The spectroscopic properties of Er3+ ion in tellurite glass of molar composition 76TeO2∙10ZnO∙9.0PbO∙1.0PbF2∙3.0Na2O∙1.0Er2O3 was investigated experimentally. The three phenomenological intensity parameters Ωk (k = 2, 4, 6) were determined from the absorption spectral intensities using the Judd-Ofelt (J-O) theory. Several radiative properties such as spontaneous transition probabilities, fluorescence branching ratios and radiative life times were determined by using these intensity parameters. The special attention was attributed to the visible emissions that could be obtained by pumping using a blue laser diode. The stimulated emission cross-section and CIE chromaticity coordinates were calculated. The latter were used to evaluate green light emitting by Er:TZPPN glass. Subsequently, the stimulated emission cross-section, around 1.5 μm, was calculated from McCumber theory. Gain cross-section for laser transition 4I13/2→ 4I15/2 of Er3+-ions was obtained. In comparison with other Er-doped laser glasses, the calculated parameters show that Er:TZPPN glass satisfies the fundamental spectral condition for laser emission around 1.5 μm. Moreover the Raman gain coefficient of the present glass was obtained from Raman scattering experiments using 532 nm excitation [(532 nm Laser type Diode-pumped, solid state (DPSS)]. The developed glass showed the widest bandwidths of gain cross section from 249 to 1,106 cm−1.
© 2014 Optical Society of America
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 , 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) . 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 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 Tg-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 . The transitions between states that meet the transition-selective rules, comprise magnetic dipole (Smd) as well as electric dipole (Sed) transitions, which can be calculated by:14] (Table 1).
The linear refractive index for TZPPN:Er was calculated using Wemple relation: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 to , , , , , and , respectively.
Such calculated and measure dielectric dipole line strengths and values of different transitions are given in Table 2. An estimation of the accuracy of the calculations of Ωt is given by the rms deviation:
The calculated spontaneous emission probabilities for electric and magneticdipole transitions, the predicted radiative lifetime 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 were estimated using the calculated intensity parameters and correcting for the refractive index. The values of all these parameters are listed in Table 3.
The branching ratios obtained for the transitions from the , , , , , and levels to the 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 4F7/2 multiplet at room temperature. The band with a maximum at 553nm is attributed to the transition from the 4S3/2 level to the 4I15/2 ground level. The next one with a maximum at represents the emission from 4F9/2 level to the 4I15/2 ground level. The band with a maximum at 844 nm is attributed to the transition from 4S3/2 level to the 4I13/2 first excited state of Er3+. The band with a maximum at 532 nm has been attributed to the 2H11/2→4I15/2 transition.
For the calculation of the emission cross-section  the Füchtbauer–Ladenburg formula is applied:
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, and , 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 can be expressed as:
The chromaticity coordinates, x, y and z are calculated from the tristimulus values according  to the equations:
Generally () 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.
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 thetransition can be calculated. The relation between the absorption cross-sections and wavelength can be expressed by using the Beer-Lambert equation :12] and the is approximately between 1,514 and 1,547nm .
The true value of Zl/Zu is not known exactly for glasses, but in the high-temperature limit, the ratio of the partition functions of the lower and upper states Zl/Zu simply becomes the degeneracy weighting of the two states [12,15]. In the following calculation, it will be assumed that the ratio Zl/Zu is equal to 16/14 [12,15]whereas the zero-phonon line is assumed to be .
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 () is about .This larger value for the emission cross-section is related to the larger value of the line strength of the transition,, and, more specifically, on the large value of the parameter found in glass under study. The value of for the studied glass is much higher than that for silicate, phosphate, germanate [18,19] and other tellurite glasses [7, 20–22].
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 () can be expressed as . The effective bandwidth is 72.5 nm. This value is similar to those of other tellurite glasses  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 emission of the Er3+ ions and to estimate the Stark splitting for the emitting and the 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 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].
Which show the ground level splits into two sublevels at around and. The excited level also splits into two sublevels (Starks levels) at around and as seen in Fig. 6 together with all of the transitions possible between these subcomponents. Thus the energy differences and are the values of the energy range of the Stark splitting of the and the 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 , silicate glasses  and tellurite glasses . 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 can be calculated from the following formula: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 at 1,532 nm, we obtain a gain coefficient equal to for Er:TZPPN glass. This value is very large than those of other tellurite glasses .
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.
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. ; which consists of three dimensional network of TeO4trigonalbipyramid (tbp) connected with asymmetric Te-eqOax-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 (Oax- Te-Oax). 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 . 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 (Teeq-O)s and the (Teeq-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  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 :
Where σT is the corrected scattering cross section at temperature T(K), λs 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: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.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.
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.
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 at 553nm, was obtained for thetransition of Er3+.Using the Judd-Ofelt parameters of in TZPPN glass, the spontaneous radiative lifetime (τr) of was calculated to be. 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 level was analyzed. We obtain a gain coefficient of, an effective bandwidth of 72.5nm for Er:TZPPN glass. Therefore, the spectroscopy investigations suggest that the TZPPN glass doped with 1mol% 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 is .The FWHM bandwidth for the band centered at is equal to. Thus, the prepared tellurite glass could be a candidate material to realize highly efficient ultra broadband fiber Raman amplifier with higher Raman gain.
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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