In this paper we explore the TeO2-Bi2O3-BaO glass family with varied TeO2 concentration for Raman gain applications, and we report, for the first time, the peak Raman gain coefficients of glasses within this glass family extrapolated from non-resonant absolute Raman cross-section measurements at 785 nm. Estimated Raman gain coefficients show peak values of up to 40 times higher than silica for the main TeO2 bands. Other optical properties, including index dispersion from the visible to the long wave Infrared (LWIR) are also summarized in this paper.
©2009 Optical Society of America
In the telecom fiber industry, Raman fiber amplification has attracted a great deal of interest. These fibers have the advantage of having a broad amplification bandwidth and a tunable maximum gain throughout their transparency window. The fiber’s ultimate performance is also only restricted by the pump wavelength and Raman active modes of the gain medium. Currently in the telecommunications industry, fused silica and germanium-doped silica fibers are the main Raman gain materials used. However, these materials are limited by low Raman gain response and limited useable spectral bandwidth [1,2].
Recent literature has highlighted the performance of tellurite glasses for these applications due to their high nonlinear optical properties as compared to fused silica [1–10]. Moreover, tellurite-based glasses have been previously shown to be promising materials for high Raman amplification due to their large Raman gain coefficients [1–3,5,6,8–10]. Their potential to satisfy the urgent need for broad bandwidth suitable for satisfying demands for both long haul and local area network data transmission is a significant part of their attraction.
In this study, the ternary TeO2-Bi2O3-BaO family of glasses was selected due to their high spontaneous Raman intensities and large spectral bandwidth as compared to fused silica. A previous study has investigated the properties of this glass family for fiber applications . The study reported a Raman spectral analysis, Ultraviolet (UV) and Infrared (IR) edge data, thermal characterization, and viscosity measurements, all of which showed a great potential for high temperature fiber applications .
In this paper, we extend the analysis of these glasses and discuss the thermal and physical characteristics of the glass as a function of the glass composition. Also, we report for the first time their complete UV/VIS/NIR/IR absorption and refractive index properties (critical to broadband spectral applications), their Raman spectra relative to fused silica and their Raman gain coefficient values.
2. Glass fabrication and experimental condition
Glasses in the system (90-x)TeO2-10Bi2O3-xBaO with x = 10, 15 and 20 were prepared from high purity raw materials: TeO2 (Alfa Aesar 99.5%), Bi2O3 (Alfa Aesar 99%), BaO (Alfa Aesar 88%). The batch mixtures were melted in a platinum crucible at a temperature range of 800-900°C, depending on the glass composition. After quenching in air, the glasses were annealed for 15 hours at 50°C below their respective glass transition temperature, Tg. The glasses were optically polished and inspected.
The glasses have been analyzed using Energy Dispersive X-ray Spectroscopy (EDS) coupled with a scanning electron microscope (SEM) to determine final glass stoichiometry as compared to the batched composition. Bulk glass density was measured by Archimedes’ principle using diethylphthalate. The accuracy was better than ± 0.02g/cm3. The glass transition temperature (Tg) and glass crystallization temperature (Tx) were determined by Differential Scanning Calorimetry (DSC) at a heating rate of 10°C/min from 40 to 475°C using a commercial apparatus (TA Instruments Inc.). The measurements were carried out in a hermetically sealed aluminum pan. The glass transition temperature was taken as the inflection point of the endotherm, obtained by taking the first derivative of the DSC curve; Tg was determined with an accuracy of ± 2°C. Tx the glass’ crystallization temperature, was determined (when present) with an accuracy of ± 2°C; however in the majority of glasses any Tx was beyond the 475°C upper temperature of our measurements.
The visible-near infrared (Vis-NIR) and infrared absorption spectra were measured at room temperature using a Perkin Elmer Lambda 900 UV/Vis/NIR Spectrometer and a Magna-IR 560 Spectrometer from Nicolet on 2 mm thick optically polished samples, respectively. In the spectra shown in Fig. 1 the Fresnel reflections have been subtracted.
The refractive index dispersion curves (real part n and imaginary part k) were obtained using a J. A. Woollam variable angle spectroscopic ellipsometer (VASE) from 0.35 to 2.2μm and from 1.75 to 30μm using a J. A. Woollam infrared VASE (IR-VASE). The complex refractive index from the UV to IR was described using a series of Gaussian oscillators. All ellipsometric data were analyzed using J.A. Woollam WVASE32 software package. The VASE and IR-VASE data acquired on each sample were simultaneously regression fit to the optical model. The refractive index was measured with an accuracy of 0.001.
The spontaneous Raman cross section measurements were conducted using a LabRamHR Horiba Jobin Yvon micro-Raman system. The 532 nm and 785 nm laser lines were used as the excitation wavelengths. In all cases, the incoming polarized (V) laser beam was focused onto the front polished surface of the sample via a 100x microscope objective, with a spatial resolution of about 2 µm. A polarizer was used to select the polarization direction (vertical, V or horizontal, H) of the scattered light. A backscattering geometry was used to collect the Raman signal, which was then spectrally analyzed with a spectrometer and a CCD detector. The Rayleigh line was reduced with a holographic notch filter. All spectra were normalized to the peak vibration of SiO2 at 440 cm−1.
3. Results and interpretation
The glasses investigated in this paper with the composition (90-x)TeO2-10Bi2O3-xBaO with x = 10, 15 and 20 were chosen because of their high tellurium content. Previous studies have shown that high TeO2-content glasses exhibit large Raman cross-sections along with the good crystallization stability (Tx-Tg values >100°C), required for good stability in optical fiber drawing . Here we present results of measurements of the glass’ Raman gain coefficient at the 1064 nm wavelength as well as their physical, thermal, optical and structural properties.
3.1 Physical and thermal properties of the glasses
The glass compositions examined in the study are listed in Table 1 . Within the error of the measurement, the density was shown to increase with increasing TeO2 demonstrating a compaction of the network with the addition of the heavy Te atoms, as expected. As also seen in Table 1, the Tg and Tx are compositionally dependent and values determined are comparable to those previously reported in the literature . The ΔT values for each of the glasses studied are over 100°C, which is in agreement with data previously reported for the three compositions studied in  confirming that the glasses are viable for fiber production.
3.2 Optical properties
Figure 1 shows the absorption spectra of the investigated glasses. The spectra were corrected to eliminate Fresnel reflections. In agreement with , the UV absorption cutoff is red shifted with increasing TeO2 concentration. It is important to mention that the UV absorption edge cutoff is near 425 nm with the tail continuing past 550 nm. This is an important consideration when conducting relative spontaneous Raman measurements to be at excitation wavelengths greater than the UV absorption tail to avoid resonance features .
As illustrated in Fig. 2 , the IR absorption spectra of the glasses show that the glasses transmit through approximately 1600 cm−1, or just past 6 µm. The spectra exhibit a large absorption band centered at 3000 cm−1 which has been as attributed to free OH molecules, and a band with lower amplitude at 2300 cm−1 band which has been associated with strong hydrogen-bonded OH- groups, as discussed in [10,11,16]. The additional feature near 2100 cm−1 is also associated with hydrogen bonded hydroxyl groups [3,11,12]. When the concentration of TeO2 increases, the amplitude of these bands increases slightly, revealing an increase in the level of OH- related impurities in the glass network which is directly related to moisture in the TeO2 starting materials. Efforts in our group are ongoing to develop melting procedures which minimize this OH content as it is imperative for fabricating low loss fiber in the near to mid-IR region . Due to the large absorption coefficient in the 2000-3500 cm−1 range seen for these glasses, some further refinement needs to be made towards OH- minimization process prior to drawing low loss fibers for mid-IR applications.
The refractive index of the investigated glasses has been measured using an ellipsometer and the index dispersion curves (n & k) are shown in Fig. 3 . The complex refractive index from the UV to IR was described using a series of Gaussian oscillators. Extracted index of refraction values for the wavelengths discussed in the paper are included in Table 2 .
The real refractive index data (n) reveals that the refractive index increases with increasing TeO2 concentration in agreement with previous studies measuring near-infrared indices on various tellurite-based glasses . It is important to mention that these refractive index values are on the high end of those of tellurite glasses and owe this increase to the content of the alkaline metal oxide, BaO [13,14]. The extinction coefficient curves (k-values) show a phonon absorption band at approximately 15 μm. This absorption feature also increases with increasing TeO2 concentration.
3.3 Spontaneous Raman spectra
Figure 4 exhibits the spontaneous Raman cross-section spectra of the investigated glasses relative to fused silica. These measurements are used to estimate the peak Raman gain coefficients at 1064 nm. The spectra were corrected for background noise and normalized to the peak intensity of SiO2.
Three bands can be seen in the spectra, located at 450, 665, 745 cm−1. These are associated with the Te-O-Te linkages, TeO4 units and TeO3+1/TeO3 units respectively, in agreement with . When the concentration of TeO2 increases, the amplitude of the bands at 450 and 665 cm−1 increases as compared to that of the band at 745cm−1. The 745 cm−1 peak development is due to the increase in the polarizability of the TeO3+1/TeO3 vibrations that occurs with the addition of BaO. With increasing TeO2 concentration, a shift from TeO3+1/TeO3 to TeO4 units is evident, indicating that the number of non-bridging oxygen in the glass decreases as the content of TeO2 increases and the network connectivity is reduced.
Also, as observed previously in , the Raman cross-section measurements in the visible (at 532 nm) show signs of resonance enhancement (Fig. 4 – solid lines), with an overall relative Raman intensity enhancement of 1.5x over the non-resonant spectra at 785 nm (Fig. 4 – dashed lines). This resonant enhancement is expected as the 532 nm Raman excitation wavelength falls in the previously noted “tail” of the UV absorption edge for these glasses shown in Fig. 1. Therefore considering this effect and its impact for the gain coefficient calculations, the non-resonant Raman cross-section spectra obtained at 785 nm were used to estimate the peak Raman gain coefficients for the TeO4 and TeO3+1/TeO3 Raman bands, at 1064 nm.
3.4 Raman gain coefficients
The procedure for determining the Raman gain spectra in bulk glass samples from spontaneous Raman cross-section measurements, has been previously described elsewhere [1,3,5] and has been confirmed as a reliable and accurate way to experimentally assess materials for the Raman gain spectrum and maximum gain coeefficient. The Raman gain spectrum of the glass was calculated using the equation:1]. In Eq. (1), ωP1 represents the pump frequency (in this particular example corresponding to the 785 nm excitation wavelength), represents the Raman vibration of the β-th mode and r-th glass constituent, and corresponds to the relative Raman intensity of the TeO2 glass normalized to SiO2, as shown in Fig. 4. Equation (1) also takes into consideration corrections to solid angle and Fresnel reflections. The spontaneous Raman spectra were also corrected using the Bose-Einstein correction factor in order to eliminate low wavenumber thermal effects [1,3]. A plot of the relative Raman gain spectra of the investigated glasses is shown in Fig. 5 .
The Raman gain values obtained in this paper are consistent with Raman gain values obtained for similar tellurite-based glass families [2,9] though the modifiers in the glass network are different. Table 2 summarizes the extrapolated peak Raman coefficient values at the 1064 nm pump wavelength for the two main Raman peaks. These calculations utilized the peak Raman gain coefficient of SiO2 (ν = 440cm−1) of 0.89x10−13m/W using 1064 nm pumping, as well as the index of refraction values of the tellurite glasses at the 532, 785, and 1064 nm wavelengths, extracted from Fig. 3.
At the 665 cm−1 vibration, the Raman gain coefficient increases with increasing TeO2 concentration, as expected from the parallel increase in spontaneous Raman intensity seen in Fig. 4. The maximum Raman gain coefficient occurs at the band of 745 cm−1 (attributed to TeO3+1/TeO3 units) and the value increases as the TeO2 concentration decreases. As explained in the previous paragraph, the increased Raman gain coefficient can be related to the less ordered TeO4 network which accompanies the introduction of BaO to the glass. With the addition of BaO into the network, the formation of TeO3+1/TeO3 units occurs, thereby increasing the band at 745cm−1. Additionally, the addition of this modifier also increases the polarizability of the glass network, thus giving rise to peak Raman gain values on the order of 40 times that of SiO2. These results, in conjunction with the glass’ superb thermal stability, confirm that these glasses are excellent potential candidates for Raman gain fiber applications.
A complete optical and structural analysis of three glass compositions in the system TeO2-Bi2O3-BaO as a function of TeO2 content has been carried out. UV-Vis-NIR absorption and FTIR spectra show that the glasses are transparent from approximately 450 nm to 6 µm and the transparency window decreases slightly with an increase of TeO2 content. Mid-infrared optical quality will be ultimately limited by the quality of OH removal that can be carried out on raw (starting) materials and maintained during melting. Spectroscopic ellipsometry measurements show that both the refractive index and extinction coefficient values increase with increasing TeO2 concentration due to an increase of TeO4 units in the glass network, as indicated using Raman spectroscopy. The calculations of the Raman gain coefficients are consistent with those previously reported for other tellurite glasses which exhibited slightly lower crystallization stability. The peak Raman gain coefficient is 40x greater than that of fused silica at 1064 nm showing that these glasses are promising candidates for Raman gain applications.
The authors would like to thank Lockheed Martin Missiles and Fire Control and the Sandia NINE program for the summer internship of the undergraduate student (JJ) participating in this work. The authors acknowledge the support of the National Science Foundation, International REU program (#ENG-0649230) whose support of this effort provided international research experiences to the undergraduate co-author (CS) and the prior student (Adam Haldeman) participating in this work.
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