Raman, green and infrared emission cross-sectionsof Er<sup>3+</sup> doped TZPPN tellurite glass

K. Damak; E. Yousef; S. AlFaify; C. Rüssel; R. Maâlej

doi:10.1364/OME.4.000597

1. Introduction

Infrared solid state and fiber lasers/optical amplifiers are of great interest for numerous applications, such as telecommunication, Raman laser amplifiers, optical parametric oscillators, vibrancies lasers, chemical sensors and medicine and atmosphere transmission. Particularly, tellurite glasses are attractive in optical fiber lasers, amplifier applications and frequency up-converters [1–7].Transparent tellurite glasses doped withTm³⁺ [4–6] and Er³⁺ [5,7] -doped have shown a great potential for optical amplifiers in the second and third telecommunications windows (at 1.3 and 1.5 µm, respectively).

Generally, tellurite glasses have a wider transmission range than silica glass. They also have much lower phonon energy, and their glass stability and corrosion resistance is superior to that of fluoride glass. Because the rheological behavior of tellurite glass is Newtonian, its viscosity does not depend on the shear rate. Consequently, the fiber drawing speed is not likely to affect the fiber quality and fiber fabrication will not be a technical challenge [1, 6].

In previously work [8], the results of differential thermal analysis (DTA) indicated that the composition 76TeO₂∙10ZnO∙9.0PbO∙1.0PbF₂∙3.0Na₂O doped with 1% Er₂O₃ (denoted as Er:TZPPN glass) has a high thermal stability and a low tendency towards crystallization. Especially the thermal stability factor is ΔT = 152 °C (the difference between crystallization and glass transition temperature) [8]. In this work, we investigate the optical transitions for this glass. The Judd-Ofelt intensity parameters of Er:TZZPN glass are calculated from the absorption spectra. The measured absorption, around 1,500nm is analyzed by McCumber theory [11, 12] in order to obtain stimulated emission cross sections and gain coefficient of ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ transition. We also present a study of photoluminescence, by excitation at 490nm, in Er:TZPPN glass. Moreover the Raman gain coefficient of the present glass is obtained from Raman scattering experiments using 532 nm excitation [(532 nm Laser type Diode-pumped, solid state (DPSS)]

2. Experimental procedure

A glass with the composition76TeO₂∙10ZnO∙9.0PbO∙1.0PbF₂∙3.0Na₂O∙1.0Er₂O₃ was prepared by mixing specified weights of raw materials. The powder mixture was given in a covered gold crucible and heated in a melting furnace to a temperature of 900 °C for 30 min; the melt was stirred from time to time. The highly viscous melt which was cast at 850 °C on a graphite mould. Subsequently, the sample was transferred to an annealing furnace and kept for 2h at 270°C (below T_g-15K). Then the furnace was switched off and the glass sample was allowed to cool.

The vertical (VV) polarized spontaneous Raman spectra of the prepared glass were acquired using a Thermo Scientific DXR Raman Microscope spectroscopy setup with 532 nm excitation [(532 nm Laser type Diode-pumped, solid state (DPSS)]. An incoming vertically surface of the bulk sample, and V-polarized Raman scattered signal collected in the back scattering geometry with a 100x microscope objective.

3. Results and discussion

3.1 Absorption spectrum and Judd-Ofelt analysis

The Judd-Ofelt theory has mostly been used to evaluate the probability of forced electric dipole transitions of rare-earth ions in various environments as well as in calculating spectroscopic parameters [8–10,13]. It has been shown that for glasses the Judd-Ofelt parameters are related to local structures in the vicinity of rare-earth ion sites, which is useful information in order to estimate the emission properties of rare-earth-doped glasses [8]. The transitions between states that meet the transition-selective rules $Δ S = Δ L = 0$ , $Δ J = 0, \pm 1$ comprise magnetic dipole (S_md) as well as electric dipole (S_ed) transitions, which can be calculated by:

S_{e d}^{c a l c} (ψ J, ψ' J') = {\sum_{k = 2, 4, 6} Ω_{k} | 〈 ψ J ‖ U^{(k)} ‖ ψ' J' 〉 |}^{2}

S_{m d}^{} (ψ J, ψ' J') = {[\frac{h}{4 π m c}]}^{2} {| 〈 ψ J ‖ L + 2 S ‖ ψ J' 〉 |}^{2}

where

Ω_{k}

represent the Judd–Ofelt parameters to be determined. We have calculated the reduced matrix elements

| 〈 ψ J ‖ U^{(k)} ‖ ψ' J' 〉 |

based on the free-ion wave functions of Er³⁺ determined by Maalej et al [14] (Table 1).

Table 1. The reduced matrix elementsof Er³⁺.

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So the measured electric dipole line strengths $S_{e d}^{m e a s} (ψ J, ψ' J')$ from the absorption spectrum can be given by [8, 13]:

S_{e d}^{m e a s} (ψ J, ψ' J') = \frac{1}{4 π ε_{0}} [\frac{9 n}{{(n^{2} + 2)}^{2}}] [\frac{3 c h}{8 π^{3} e^{2}} \frac{(2 J + 1)}{N \bar{λ}} \times \frac{2.303}{ℓ} \int_{J \to J'} O D (λ) d λ - n S_{m d}]

where

\bar{λ}

is the mean wavelength of the absorption band, n is the refractive index of the host with respect to

\bar{λ}

,

ℓ

( = 10 mm) is the thickness of the studied sample, N

(= 4 .420 \times 10^{20} i o n / c m^{3})

is the Er³⁺ ions concentration, e is the electron charge and

\int_{J \to J'} O D (λ) d λ

represents the experimental integrated optical density in the wavelength range and can be obtained by calculating the total area under the absorbed band. S, L, and J are spin angular momentum, orbit angular momentum, and total angular momentum of the initial state, respectively.

The linear refractive index for TZPPN:Er was calculated using Wemple relation:

\frac{1}{n^{2} (E) - 1} = \frac{E_{s}}{E_{d}} - \frac{E^{2}}{E_{s} E_{d}}

The E_d and E_s, equal to 20.52 eV and 6.59 eV respectively, obtained in previous work [8].

Figure 1 shows the absorption spectrum of Er³⁺ doped TZPPN glass. Er³⁺ doped TZPPN glass has numerous absorption bands located at 1530, 975, 803, 654, 545, 523 and 489nm, which correspond to the transitions from ${}^{4}I_{15 / 2}$ to ${}^{4}I_{13 / 2}$ , ${}^{4}I_{11 / 2}$ , ${}^{4}I_{9 / 2}$ , ${}^{4}F_{9 / 2}$ , ${}^{4}S_{3 / 2}$ , ${}^{2}H {(21)}_{11 / 2}$ and ${}^{4}F_{7 / 2}$ , respectively.

Fig. 1 Optical density for the TZPPN glass doped 1%Er₂O₃ from ${}^{4}I_{15 / 2}$ level.

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The three intensity parameters $Ω_{k} (k = 2, 4, 6)$ of Er³⁺ ions in TZPPN glass were calculated by using Eqs. (1-3). The Judd-Ofelt parameters are found to be $Ω_{2} = 4 .28 \times 10^{- 20} c m^{2}$ , $Ω_{4} = 1 .68 \times 10^{- 20} c m^{2}$ , $Ω_{6} = 1 .38 \times 10^{- 20} c m^{2}$ by using the least squares method.

Such calculated and measure dielectric dipole line strengths $S_{e d}^{c a l}$ and $S_{e d}^{m e a s}$ values of different transitions are given in Table 2. An estimation of the accuracy of the calculations of Ω_t is given by the rms deviation:

δ_{r m s} = {(\frac{\sum {(S_{e d}^{m e a s} - S_{e d}^{c a l})}^{2}}{(p - 3)})}^{1 / 2}

Where p is the number of spectral bands analyzed. The value of the rms deviation calculated in our case was

0.20 \times 10^{- 20} c m^{2}

, which denotes a good agreement between the calculated and the experimental data and, consequently, a good precision in the determination of the intensity parameters.

Table 2. Average wavelengths, refractive indexes and electric and magnetic dipole line strengths for Er³⁺ doped TZPPNglass.

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The calculated spontaneous emission probabilities for electric $A_{e d}$ and magnetic $A_{m d}$ dipole transitions, the predicted radiative lifetime $τ_{r}$ for any specific emitting state (which is an important parameter in consideration of the pumping requirement for laser action threshold) and the intermultiplet luminescence branching ratios $β$ were estimated using the calculated intensity parameters and correcting for the refractive index. The values of all these parameters are listed in Table 3.

Table 3. Electric and magnetic dipole line strengths $(S_{e d}^{c a l})$ and $(S_{e d}^{m e a s})$ , electric dipole transition probabilities $(A_{e d})$ , magnetic dipole transition probabilities $(A_{m d})$ , radiative branching ratios $(β)$ and radiative lifetimes $(τ_{r})$ of the energy levels of Er³⁺ doped TZPPN glass.

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The branching ratios obtained for the transitions from the ${}^{4}I_{13 / 2}$ , ${}^{4}I_{11 / 2}$ , ${}^{4}I_{9 / 2}$ , ${}^{4}F_{9 / 2}$ , ${}^{4}S_{3 / 2}$ , and ${}^{4}F_{7 / 2}$ levels to the ${}^{4}I_{15 / 2}$ ground state are larger than 0.74, which predicts efficient emissions from such levels under suitable excitation conditions.

3.2 Fluorescence properties

Figure 2 shows the fluorescence spectrum of the Er³⁺ ion which is excited with 490nm to the ⁴F_7/2 multiplet at room temperature. The band with a maximum at 553nm is attributed to the transition from the ⁴S_3/2 level to the ⁴I_15/2 ground level. The next one with a maximum at $666 n m$ represents the emission from ⁴F_9/2 level to the ⁴I_15/2 ground level. The band with a maximum at 844 nm is attributed to the transition from ⁴S_3/2 level to the ⁴I_13/2 first excited state of Er³⁺. The band with a maximum at 532 nm has been attributed to the ²H_11/2→⁴I_15/2 transition.

Fig. 2 Fluorescence spectrum of the Er:TZPPN glass excited by 490 nm.

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For the calculation of the emission cross-section [15] the Füchtbauer–Ladenburg formula is applied:

σ_{e m}^{} (λ) = \frac{β λ^{5} I (λ)}{8 π n^{2} c τ_{R} \int_{} λ I (λ) d λ}

where

I (λ)

is the intensity of the emission spectrum, n is the refractive index of the material,

τ_{R}

and

β

are the radiative lifetimes and luminescence branching ratios, determined from the J-O theory. The highest value of the emission cross-section,

σ_{e m}^{}

, equal to

6.61 \times 10^{- 21} c m^{2}

at 553 nm is obtained for the

{}^{4}S_{3 / 2} \to {}^{4}I_{15 / 2}

transition of Er³⁺.

3.3 CIE Chromaticity Coordinates

The assessment and quantification of color is referred to colorimetry or the ‘science of color’. The CIE (Commission International de l’Eclairage) system is the most common method to describe the compositions of any color in terms of three primaries $\bar{x} (λ)$ , $\bar{y} (λ)$ and $\bar{z} (λ)$ , which are called color matching functions [16,17]. Artificial “colors,” denoted by X, Y and Z, also called tristimulus values, can be added to produce real spectral colors [16,17]. The degree of simulation required to match the color of given power spectral density $(P (λ))$ can be expressed as:

X = \int_{λ} \bar{x} (λ) P (λ) d λ

Y = \int_{λ} \bar{y} (λ) P (λ) d λ

Z = \int_{λ} \bar{z} (λ) P (λ) d λ

The chromaticity coordinates, x, y and z are calculated from the tristimulus values according [17] to the equations:

x = \frac{X}{X + Y + Z}

y = \frac{Y}{X + Y + Z}

z = \frac{Z}{X + Y + Z}

Generally ( $x, y$ ) coordinates are used to represent the color. The locus of all monochromatic color coordinates makes the perimeter of CIE1931 chromaticity diagram. All the multi-chromatic wavelengths will lie within the area of the chromaticity diagram.

The CIE coordinate by using suitable software is calculated for Er³⁺doped tellurite glass upon excitation at 490nm and is found to be (x = 0.308, y = 0.684)which lies within the green region (Fig. 3). Because of the above reason, present glass gives emission in the green region with appreciable intensity for display applications, light emitting diodes and laser action.

Fig. 3 The CIE coordinates for the Er:TZPPNglass upon excitation at490 nm

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3.4 Stimulated emission cross-section and gain coefficient at 1.5μm

According to the measured absorption spectra shown in Fig. 1, the absorption cross-sections of Er³⁺ ion for the ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ transition can be calculated. The relation between the absorption cross-sections $σ_{a b s} (λ)$ and wavelength can be expressed by using the Beer-Lambert equation [13]:

σ_{a b s} (λ) = \frac{\ln (I_{0} (λ) / I (λ))}{N ℓ} = \frac{2.303 O D (λ)}{N ℓ}

where

I_{0} (λ)

is the incident optical intensity,

I (λ)

is the optical intensity throughout the sample,

O D (λ)

is the optical density,

ℓ

is the thickness of the sample, and N is the concentration of Er³⁺.

The emission cross section $σ_{e m} (λ)$ of Er³⁺ for ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ can be calculated from the absorption cross-section by using the McCumber formula to evaluate the possibility of laser effect [11,12]:

σ_{e m} (λ) = σ_{a b s} (λ) \frac{Z_{l}}{Z_{u}} \exp [\frac{h c}{k T} (\frac{1}{λ_{Z L}} - \frac{1}{λ})]

Where Z_l and Z_u are the partition functions, respectively, for the lower and the upper levels involved in the considered optical transition, T is the temperature (here is the room temperature), k is the Boltzmann constant and

λ_{Z L}

is the wavelength for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets (zero-phonon line). Generally, the room temperature value of the Z_l/Z_u ratio is typically in the range of 0.757–1.045 for the most Er-doped crystals [12] and the

λ_{Z L}

is approximately between 1,514 and 1,547nm [12].

The true value of Z_l/Z_u is not known exactly for glasses, but in the high-temperature limit, the ratio of the partition functions of the lower and upper states Z_l/Z_u simply becomes the degeneracy weighting of the two states [12,15]. In the following calculation, it will be assumed that the ratio Z_l/Z_u is equal to 16/14 [12,15]whereas the zero-phonon line is assumed to be $λ_{Z L} = 1, 531 n m$ [7].

Figure 4 shows the calculated absorption and emission cross-sections for the glass. The emission cross-sections are very similar to those calculated for other Er doped glasses [5,7]. The peak of stimulated emission cross-section ( $σ_{e m}^{p e a k}$ ) is about $1.03 \times 1 0^{- 20} c m^{2}$ .This larger value for the emission cross-section is related to the larger value of the line strength of the ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ transition, $S ({}^{4}I_{13 / 2}, {}^{4}I_{15 / 2}) = 0.0 19 Ω_{2} + 0. 117 Ω_{4} + 1.432 Ω_{6}$ , and, more specifically, on the large value of the $Ω_{6}$ parameter found in glass under study. The value of $σ_{e m}^{p e a k}$ for the studied glass is much higher than that for silicate, phosphate, germanate [18,19] and other tellurite glasses [7, 20–22].

Fig. 4 Absorption cross-sections $σ_{a} (λ)$ and stimulated emission cross-section $σ_{e} (λ)$ for the Er:TZPPNglass.

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The full width at half maximum (FWHM) is also a critical parameter that is used to evaluate the gain bandwidth properties of the optical amplifiers. The full width at half maxima (FWHM) of the emission peak is 46nm for Er³⁺-doped TZPPN glass.

Due to the large overlap of the absorption and emission spectrum of Er³⁺ ions at 1.5 µm, reabsorption will occur and cause a change in the fluorescence spectrum. Thus, due to the asymmetric profile of the emission line, it is more reasonable to calculate an effective bandwidth, instead of the FWHM. The effective bandwidth ( $Δ λ$ ) can be expressed as $Δ λ = \int σ_{e m} (λ) d λ / σ_{e m}^{p e a k}$ . The effective bandwidth is 72.5 nm. This value is similar to those of other tellurite glasses [10] and it is very large with respect to those of silicate, phosphate, germanium glasses and boro-tellurite glasses [22–24].

In order to understand the band profile of the ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ emission of the Er³⁺ ions and to estimate the Stark splitting for the ${}^{4}I_{13 / 2}$ emitting and the ${}^{4}I_{15 / 2}$ ground levels in the tellurite glass under study, a Gaussian deconvolution of the 1.5 µm band developed assuming a simplified model of 4 Stark levels system for the first two levels of the Er³⁺ ions in the tellurite glass.Fig. 5 shows the emission spectra due to the ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ transition of Er³⁺ ions and the deconvoluted Gaussian amplitude peaks obtained from the fitting of the emission spectra of the Er:TZPPN glass (dotted lines). Peak positions and the widths of these subcomponent peaks are labeled as A, B, C and D and tabulated in Table 4. In order to explain 1.5 μm emissions of the Er³⁺ ions, an equivalent model of four levels system is shown in Fig. 6 [19,25–27].

Fig. 5 Emission spectra of the Er:TZPPN glass and deconvolution into Gaussian peaks.

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Table 4. Peak positions (λ) and the half maximum(W) of the A-D subcomponents.

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Fig. 6 An equivalent model of four level system for describing 1.5 µm emission of Er³⁺.

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Which show the ground ${}^{4}I_{15 / 2}$ level splits into two sublevels at around $0 c m^{- 1}$ and $227 c m^{- 1}$ . The excited ${}^{4}I_{13 / 2}$ level also splits into two sublevels (Starks levels) at around $6, 530 c m^{- 1}$ and $6, 650 c m^{- 1}$ as seen in Fig. 6 together with all of the transitions possible between these subcomponents. Thus the energy differences $Δ E_{1} = 227 - 0 = 227 c m^{- 1}$ and $Δ E_{2} = 6, 650 - 6, 530 = 120 c m^{- 1}$ are the values of the energy range of the Stark splitting of the ${}^{4}I_{15 / 2}$ and the ${}^{4}I_{13 / 2}$ multiplets, respectively. The ground state presents a larger Stark splitting than the emitting level for the tellurite glass under study, in a similar way to what has been found in Er³⁺ doped phosphate glasses [27], silicate glasses [28] and tellurite glasses [29]. The results also indicate that the bandwidth is strongly dependent on the overall extent of the Stark splitting.

Optical gain coefficient is an important factor for evaluating the performance of a laser media. If the absorption and emission cross sections for the transitions between two laser operating levels are obtained, the optical gain coefficient $g (λ)$ can be calculated from the following formula:

g (λ) = N [P . σ_{e m} (λ) - (1 - P) . σ_{a b s} (λ)]

where

P

is the population inversion between the upper and the lower levels. Figure 7 shows the gain cross-section as a function of the wavelength under different population inversions. It can be seen that the peak gain cross-section increases and the gain band extends to the short-wavelength side as P increases for both transitions. Higher P gives rise to both broader bandwidth and higher peak value of the gain cross-section. In the case of total inversion

(P = 1)

at 1,532 nm, we obtain a gain coefficient equal to

4.54 c m^{- 1}

for Er:TZPPN glass. This value is very large than those of other tellurite glasses [30].

Fig. 7 The gain coefficient for the ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ transition of the Er:TZPPNglass.

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3.5 Raman spectra

Raman bands are obtained in present glasses in Fig. 8.These bands are deconvoluted into five symmetrical Gaussian peaks at about 382, 484, 560, 710 and 803 cm⁻¹ in the following are denoted as peak A, B, C, D and E, respectively.

Fig. 8 Deconvolution of the Raman spectra of the Er:TZPPN glass. Experimental spectra: symbol; fitted curves: dashed lines.

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The assignment of the deconvoluted peaks is performed based on the literature on the tellurite glasses [31–34]. The phase structure α-TeO₂ is similar to those tellurite glasses estimated by Sekiya et al. [35]; which consists of three dimensional network of TeO₄trigonalbipyramid (tbp) connected with asymmetric Te-_eqO_ax-Te bond. The Raman bands in the low frequency region at 123 and 152 cm⁻¹ correspond to the intra-molecular asymmetric motion of the Te-O bonds. These bands are not appeared in the present glass. When added of metal oxide to TeO₂ leads to a break of the axial Te-O bonds because of the strong polarizabilty of the tellurium lone pair electrons and the formation of non-bridging Te-O bonds. Moreover the TeO₄ units are transformed into TeO₃₊₁ and TeO₃ polyhedra. In the present glass,the band at 382 cm⁻¹ (labeled as A) can be contributed to the axial bending vibration mode (O_ax- Te-O_ax). A strong band labeled B at 482 cm⁻¹can be attributed stretching vibrations of Te-O^- and Te = O bonds containing non-bridging oxygen in TeO₃tps and TeO₃₊₁ polyhedral. Furthermore this band is not observed in pure TeO₂ glass, when the addition of network modifiers results in a cleavage of Te-O-Te linkages of the initially polymerized structure and in the transformation of TeO₄tbp into TeO₃₊₁polyhedrawithnon bridging oxygen (NBO) or into TeO₃tp with even more NBO atoms [36]. The weak band centered around 560 cm⁻¹(labeled C) and 803 cm⁻¹ (labeled E)observed in the present glass are attribute able to the (Te_eq-O)_s and the (Te_eq-O)_as vibrational modes of TeO₃₊₁polyhedra and/or TeO₃trigonal pyramids. Furthermore, a strong and broad band with the peak at 710 cm⁻¹ (peak D)is observed. We suggest that this band is due to the symmetric stretching vibrations of Te-O-Te in which both the Te-O bonds have lengths of about 2.0 Å. Hoppe et al [37] determined Te-O and Zn-O coordination in zinc tellurite glasses by X-ray and neutron scattering measurements. They concluded from radial distribution function analyses of neutron scattering data that increasing the ZnO concentration to 10 mol% ZnO in the TZPPN glass, leads to a decrease in the mean Te-O coordination number due to a conversion of TeO₄ into TeO₃₊₁ and TeO₃ structural units.

The intensities of various peaks in the Raman spectra of the prepared Er₂O₃ doped glass are higher than of other tellurite glasses reported in the literature [31–34]. This indicates the transformation of TeO₄ tbp into TeO₃₊₁/TeO₃ tp in the studied glasses seen from the high intensity of the band at 710 cm⁻¹.

3.6 Stimulated Raman gain coefficient

We can calculate the Raman gain coefficient, G, of the present glass using the equation [38]:

G = \frac{σ_{T} λ_{S}^{3}}{c^{2} h n^{2} [N (w, T) + 1]}

Where σ_T is the corrected scattering cross section at temperature T(K), λ_s is the Stokes wavelength, c is velocity and n is the refractive index at the excitation wavelength. N(w,T) is the Bose-Einstein factor:

N (w, T) = \frac{1}{\exp (\frac{ℏ w}{K_{B} T}) - 1}

the gain coefficient of silica glass at 440 cm⁻¹for a pump wavelength of 532 nm is

1.86 10^{- 13} m / W

[39].The shape of the Raman gain spectrum of76TeO₂∙10ZnO∙9.0PbO∙1.0PbF₂∙3.0Na₂O∙1.0Er₂O₃glass is shown in Fig. 9.The Raman gain peak for the prepared glass at about 705 cm⁻¹ is equal to

9.3 \times 10^{- 11} m / W

.This value was ~500 times larger compared to that silica glass at 532 nm. For selecting a material for Raman amplifiers, the gain bandwidth is an additional important parameter which can be obtained from the wave number dependency of the Raman gain coefficient. The FWHM bandwidth for the band centered at 705 cm⁻¹ is equal to 161 cm⁻¹.

Fig. 9 Raman gain spectra for the Er:TZPPN glass calculated from absolute spontaneous Raman cross-section.

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Figure 10 shows the Raman cross section spectra of the prepared glass and its deconvolution within the wave number range from 249 to 1106 cm⁻¹. The shows a decrease in the cross section and Raman gain coefficient at around 434 and 880 cm⁻¹ related to vibration of Te-O-Te bridges and TeO₄polyhedra. Otherwise, the decrease of the number of TeO₄ units leads to the formation TeO₃₊₁ or TeO₃ structural units and hence to depolymerization of the tellurite glass matrix and results in increasing Raman gain coefficient and cross sections at 705 cm⁻¹. In conclusion, the introduction of Er³⁺ ions has a major effect by the depolymerization of the tellurite network leads to local polarizability/hyperpolarizability and highest Raman gain compared with other oxide tellurite glass without doping by Er³⁺ [40–42]. Therefore, the prepared material could be a promising candidate for ultrabroadband Raman amplifier.

Fig. 10 Deconvolution of the corrected Raman cross section spectra of the Er:TZPPN glass.

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4. Conclusion

The optical transitions of Er³⁺ in tellurite glass were investigated. The CIE coordinate was calculated for the glass upon excitation at 490 nm and was found about (x = 0.3080,y = 0.6840) that indicated the purity of the emission spectra for green band. Using the Füchtbauer-Ladenburg formula, the highest value of the emission cross-section in visible region, equal to $6.61 \times 10^{- 21} c m^{2}$ at 553nm, was obtained for the ${}^{4}S_{3 / 2} \to {}^{4}I_{15 / 2}$ transition of Er³⁺.Using the Judd-Ofelt parameters of $E r^{3 +}$ in TZPPN glass, the spontaneous radiative lifetime (τ_r) of ${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$ was calculated to be $2.723 m s$ . Stimulated emission cross section in the 1.5 µm region was obtained by using McCumber theory and the optical gain coefficient to the population inversion of the ${}^{4}I_{13 / 2}$ level was analyzed. We obtain a gain coefficient of $4.54 c m^{- 1}$ , an effective bandwidth of 72.5nm for Er:TZPPN glass. Therefore, the spectroscopy investigations suggest that the TZPPN glass doped with 1mol% $E r^{3 +}$ ions might be a promising material for broadband amplification in the third telecommunications window as well as to generate green light in color display devices.

In addition, the Raman gain coefficient for a pump wavelength of 532 nm at about $705 c m^{- 1}$ is $9.3 \times 10^{- 11} m/W$ .The FWHM bandwidth for the band centered at $705 c m^{- 1}$ is equal to $161 c m^{- 1}$ . Thus, the prepared tellurite glass could be a candidate material to realize highly efficient ultra broadband fiber Raman amplifier with higher Raman gain.

Acknowledgments

This research was supported by a Grant of King Abdulaziz City for Science and Technology (Code Number: 10-ADV1160-07) from King of Saudi Arabia.

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Transitions	${‖ U^{2} ‖}^{2}$	${‖ U^{4} ‖}^{2}$	${‖ U^{6} ‖}^{2}$
${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$	0.0194	0.1173	1.4322
${}^{4}I_{11 / 2} \to {}^{4}I_{15 / 2}$	0.0245	0.0001	0.3985
$\to {}^{4}I_{13 / 2}$	0.0330	0.1694	0.0101
${}^{4}I_{9 / 2} \to {}^{4}I_{15 / 2}$	0	0.1694	0.0101
$\to {}^{4}I_{13 / 2}$	0.0003	0.0103	0.7400
$\to {}^{4}I_{11 / 2}$	0.0014	0.0745	0.1606
${}^{4}F_{9 / 2} \to {}^{4}I_{15 / 2}$	0	0.548	0.4769
$\to {}^{4}I_{13 / 2}$	0.0092	0.1598	0.0855
$\to {}^{4}I_{11 / 2}$	0.0644	0.0090	1.2651
$\to {}^{4}I_{9 / 2}$	0.1266	0.0056	0.0268
${}^{4}S_{3 / 2} \to {}^{4}I_{15 / 2}$	0	0	0.2369
$\to {}^{4}I_{13 / 2}$	0	0	0.3498
$\to {}^{4}I_{11 / 2}$	0	0.0038	0.0875
$\to {}^{4}I_{9 / 2}$	0	0.0645	0.2488
$\to {}^{4}F_{9 / 2}$	0	0.0002	0.0270
${}^{2}H_{11 / 2} \to {}^{4}I_{15 / 2}$	0.8120	0.4669	0.0951
$\to {}^{4}I_{13 / 2}$	0.0304	0.0826	0.0294
$\to {}^{4}I_{11 / 2}$	0.0328	0.1408	0.0401
$\to {}^{4}I_{9 / 2}$	0.2168	0.0682	0.2624
$\to {}^{4}F_{9 / 2}$	0.4292	0.0305	0.0001
$\to {}^{4}S_{3 / 2}$	0	0.2207	0.0103
${}^{4}F_{7 / 2} \to {}^{4}I_{15 / 2}$	0	0.1475	0.6348
$\to {}^{4}I_{13 / 2}$	0	0.3346	0.0001
$\to {}^{4}I_{11 / 2}$	0.0032	0.2707	0.1717
$\to {}^{4}I_{9 / 2}$	0.0153	0.1015	0.4309
$\to {}^{4}F_{9 / 2}$	0.0143	0.0379	0.0117
$\to {}^{4}S_{3 / 2}$	~0	0.0046	0
$\to {}^{2}H_{11 / 2}$	0.1342	0.0250	0.3856

Transition from ${}^{4}I_{15 / 2}$ to	$\bar{λ} (n m)$	N	$S_{e d}^{m e a s} (p m^{2})$	$S_{m d} (p m^{2})$	$S_{e d}^{c a l} (p m^{2})$
${}^{4}I_{13 / 2}$	1529.8	2.0405	2.4275	0.6998	2.2673
${}^{4}I_{11 / 2}$	975.4	2.0582	0.6532	0	0.6580
${}^{4}I_{9 / 2}$	803.2	2.0728	0.4124	0	0.2994
${}^{4}F_{9 / 2}$	654.5	2.0968	1.6029	0	1.5853
${}^{4}S_{3 / 2}$	545.4	2.1300	0.1616	0	0.3285
${}^{2}H {(21)}_{11 / 2}$	523	2.1399	4.3971	0	4.4008
${}^{4}F_{7 / 2}$	490	2.1579	0.8182	0	1.1289

Transitions	Average wave number $(c m^{- 1})$	n	$S_{e d} (p m^{2})$	$S_{m d}^{} (p m^{2})$	$A_{e d} (s^{- 1})$	$A_{m d} (s^{- 1})$	$β (%)$	$τ_{r} (m s)$
${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$	6537	2.040	2.267	0.7	281.54	85.713	100	2.723
${}^{4}I_{11 / 2} \to {}^{4}I_{15 / 2}$	10252	2.058	0.658	0	379.72	0	84.025	2.212
$\to {}^{4}I_{13 / 2}$	3715	2.032	1.972	0.791	51.684	20.050	15.975	2.212
${}^{4}I_{9 / 2} \to {}^{4}I_{15 / 2}$	12450	2.072	0.299	0	381.27	0	73.353	1.923
$\to {}^{4}I_{13 / 2}$	5913	2.038	1.045	0	133.97	0	25.774
$\to {}^{4}I_{11 / 2}$	2198	2.030	0.354	0.348	2.2979	2.237	0.872
${}^{4}F_{9 / 2} \to {}^{4}I_{15 / 2}$	15278	2.096	1.585	0	3894.1	0	91.449	0.234
$\to {}^{4}I_{13 / 2}$	8741	2.05	0.427	0	180.8	0	4.246
$\to {}^{4}I_{11 / 2}$	5026	2.035	2.045	0.172	160.24	13.385	4.077
$\to {}^{4}I_{9 / 2}$	2828	2.030	0.589	0.112	8.1555	1.538	0.227
${}^{4}S_{3 / 2} \to {}^{4}I_{15 / 2}$	18336	2.13	0.328	0	3700.2	0	70.41	0.190
$\to {}^{4}I_{13 / 2}$	11799	2.068	0.485	0	1303.6	0	24.807
$\to {}^{4}I_{11 / 2}$	8084	2.046	0.127	0	106.3	0	2.022
$\to {}^{4}I_{9 / 2}$	5886	2.038	0.453	0	143.42	0	2.729
$\to {}^{4}F_{9 / 2}$	3058	2.031	0.038	0	1.664	0	0.031
${}^{2}H_{11 / 2} \to {}^{4}I_{15 / 2}$	19121	2.139	4.400	0	19067	0	95.329	0.050
$\to {}^{4}I_{13 / 2}$	12584	2.073	0.310	0.116	341.13	124.96	2.33
$\to {}^{4}I_{11 / 2}$	8869	2.050	0.433	0.086	159.85	31.254	0.955
$\to {}^{4}I_{9 / 2}$	6671	2.041	1.408	0.030	217.12	4.626	1.108
$\to {}^{4}F_{9 / 2}$	3843	2.032	1.892	0.009	54.926	0.256	0.275
$\to {}^{4}S_{3 / 2}$	785	2.028	0.386	0	0.094	0	0.0004
${}^{4}F_{7 / 2} \to {}^{4}I_{15 / 2}$	20436	2.157	1.128	0	9245.8	0	80.295	0.086
$\to {}^{4}I_{13 / 2}$	13899	2.084	0.563	0	1274.7	0	11.07
$\to {}^{4}I_{11 / 2}$	10184	2.057	0.708	0	600.43	0	5.214
$\to {}^{4}I_{9 / 2}$	7986	2.046	0.834	0.051	334.23	20.329	3.079
$\to {}^{4}F_{9 / 2}$	5158	2.036	0.141	0.212	15.014	22.198	0.323
$\to {}^{4}S_{3 / 2}$	2100	2.029	0.008	0	0.057	0	0.0005
$\to {}^{2}H_{11 / 2}$	1315	2.029	1.152	0	1.997	0	0.0173

FWHM (nm)	Peak A		Peak B		Peak C		Peak D
	λ_A	W_A	λ_B	W_B	λ_C	W_C	λ_D	W_D
46	1504.8	42.1	1530.6	16.6	1551.5	35	1593.3	41.1

Transitions	${‖ U^{2} ‖}^{2}$	${‖ U^{4} ‖}^{2}$	${‖ U^{6} ‖}^{2}$
${}^{4}I_{13 / 2} \to {}^{4}I_{15 / 2}$	0.0194	0.1173	1.4322
${}^{4}I_{11 / 2} \to {}^{4}I_{15 / 2}$	0.0245	0.0001	0.3985
$\to {}^{4}I_{13 / 2}$	0.0330	0.1694	0.0101
${}^{4}I_{9 / 2} \to {}^{4}I_{15 / 2}$	0	0.1694	0.0101
$\to {}^{4}I_{13 / 2}$	0.0003	0.0103	0.7400
$\to {}^{4}I_{11 / 2}$	0.0014	0.0745	0.1606
${}^{4}F_{9 / 2} \to {}^{4}I_{15 / 2}$	0	0.548	0.4769
$\to {}^{4}I_{13 / 2}$	0.0092	0.1598	0.0855
$\to {}^{4}I_{11 / 2}$	0.0644	0.0090	1.2651
$\to {}^{4}I_{9 / 2}$	0.1266	0.0056	0.0268
${}^{4}S_{3 / 2} \to {}^{4}I_{15 / 2}$	0	0	0.2369
$\to {}^{4}I_{13 / 2}$	0	0	0.3498
$\to {}^{4}I_{11 / 2}$	0	0.0038	0.0875
$\to {}^{4}I_{9 / 2}$	0	0.0645	0.2488
$\to {}^{4}F_{9 / 2}$	0	0.0002	0.0270
${}^{2}H_{11 / 2} \to {}^{4}I_{15 / 2}$	0.8120	0.4669	0.0951
$\to {}^{4}I_{13 / 2}$	0.0304	0.0826	0.0294
$\to {}^{4}I_{11 / 2}$	0.0328	0.1408	0.0401
$\to {}^{4}I_{9 / 2}$	0.2168	0.0682	0.2624
$\to {}^{4}F_{9 / 2}$	0.4292	0.0305	0.0001
$\to {}^{4}S_{3 / 2}$	0	0.2207	0.0103
${}^{4}F_{7 / 2} \to {}^{4}I_{15 / 2}$	0	0.1475	0.6348
$\to {}^{4}I_{13 / 2}$	0	0.3346	0.0001
$\to {}^{4}I_{11 / 2}$	0.0032	0.2707	0.1717
$\to {}^{4}I_{9 / 2}$	0.0153	0.1015	0.4309
$\to {}^{4}F_{9 / 2}$	0.0143	0.0379	0.0117
$\to {}^{4}S_{3 / 2}$	~0	0.0046	0
$\to {}^{2}H_{11 / 2}$	0.1342	0.0250	0.3856

Raman, green and infrared emission cross-sectionsof Er³⁺ doped TZPPN tellurite glass

Abstract

1. Introduction

2. Experimental procedure

3. Results and discussion

3.1 Absorption spectrum and Judd-Ofelt analysis

3.2 Fluorescence properties

3.3 CIE Chromaticity Coordinates

3.4 Stimulated emission cross-section and gain coefficient at 1.5μm

3.5 Raman spectra

3.6 Stimulated Raman gain coefficient

4. Conclusion

Acknowledgments

References

Cited By

Figures (10)

Tables (4)

Equations (17)

Optical Materials Express