrite Eq. (6) as KCu–Nd=ȕ[Cu] (7) Because of the linear correlation between the total decay rate 1/IJ and [Cu] as shown in Fig. 5, the decay rate can be described as follows: 1 =KCu[Cu]+ 1 (8) W W
Where KCu is the slope of the fitting curve and 1/IJ0 is the decay rate in the absence of Cu impurity. The comparison of Eq. (5) with Eq. (8) leads to 1
W
1
W
=Krad+Kother
(9)
=ȕ[Cu]+ 1 W
(10)
From Eqs. (8) and (10), we can also get KCu–Nd=KCu[Cu]= ȕ[Cu]
(11)
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