Er3+-doped K-Ca-Al fluorophosphate glasses were prepared by melt quenching technique and their thermal and optical properties were studied. The thermal stability factor was obtained to be 131 °C. The gain bandwidth, lifetime and quantum efficiency of the 4I13/2→4I15/2 transition were found to be 96.14 × 10−27 cm3, 5.41 ms and 61%, respectively for 1.0 mol% Er2O3-doped glass. The quenching concentration has been evaluated (Q = 2.30 mol%) and found to be higher compared to other reported glasses. These results clearly indicate that present glasses are suitable for laser as well as optical amplifiers in the 1.53 μm region.
© 2015 Optical Society of America
Now-a-days trivalent rare-earth (RE3+)-doped materials have attracted much attention to develop optical devices such as solid state lasers, optical sensors and optical amplifiers to meet the incessant demands in basic science research, medicine, and telecommunications. For the past decades, the near-infrared (NIR) emission of the trivalent erbium (Er3+) ion at around 1.53 μm corresponding to 4I13/2→4I15/2 transition has been used in telecommunication for optical amplification applications [1,2 ]. Present commercial erbium doped fiber amplifiers (EDFA) are made of silica glass, exhibiting a relatively narrow bandwidth (∼35 nm) that limits broadband transmission . Therefore, it is necessary to search for a new host matrix for Er3+ ions so as to exhibit relatively higher bandwidth and longer lifetimes for optical amplification in telecommunication. For this, glasses doped with RE3+ ions are attractive due to their ease and low cost of fabrication, good mechanical and thermal stability, and relatively high RE3+ ion solubility. However, the selection of glass composition with desirable optical properties often is a challenging task. One of the crucial parameters for the development of RE3+ based systems is the concentration quenching that is mainly dependent on glass composition. Hence, rigorous search is needed to find suitable glass composition with good thermal stability, chemical durability and superior optical properties for specific applications.
Phosphate based multicomponent glasses have received enormous interest because of their high gain due to high RE3+ ion solubility. Moreover, addition of fluoride content to these phosphate glasses not only brings the advantages of both fluoride and phosphate glasses, such as excellent transparency from UV to IR, good thermal stability and chemical durability, etc., but also reduces the OH presence which plays a vital role in the quenching of the radiative emission of excited levels of Er3+ ions. In fact, an efficient coupling is possible between the fundamental stretching vibration of OH groups (2500-3600 cm−1) and energy gap between the 4I13/2 level and 4I15/2 ground level (around 6600 cm−1) [3,4 ]. In the present glasses, the OH quenching effect is expected to be reduced due to addition of fluoride (∼650 cm−1), thus it may enhance the lifetime value of the corresponding 4I13/2 → 4I15/2 transition.
In the present work, optical properties of Er3+-doped fluorophosphate glasses (P2O5 + K2O + Al2O3 + CaO + CaF2) for different Er2O3 concentrations, have been studied using the Judd-Ofelt (JO) theory [5,6 ]. The radiative properties for the luminescent levels of Er3+ ion, have been obtained from the absorption spectra of 1.0 mol% of Er2O3-doped glass. The luminescence properties of 4I13/2 → 4I15/2 transition have been derived from the emission spectra. Decay curves for the 4I13/2 level of Er3+ ion were measured and analyzed for the determination of lifetime.
2. 2. Experimental details
2.1. Glass preparation
The fluorophosphate glasses with the composition (mol%) of 41 P2O5 + 17 K2O + (10-x) CaO + 8 Al2O3 + 24 CaF2 + x Er2O3, where x = 0.1, 0.5, 1.0, and 2.0, referred to as PKCFAEr01, PKCFAEr05, PKCFAEr10, and PKCFAEr20, respectively, were prepared by conventional melt quenching technique. High-purity reagent grade oxide and fluoride chemicals of Al(PO3 ) 3, KPO3, CaCO3, CaF2, and Er2O3 were used as starting materials to prepare the glasses. These precursor oxides and fluorides were thoroughly mixed using an agate mortar and the homogeneous mixture was transferred into a platinum crucible and kept in an electric furnace and then heated at around 1100-1200 °C for 1 hour and then the melt was cast onto a preheated brass mold at around 400 °C. The obtained glasses were annealed at around 450 °C for 12 hour to remove strain and stress. Finally, the prepared glasses were cut and polished with alumina and cerium oxide powders to achieve optical quality for optical measurements. The dimensions of the glasses were 11.78 × 9.70 × 3.43 mm, 11.83 × 9.50 × 3.74 mm, 11.50 × 9.90 × 3.54 mm, and 11.95 × 9.89 × 3.57 mm for the PKCFAEr01, PKCFAEr05, PKCFAEr10, and PKCFAEr20 glasses, respectively.
2.2. Characterization techniques
A prism coupler system (Metricon model 2010/M) was used to measure the linear refractive index ( ± 0.001) of the PKCFAEr glasses at 632.8 nm. The density was determined by the Archimedes’ method with water as an immersion liquid. Differential scanning calorimetry (DSC, SDT Q600) measurement was carried out in the range of 0 °C to 700 °C with alumina pans without sealing. Absorption spectra (300-1800 nm) were measured using a Carry Series UV-Vis-NIR spectrophotometer. The visible emission spectra in the range of 500-700 nm were measured using an Ar+ ion laser (488 nm) as the pump and with a photomultiplier tube (PMT, model PD-471) as the detector. The near infrared emission (NIR) spectra were recorded by exciting the samples at 980 nm with a continuous wave laser diode (CWLD). The CWLD was focused onto a sample with a 5 cm focal-length lens. The emitted signal was focused onto a PC-controlled SP-2357 spectrograph (Acton Research) and detected by an InGaAs detector (Thorlabs DET10C). The system was controlled with a PC where emission spectra were obtained. The decay curves were measured by exciting the samples with a laser diode (at 980 nm) with pulse width of 20 ms and frequency of 10 Hz and detected with an InGaAs detector and a digital oscilloscope. All the measurements were carried out at room temperature.
3. Results and discussion
3.1. Physical and thermal properties
The physical properties that include refractive index, density, concentration, polaron radius, interionic distance, field strength, reflection loss, dielectric constant, molar volume, molar refractivity, and electronic polarizability, were calculated for the present PKCFAEr glasses with varying Er3+ ion concentration by the procedure described elsewhere , and are presented in Table 1 .
The thermal stability of a glass sample plays a significant role in the fabrication of optical fiber. Since fiber drawing is a reheating process, any crystallization during this process will increase the scattering loss of the fiber and then degrade its optical performance . The thermal stability (ΔT) is generally characterized by difference of glass transition temperature (Tg) and onset of crystallization temperature (Tx) i.e., ΔT = Tx-Tg. Therefore, it is desirable for a glass host to have ΔT as large as possible to draw the fiber. Figure 1 shows the DSC thermograph for the present host glass composition. The Tg, Tx and ΔT for the present host composition were obtained to be 504, 635 and 131 °C, respectively and compared to the other host matrices presented in Table 2 . As can be seen from Table 2, the ΔT of the present host composition (131 °C) was found to be larger than that of fluorophosphate , borate , borophosphate , and silicate , but smaller than that of phosphate , germanate , and antimony  glasses. It is reported that glasses exhibiting ΔT >100 °C can satisfy the requirement of a conventional fiber drawing process [16,17 ]. Hence, the prepared glasses are thermally stable and may be suitable for fiber drawing and other device applications.
3.2. Absorption spectra: Judd-Ofelt theory
Figure 2 shows the absorption spectra of the 1.0 mol% Er2O3-doped glass in the range of 300-1850 nm. As can be seen, 13 absorption bands were observed, corresponding to the transitions from the ground state 4I15/2 to the various excited states that are 4I13/2, 4I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, 4F3/2, 2G9/2, 4G11/2, 4G9/2, and 2K15/2 of the Er3+ion, respectively. The oscillator strengths, JO intensity parameters, radiative transition probabilities, branching ratios, and radiative lifetimes of different transitions of the Er3+ ion can be predicted from the absorption spectra using the JO theory [5,6 ].
The experimental oscillator strengths () for each absorption band, can be calculated by the following expression:
The JO parameters have been evaluated by a least square fitting approach using Eq. (2) and experimental oscillator strengths (fexp). The obtained JO parameters in turn were used to get calculated oscillator strengths (fcal). The fexp and fcal of the various absorption bands of the PKCFAEr10 glass along with the JO parameters are shown in Table 3 . Root-mean-square deviation (σrms) was used to know the quality of the fitting between the experimental and calculated oscillator strengths. The small value of σrms ( ± 0.59 × 10−6) indicates a good agreement between the experimental and calculated oscillator strengths. Some transitions are very sensitive to small changes of environment around RE3+ ions and are called hypersensitive transitions (HSTs), which follow the selection rules, |ΔS| = 0, |ΔL| ≤ 2 and |ΔJ| ≤ 2. The 4I15/2→4G11/2 and 4I15/2→2H11/2 absorption transitions are considered as hypersensitive transitions. As shown in Table 3, it is interesting to note that oscillator strengths of these HSTs are larger than those of the other transitions, indicating higher asymmetry in the vicinity of Er3+ ions in the present glass composition.
The JO parameters Ωλ (λ = 2, 4 and 6) are important for investigation of bonding nature in the vicinity of RE3+ ions . The obtained JO parameters for the PKCFAEr10 glass were Ω2 = 9.15, Ω4 = 3.59 and Ω6 = 0.76 ( × 10−20 cm2). Table 4 shows the JO parameters and their trend in various Er3+-doped systems that include fluorophosphate , borate , borophosphate , silicate , phosphate , germanate , antimony , 58.5P2O5-17K2O-14.5BaO-9.0Al2O3-1.0Er2O3 (phosphate) , 84TeO2-5.0WO3-10CdO- 1.0Er2O3 (tellurite) , 1.5P2O5-10K2O-15.5SrO-12Al2O3-1.0Er2O3 (phosphate) , 50(NaPO3)6-10TeO2-20AlF3-19KF-1.0Er2O3 (fluorophosphate) , 20Sr(PO3)2-80[MgF2,CaF2,SrF2,AlF3,ErF3,YbF3] (fluoride phosphate) , and 76TeO2-10ZnO-9.0PbO-1.0PbF2-3.0Na2O-1.0Er2O3 (tellurite)  glasses. Of these parameters, Ω2 is sensitive to the local environment of RE3+ ions and is often related to the asymmetry of the coordination structure, the polarizability of ligand ions, and the nature of bonding. Ω6 is inversely proportional to the covalency of the Er-O bonds which can be adjusted by the composition or structure of the glass host, whereas Ω4 depends on bulk properties such as viscosity and rigidity of the glass matrix. As shown in Table 4, the present glass exhibits higher covalency between Er-O bonds and asymmetry around the Er3+ ions when compared to those of the previously reported glasses [9–15,19–24 ]. On the other hand from the values of Ω4, the present glass was rigid compared to the previously reported glasses [9–15,19–24 ]. It is also noticed that the variation in trends of the JO parameters arises due to the large and more sensitive values of the oscillator strengths of the two HSTs (4I15/2 → 4G11/2 and 4I15/2 → 2H11/2) (Table 3). The variation of JO parameters Ω2, Ω4 and Ω6 due to changes in the host composition have been studied by Takebe et al.  for Er3+-doped silicate, borate and phosphate glasses. Therefore, different reported host matrices have been chosen to see the effect of JO parameters on radiative properties. The concentration dependence of JO intensity parameters has also been reported and revealed that they are less sensitive to the active ion concentration but depend only on the glass composition [26,27 ].
Once the JO parameters have been obtained, they can be used to derive the spontaneous emission probabilities, radiative lifetimes, and branching ratios . The spontaneous emission probability (A) of an electric-dipole transition is given by the following equation:
A and τrad for the 4I13/2 → 4I15/2 transition were found to be 112.65 s−1 and 8.88 ms, respectively. The τrad for the 4I13/2 level of Er3+ ion for the PKCFAEr10 glass was also shown in Table 4 along with the other Er3+-doped glasses [9–15,19–24 ]. It is noted that τrad is found to be higher for the present glass than those of the other Er3+-doped glasses except for fluorophosphate glass  (Table 4). Higher τrad is essential to achieve population inversion. Hence, the present PKCFAEr10 glass can be used as optical amplifier at 1.53 μm region.
3.3. Luminescence properties
Figure 3 shows the NIR luminescence spectra for all the samples with varying Er2O3 concentration, obtained with 980 nm diode laser excitation. The emission spectra consist of the characteristic band at around 1534 nm attributed to the 4I13/2 → 4I15/2 transition. As seen from the figure, the intensity of the emission band increased with increasing Er2O3 concentration for all the studied glasses. The full width at half maximum (FWHM), peak stimulated emission cross-section (σe(λ)) and gain bandwidth (σe(λ) × FWHM) were determined for the 4I13/2→4I15/2 transition of the PKCFAEr10 glass. The σe(λ) has been calculated for the 4I13/2→ 4I15/2 transition using the McCumber relation given by Fig. 4 .
The obtained values of the FWHM, σe(λ) and σe(λ) × FWHM of the 4I13/2→4I15/2 transition for the PKCFAEr10 glass, are presented in Table 5 along with those of the reported Er3+:glasses such as fluorophosphate , borate , silicate , phosphate , antimony , phosphate , tellurite , phosphate , fluorophosphate , fluoride phosphate , tellurite , 35AlF3-8RF2(R = Mg,Ca,Sr,Ba)-9YF3-5Al(PO3)3-7KF-6ErF3 (fluorophosphate) , and 29.8AlF3-3.5MgF2-19.8CaF2-10.9SrF2-12.8BaF2-8.4YF3-9.8ZrF4-4.0NaPO3-1.0ErF3 (fluoride) . As shown in Table 5, it is interesting to note that the present PKCFAEr10 glass possesses higher σe(λ) than those of the reported Er3+-doped glasses [9,10,12,13,15,19–24,30 ]. It is known that the transition with large σe(λ) exhibits low threshold and high gain laser operation. Hence, the PKCFAEr glasses with relatively larger σe(λ) values are better hosts for broadband amplifiers and laser applications in the NIR region. Both FWHM and σe(λ) are very important parameters to achieve broadband and high gain amplification. In fact, the gain bandwidth of an amplifier can be estimated from the σe(λ) × FWHM product, so that the larger the product, the larger the gain bandwidth is. From Table 5, it can be seen that the gain bandwidth obtained for the present PKCFAEr10 glass was larger than the previously reported Er3+:glasses [9,10,12,13,15,19–24,30 ]. The variation of the σe(λ) and σe(λ) × FWHM in different Er3+-doped glasses including the present glass is also shown in Fig. 5 . The figure of merit given by σe(λ) × τexp is one of the desirable parameters for lasers and optical amplifiers and should be as large as possible to attain high gain. In the present study, it was found to be larger compared to the previously reported glasses [9,10,15,19,21–23,30 ] (Table 5).
In the case of the 4I13/2→4I15/2 transition of Er3+, due to the difference of total angular momentum ΔJ equals 1, there exists a contribution of the magnetic-dipole transition Smd . Consequently, in order to get broadband and flat 1.5 μm emission spectra, it is effective to increase the contribution of the electric-dipole transition Sed . Because Smd is independent of the ligand field, but Sed is a function of ligand fields and should be improved by modifying the structure and matrix composition [5,6,33 ]. According to JO theory, the line strength of Sed of the 1.53 μm corresponding to 4I13/2→4I15/2 transition of Er3+ is given by 33]. For the 4I13/2 → 4I15/2 transition of the PKCFAEr10 glass, the calculated values of the Smd and Sed were 69 × 10−22 cm2 and 168 × 10−22 cm2, respectively. The calculated line strength ratio Sed/(Sed + Smd) was found to be 0.71. This indicates that the present glasses are beneficial as a gain host for broadband fiber amplifier as we have got the adequate Ω6 parameter in the present glass system (see Table 4).
The visible emission spectra were obtained for all the samples for different Er2O3 concentrations under 488 nm laser excitation in the range of 500-700 nm and are shown in Fig. 6 . Two strong green bands in the spectral regions 515-538 and 538-571 nm corresponding to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively and the weaker red band in the region 644–677 nm corresponding to the 4F9/2 → 4I15/2 transition, were observed. It was noticed that with the increase in concentration of Er3+ ions from 0.1 to 1.0 mol%, there was a continuous increase of luminescence intensity and then it slightly decreased with further increase in concentration (> 1.0 mol%). The spontaneous transition probability, radiative lifetime and branching ratio for the 4S3/2 → 4I15/2 transition of the Er3+ ion, were obtained using the JO theory and were found to be 609 s−1, 1069 µs and 65%, respectively. The σe(λ) for the 4S3/2 → 4I15/2 transition can be calculated from the Fuchtbauer-Ladenburg (FL) formula given by 
Gain cross-section spectrum can be obtained as a function of the population inversion using the relation given by Figure 7 shows the gain cross-section spectra of the PKCFAEr10 glass as a function of the population inversion. The gain will be positive when the population inversion is larger than 0.4. Thus, it can be concluded that for a normal population inversion above 40%, the sample has a flat gain bandwidth in the range of 1508-1586 nm, which covers the C band (1530–1565 nm) in the optical communication window.
Longer lifetime and high bandwidth of the metastable state are essential for Er3+-doped fiber amplifiers in optical communications since longer lifetime permits the required high population inversion to have high and broad amplification. The decay curves for the 4I13/2 level of Er3+ ion have been obtained upon 980 nm diode laser excitation for all the studied glasses with different Er2O3 concentrations and are depicted in Fig. 8 . It is interesting to note that all the decay curves exhibited single exponential nature irrespective to the Er2O3 concentration. Hence, the experimental lifetimes (τexp) of the 4I13/2 level have been determined by single exponential fitting and were found to be 7.21, 5.83, 5.41, and 3.95 ms for 0.1, 0.5, 1.0, and 2.0 mol% of the Er2O3-doped glasses, respectively. It was found that with the increase of Er3+ concentration, the lifetime of the 4I13/2 level decreased. The shortening of lifetimes of the 4I13/2 level at higher Er3+ concentration was likely due to the enhancement of non-radiative energy transfer processes through energy diffusion . Table 5 tabulates the lifetime of the PKCFAEr10 glass along with the reported glasses [9–11,13–15,19,21–23,30,31 ]. It can be seen that longer lifetime has been noticed for present glass compared to Er3+-doped borate , borophosphate , phosphate , germanate , antimony , phosphate  and phosphate  glasses except for fluorophosphate , fluoride phosphate , fluorophosphate , and fluoride  glasses. Hence, the present glasses are suitable hosts for the laser as well as optical amplifiers at 1.53 μm.
The fluorescence lifetime of the 4I13/2 level with increasing Er2O3 concentration, obeys the following empirical formula [37,38 ] given byFigure 9 shows the measured luminescence lifetime of the 4I13/2 level for the studied glasses as a function of Er2O3 concentration. As can be seen, the observed luminescence lifetime decreased with increasing dopant concentration.
Fitting the experimental data with Eq. (10), the quenching curve (Fig. 9) yields the following parameters: τ0 = 7628 µs, Q = 2.30 mol% and p = 0.82. The quenching concentrations in Er-doped glasses have been reported by Orignac et al.  (τ0 = 6445 µs, Q = 0.62 at.% and p = 1.27) and Goncalves et al.  (τ0 = 6.8 ms, Q = 0.81 mol% and p = 1.3). It is interesting to note that the present glass system exhibits a higher quenching concentration than those of the previously reported Er3+-doped glasses [37,38 ]. Therefore, the optimum Er2O3 concentration is 2.30 mol%.
The luminescence quantum efficiency (η) can be determined using the equation given by
The η values for the 4I13/2 level of Er3+ ions in the PKCFAEr01, PKCFAEr05, PKCFAEr10, and PKCFAEr20 glasses were determined to be 81, 66, 61, and 44%, respectively as shown in Fig. 9. The decrease in η with the increase in Er2O3 concentration may be due to energy transfer among Er3+ ions and from Er3+ ions to luminescence quenching centers.
Er3+-doped K-Al-Ca fluorophosphate glasses have been prepared and characterized by DSC, optical absorption, luminescence, and decay studies. The present glasses are thermally stable against crystallization/devitrification. The intensity parameters and radiative properties for the important luminescent levels of Er3+ ion have been determined using the JO theory for 1.0 mol% Er2O3-doped glass. The larger Ω2 and the smaller Ω6 parameters of the present glass indicate the larger degree of covalency of the Er-O bond and/or asymmetry of the Er3+ sites. The peak stimulated emission cross-section, gain bandwidth, and figure of merit for the 4I13/2→4I15/2 transition of the present glasses were found to be larger than those of the previously reported glasses. The lifetime for the 4I13/2 level was adequate when compared to that of the previously reported glasses. The quenching concentration (Q = 2.30 mol%) was found to be higher compared to the reported glasses. These results make these glasses attractive for laser and broadband amplifiers at 1.53 μm region.
Dr. Venkatramu is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi for the sanction of major research project (No. 03(1229)/12/EMR-II, dated:16th April, 2012). Prof. C.K. Jayasankar acknowledges the support from the DST, Govt. of India for the sanction of Indo-Mexican Joint Research Project No.DST/INT/MEXICO/P-102012. This work was partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2013R1A1A2063250), the Korea government (MSIP) (No. 2011-0031840) and the Brain Korea-21 Plus Information Technology Project through a grant provided by the Gwangju Institute of Science and Technology, South Korea.
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