Er3+ doped tellurite fibers have been fabricated using a convenient suction technique. Different scanning calorimetry (DSC), Electro-probe micro-analyzer (EPMA), Raman, steady and dynamic luminescence spectra were performed to investigate their thermal, structure, and luminescent properties. Broadband 2.7 μm amplified spontaneous emission (ASE) emission with concurrent 1.5 μm and visible up-conversion emissions have been achieved in fibers with various lengths upon the excitation of 980 nm laser diode (LD). A quantitative study of the spectroscopic properties has been further analyzed in detail according to the Jude-Ofelt model, rate equation, and Dexter’s theory to evaluate the competitive relationship among them. The desirable spectroscopic characteristics associated with excellent thermal stability and mechanical properties indicate that the Er3+ doped tellurite fibers are excellent host matrices for 2.7 μm lasing and may provide applications in mid-infrared fiber lasers and amplifiers.
© 2015 Optical Society of America
Rare earth (RE) ions doped photonic glasses that exhibit laser emission at mid-infrared (MIR) wavelength longer than 2.0 μm are of interest for tremendous applications, including non-invasive medical diagnostics, atmospheric pollution monitoring, remote sensing, eye-safe radars, military weapons, and pump resources [1–3]. In this respect, many works have been devoted to investigating the emission properties of Er3+ at 2.7 μm. However, options for MIR fiber lasers are often restricted by the difficulty in finding glass host with adequate transparency and sufficient robustness. To date, lasing at 2.7 μm has been achieved in ceramics (Y2O3, CaF2, etc.), crystals (YSGG, LiYF4, etc.) as well as nonoxide glasses (e.g., ZBLAN glass) [4–8]. The most developed mid-infrared fiber lasers are based on the RE doped fluoride glass. The highest output power at 2.7 μm ever reported from Er3+ doped ZBLAN fiber laser is 24 W upon the excitation of 975 nm . Although laser oscillation at wavelength as long as 3.9 μm and ultrabroadband supercontinuum spectra from deep-ultraviolet to MIR have been successfully demonstrated in fluoride glasses, they are still not been widely accepted by the industry due to their relatively inferior stability and fragility [9,10]. Chalcogenide glass is another well-known infrared transmitting material, which exhibits favorable properties for RE doped fiber lasing such as high refractive index resulting in large absorption and emission cross-sections and generally low phonon energy for efficient radiative processes. Significant efforts have been made to develop the RE doped chalcogenide glass fiber, but achieving laser emission beyond 1.1 μm is still a great challenge . Recently researchers pay more attention to the multicomponent oxide, oxyfluoride glasses or glass ceramics as MIR host materials [12–15]. However, until now, all of these studies were mainly limited to bulk glass. As an alternative, tellurite glass possesses lower phonon energy and better mechanical property among all of the oxide glass . Moreover, lasers operating at 1.0, 1.5, and 2.0 μm based on the tellurite fibers have been realized in the past decades [17–19]. Therefore, it is extremely essential to extend the working range further into the longer wavelength region in this promising glass host. More recently, Fan et al. investigated an enhanced 2.7 μm emission from the Er3+ doped tellurite bulk glass, but only up-conversion emission was studied from the glass fiber . The barium tellurite is chosen in the present work may possesses outstanding thermal stability against crystallization and high glass transition temperature compared with the typical TeO2-ZnO-Na2O system. Meanwhile, it owns lower phonon energy than the tungsten tellurite glass, which may beneficial for high radiative decay rate of Er3+:4I11/2 → 4I13/2 transition and corresponding 2.7 μm mid-infrared emission.
In this work, high quality Er3+ doped tellurite fibers were designed and fabricated by a convenient suction technique. The electro-probe micro-analyzer (EPMA) images, DSC curves, Raman spectrum were measured to characterize the element distribution and physical properties of the glass and fiber. Broadband 2.7 μm amplified spontaneous emission (ASE) with concurrent 1.5 μm and visible up-conversion emissions were achieved in the Er3+ doped tellurite fiber upon the excitation of 980 nm LD. The dynamic luminescence spectra were also studied for further understanding the competitive relationship among them. The higher spontaneous transition probability, emission cross section, and figure of merit together with outstanding thermal property indicate that 2.7 μm laser output is very expected in this kind of tellurite glass fiber.
Tellurite fibers with a nominal cladding of 82TeO2–10BaF2–5BaO–3La2O3 and core compositions of 80TeO2–10BaF2–5BaO–5La2O3 (in mol%) were prepared from high purity (≥99.99%) starting chemicals using a convenient suction technique. Here we introduced 10 mol% BaF2 to reduce the OH– contents which exist in the glass. This is in favor of reducing the multi-phonon decay rate of Er3+ and realizing efficient 2.7 µm luminescence in the tellurite fibers. According to the literature, the core glass was doped with an additional 2.0 wt.% Er2O3 . The well mixed core (40 g) and cladding (100 g) glass batches were melted in separate vertical cylindrical furnaces at 850 °C for 3 h. The glass melts were kept well stirred by a quartz glass rod for several times to achieve homogeneous mixing during the melting process. Subsequently the melt was cast into a preheated cylindrical copper mold and annealed before they were cooled to room temperature. In order to compare the optical properties of the bulk glass and fiber, the rest of the glass melts were poured onto a stainless plate and then cut and polished. Figure 1 shows the detailed process of suction method for preparing Er3+ doped tellurite glass fibers. The photographs of the annealed glass perform and fibers were also shown in the Figs. 1(d)-1(e). The fiber was arranged into a circle with the diameter of approximately 15 cm showing its excellent flexibility.
Absorption spectra were performed on Perkin-Elmer Lambda 900 UV/VIS/NIR double beam spectrophotometer (Waltham, MA) with a resolution of 1 nm. Infrared transmittance spectra were measured using a Vector 33 Fourier transform infrared (FTIR) spectrophotometer (Bruker, Switzerland). The characteristic temperatures of glass transition and crystallization were carried out by a Netzsch STA 449 C Jupiter Different scanning calorimetery (DSC) at a heating rate of 10 °C/min from 25 °C to 700 °C under N2 atmosphere. The Raman spectra were obtained by a Raman spectrometer (Renishaw in Via, Gloucestershire, UK) and using a 532 nm laser as the excitation source. The element distributions of the fiber were determined by an electro-probe micro-analyzer (EPMA) system (EPMA-1600, Shimadzu, Kyoto, Japan). Emission spectra were employed on computer controlled Triax 320 spectrofluorimeter (Jobin-Yvon Corp.) equipped with a 980 nm LD as exciting source. The visible and NIR emission were measured by detectors equipped with R928 photomultiplier tube (Products for research Inc., Danvers, MA) and InGaAs photodetectors, respectively. A PbSe photodetector assembled with lock-in amplifier (Stanford Research Systems, Sunnyvale, CA) and chopper for MIR emission. Luminescence decay curves were recorded by a Tektronix TDS 3012c Digital Phosphor Oscilloscope with pulsed 808 nm LD. All the measurements were carried out at room temperature.
3. Results and discussions
3.1 Optical absorption, thermal stability, structure, and Judd-Ofelt analysis
Figure 2(a) presents the absorption spectrum of Er3+ doped tellurite glass in the range of 400–1800 nm. The glass sample exhibits Er3+ absorption bands at 1533, 976, 800, 652, 546, 522, 489, and 452 nm corresponding to the typical electronic transitions from 4I15/2 ground state to 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, and 4F5/2 excited states, respectively. The 800 and 980 nm absorption bands of Er3+ coincide with widely available high power LDs. It can be found that the absorption coefficients at these two bands reach to 0.06 and 0.24 cm–1, respectively, which means more efficient pump absorption will be achieved upon excitation of 980 nm LD. Figure 2(b) compares the FTIR absorption coefficient spectra of the core and cladding glasses. Two prominent broad absorption bands centered at 3287 (3042 cm–1) and 4394 nm (2276 cm–1) can be ascribed to the stretching of OH– vibration. The OH– absorption bands in oxide glasses have been classified into free OH– groups (at ~3500 cm–1), strongly hydrogen-bonded OH– groups (at ~2650 cm–1), and very strongly hydrogen-bonded OH– groups (at ~2300 cm–1) . Therefore, it is clear that only the free OH– groups and very strongly hydrogen-bonded OH– groups exist in this kind of tellurite glass. The lower OH– absorption coefficient is expected to be in favor of reducing the multi-phonon decay rate of Er3+ and realizing efficient 2.7 µm luminescence in tellurite fibers.
Thermal stability is one of the most important properties for glasses and fiber drawing. It can be estimated based on the characteristic temperatures including Tg, Tx, and Tp, which are determined from the DSC curves. Parameter △T has been frequently used as an approximate estimation of the glass thermal stability. Generally, a large △T = Tx–Tg above 100 °C will facilitate the fiber drawing. Figure 3(a) displays the DSC curve of tellurite glass. It is found that the △T is much larger than 100 °C, which suggests a wide operating temperature range and excellent glass stability against crystal nucleation and growth during the fiber drawing process. Furthermore, it is noticed that the Tg of the present glass is higher than the classical TeO2–ZnO–NaO (~300 °C) glass but smaller than the TeO2–WO3–La2O3 (~450 °C) glass . The structural units and information of the glass network can be analyzed with the help of Raman spectra, as shown in Fig. 3(b). All of these peaks are ascribed to the vibrations of the coordination polyhedral tellurium . From Fig. 3(b), it can be clearly seen that the largest phonon energy of the present glass only extends to 770 cm–1, which is much lower than that of tungsten tellurite glasses (~920 cm–1) and germanate glasses (~900 cm–1) . In general, the highest phonon frequencies of the matrix should be about 0.2–0.25 times less than the light frequency in order to emit at long wavelengths . For ~3.0 μm fluorescence, the maximum phonon frequency of the host medium should be smaller than 833 cm–1. Therefore, this tellurite glass with smaller phonon energy could reduce the non-radiative relaxation probability of Er3+ efficiently and thus be very conducive to Er3+ 2.7 μm luminescence.
Table 1 lists the basic physical property data of the Er3+ doped tellurite glass. The Er3+ concentration and refractive index were used to evaluate the Judd-Ofelt (J-O) intensity parameters. The non-linear refractive index and third-order nonlinear susceptibility are essential parameters for Raman amplifier and ultra-fast optical switching in optical communication systems . From the absorption spectra and parameters in Table 1, the J-O intensity parameters Ωt (t = 2, 4, 6), spontaneous transition probabilities A, radiative lifetimes τrad, and fluorescence branching ratios β of the related transitions are calculated. The detailed procedure and matrix elements have been described elsewhere [24,25]. In this process, six absorption bands 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, and 4F7/2 were used for the calculation. Table 2 shows the J-O intensity parameters of Er3+ doped tellurite glass in comparison with other glass systems [26–29]. The root-mean-square deviation δrms in this case is 0.26 × 10−6, indicating the results are reliable. Larger Ω2 in the tellurite glass means it has higher asymmetry and covalent environment around Er3+ ions than that of fluoride, fluorophosphate, and germanate glasses. In addition, it is found that the studied glass matrix possesses a larger spectroscopic quality factor of Ω4/Ω6. Larger Ω4/Ω6 is favorable for stimulated emission in a laser active medium . The spontaneous emission probabilities, branching ratios, and radiative lifetimes of several levels are presented in Table 3. It is noted that the spontaneous radiative probability for the 4I11/2 → 4I13/2 transition is 50.84 s–1, which is much higher than that of ZBLAY (28.92 s–1) and similar with chalcogenide glass (48.40 s–1) [26,31].
3.2 Element distribution and transmission loss of the fiber
Figure 4 shows the optical micrograph and EPMA measurement of the tellurite fiber cross section. The diameters of fiber core and cladding are about 60.5 and 254.6 μm, respectively. The boundary between them is well defined. EPMA measurement was performed to describe the element distribution in the core and cladding after a reheating process of fiber drawing. From Fig. 4(c-e), it can be found that the relative concentration of Te and La show an abrupt change due to the different concentration in the core and cladding, as designed in the Experimental section. Meanwhile, the Er3+ ions are uniformly distributed in the core and hardly diffuse into the cladding. The distribution boundary of each element forms a circle which is of the similar size to the fiber core shown in Fig. 4(b). These results indicate that the fiber structure is preserved completely and no obvious elements internal diffusion between the core and cladding occurs during the whole fiber fabrication.
The measured refractive indices of the core and cladding glasses have been given in Table 1. The experimental data are used to determine the coefficients in the Sellmeier dispersion equation by using a least-squares fitting code to establish S and λ0 in the following expression :
The Sellmeier coefficients S and λ0 are calculated to be 2.940 and 174.8 nm for the core glass, and 2.916 and 178.0 nm for the cladding glass. Based on the fitted Sellmeier parameters, a continuous refractive indices data as a function of wavelength was obtained, as shown in Fig. 5(a). The numerical aperture (NA) of the fiber can be determined through Eq. (2) :Figure 5(b) shows the cutback measurement of Er3+ doped tellurite fiber. The transmission loss of the fiber at 1310 nm was measured to be 3.46 dB/m, which is lower than that of Tm3+ doped germanate (5.00 dB/m) and silicate glasses (7.00 dB/m) fabricated by rod-in-tube or stacking methods [34,35]. The loss of the fiber mainly includes the absorption and scattering caused by impurities such as OH–, transition metals and so on, which may introduced by the raw materials and ambient environment during the glass melting and fiber fabrication process.
3.3 ASE spectra and luminescence decay dynamics of the Er3+ doped tellurite fiber
Figure 6(a) compares the ASE spectra of Er3+ doped tellurite fibers with various lengths upon excitation of 980 nm LD. An intense emission peak at 2720 nm with a full width at half maximum (FWHM) of 112 nm originated from Er3+: 4I11/2 → 4I13/2 transition is obtained in the fibers. With the increment of the fiber length, the 2.7 μm emission band increases gradually and reaches to a maximum value at around 6 cm, and then decreases due to the enhanced energy transfer toward unidentified impurities and fiber loss. At the same time, it can be found that the shape of 2.7 μm emission becomes sharper and asymmetry. The intensity of 1.5 μm emission shows a similar tends, as shown in Fig. 6(b). Unlike the 2.7 and 1.5 μm emissions, the visible up-conversion emissions at 528, 546, and 663 nm keep gradually increasing all the time, corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions through energy transfer up-conversion processes (ETU1: 4I11/2 + 4I11/2 → 4F7/2 + 4I15/2 and ETU2: 4I13/2 + 4I13/2 → 4I9/2 + 4I15/2). The 980 nm emission of Er3+ is also studied upon excitation of 808 nm LD in order to get the luminescent decay dynamics of 4I11/2 level, as shown in Fig. 6(d). Overall this emission exhibits a red shift from 978 to 990 nm as the length of the fiber increases, originating from the radiation trapping where light is absorbed from the ground state to the 4I11/2 level and then reemitted.
Figure 7(a) illustrates the luminescence decay dynamics of Er3+ upon excitations of 808 and 980 nm LDs. All of these decay curves of Er3+ exhibit a single-exponential decay. The calculated luminescent lifetimes of Er3+ are displayed in Fig. 7(b). The lifetimes of 4I13/2 level prolong significantly with increasing fiber length, which may be due to radiative trapping effect . In sharp contrast, the 4I11/2, 4F9/2, 4S3/2 levels keep a slight variation. Meanwhile, it is found that the lifetime of 4I11/2 upper level is much shorter than that of 4I13/2 lower level and still not overcome the bottleneck. The shorter lifetime is not necessary for laser operation but beneficial to the obtainment of high performance MIR fiber lasers .
Figure 8(a–b) illustrates the absorption and emission cross sections around the 2.7 and 1.5 μm in Er3+ doped tellurite glass. The absorption and emission cross sections of Er3+ at 2.7 μm in the present glass are 0.38 × 10−20 and 0.79 × 10−20 cm–2, respectively. It is comparable with the values of ZBLAN glass (0.65 × 10−20 and 0.98 × 10−20 cm–2), chalcogenide (0.60 × 10−20 and 0.66 × 10−20 cm–2), but smaller than the germanate glasses (1.06 × 10−20 and 1.24 × 10−20 cm–2) [25,30,37]. The gain cross coefficient of Er3+ can be determined by the following Eq :Fig. 8(c-d). The maximum gain coefficients at 2.7 and 1.5 μm are as high as 2.85 and 2.31 cm–1, respectively. The gain coefficients become positive once the population inversion levels reach to 40% and 20%, respectively, indicating that a low pumping threshold can be achieved for the Er3+: 4I11/2 → 4I13/2 and 4I13/2 → 4I15/2 transitions laser operation in Er3+ doped tellurite fiber. For a population inversion level of 0.6, it is possible to obtain smooth tuning in rather wide spectral region 2600–2800 nm. The tunable emission properties of Er3+ doped tellurite glasses show potential applications in tunable lasers around 2.7 μm.
3.4 Multi-phonon relaxation, rate equation, and energy transfer mechanism
Multi-phonon relaxation rates (MPR, Wmp) are one of the most important properties that determine the lifetime and quantum efficiency of RE ions. MPR rates were calculated from the measured (τm) and calculated (τrad) lifetimes of several energy states in RE ions using Eqs. (4-5) :Figure 9(a) displays the dependence of the MPR on the energy gap for the present glass using Eqs. (4-5). With a phonon energy of 770 cm–1 from the Raman spectrum we obtained values of 5.83 × 105 s–1 and 1.35 × 10−3 cm for the parameters W0 and α, which are significantly different from the values of 6.3 × 1010 s–1 and 4.7 × 10−3 cm published by Reisfeld and Eyal . The error in the determination of the MPR would be a measured lifetime which is shorted by ion-ion interactions giving a higher rate than the actual one. Then the electron-phonon coupling constant γ can be determined by the following Eq :38].
For quantitatively understanding the ETU2 process, energy transfer coefficient (CETU2) can be computed by the rate equation [12,14,37]:12,14,37]:Eq. (8) in the steady-state condition can be solved as [12,14,37]:Fig. 9(b), which can be expressed as [14,37]:
Combine with Eqs. (9-10), we calculate a CETU2 of 0.14 × 10−13 cm3·s–1. The fitting procedure of ETU1 is similar to the ETU2 process, and the CETU1 equals to 0.66 × 10−16 cm3·s–1. It is obvious that the coefficient of CETU2 is about 212 times greater than CETU1. Larger CETU2 is more conducive to the population inversion between the 4I11/2 and 4I13/2 levels.
Energy transfer parameters can be determined according to the absorption and emission cross sections of Er3+. For the dipole-dipole interaction, when phonon-assistance is taken into account, the energy transfer parameter (CDA) can be estimated by the Eq :Fig. 8. The well overlapped between them suggesting efficient energy transfers of 4I13/2 → 4I13/2 transition. The CDA of 4I13/2 level is calculated to be 2.32 × 10−39 cm6·s–1. All of these basic spectral parameters of Er3+ doped tellurite glass are summarized in Table 4.
Based on the discussions mentioned above, the mechanism of the infrared and visible emissions can be explained by the simplified energy level diagram in Fig. 10. Upon excitation of 980 nm LD, the ions in Er3+: 4I15/2 level can be excited to the 4I11/2 by ground state absorption process. After that, the ions in 4I11/2 level decay to 4I13/2 level and generate 2.7 μm emission. Finally, ions in 4I13/2 state relax to the ground state and 1.5 μm emission occurs. One the other hand, the 4F7/2 level can be populated due to the energy transfer up-conversion (ETU1: 4I11/2 + 4I11/2 → 4F7/2 + 4I15/2) from 4I11/2 level. Because of the small energy gaps among 4F7/2, 2H11/2, 4S3/2, and 4F9/2 levels, ions in 4F7/2 state decay nonradiatively to the excited states below it. Then green (528 and 546 nm) and red (663 nm) emissions take place via 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions, respectively. Besides, the ions in 4I13/2 level can also experience an ETU2 process (4I13/2 + 4I13/2 → 4I9/2 + 4I15/2), which is very favorable to population inversion between the 4I11/2 and 4I13/2 levels.
In summary, Er3+ doped tellurite fibers have been prepared using a convenient suction technique. The transmission loss of the fiber at 1310 nm is measured to be 3.46 dB/ m by the cutback method. Upon excitation of 980 nm LD, intense 2.7 μm amplified spontaneous emission from Er3+ doped tellurite fiber is observed with various fiber lengths. The higher spontaneous transition probability (50.84 s–1), emission cross section (0.79 × 10−20 cm–2), and figure of merit (3.18 × 10−24 cm–2·s) give evidence of intense 2.7 μm emission. The excellent thermal stability (△T = 146 °C), lower OH– coefficients (less than 1 cm–1) and phonon energy (about 770 cm–1), as well as the desirable spectroscopic characteristics indicate that the Er3+ doped tellurite fiber is a promising candidate for 2.7 μm fiber lasers and amplifiers.
This work is financially supported by National Natural Science Foundation of China (NSFC) (Grant Nos. 51125005, 51472088 and 51302086), and the Fundamental Research Funds for the Central Universities, SCUT.
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