1. Introduction

Development of superbroadband near-infrared luminescence sources for broadband optical amplifiers and tunable lasers covering entirely the expanded low-loss telecommunication window (~1.2-1.7 μm) attracts considerable attention, in particular, after the progress made in the production of hydroxyl (OH^–)-free silica fibers (dry optical fibers) [1,2]. Previous investigations were focused on the bismuth (Bi) [3–6], and then an extension to other heavy metal (HM) and transition metal (TM) ions, such as nickel (Ni), chromium (Cr), lead (Pb), etc. [7–9]. However, the bandwidth and peak wavelength of the broadband emissions from HM/TM ions depend sensitively on the host matrix and the excitation wavelength, and also the luminescence origin for some of them requires further studies [10]. To date, there has been little work reports on the superbroadband luminescence from rare earth (RE) doped systems, although they play crucial roles as optical amplifiers and laser sources in telecommunication systems [11]. Typical RE emissions/amplifications cover only separate C-, L-, S-, E-, and O-bands. Thus, novel host matrix, dual-pump configurations, nanostructures, and REs codoping schemes have been investigated to further improve the bandwidth and the quantum efficiency [11]. Recently, broadband emissions located at 1.20 and 1.47 μm were observed in thulium (Tm³⁺)-doped glasses [12,13], and a broader emission band from 1.0 to 1.7 μm was obtained using Tm-Bi codoping scheme by taking advantage of the Bi 1.3 μm emission [13].

Praseodymium (Pr³⁺) shows promising to achieve some novel near-infrared emissions due to the rich multiple energy levels. Apart from the well-known 1.3 μm emission (¹G₄→³H₅ transition) [14], an emission around 1.6 μm from the ³F_3,4→³H₄ transition was also observed in Pr³⁺-doped selenide glass [15]. We recently observed superbroadband near-infrared luminescence in Pr³⁺-doped bismuth gallate glass [16]. However, the low transmission of bismuth gallate glass in the blue region resulted in a depression of the pump efficiency using blue light, at which Pr³⁺ possesses intense absorption bands. In addition, the Pr³⁺ near-infrared emission, especially at the longer wavelength side, was seriously quenched by the cross relaxation [¹D₂, ³H₄]→[¹G₄, ³F_3,4] because of the intense ground-state absorption ³F_3,4←³H₄ which is overlapped with the Pr³⁺ emission.

In the present work, we propose Pr³⁺-Er³⁺ codoping scheme to achieve the superbroadband near-infrared emission for the first time to our best knowledge. Efficient Er³⁺ 1.53 μm emissions/amplifications have already been demonstrated under 488 nm excitation (into the absorption band Er³⁺: ⁴F_7/2←⁴I_15/2) [11]. Fluorotellurite glasses were selected as host because of their broad transmission window, good mechanical properties and chemical durability, and optical amplification and laser operation have been achieved in tellurite glass fibers [17].

2. Experimental

Glass samples were prepared by melting well-mixed high purity materials of lanthanum fluoride (LaF₃), barium fluoride (BaF₂), barium carbonate (BaCO₃), zinc oxide (ZnO), and tellurium oxide (TeO₂) following a standard melt-quenching procedure as described in [12]. Praseodymium fluoride (PrF₃) and erbium fluoride (ErF₃) were used as RE dopants with doping levels of xPrF₃-yErF₃ mol% where x = 0, 0.1, 0.2, 0.3; y = 0.1, and x = 0.3; y = 0, 0.05. LaF₃ and BaF₃ were added to modify the ligand field between the RE sites and the matrix. The as-prepared glasses were cut and optically polished for optical measurements.

The refractive index of the glass samples was measured using a Metricon 2010 prism coupler. The Raman spectrum of undoped glass sample was measured using a HORIBA Jobin Yvon HR800 Raman spectrometer with a 488 nm laser excitation source. The absorption spectra were recorded using a Perkin Elmer UV-VIS-NIR Lambda 19 double beam spectrophotometer. The visible and infrared emission spectra were recorded using an Edinburgh Instruments FLSP920 spectrofluorometer. The wavelengths of excitation sources were tuned from a continuous xenon lamp by a monochromator. The excitation spectra were recorded using the same setup with a continuous wavelength xenon lamp as the excitation source. The emission decay curves were recorded using the same setup with a flash xenon lamp as the excitation source. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows the absorption spectra of Pr³⁺-, Er³⁺-singly doped and Pr³⁺-Er³⁺ codoped samples. All the absorption bands observed are due to the electronic transitions from the ground-state to the respective excited states as indicated in Fig. 1. The transparency at the blue wavelength region is much higher than bismuth gallate glass, which would allow efficient pumping with blue light sources. It is interesting to observe that both Pr³⁺ and Er³⁺ possess absorptions around 488 nm [see inset (a) of Fig. 1]; they correspond to the Pr³⁺: ³P₀←³H₄ and Er³⁺: ⁴F_7/2←⁴I_15/2 transitions, respectively. This resonant energy-level matching makes it possible to achieve the superbroadband emission in the Pr³⁺-Er³⁺ codoping scheme by pumping with a single-wavelength light source. This is in agreement with the excitation spectra, as shown in inset (b) of Fig. 1.

 

Fig. 1 Absorption spectra of (0.3 mol%)Pr³⁺-doped, (0.1 mol%)Er³⁺-doped, and (0.3mol%)Pr³⁺ -(0.1mol%)Er³⁺ codoped fluorotellurite glasses. Inset (a) shows the details of absorption spectra in wavelength region 410-560 nm; Inset (b) shows the excitation spectra monitored at 1.42 and 1.53 μm wavelengths from Pr³⁺- and Er³⁺-singly doped samples, respectively.

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Using the absorption spectra, Judd-Ofelt analysis was performed [18,19]. The values of intensity parameters Ω_t (t = 2, 4, 6) are calculated to be (3.57, 6.60, 5.18) × 10⁻²⁰ cm², and (5.62, 1.12, 1.78) × 10⁻²⁰ cm² for Pr³⁺- and Er³⁺-singly doped samples, respectively, using a least-squares fitting of the experimental and theoretical electric dipole oscillator strengths. The larger Ω₂ value of Pr³⁺ in fluorotellurite than those other fluoride contained oxide glasses indicates a stronger asymmetry and covalent environment of Pr³⁺ in fluorotellurite glass [20,21]. A similar result is also obtained for Er³⁺-doping, in which the value of Ω₂ is larger than those ZnF₂ contained tellurite glasses [22]. The spontaneous transition properties of Pr³⁺ and Er³⁺ are listed in Table 1 . The spontaneous transition probability of Pr³⁺: ¹D₂→¹G₄ is 880.1 s^–1 (with branch ratio of 11.1%), which is comparable to that of Pr³⁺ in bismuth gallate glass [16]. Regarding the Er³⁺: ⁴I_13/2→⁴I_15/2, the spontaneous transition probability (454.1 s^–1) is larger than that in ZnF₂ contained and oxide-based tellurite glasses [22,23].

Table 1. Spontaneous transition parameters of Pr³⁺ and Er³⁺ in fluorotellurite glass

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Figure 2 compares the near-infrared emissions of Pr³⁺-Er³⁺ codoped samples under 488 nm excitation for different Pr³⁺ concentrations. Compared with the narrower Er³⁺ 1.53 μm emission band (Er³⁺: ⁴I_13/2→⁴I_15/2 transition), the emission in Pr³⁺-Er³⁺ codoped samples shows an obvious extension and enhancement in the short wavelength side, resulting in a superbroad emission band at 1.3-1.68 μm range. The broad emission at short wavelength region is contributed by the Pr³⁺: ¹D₂→¹G₄ transition, which leads to a broad near-infrared emission band under blue excitation. This is in agreement with the Pr³⁺-doped bismuth gallate glass [16]. Figure 2 inset shows the normalized emissions of the Pr³⁺-Er³⁺ codoped samples with different Er³⁺ concentrations. The composite near-infrared emission at the longer wavelength side is enhanced by incorporation of Er³⁺. The emission observed at 1.23 μm is due to the Er³⁺: ⁴S_3/2→⁴I_11/2 transition, which shows a relative increasing trend with the increase of Pr³⁺.

 

Fig. 2 Normalized near-infrared emission spectra of Er³⁺-doped and Pr³⁺-Er³⁺ codoped samples with respect to the Er³⁺ 1.53 μm emission. Inset shows the normalized emission spectra with respect to the Pr³⁺ emission at 1.42 μm wavelength. The excitation wavelength is 488 nm.

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Figure 3(a) shows the decay curves of Pr³⁺: ¹D₂ measured at a monitoring wavelength of 1.42 μm. This wavelength is significantly away from the Er³⁺ 1.53 μm emission band, and hence the effect of the Er³⁺ 1.53 μm emission on the decay measurement can be ignored. Since the decay curves recorded deviate slightly from the single exponential function, the lifetime is then determined by ${\tau }_m={\displaystyle \int t\phi \left(t\right)}/{\displaystyle \int \phi \left(t\right)}$, where $\phi \left(t\right)$ is the decay as a function of time t. The mean lifetime decreases from 55.5, 49.1, to 47.8 μs after the presence of Er³⁺ and further increase from 0.05 to 0.10 mol%. This can be attributed to the cross relaxation [Pr³⁺(¹D₂), Er³⁺(⁴I_15/2)]→[Pr³⁺(¹G₄), Er³⁺(⁴I_13/2)], by which some of the energy in Pr³⁺:¹D₂ were transferred to neighboring Er³⁺ in the ground state due to the broad spectral overlap between them [see inset of Fig. 3(a)], resulting in a depleting of the Pr³⁺:¹D₂. Another possible ET from Pr³⁺:¹D₂ to Er³⁺:⁴F_9/2 can be ignored considering the large energy gap between them (~1490 cm^–1) and the relatively low phonon energy of the host (the maximum phonon energy is ~770 cm^–1 according to the Raman spectrum). Indeed no emission from relevant Er³⁺ levels such as ⁴F_9/2 is observed in the Pr³⁺-Er³⁺ codoped sample when excited at 594 nm (into the Pr³⁺:¹D₂ absorption band). The overall efficiency can be evaluated by the quantum efficiency η_QE of the emitting level Pr³⁺:¹D₂ which is calculated by η_QE = τ_m/τ_r, where τ_r is the radiative lifetime obtained by the Judd-Ofelt calculation [14]. The value of η_QE, which is 44.0% in the (0.3 mol%)Pr³⁺-singly doped sample, shows a slight decrease after the codoping of Er³⁺ but remains above 38.0% in the (0.3 mol%)Pr³⁺-(0.1 mol%)Er³⁺ codoped sample.

 

Fig. 3 Decay curves monitored at (a) 1420 nm and (b) 495 nm under 488 nm excitation. Inset of (a) shows the spectral overlaps between normalized (i) Pr³⁺: ¹D₂→¹G₄ emission band, (ii) Pr³⁺: ³F_4,3←³H₄ and (iii) Er³⁺: ⁴I_13/2←⁴I_15/2 absorption bands. Inset of (b) shows the Pr³⁺: ³P₀→³H₄ emission band under 488 nm excitation. (c) Visible emissions of Pr³⁺-Er³⁺ codoped sample under 525 nm excitation and Pr³⁺-doped sample under 488 nm excitation. Inset of (c) shows the Pr³⁺ 1.3 μm emission from Pr³⁺-Er³⁺ codoped sample under 980 nm excitation. (d) Er³⁺ 1.53 μm emissions from Er³⁺-singly doped and Pr³⁺-Er³⁺ codoped samples under 980 nm excitation.

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To further understand the phenomena observed, it is necessary to consider other energy transfer (ET) processes involved between Pr³⁺ and Er³⁺. To investigate the ET process from Pr³⁺:³P₀ to Er³⁺:⁴F_7/2, we measured the Pr³⁺: ³P₀→³H₄ emission as well as its decay curves at monitoring wavelength of 495 nm under 488 nm excitation, and they are shown in Fig. 3(b) and inset. The lifetime shows a decrease for the Pr³⁺-doped samples after the addition of Er³⁺. This is in accordance with the decrease of 526, 611, and 643 nm emissions which originate from the common level Pr³⁺:³P₀, confirming the occurrence of the ET process Pr³⁺(³P₀)→ Er³⁺(⁴F_7/2). The ET from Er³⁺:⁴S_3/2 to Pr³⁺:¹D₂ can be taken no consideration because the energy mismatch between them is as large as ~1580 cm^–1. This is also confirmed by the emission spectrum measured under excitation of 525 nm [see Fig. 3(c)] at which the excitation energy is absorbed only by Er³⁺(²H_11/2). In Fig. 3(c), the Er³⁺ green emissions from Er³⁺: (²H_11/2,⁴S_11/2)→⁴I_15/2 transition are clearly observed while no Pr³⁺ relevant emission is observed. The Er³⁺:⁴I_11/2 and Pr³⁺:¹G₄ are quasi-resonant levels, and the ET process Er³⁺(⁴I_11/2)→Pr³⁺(¹G₄) occurred, which is confirmed by the observation of the Pr³⁺ 1.3 μm emission in Pr³⁺-Er³⁺ codoped sample under 980 nm excitation, while there is no 1.3 μm emission observed in Pr³⁺-singly doped sample under the same excitation. This ET was also observed in Er³⁺-Pr³⁺ codoped ZBLAN glasses [24,25], and the 1.3 μm emission/amplification was obtained in Pr³⁺-Yb³⁺ codoped fiber using a 980 nm laser diode pump [26]. As for the ET from the resonant levels of Er³⁺:⁴I_13/2 to Pr³⁺:³F_4,3, it is confirmed by the significant decrease of the Er³⁺ 1.53 μm emission after the addition of Pr³⁺, as shown in Fig. 3(d). By depleting the terminal level of Er³⁺: ⁴I_11/2→⁴I_13/2 transition through this resonant ET process, enhanced gain at 2.7-2.8 μm range was obtained in Er³⁺-Pr³⁺ codoped ZBLAN fiber [27]. The relative increase of the Er³⁺ 1.23 μm emission compared with of 1.53 μm [see Fig. 2] can be ascribed to the improved population inversion between the upper and lower levels besides the decrease of 1.53 μm emission itself due to the ET3 process. Under 488 nm excitation, the emitting level Er³⁺:⁴S_3/2 is populated via the multi-phonon relaxations from Er³⁺:⁴F_7/2 followed the ET1 process and meanwhile the terminal level depleted by the ET from it to Pr³⁺:¹G₄. The both contribute to an increase of the population inversion and thereafter the relatively enhanced 1.23 μm emission. All the ET processes involved are schematically illustrated in Fig. 4 .

 

Fig. 4 Schematic energy-level diagram of Pr³⁺ and Er³⁺, and possible energy transfer routes involved. ETi (i = 1,2,3), CRi (i = 1,2,3), and MPR represent energy transfer, cross relaxation, and multi-phonon relaxation processes, respectively.

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The stimulated emission cross-section σ_em for the Pr³⁺: ¹D₂→¹G₄ transition was determined to be 0.90 × 10⁻²⁰ cm² by scaling the emission spectrum through the Füchtbauer-Ladenburg approach [28] because this transition occurs between the two excited states. Theσ_em of Pr³⁺ emission in fluorotellurite is larger than in bismuth gallate glass (0.70 × 10⁻²⁰ cm²) [16] and of Ni²⁺ in glass-ceramics (0.63 × 10⁻²⁰ cm²) [8], although smaller than the active Bi ions in germanate glass (1.59 × 10⁻²⁰ cm²) [4]. The σ_em for the Er³⁺: ⁴I_13/2→⁴I_15/2 transition was derived to be 1.06 × 10⁻²⁰ cm² using the McCumber relationship [29]. It is larger than those in TeZnNa (0.85 × 10⁻²⁰ cm²) [30] as well as many other types of oxides such as germanate (0.58 × 10⁻²⁰ cm²), silicate (0.73 × 10⁻²⁰ cm²), and phosphate (0.76 × 10⁻²⁰ cm²) [29].

4. Conclusions

Superbroadband near-infrared emission covering the expanded low-loss telecommunication window was achieved in Pr³⁺-Er³⁺ codoped fluorotellurite glasses under 488 nm excitation. Er³⁺ is demonstrated to be a good candidate to compensate the quenching of Pr³⁺ near-infrared emission resulted from the cross relaxation process [¹D₂, ³H₄]→[¹G₄, ³F_3,4]. The results confirm that Pr³⁺-Er³⁺ codoped fluorotellurite glass is promising for the superbroadband amplified spontaneous emission sources, optical amplification, and tunable lasers applications. Further investigations and experiments are underway.