1. Introduction

Over the past several decades, considerable work has been carried out on the RE ions doped fiber lasers due to their extensive applications in optical communication, remote sensing, laser surgery, and laser glass processing, etc [1–4]. Recently, visible fiber lasers especially in the green wavelength region have gained significant attention because of the rapid development of optical data storage, medicine, and visible light communication technologies [5–8]. Among the RE ions, Er³⁺ is well-known for the generation of green emission based on the up-conversion luminescence process through the transition of Er³⁺: ⁴S_3/2 → ⁴I_15/2, pumped by an 808 nm or 980 nm laser diode (LD) [9]. Already, Er³⁺ doped fluoride glass fibers with low phonon energy have been achieved for green laser [9–11]. However, much energy was lost in the non-radiation relaxation processes due to the intrinsic disadvantages of the up-conversion luminescence process [5]. Thus, a high pump power is required to realize a population inversion between the ⁴S_3/2 and ⁴I_15/2 levels of Er³⁺ [12]. In addition, the applications of the fluoride glass fibers are restricted due to their very obvious defects of inferior mechanical and chemical properties and low laser damage threshold [2]. In order to design new, efficient and improved optical devices for specific applications with enhanced performance, active work is being carried out by selected appropriate hosts glass of RE ions.

Compared with Er³⁺, Tb³⁺ is of special interests for laser materials in the green region around 540 nm based on its ⁵D₄ → ⁷F₅ transition, which requires lower threshold pump power [12–14]. Therefore, Tb³⁺ doped glass fiber is considered to be an attractive candidate for a gain medium in the green fiber lasers during the past years. And Tb³⁺ doped different hosts glass fibers, including fluoride, silicate, phosphate, phosphosilicate, and germanosilicate glass, have been reported [5,8,14–16]. It is noting that phosphate glass is a promising host glass for laser gain material due to its excellent network-forming glass and optical properties, higher RE-ions solubility, and good mechanical property [17–19]. What is more, alkali, alkaline earth, and transition metal oxides are usually added to the phosphate glass to improve its RE-ions solubility and chemical stability [20]. Recently, luminescence properties of Tb³⁺ singly doped and Ce³⁺/Tb³⁺ co-doped phosphate glasses have been systematically studied [5,12,21–24]. However, heavily Tb³⁺ doped phosphate glass fibers were rarely reported. In Ref [25], complicated techniques related to pumping schemes were proposed to enhance the laser efficiency. However, heavily RE ions doped glass fiber make it possible to obtain high gain and high output power per unit length, which can reduce the required fiber length in fiber lasers and contributes to background loss reduction, nonlinear effect mitigation and narrow emission linewidth [18,26,27]. Therefore, heavily RE ions doped glass fibers are promising in applications that require high gain and high power from a short piece of active optical fiber. Although no fluorescence quenching was observed when Tb³⁺ concentration reached 15 mol % in the phosphate glass, only 5 mol % Tb³⁺ doped phosphate glass fibers have been successfully drawn by the conventional rod-in-tube method [5]. The biggest challenge to fabricate heavily Tb³⁺ doped glass fibers is the higher RE ions doping concentration will cause crystallization during the fiber drawing process, since the core glass will go through twice thermal drawing process by using conventional rod-in-tube technique [18]. Therefore, an effective method is urgent needed to fabricate heavily Tb³⁺ doped glass fibers.

More recently, the molten core drawing method has attracted much attention due to its versatile, practical, and yields long lengths of glass-clad multimaterial fibers, which contain unconventional core materials such as semiconductor materials, new dielectric materials, and metal materials [28–31]. The glass cladding draws directly into fiber and the molten of core precursor phase goes “along for the ride” and ultimately quickly solidifies as the fiber cools [29]. The fiber core can maintain amorphous state due to high cooling rate of the thin core glass during this drawing process. In our previous study, it has been proved that the molten core technique can be used to effectively solve the crystallization problem in fiber drawing process, which caused by the heavily RE ions doped concentration [18]. In this work, no fluorescence quenching was observed in the prepared multi-component phosphate glasses even when Tb³⁺ doping concentration reached 16 mol %. More importantly, silicate-clad heavily Tb³⁺ doped (16 mol %) multi-component phosphate glass core optical fibers were first successfully drawn by the molten core method. An intense 540 nm emission with fluorescence lifetime as long as 1.85 ms was obtained from the as-drawn fibers. The results suggest that the heavily Tb³⁺ doped multi-component phosphate glass fibers have promising application in green fiber laser. To the best of our knowledge, 16 mol % is the record high Tb³⁺ ion in phosphate glass fibers for green laser.

2. Experimental

Multi-component phosphate glasses with the molar compositions of (66.5-x) P₂O₅-13 K₂O-14.3 BaO-2.2 Al₂O₃-4 La₂O₃-x Tb₂O₃ (x = 0.5, 4, 8, 10, 12, 14) were prepared by the conventional melting-quenching method. High purity reagents (99.99% minimum) of P₂O₅, K₂CO₃, BaCO₃, Al₂O₃, La₂O₃, and Tb₄O₇ were used as starting materials. Well-mixed raw materials (50 g) were melted in a covered alumina crucible at 1250 °C for 1 h with stirring by a quartz glass rod. Also the melt was bubbled with high purity O₂, maintained at 0.1 L/h for 10 min. Finally, the melt was poured into a preheated steel mold and annealed at 480 °C for 2 h before they were cool to room temperature. The well annealed glasses were cut and optically polished into a shape of 15 × 15 × 1.5 mm³ for optical measurements including Raman, absorption, excitation, emission spectra, and lifetime fluorescence. The core glass and fibers were separately ground into powder for the emission spectra measurement.

For fiber fabrication, core glass with the molar composition of 58.5 P₂O₅-13 K₂O-14.3 BaO-2.2 Al₂O₃-4 La₂O₃-8 Tb₂O₃ was prepared. Well-mixed raw materials (500 g) were melted in a covered alumina crucible at 1250 °C for 4 h, and bubbled with high purity O₂, maintained at 0.1 L/h for 2 h. Then the melt was stirred and clarified to remove bubbles and stripes. The fine annealed bulk glass was cold worked into cylindrical rods with diameter of 3.5 mm in lathe. A bulk silicate glass (BK7 Schott) was also worked into cylindrical tube with one end closed in lathe. The silicate glass tube has an inner diameter of 3.6 mm and outer diameter of 25.0 mm. The surface of the rod and hole was polished and etched by acid in order to remove the contaminated surface layer. Then the core glass rod was inserted into the silicate glass (BK7 Schott) tube to form a preform. Finally, the preform was drawn into the fibers in the furnace of the fiber drawing tower. At the drawing temperature of 900 °C, the cladding glass became softened while the core glass has been turned into melt. And the fiber core maintains amorphous state due to high cooling rate of the thin core glass during this drawing process.

The transition temperature (T_g) of the glass samples were measured by using a Netzsch SAT 449C Jupiter differential scanning calorimeter (DSC) under argon atmosphere at a heating rate of 10 °C/min. The refractive indexes of the glass samples were measured by a prism coupling apparatus (Metricon Model 2010). Raman spectra were employed on a Raman spectrometer (Bruker, Switzerland) with a 532 nm laser as the excitation source. The absorption spectra were measured on a Perkin-Elmer Lambda 900 UV-Vis-NIR double beam spectrophotometer (Waltham, MA). The excitation spectra, emission spectra, and lifetime were measured on an Edinburgh FLS 920 spectrofluorometer (Edinburgh Instruments, Wales, UK). The fiber cross section and the distribution of elements spatially across the core/clad interface were obtained by an electro probe X-ray micro-analyzer (EPMA-1600, Shimadzu, Japan). All the measurements were made at room temperature.

3. Results and discussions

Figure 1 shows the DSC curves of the multi-component phosphate glasses doped with different Tb³⁺ concentration. The T_g and crystallization peak are not obvious for low Tb³⁺ concentration (≤ 8 mol %) doped multi-component phosphate glasses. The T_g increases slightly with the increase of Tb³⁺ concentration from 16 mol % to 28 mol %. Similarly, the crystallization peak becomes more obvious in high Tb³⁺ concentration (> 8 mol %) doped glasses, indicating the anti-crystallization ability of the glass samples is weakened. Therefore, the heavily Tb³⁺ (> 8 mol %) doped multi-component phosphate glass is easier crystallization during the fiber drawing process by using conventional rod-in-tube technique since the core glass will go through twice heat drawing process.

 

Fig. 1 DSC curves of the Tb³⁺ doped multi-component phosphate glasses.

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The Raman spectra of the glass samples are presented in Fig. 2. Two broad Raman bands centered at 339 and 560 cm⁻¹ are in general related to different network bending modes, which are assigned to the vibrations of Tb-O-Tb and Tb-O [5]. The Raman peak at about 697 cm⁻¹ is ascribed to the P-O-P symmetric stretch vibrations of Q² and Q¹ units [32]. The Raman band at 944 cm⁻¹ is due to the vibrations of (PO4)^3- units. And the vibration at about 1094 cm⁻¹ is corresponding to dimer (P₂O₇)^4- units [33]. The highest Raman activity was observed at about 1195 cm⁻¹ and the shoulder peak at 1280 cm⁻¹, which are attributed to the vibrations of (PO₃)^- units [5]. It is obviously observed that the intensity of the Raman bands at 339 and 560 cm⁻¹ increases, while the Raman peaks at 944, 1094, and 1280 cm⁻¹ disappear slowly with increasing Tb³⁺ concentration, showing that the Tb-O-Tb and Tb-O groups increase and the (PO4)^3-, (P₂O₇)^4-, and (PO₃)^- units decrease with increasing Tb³⁺ concentration. In addition, the maximum phonon energy decreases with increasing Tb³⁺ concentration, which is beneficial to reduce non-radiative relaxation transition.

 

Fig. 2 Raman spectra of the Tb³⁺ doped multi-component phosphate glasses.

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The absorption spectra of the Tb³⁺ doped multi-component phosphate glasses are shown in Fig. 3. Six characteristic absorption bands located at 2210, 1893, 482, 377, 351, 317 nm can be attributed to the typical transitions from ground level ⁷F₆ to the excited states ³F₃, ⁷F₁, ⁵D₄, ⁵D₃, ⁵G₃, and ⁵H₇ of Tb³⁺, respectively. It is observed that there is no obvious change in the position of the absorption peaks of Tb³⁺, while the absorption intensity increase with the monotonically increase of Tb³⁺ concentration.

 

Fig. 3 Absorption spectra of the Tb³⁺ doped multi-component phosphate glasses.

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Figure 4(a) shows the photoexcitation (PLE) spectra of the Tb³⁺ doped multi-component phosphate glasses. 290-500 nm emitted by Xenon lamp was used as the excitation wavelength for measuring PLE spectrum. A series of sharp overlapping PLE bands in the near ultraviolet region with peaks at 302, 318, 340, 350, 358, 368, and 376 nm, and another sharp PLE locate in the blue with a maximum at 484 nm were observed in the PLE spectra, which are full consisted with the optical absorption data of Tb³⁺ [34]. These bands can readily be ascribed to the intra-configurational parity-forbidden 4f⁸ → 4f⁸ electronic transitions of Tb³⁺ ions from the ground level ⁷F₆ to the labeled excited states in Fig. 4(a). The strongest PLE peak is the transition ⁷F₆ → ⁵D₃ at 376 nm, used in the following as excitation wavelength to measure the emission (PL) spectra, which are shown in Fig. 4(b). And the energy level diagram of Tb³⁺ is shown in the inset of Fig. 4(b). Five typical PL bands centered at 486, 540, 546, 582, and 618 nm were obtained, which are originated from the transitions of Tb³⁺ from⁵D₄ level to the ⁷F₆, ⁷F₅, ⁷F₅, ⁷F₄, and ⁷F₃ states, respectively. The corresponding transitions related to these peaks are marked in the inset of Fig. 4(b). The whole emission band at visible region are all transited from the intra-configurational parity-forbidden ⁴f₈ → ⁴f₈ transitions from ⁵D₄ to the ⁷F_J (J = 6, 5, 4, 3, and 2) multiplet [34,35]. It is noted that the green PL band was stark-split into two peaks at 540 and 546 nm due to the distorting effect of the disordered glass network on the Tb³⁺ ions [34,36]. It is worth noting that no fluorescence quenching was observed even Tb³⁺ concentration reaches up to 16 mol %, which is higher than that in the other Tb³⁺ doped phosphate glass (15 mol %), silicate glass (3 mol%), and GeO₂-B₂O₃-Al₂O₃-Ga₂O₃ glass (14 mol %) before concentration quenching [5,34]. The concentration quenching effect here is related to an increasing probability for the formation of Tb-O-Tb entities in the first coordination shell of Tb³⁺ [37]. And the higher Tb³⁺ doping concentration in the prepared multi-component phosphate glass are vital for high performance green laser fiber materials. The Tb³⁺ ions are easily isolated by PO₄ tetrahedrons and reach charge balance in phosphate glass, while they tend to form clusters in silicate glass [5]. Therefore, the multi-component phosphate glass doped with 16 mol % Tb³⁺ was chosen as the core glass.

 

Fig. 4 (a) Excitation and (b) emission spectra of the Tb³⁺ doped multi-component phosphate glasses. The inset of Fig. 4(b) shows the energy level diagram of Tb³⁺.

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The normalized decay curves of photoluminescence from Tb³⁺ doped multi-component phosphate glasses as a function of Tb³⁺ concentration are shown in Fig. 5. The inset of Fig. 5 shows the dependence of lifetime on Tb³⁺ concentration. The fluorescence lifetime of the Tb³⁺: ⁵D₄ → ⁷F₅, defined as the time when the emission intensity decays to 1/e of initial intensity, was measured to be 2.85-2.46 ms. It is found that the lifetime decreases with increasing Tb³⁺ concentration, and the lifetime of the core glass (16 mol % Tb³⁺ doped glass sample) (2.61 ms) is longer than that of Tb³⁺ doped GeO₂-B₂O₃-Al₂O₃-Ga₂O₃ glass (~2.23 ms) [34] and B₂O₃- GeO₂-Gd₂O₃ glass (~1.80 ms) [38] before concentration quenching. The longer fluorescence lifetime of Tb³⁺ in core glass can reduce the pump threshold and be favorable to achieve green laser output. In addition, the lifetime of the 28 mol % Tb³⁺ doped glass sample is about 2.46 ms, showing that relatively weak concentration quenching occurs in the prepared multi-component phosphate glasses.

 

Fig. 5 Normalized decay curves of photoluminescence from the Tb³⁺ doped multi-component phosphate glasses. The inset shows the fluorescence lifetime of the multi-component phosphate glasses doped with different Tb³⁺ concentration.

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For fiber fabrication, we firstly tried to use the conventional rod-in-tube method to fabricate heavily (16 mol %) Tb³⁺ doped multi-component phosphate glass fibers. The small core glass rods with diameter about 3 mm were drawn from the polished cylindrical core glass in a drawing tower, which is usually referred to as first drawing. Figure 6 shows the core glass rod fabricated by thermal drawing process. It can be seen that the core glass rod is completely crystallization into milky white due to the heavily Tb³⁺ doping concentration. No doubt it cannot withstand the secondary thermal drawing process. Inasmuch as drawing glass fibers by the conventional rod-in-tube technique contains two heating processes, the core glass with inferior anti-crystallization ability will easily precipitate crystals resulting in scattering centers and high fiber loss [2]. Therefore, the conventional rod-in-tube technique is not suitable for drawing heavily Tb³⁺ doped multi-component phosphate glass fibers. Then, another fine annealed bulk glass was cold worked into cylindrical rods with diameter of 3.5 mm in lathe, which will avoid a thermal drawing process. The surface of the core glass rod was polished and etched by acid in order to remove the contaminated surface layer. Finally, the core glass rod was inserted into the silicate glass (BK7 Schott) tube to form a preform. The assembled perform was suspended in the furnace of a commercial optical fiber drawing tower. Considering the different coefficient of thermal expansion between the silicate glass cladding and the multi-component glass core, the heating temperature was kept gradual and slow proceeded. Then continuous and crystal-free silicate-clad heavily Tb³⁺ doped multi-component phosphate glass core multimaterial fibers were successfully drawn at about 900 °C. In this temperature, the core glass has been turned into melt while the cladding glass became softened. Then the glass cladding was directly drawn into fibers and the molten of core quickly solidified as the fibers cooled. What is more, the fiber core can maintain amorphous state due to high cooling rate of the thin core glass during this fiber drawing process.

 

Fig. 6 Core glass rod fabricated by thermal drawing process.

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The electron micrograph of the as-drawn fiber in cross section is shown in Fig. 7(a). It is observed that the core-clad structure of the fiber is preserved completely due to the reasonable difference of softening point between the cladding glass and core glass. And the fiber has an outer diameter of about 200 μm and core diameter of about 34 μm. The fiber core shows a brighter color in the backscattered electron image when compared with the fiber cladding. And there are no discontinuities at the core/clad interface, no microcracks and other defects in the core. The refractive indexes of the core glass (n_core) and cladding glass (n_cladding) were measured to be 1.562 and 1.515 at 633 nm, respectively. The numerical aperture (NA) can be calculated through the Eq. (1) [39]:

 

Fig. 7 (a) Electron micrograph of the as-drawn fiber in cross section. (b)-(e) EPMA images of the marked area in (a).

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The NA of the as-drawn fibers was calculated to be 0.380 at 633 nm. When the normalized frequency V of a fiber is < 2.405, only a fundamental mode can be propagated in the gain fiber and the single-mode operation occurs. The V of a fiber is expressed as (2) [27]:

The two-dimensional energy-dispersive X-ray mapping distributions of P, Si, Al, and Tb are illustrated in Figs. 7(b)-(e), respectively. The distribution boundary of each element forms a circle, and the Si is mainly distributed in the cladding region, while the P and Tb are mainly distributed in core area. According to the color bar, it is found that a higher concentration of Al in the core than that in the cladding, which was also confirmed by the elemental profile across the core/clad interface of the as-drawn fiber.

Figure 8 displays the elemental profile (P, Tb, Al, and Si) across the core/clad interface of the as-drawn fiber. The zero relative distance represents the middle of the core. The Si, P, and Tb elements show an abrupt change in the boundary of the core and cladding because the P and Tb elements are only observed in the core and hardly diffuse into the cladding, while the Si element only detected in the cladding. The Al element is distributed in both core and cladding, which is agreement with the initial compositions of the core and cladding. These results suggest that the silicate glass is a suitable cladding for the heavily Tb³⁺ doped multi-component phosphate glass core and can be successfully drawn into fibers by the molten core method.

 

Fig. 8 Elemental profiles (relative elemental composition as a function of position across the fiber) for the as-drawn fiber.

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The emission spectra of the core glass and the as-drawn fibers upon excitation of 376 nm are shown in Fig. 9. Intensive emission peaks at 540 and 546 nm originated from the transition ⁵D₄ → ⁷F₅ of Tb³⁺ were obtained from the core glass and the as-drawn fibers. Other typical emission peaks of Tb³⁺ in the wavelength from 450 to 650 nm were also observed. It can be found that there is no obvious change in the emission peaks between the core glass and the as-drawn fibers. In order to avoid the delay of the lifetime due to re-absorption from unexcited active ions, the fluorescence spectra were collected at 90 degree from the incident pumping light. And the pump power and the other test conditions were kept the same. Therefore, the comparison of luminescence intensity and lifetime of glass samples are considered credible. The inset of the Fig. 9 shows the fluorescence decay curves of Tb³⁺: ⁵D₄ → ⁷F₅ of the as-drawn fibers. Lifetimes were determined from the first e-folding time of emission intensities in the decay curves. The decay curve in the inset of the Fig. 9 was best fit to a single-exponential decay function. In order to better read out the value of lifetime, the ordinate was replaced by a logarithmic coordinate. The fluorescence lifetime of the 540 nm of the as-drawn fibers was measured to be 1.85 ms, which is shorter than that of the core glass (2.61 ms). Both of the conventional rod-in-tube technique and the molten core method have the inherent problems such as impurities introduced during the core glass rod processing, core-clad interface irregularities, and so on. The impurities introduced during the core glass rod processing and fiber drawing process may shorten the luminescence lifetime in the as-drawn fibers. Despite this, the fluorescence lifetime of the as-drawn fibers is comparable to that of the heavily Tb³⁺ doped B₂O₃- GeO₂-Gd₂O₃ glass (~1.80 ms) before concentration quenching [38]. The (amplified spontaneous emission) ASE is a good proof of a new material for potential laser application. Unfortunately, we are currently lacking the corresponding optical devices for ASE experiments. However, the excellent emission properties suggest that the as-drawn fibers are promising fiber materials for green laser, and the molten core method is highly suitable for fabricating heavily RE ions doped glass fibers. In the future, laser experiments will be conducted to further verify the performance of the as-drawn fibers.

 

Fig. 9 Emission spectra of the core glass and the as-drawn fibers. The inset shows the fluorescence decay curves of the as-drawn fibers.

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4. Conclusion

In conclusion, heavily Tb³⁺ doped multi-component phosphate glasses were prepared by the conventional melt-quenching method. The absorption spectra, thermal properties, Raman spectra, luminescence spectra, and luminescence lifetime of the glass samples were systematically investigated. No fluorescence quenching was observed in the prepared multi-component phosphate glasses even when Tb³⁺ doping concentration reached 16 mol %. Continuous and crystal-free silicate-clad heavily Tb³⁺ doped multi-component phosphate glass core fibers were first successfully drawn by a molten core method. Intensive fluorescence emission at green light band and long fluorescence lifetime were obtained from the as-drawn fibers, showing potential interest for further investigation as green laser fiber materials. To the best of our knowledge, 16 mol % is the record high Tb³⁺ ion in multi-component phosphate glass fibers for green laser. And this work is also a proof that the molten core method is highly suitable for fabricating heavily RE ions doped glass fibers.