1. Introduction

Research in developing optical fibers with broadband emissions for applications, such as fiber lasers and broadband fiber amplifiers, has experienced significant growth over the past decades [1–4]. Until recently, rare earth-doped optical fibers working in the near-infrared (NIR) region have been widely used in optical telecommunications, material processing, biomedical applications, etc [5]. Owing to the 4f–4f electronic transition nature of rare-earth ions and their spectral characteristics, it is difficult for rare-earth ion-doped fiber to realize broadband emissions [6]. Therefore, there is great demand for specially designed and engineered active materials that can provide wavelength-tunable emissions for fiber lasers and broad gain bandwidth for fiber amplifiers. Recently, note that quantum dots (QDs) have emerged as candidate gain medium of fiber lasers and broadband fiber amplifiers [7–9]. Attractive features of QD-doped materials include the ease of tuning their band-gap and photoluminescence (PL) profile by controlling over their size and size distribution, as well as the wealth of possibilities for their incorporation into micro- and nano-cavities and material matrices [10–12]. Among most II–VI and III–V QDs, PbS QDs can achieve strong quantum confinement effect, due to their large exciton Bohr radii (18 nm) and nearly equivalent electron and hole effective masses, exhibiting intense NIR emissions in the region from 1 to 2 μm [13–15]. In our previous studies, tunable NIR emissions and optical amplification at 1330 nm and 1550 nm have been demonstrated [16–19]. Hence, it is believed that such PbS QD-doped optical glass fibers exhibit potential in meeting these requirements towards applications in the fiber lasers and broadband fiber amplifiers.

Active optical glass fibers are one of the most efficient laser media that have potential to achieve efficient solid-state lasers with excellent beam quality. Usually, glass fibers are fabricated by conventional chemical vapor deposition (CVD) technique and rod-in-tube technique, etc [20, 21]. Few studies have been done to fabricate PbS QD-doped silicate optical fiber, such as using atomic layer deposition (ALD) [22, 23]. Although PL can be obtained from PbS QD-doped silicate fiber, the ALD technique process is complicated and hard to fabricate high-quality all solid-state QD-doped glass fibers with controllable composition, structure and broadband NIR emission. For rod-in-tube technique, both the core and cladding glasses of preform are heated and then drawn near the softening temperature. However, QD is so temperature-sensitive that small variations over its precipitation temperature would cause rapid abnormal crystallization. Therefore, the PL emission from QD-doped glass fibers drawn by rod-in-tube technique, whose fiber-drawing temperature is usually hundreds of degrees higher than the temperature for precipitation of QDs, is strongly suppressed.

A new ‘melt-in-tube’ technique, which was previously used to fabricate metal core (such as silicon, germanium, etc.) glass fibers, etc., is used to try to solve this issue [24–26]. In fact, the ‘melt-in-tube’ technique is evolved from rod-in-tube technique. The feature of ‘melt-in-tube’ technique is that, by a careful composition design between low-melting-temperature core and high-melting-temperature cladding glass of preform, the core glass is completely melted while the glass cladding tube is softened during fiber-drawing process. The QDs, which are precipitated during temperature-rise period, melt into core glass again. Then, after rapidly fiber-drawing process, uncontrollable growth of QDs that usually occurs in the fibers fabricated by rod-in-tube technique can be avoided. Furthermore, the QDs can be controllably precipitated in glass fiber core successfully by annealing the precursor fibers at a much lower temperature (between glass transition temperature and crystallization peak temperature).

In this paper, all solid-state PbS QD-doped glass fibers with broadband NIR emissions were successfully fabricated by melt-in-tube technique. However, we found that elements of PbS QD-doped glass fibers migrated between core glass and cladding glass in different extent, due to the vast difference between the melting temperature of core glass (lower than 1400 °C) and the softening temperature of cladding glass (higher than 1800 °C). Besides, volatilization of sulfur element happened during both glass-melting and fiber-drawing process, thus causing inhomogeneous precipitation of PbS QDs in the fibers. Herein, we divided the fiber samples into three parts correspond to different fiber length abscissas: beginning of drawing for 0 to 15 m, middle for 150 to 165 m and end for 300 to 315 m, and each was named as FB, FM and FE for beginning, middle and end of drawing, respectively). Electro-probe micro-analyzer (EPMA) images, Raman spectra and high-resolution transmission electron microscopy (HRTEM) images were measured to cooperatively characterize the element distribution and microstructures of the fibers, and the element-migration pathways along the fiber length abscissas were revealed. The broadband NIR emissions of PbS QD-doped glass fibers along the fiber length abscissas were also studied in detail, which indicated that the PbS QD-doped glass fibers drawn by melt-in-tube technique were expected to be applied as gain medium of broadband fiber amplifiers and fiber lasers.

2. Experimental section

In the previous studies, the SiO₂-B₂O₃-K₂O-BaO-ZnO glass system has been demonstrated as an excellent matrix to form PbS QDs by heat treatment [16, 17]. In this work, the normalized mole composition of core glass used for precursor fiber was 66SiO₂-8B₂O₃-18K₂O-6ZnO-2ZnS-1PbO (mol%). ZnS and PbO were used as source materials to precipitate PbS QDs in the glass fibers. A 100 g reagent grade stoichiometric mixture of SiO₂, B₂O₃, K₂O, ZnO, ZnS and PbO was well mixed in an agate mortar and melted in a covered corundum crucible at 1400 °C for 60 min in air. The melt was cast in to a preheated mold, forming amorphous solids. The as-synthesized block glass underwent a further annealing process to relieve inner stress. The block glass was then cold worked into a cylindrical rod with diameter of 4.0 mm in lathe. The surfaces of the rod were polished carefully and etched by acid in order to remove the contaminated surface layer. The glass rod was inserted into a high-purity (99.999%) silica glass cylindrical tube with inner diameter of 4.1 mm and external diameter of 30.0 mm as shown in Fig. 1(b). The bottom of glass tube was sealed to form a fiber preform. Then the preform was suspended in a resistance furnace heating at about 1830 °C with a heating rate of 10 °C/min. The cladding glass became softened while the core glass was presented as a melt at this temperature. By quickly drawn at a speed about 15 m/min, the precursor fibers were prepared. Owing to the high cooling speed of thin fiber core, the core glass maintained amorphous state easily. Finally, 30 cm long precursor fibers were put into sealed quartz tubes and then heat treated at 560 °C to 600 °C for 10 h to induce the controllable precipitation of PbS QDs in glass fibers. Detailed schematic diagram of the modified melt-in-tube technique was shown in Fig. 1(a). To study the formation of QDs and NIR emissions of PbS QD-doped glass fibers obtained at different stages, three kinds of glass fibers were selected according to the distance along the drawn fiber. The fibers were collected from the same preform after the beginning of the drawing. As the obtained fibers were about 350 m long in total, we selected the fiber samples as three parts correspond to different fiber length abscissas: beginning of drawing for 0 to 15 m, middle for 150 to 165m and end for 300 to 315 m, and each was named as FB, FM and FE for beginning, middle and end of drawing, respectively).

 

Fig. 1 (a) Schematic diagram of melt-in-tube technique. And digital photographs of glass preform (b) before and (c) after drawn by melt-in-tube technique. (d) HRTEM image of the PbS QD-doped glass fiber FB heat treated at 560 °C for 10 hours. (e) HRTEM of a single QD in the PbS QD-doped glass fibers in (d). (f-h) TEM images of the PbS QD-doped glass fiber FB, FM and FE heat treated at 560 °C for 10 h. The insets in (f-h) showed the size distribution of PbS QDs in glass fiber corresponding to TEM images.

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The morphology and size distribution of the PbS QDs in the glass fibers were characterized using HRTEM (Tecnai G2, FEI, Amsterdam, Netherlands) by grinding the fiber samples into powders. The element distribution of the fiber cross section was performed by an EPMA system (EPMA-1600, Shimadzu, Kyoto, Japan). Micro-Raman spectra of fibers and glasses were recorded by a Raman spectrometer (Renishaw in Via, London, UK) with an excitation of 785 nm laser. Besides, for further determining the distribution of PbS QDs at the fiber cross section, Raman spectra of different positions at the fiber cross section were performed by the Raman spectrometer. The NIR PL spectra of PbS QD-doped fibers were recorded by a spectrometer (Zolix, Omni k3007, Beijing, China) equipped with an AsGain photodetector (Hamamatsu R5108, Shizuoka-ken, Japan), SR830 Stanford Research lock-in amplifier (SRS, SR830, Sunnyvale, CA) and 808 nm laser (LEO Photoelectric, China). 2 cm long fiber samples were used for PL measurement. The PL of PbS QD-doped glass fibers was collected longitudinally at the end face of the fibers after an 808 nm laser with ~1 w pumping power was injected into the fiber core. All the measurements were performed at room temperature.

3. Results and discussions

Figures 1(b) and 1(c) showed the digital photographs of glass fiber preform before and after fiber-drawing process. Unlike PbS QD-doped bare glass fibers obtained by directly drawing fiber preform [9, 14], the fiber core in this paper maintained transparent after fiber-drawing process due to the high cooling rate of the melting of core glass, preventing precipitation of PbS QDs from core glass. It showed that such fibers were of higher quality, which inevitably led to completely dark and opaque fibers due to the uncontrollable crystallization. In the pursuit of achieving PbS QD-doped glass fibers with high transmission and low loss, the size of QDs is significant because large transmission loss will be caused by light scattering of QDs with different refractive index in glass matrix. As a result, the precipitation of QDs in the glass fibers must be controlled accurately to form homogeneous and small QDs.

Figure 1(d) showed the TEM image of fiber FB heat treated at 560 °C for 10 h, which indicated that homogeneous quasi-spherical PbS QDs were precipitated in the core of fiber. The HRTEM image shown in Fig. 1(e) displayed a clearly resolved lattice fringe with a constant spacing of 0.21 nm, corresponding to the (220) plane of PbS crystal. These structural characterizations highlighted that the successful precipitation of PbS QDs in the core of fibers. Figures 1(f)-1(h) were the TEM images and size distribution of the PbS QD-doped glass fiber FB, FM and FE heat treated at 560 °C for 10 h. Quasi-spherical PbS QDs dispersed in the glass fiber with average size of ~2.61 nm for fiber FB, ~2.55 nm for fiber FM and ~2.36 nm for fiber FE. The size distribution of QDs became smaller and narrower as the fiber samples were collected along different fiber abscissas: beginning, middle and end of drawing.

The fiber core can be as small as a dozen microns in magnitude, and the confocal micro-Raman spectra can detect a region within 1 μm, which is beneficial to study the microstructure in fibers. Figure 2(a) showed the micro-Raman spectra of fibers cross section and bulk glasses. It is noticed that the Raman spectra of both bulk PbS QD-doped glass (Fig. 2(a), line 1) and PbS QD-doped fiber core (Fig. 2(a), line 2) presented intense broad bands, which were attributed to the fluorescence background stemmed from the formation of PbS QDs during annealing [27]. As a result, PbS QDs were confirmed to be precipitated in fiber core successfully. The difference of the bands between PbS QD-doped glass (around 2660 cm⁻¹) and fiber core (around 2376 cm⁻¹) with the same heat treatment schedule was perhaps ascribed to the slight evaporation of PbS during fiber-drawing process. For the as-prepared bulk glass (Fig. 2(a), line 3), as-prepared precursor fiber core (Fig. 2(a), line 4) and fiber cladding (Fig. 2(a), line 5), there are four main Raman peaks at 460, 600, 790 and 1097 cm⁻¹. The four peaks were all attributed to the characteristic Raman peaks of Si-O-Si network [28]. Another two peaks were found in the Raman spectra of as-prepared glass (line 3) and fiber core (line 4). The band at 625 cm⁻¹ was ascribed to O-B-O bond bending vibrations [29]. And the band at 1370 cm⁻¹ was attributed to BØ₂O^- triangles linked to $\text{B}{Ø}_4^{-}$ units (Ø: an oxygen atom bridging two boron atoms, and O^-: a non-bridging oxygen atom) [29, 30]. Besides, no obvious fluorescence background of PbS QDs could be observed, indicating that no PbS QDs were precipitated in precursor fiber during fiber-drawing process. Thus, the melt-in-tube technique can effectively avoid the crystallization of PbS QDs during fiber-drawing process, which is beneficial to make the following crystallization of PbS QDs completely controllable by heat treatment schedule. As shown in Fig. 2(b), the fluorescence band of PbS QD-doped fibers shifted to longer region with heat treatment temperature increasing from 560 to 600 °C, indicating that the size of QDs enlarged with the increase of heat treatment temperature. These results mean that the crystallization of PbS QDs in fibers can be controlled finely through changing the heat treatment schedules. However, we found that fiber FB, FM and FE presented different Raman bands under the same heat treatment schedule. And the variation became more apparent with the heat-treated temperature increasing, meaning that QDs in different sizes were formed in fibers along the fiber length abscissas. After deep thinking, we suspected that sulfur volatilization probably happened during fiber-drawing process, and the most serious volatilization of sulfur happened at the end of the fiber. Thus, sulfur concentration in fiber FM and FE was relatively low and not sufficient for crystallization of PbS QDs after heat treatment. For further studies of element-migration in the fibers, EPMA and Raman spectra were conducted in order to provide detailed information regarding the formation and evolution of PbS QDs in the fibers.

 

Fig. 2 (a) Raman spectra of different regions in as-prepared bulk glass and precursor fiber core, bulk PbS QD-doped glass and PbS QD-doped glass fiber core and cladding, (line 1: bulk PbS QD-doped glass heat treated at 580 °C for 10h, line 2: PbS QD-doped fiber core heat treated at 580 °C for 10h, line 3: as-prepared bulk glass, line 4: as-prepared precursor fiber core, line 5: PbS QD-doped fiber cladding heat treated at 580 °C for 10h). (b) Raman shift corresponding to fluorescence band from PbS QDs of fiber FB, FM and FE collected as a function of heat treated temperature.

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Melt-in-tube method can avoid uncontrollable crystallization during fiber-drawing process, because the core glass is melted while the cladding glass is softened at fiber-drawing temperature. Nevertheless, element-migration maybe occur simultaneously. So, elemental analysis was conducted by EPMA mapping to present the spatial elements distribution in the cross section of fiber FB heat treated at 580 °C for 10 h (shown in Fig. 3). As can be seen in the backscattered electron image of fiber FB (Fig. 3(a)), the fiber and fiber core were both fine circle with diameter about 125.5 and 18.1 μm, respectively, which matched well with the original proportion of core-cladding in perform. As a result, the fiber dimensions including the fiber and core diameter can be well preserved along the fiber length. Figure 3(b)-3(f) were images of distribution in fiber FB cross section of K, Zn, Si, Pb, S elements. It can be seen that all the elements exhibited diffusion to some extent. Furthermore, the line scanning EPMA across diameter of fiber FB was measured, shown in Fig. 3(g). Similar to the EPMA mapping characterization, element-migration was also detected. K, Zn and Si elements exhibited slopy change, indicating that element-migration happened. Besides, it was noteworthy that the concentration of Pb and S elements used to form PbS QDs underwent slopy change from core glass to cladding. Thus, PbS QDs may be precipitated outside the fiber core. All these results above showed that element-migration happened during fiber-drawing process and the molten core glass composition entered into the softened cladding glass. The similar phenomena occurred in the fabrication of single-crystal germanium core optical fiber [31] and yttrium aluminosilicate glass optical fiber [32] using melt-in-tube method. As element-migration is a thermally activated process, the huge difference between the melting temperature of core glass (lower than 1400 °C) and the softening temperature of cladding glass (higher than 1800 °C) probably take responsible to the element-migration.

 

Fig. 3 (a) backscattered electron image of cross section of fiber FB heated at 580 °C for 10 h. (b-f) The EPMA images of different elements from the fiber FB cross section heat treated at 580 °C for 10 h. (g) The distribution of representative K, Zn, Pb, S, Si elements by line scanning EPMA across the diameter of fiber FB heat treated at 580°C for 10 h. The box in (a) is guide marks for the EPMA mapping zone.

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Compared with the EPMA, the confocal micro-Raman spectra are more sensitive to the formation of PbS QDs due to their intense fluorescence bands. Thus, Raman spectra were used to study the distribution of PbS QDs in the cross section of fibers along the fiber length abscissas under the same heat treatment schedule. We supposed that the change from the center of fiber core to the outer cladding would be homogeneous in each direction. The interacted dots detected in the optical images in Fig. 4 are based on the distance from the center of the fibers. The confocal center was marked as position 1, and the other dots were set at 3 μm intervals across the radius of the fiber cross section. We took fiber FB, FM and FE heat treated at 580°C for 10 h as examples. As presented in Fig. 4(a), intense fluorescence bands were detected beyond the fiber core (position 1 to 3), whereas they still appeared at position 4 to 7 which already belonged to fiber cladding, implying the formation of PbS QDs in fiber cladding. The fluorescence background of QDs disappeared after 21 μm (position 8) away from the center of fiber and the characteristic Raman peaks of silicate glass appeared completely, consistent with the Raman spectrum of fiber cladding in Fig. 2(a), line 5. The change in the Raman spectra, showing distance-related characteristic, was more apparent compared with line scanning EPMA (Fig. 3(g)). It can be deduced that the composition of the core glass gradually entered into the fiber cladding and PbS QDs were precipitated in the cladding as well.

 

Fig. 4 (a-c) Normalized Raman spectra at different positions of cross section of PbS QD-doped glass fibers heat treated at 580 °C for 10 h (a: fiber FB, b: fiber FM and c: fiber FE). The inset of (a-c) are the images of cross section of PbS QD-doped glass fibers FB, FM, FE heat treated at 580 °C for 10 h, respectively. (d) The normalized intensity (after deducting fluorescence background of QDs) of Raman peak location at 460 cm⁻¹ of fiber FB, FM and FE heat treated at 580 °C for 10 h collected as a function of position of cross section. (e) Schematic illustrations of element-migration pathways.

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Figures 4(b) and 4(c) are the Raman spectra at different positions of cross section of fiber FM and FE heat treated at 580 °C for 10 hours. Parallel to the Raman spectra of fiber FB (Fig. 4(a)), the changes also showed distance-related characteristic. The fluorescence bands in fiber FM (Fig. 4(b)) disappeared completely after position 7 while it disappeared after position 6 in fiber FE (Fig. 4(c)). Obviously, the change of the Raman spectra and the fiber length abscissas showed a significant correlation, i.e. the larger the fiber length abscissas was, the more sulfur volatilized. The optical images in the Figs. 4(a)-4(c) showed the core section of fiber FB, FM and FE heat treated at 580 °C for 10 h, respectively. The unchanged fiber and core diameter of fiber FB, FM and FE meant the good homogeneity of the obtained fiber. It is clear that the color of fiber core changed from dark brown to pale yellow, indicating the decrease of PbS QDs along the fiber length abscissas. These results were consistent with the Raman spectra in Figs. 4(a)-4(c). It is noteworthy that the normalized intensity of the Raman peak at 460 cm⁻¹, which was assigned to the Si-O-Si bending vibration modes of [SiO₄] unit [28], was a function of the distance from the fiber center. As the distance increased, the normalized intensity of the peak at 460 cm⁻¹ enhanced. The curves of the normalized intensity of the peak at 460 cm⁻¹ of fiber FB, FM and FE heat treated at 580 °C collected as a function of position were summarized in Fig. 4(d). Here we see that the normalized intensity of the 460 cm⁻¹ peak of fiber FB increased abruptly just after the interface of the fiber core and cladding (9 μm away from the center, position 4) and reached the top at position 8 (21 μm away from the center). The same tendency was also observed in the fiber FM and FE, however, the positions of the abrupt increase (position 3 for FM and position 2 for FE) and those of highest peak (position 6 for FM and position 5 for FE) occurred earlier. These results can be explained by the diffusion and volatilization of sulfur that happened simultaneously during the fiber fabrication (Fig. 4(d)). Pb²⁺ was glass network modifier as its relatively low concentration in glass and the single-bond strength of Pb-O was as low as 36 kcal g⁻¹ atom [19], which caused easiness of Pb²⁺ migration into cladding glass under thermal effect. Meanwhile, S^2- tended to bond with Pb²⁺ to form PbS QDs, resulting in S^2- diffusion away from core glass [19, 33]. In addition, the volatilization of S element increased along the fiber length abscissas, resulting in the lack of S element to form PbS QDs in fiber FE.

To obtain wavelength-tunable NIR emissions from PbS QD-doped glass fibers, the precursor fibers were carefully heat treated at 560 to 600 °C for 10 h to induce the precipitation of QDs with different sizes. Figures 5(a)-5(c) show the PL spectra of fiber FB, FM and FE heat treated at various temperature under the excitation of an 808 nm laser. Broadband emissions appeared in the spectra of fiber FB, FM and FE, which were attributed to the recombination of the electron-hole pairs of PbS QDs induced by the excitation photon. With the heat-treated temperature increasing, all the emission bands of fiber FB, FM, and FE red shifted and broadened due to the growth of PbS QDs. Surprisingly, slight shifts of emission bands and different full width at half-maximum (FWHM) of fiber FB, FM and FE under the same heat treatment were detected. The FWHM and PL peak of fiber FB, FM and FE with different heat-treated temperature were summarized in Fig. 5(d). The PL peaks and FWHM presented obvious temperature-related characteristic. The change of PL peaks was consistent with the change of Raman peaks shown in Fig. 2(b). When the heat-treated temperature increased from 560 to 600 °C, FWHM broadened from about 200 nm to over 270 nm, which was ascribed to the broadening of size distribution of PbS QDs. Such broad FWHM bandwidth was in favor of the development for fiber amplifiers with broader and flatter gain band. Under the same heat treatment schedule, the PL peak and FWHM gradually decreased along the fiber length abscissas, which were consistent with the change of QD sizes and size distribution shown in Figs. 1(f)-1(h). These results further confirmed that volatilization of S element happened during fiber-drawing process owing to the vast difference between the melting temperature of core glass and the fiber-drawing temperature.

 

Fig. 5 (a-c) Normalized PL spectra of PbS QD-doped glass fibers heat treated at different temperature for 10 h (a: fiber FB, b: fiber FM and c: fiber FE) excited by an 808 nm laser. (d) FWHM and PL peak of fiber FB, FM and FE collected as a function of heat treated temperature.

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4. Conclusions

All solid-state PbS QD-doped glass fibers were successfully fabricated by using the novel melt-in-tube technique. The fiber core glass was melted while the cladding was softened at fiber-drawing temperature. By the virtue of melt-in-tube technique, the rapid uncontrollable growth of QDs in fibers was suppressed. Raman spectra confirmed that no PbS QDs precipitated after fiber-drawing process. And by further heat treatment schedules, controllable PbS QDs with different sizes were formed in the fibers. Excited by an 808 nm laser, broadband NIR emissions were obtained from PbS QD-doped glass fibers. However, owing to the huge difference between the melting temperature of core glass and the fiber-drawing temperature, element-migration and volatilization of sulfur happened during the fiber-drawing process. To suppress the element-migration and volatilization during fiber-drawing process, the fiber-drawing temperature should be lowered. In order to solve the problem, a new core glass for inducing the formation of PbS QDs with low-melting-temperature, which precisely matches well with low-softening-temperature of a new cladding glass, should be collaboratively developed. In the future, PbS QD-doped glass fibers with excellent emission property and fine waveguide structure are expected to be fabricated by melt-in-tube method. And this kind of PbS QD-doped glass fiber will be a promising active medium for fiber lasers and broadband fiber amplifiers. More importantly, the melt-in-tube method exhibits a feature of completely controllable crystallization in fiber formation process, which would open a new way for fabricating various functional QD-doped glass fibers.