1. Introduction

Wideband optical fiber amplifiers are essential for increasing the scale and performance of communications systems [1,2]. Among optical fiber amplifiers, Pr³⁺-doped fluoride fibers are used as the gain media for realizing O-band optical amplifiers in which a 1017 nm laser diode is used as the pump source, since the transition ¹G₄→³H₅ of Pr³⁺ can produce O-band emissions and low phonon energy of fluoride fibers is required for reducing the non-radiative transitions from the ¹G₄ level to the neighbored levels [3]. At present, Pr³⁺-doped fluoride fiber amplifiers have already been commercial and widely used in optical transmission systems [4,5]. Recently, much efforts have been devoted to further expand the bandwidth of Pr³⁺-doped optical amplifiers. In 2011, B. Zhou et al. reported broadband near-infrared emission ranging from 1250 to 1680 nm in Pr³⁺-doped bismuth gallate glasses excited at 488 nm or 595 nm, in which the transitions ¹D₂→¹G₄ and ¹G₄→³H₅ of Pr³⁺ produce such broadband emissions [6]. In 2012, B. Zhou et al. demonstrated broadband emission extending from 1.3 to 1.67 µm in Pr³⁺-doped fluorotellurite glasses under 488 nm or 590 nm excitation [7]. In 2016, J. Pisarska et al. investigated influence of BaF₂ and activator concentration on broadband near-infrared luminescence of Pr³⁺ ions in gallo-germanate glasses for achieving broadband near-infrared amplifiers [8]. Although broadband near-infrared emission can be obtained by using the transitions ¹D₂→¹G₄ and ¹G₄→³H₅ of Pr³⁺, the efficiency of the transition ¹G₄→³H₅ is largely affected by the phonon energy of the matrix since the energy difference between the ¹G₄ level to the neighbored state ³F₄ is just 3000 cm^-1 and the nonradiative transition between them can quench the O-band emission [1]. It indicates that efficient broadband near-infrared emission might be obtained in Pr³⁺-doped fluoride glasses with low phonon energy. Among fluoride glasses, ZnF₂-based glasses have the relatively low phonon energy (∼ 420 cm^-1) [9]. However, until now, broadband near-infrared emission properties in Pr³⁺-doped ZnF₂-based glasses have not yet been investigated, and the effects of the phonon energy of the matrix on near-infrared emission of Pr³⁺ were also not clarified.

In this paper, we reported intense O + E + S-band emission in Pr³⁺-doped ZnF₂-based glasses and the effects of the phonon energy of the matrix on the O + E + S-band emission of Pr³⁺. Pr³⁺ doped ZnF₂-based glasses were prepared by using a melt-quenching method in a glove box filled with dry N₂. With the excitation of a 588 nm LED, broadband emissions from 1245 to 1640 nm were observed in the glasses, which originate from the transitions ¹D₂→¹G₄ and ¹G₄→³H₅ of Pr³⁺. With an increase of the doping concentration of Pr³⁺, the emissions in E + S-band were quenched by the cross relaxation process [¹D₂, ³H₄]→[¹G₄,³F₄,₃], and the emission spectrum with a full width at half maximum (FWHM) of 215 nm (1289 nm-1504 nm) was obtained in the glass at a doping concentration of 5000 ppm. In addition, a comparison of emission spectra of Pr³⁺-doped ZnF₂-based (the phonon energy: ∼ 420 cm^-1), InF₃-based (∼ 507 cm^-1) and ZrF₄-based (∼ 579 cm^-1) glasses at a same doping concentration of 5000 ppm was given, the emissions in O-band increased with a decrease of the phonon energy of the glass due to the reduction of the nonradiative transition from ¹G₄ level to the neighbored state ³F₄, which is beneficial for achieving broadband O + E + S-band emission. The above results indicated that Pr³⁺-doped ZnF₂-based glasses might be used as the gain medium for constructing O + E + S-band lasers and optical amplifiers.

2. Experiments and results

In our experiments, Pr³⁺-doped ZnF₂-based glasses with a composition of 60ZnF₂-15BaF₂-10SrF₂-(15-x)YF₃-xPrF₃ (x = 0, 0.1, 0.3, 0.5, 0.7, 1, 2 and 3, respectively) were fabricated by using a conventional melt-quenching method in a glove box filled with dry N₂. The detailed procedures could be seen in [10]. The starting materials were high-purity (99.99%) anhydrous fluorides. The mixed batches were melted in a platinum crucible at 950 °C for 2 h. Then the glass melts were injected into a copper mold and annealed near the glass transition temperature for 5 h. The glass samples were cut and polished to 10 × 5 × 0.43 mm for optical measurements. By using a Seiko TG/DTA6200 analyzer, the transition temperature (T_g) and the onset crystallization temperature (T_x) of the ZnF₂-based glass were measured to be ∼315 and ∼369 °C, respectively.

Figure 1 shows the transmission spectrum of Pr³⁺-doped ZnF₂-based glass at a doping concentration of 30000 ppm (or 3 mol%) measured with a Shimadzu UV3600 spectrometer (180-2500 nm) and Nicolet 6700 FTIR spectrophotometer (2500-14000 nm). ZnF₂-based glass has a transmission window from 200 to 12000nm, which is wider than that of InF₂-based, ZrF₄-based and AlF₃-based glasses, due to the relatively low phonon energy (∼ 420 cm^-1) of ZnF₂-based glass. The absorption peaks at 442, 467, 480, 588, 1015, 1442, 1535, 1942, 2250 and 4705 nm are caused by the transitions from the ground level ³H₄ to highly excited levels ³P₂, ¹I₆, ³P₀, ¹D₂, ¹G₄, ³F₄, ³F₃, ³F₂, ³H₆ and ³H₅ of Pr³⁺ ions, respectively [11]. In addition, no obvious hydroxyl (OH⁻) absorption at ∼3 µm was observed in the transmission spectrum of the ZnF₂-based glass, which showed a quite low content of residual OH⁻ absorption in the ZnF₂-based glass. Such a relatively low OH⁻ content is good for obtaining efficient near-infrared emissions from Pr³⁺-doped ZnF₂-based glasses since the existence of OH⁻ may quench near-infrared emissions of Pr³⁺ ions significantly.

 

Fig. 1. Optical transmittance spectrum of Pr³⁺-doped ZnF₂-based glass (30000 ppm). Inset shows the detail of the transmittance bands from 320 to 5600 nm.

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Figure 2 shows the experimental setup used in the experiments for measuring the emission spectra from the Pr³⁺ doped ZnF₂-based fluoride glasses. A 588 nm LED was used as the pump source. The pump light was launched to the surface of the Pr³⁺ doped ZnF₂-based fluoride glasses through a couple of silica lens, and the emission spectra were recorded by using a grating spectrometer (SPEX 1000 M, Horiba) equipped with an InGaAS avalanche photodiode (APD, Hamamatsu). To avoid the effect of high order diffraction, a 1050 nm long pass filter was used for measuring the emission spectra in the range of 1100-1650 nm. Figures 3(a) and 3(b) show the measured emission spectra of Pr³⁺-doped ZnF₂-based glasses with various doping concentration. The energy level diagram of Pr³⁺ ions is shown in Fig. 5. As the doping concentration of Pr³⁺ is ∼ 1000 ppm, several emission peaks at 800, 840, 1026, 1335 and 1464 nm were observed in Pr³⁺-doped ZnF₂-based glass, which were attributed to the transitions ¹D₂→³H₆, ¹D₂→³F₂, ¹D₂→³F_3,4, ¹G₄→³H₅ and ¹D₂→¹G₄ (shown in Fig. 5), respectively [12]. Figure 3(c) shows the dependence of the intensity of these emission bands at 800, 840, 1026, 1335 and 1464 nm on the doping concentration of Pr³⁺ ions. With an increase of the doping concentration of Pr³⁺ ions, the intensity of the emission bands at 800, 840, 1026 and 1464 nm decreased monotonically owing to the occurrence of the cross relaxation process [¹D₂, ³H₄]→[¹G₄,³F₄,₃], which depopulated the energy level ¹D₂ of Pr³⁺ ions and made the transitions from the energy level ¹D₂ to the other energy levels quenched significantly [7]. In addition, the concentration quenching and reabsorption effects in high concentration Pr³⁺ doped ZnF₂-based fluoride glasses can also result in the decrease of the intensity for emissions at 800, 840, 1026 and 1464 nm. Interestingly, the intensity of the emission band at 1335 nm increased with an increase of the doping concentration of Pr³⁺ ions from 1000 to 5000 ppm due to the occurrence of the cross relaxation process [¹D₂, ³H₄]→[¹G₄,³F₄,₃], which populated the energy level ¹G₄ of Pr³⁺ ions and further made the intensity of the emission band at 1335 nm (the transition ¹G₄→³H₅) enhanced (shown in Fig. 5). However, as the doping concentration of Pr³⁺ ions was larger than 5000 ppm, the intensity of the emission band at 1335 nm decreased due to the occurrence of the cross relaxation process [¹G₄, ¹G₄]→[³H₅,¹D₂] and other processes (such as the concentration quenching and reabsorption processes), which depopulated the energy level ¹G₄ of Pr³⁺ ions and further made the intensity of the emission band at 1335 nm decreased (shown in Fig. 5) [13]. The above results showed that the shape of the emission spectra in O + E + S-band could be tailored by varying the concentration of Pr³⁺ ions. Figure 3(d) shows the dependence of the FWHM of the emission bands at 1335 and 1464 nm on the doping concentration of Pr³⁺ ions. As the doping concentration of Pr³⁺ ions was ∼ 5000 ppm, emission spectra with the FWHM of 215 nm (1289 nm-1504 nm) was obtained in the Pr³⁺-doped ZnF₂-based glass, which almost covered O + E + S-band. Those results indicated that broadband O + E + S-band emissions could be obtained in Pr³⁺-doped ZnF₂-based glasses. Lifetimes of the ¹D₂ and ¹G₄ levels of Pr³⁺ doped in ZnF₂-based glasses were also measured by monitoring the emissions at 1464 and 1335 nm, shown in Fig. 4. An optical parametric oscillator (OPO) nanosecond pulsed laser (Continuum SLIII-EX) was used as the pump source (588 and 980 nm were used). For the ZnF₂-based glass with a Pr³⁺ doping concentration of 1000 ppm, the measured lifetime of the ¹D₂ and¹G₄ levels were about 354 and 183 µs, respectively, which are much longer than that obtained in Pr³⁺ doped gallo-germanate glasses (∼110 µs for the ¹D₂ level) [8] and Pr³⁺ doped ZrF₄-based glasses (∼110 µs for the ¹G₄ level) [13]. With the increase of Pr³⁺ doping concentration to 30000 ppm, the lifetimes of the ¹D₂ and¹G₄ levels were dramatically decreased to about 4 and 3 µs, respectively.

 

Fig. 2. Schematic diagram of experimental setup for measuring the emission spectra of Pr³⁺ doped ZnF₂-based fluoride glasses.

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Fig. 3. Measured emission spectra in the range (a) 750-900 nm and (b) 900-1650 nm of Pr³⁺-doped ZnF₂-based glasses with various doping concentration. (c) Dependence of the intensity of these emission bands at 800, 840, 1026, 1335 and 1464 nm on the doping concentration of Pr³⁺ ions. (d) Dependence of the FWHM of the emission bands at 1335 and 1464 nm on the doping concentration of Pr³⁺ ions.

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Fig. 4. Dependence of the measured lifetimes of the ¹D₂ and¹G₄ levels on the Pr³⁺ doping concentration in ZnF₂-based glasses.

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Fig. 5. Energy level diagram of Pr³⁺ ions in the ZnF₂-based glass.

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Furthermore, according to the Füchtbauer-Ladenburg (FL) theory and the emission spectra in O + E + S-band of Pr³⁺-doped ZnF₂-based glass, the stimulated emission cross sections (σ_emi) are calculated by the following equation [14]:

Table 1. Calculated radiative transition probability (A) and branching ratio (β) for the transitions from the energy levels ¹D₂ and ¹G₄ to lower levels of Pr³⁺ ions.

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Fig. 6. Calculated absorption and emission cross sections of (a) Pr³⁺: ³H₅ ↔ ¹G₄ and (b) Pr³⁺: ¹G₄ ↔ ¹D₂ transitions. (c) Calculated gain cross sections of Pr³⁺: ³H₅ ↔ ¹G₄ and (d) Pr³⁺: ¹G₄ ↔ ¹D₂ transitions in ZnF₂-based glass. From bottom to top, the population inversion ratios (P) are 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively.

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In addition, to investigate the effects of the phonon energy of the glass on near-infrared emission of Pr³⁺ ions, Pr³⁺-doped InF₃-based and ZrF₄-based glasses with a composition of 15ZnF₂-25.5InF₃-11.5GaF₃-18BaF₂-8SrF₂-5LiF-(2.5-0.5x)LaF₃-(2.5-0.5x)YF₃-12PbF₂-xPrF₃ and 50ZrF₄-33BaF₂-7AlF₃-(10-x)YF₃-xPrF₃ (x = 0.5) were fabricated by using a conventional melt-quenching method in a glove box filled with dry N₂. The glass samples were cut and polished to 10 × 10 × 2 mm for optical measurements. Figure 7(a) shows the measured Raman scattering spectra of undoped ZnF₂-based, InF₃-based and ZrF₄-based glasses. The Raman scattering spectra of the fluoride glasses were measured with a confocal Raman microscope (LabRAM HR Evolution, HORIBA), and a 633 nm laser was selected as the pump source. The Raman shift for main Raman peaks of ZnF₂-based, InF₃-based and ZrF₄-based glasses were 420, 507 and 579 cm^-1, respectively, which correspond to the phonon energy of those three types of fluoride glasses [21]. Then we measured the near-infrared emission spectra of Pr³⁺ ions in ZnF₂-based, InF₃-based and ZrF₄-based glasses with a same doping concentration of 5000 ppm under the excitation of a same 588 nm LED and the measured results were shown in Figs. 7(b) (normalized at 1464 nm). Two emission peaks at 1335 and 1464 nm were all observed in Pr³⁺-doped ZnF₂-based, InF₃-based and ZrF₄-based glasses, which were attributed to the transitions ¹G₄→³H₅ and ¹D₂→¹G₄ (shown in Fig. 5), respectively. Interestingly, with a decrease of the phonon energy of the glass, the relative intensity of the emission peak at 1335 nm (the transition ¹G₄→³H₅) increased significantly since low phonon energy is beneficial for reducing the nonradiative transition from the ¹G₄ level to the neighbored state ³F₄ and further make the intensity of the emission band at 1335 nm enhanced significantly [22,23]. In the future, we will systematically investigate the effect of cross relaxation, concentration quenching and pumping intensity on the emission intensities and intensity ratio between 1335 nm and 1464 nm. The above results showed that Pr³⁺-doped ZnF₂-based glasses with low phonon energy are preferable for constructing O + E + S-band lasers and optical amplifiers.

 

Fig. 7. (a) Measured Raman scattering spectra of undoped ZnF₂-based, InF₃-based and ZrF₄-based glasses. (b) Normalized near-infrared emission spectra of Pr³⁺ ions in ZnF₂-based, InF₃-based and ZrF₄-based glasses with a same doping concentration of 5000 ppm under the excitation of a same 588 nm LED.

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3. Conclusions

In conclusion, we reported intense O + E + S-band emission in Pr³⁺-doped ZnF₂-based glasses. With the excitation of a 588 nm LED, broadband emissions from 1245 to 1640 nm were observed in the glasses, which originated from the transitions ¹D₂→¹G₄ and ¹G₄→³H₅ of Pr³⁺. The shape of the emission spectra could be tailored by varying the concentration of Pr³⁺. Emission spectra with the FWHM of 215 nm (1289 nm-1504 nm) was obtained in the ZnF₂-based glass at a doping concentration of 5000 ppm. The effects of the phonon energy of the matrix on O + E + S-band emission were also investigated. Our results showed that Pr³⁺-doped ZnF₂-based glasses with low phonon energy might be used as the gain medium for constructing O + E + S-band lasers and optical amplifiers.