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The fluorescence enhancement in broadband Cr-doped fibers (CDFs) fabricated by a drawing tower with a redrawn powder-in-tube preform is proposed and demonstrated. The CDFs after heat treatment exhibited Cr4+ emission enhancement with spectral density of 200 pW/nm, verified by the formation of α-Mg2SiO4 nanocrystalline structures in the core of CDFs. The high fluorescence achievement in the CDFs is essential to develop a broadband CDF amplifier for next-generation optical communication systems.
©2013 Optical Society of America
1. Introduction
Recently, chromium-doped fibers (CDFs) have been fabricated by a drawing-tower method with rod-in-tube (RIT) [1–3] or powder-in-tube (PIT) [4] techniques. The single-mode characteristics of CDFs were achieved by RIT with broadband emissions mainly in NIR to IR regions (800-1200 nm and 1100-1600 nm) [3]. However, the concentration of Cr in Cr:YAG rod was low because of the thermal expansion mismatch between Cr:YAG rod and glass clad [5]. Therefore, the fabricated CDFs using drawing tower with RIT technique showed the limitation of Cr-ion concentrations in CDFs. In contrast to the RIT method [3], the fluorescence intensity of CDFs fabricated by PIT method was improved by about two orders of magnitude [4], although the transmission loss and the non-circularity of core were high. However, the high-temperature process could degrade the fluorescent intensity owing to chromium oxide was highly volatile, and Cr:YAG was highly reactive to SiO2 at high drawing temperatures, resulting in the inter-diffusion between core and cladding. Recently, a new formula of Cr-doped powder based on magnesium silicate has been applied to improve the fluorescence of the Cr-doped glass ceramics [6]. It was shown that the fluorescence of the Cr-doped glass ceramics could be enhanced after a heat-treatment process. Based on bulk material of the magnesium silicate to enhance Cr4+ fluorescence, it is a strong motivation to apply the thermal process to fiber-type material for fluorescence enhancement in broadband CDFs. Such high fluorescence in the CDFs is important to develop a broadband CDF amplifier for next-generation optical communication systems [7, 8].
In this study, the CDFs employed magnesium silicate powder-in-tube integrated with multiple steps design by drawing tower were fabricated [9]. After heat treatment, the fabricated CDFs significantly exhibited Cr4+ emission enhancement with spectral density of 200 pW/nm at a central wavelength of 1050 nm, which was verified by the formation of α-Mg2SiO4 nanocrystalline structures in the core of CDFs. For optical coherence tomography (OCT) application, such a broadband emission represents an isotropic resolution of 5 µm in air. Though the present spectral density is good for micro-structured thin film characterization, further improvement on power yield can facilitate its use for biomedical applications.
2. Fabrication of Chromium-doped fibers
A redrawing method was employed to assemble the Cr-doped powder/silica preform. The uniformity of Cr-doped powder is necessary to avoid bubble produced in the high process temperature around 2000°C. A composition of magnesium silicate powder, SiO2-Al2O3-MgO-K2O-TiO2-Cr2O3, was selected [6]. Regarding optical fiber fabrication process, the inter-diffusion between core and cladding is an inevitable issue at high fiber-drawing temperature. The SiO2 of cladding may diffuse into the core region and become one of the new compositions in the core. Accordingly, the composition of non-silica magnesium silicate powder was poured into the center of the silica tube with outer diameter of 20 mm and inner diameter of 8 mm. The powder compositions in weight percentage are listed in Table 1 . Figure 1 shows a schematic diagram of a Cr-doped powder preform. The powder melted in the silica tube during the drawing process. This process helped to eliminate the crucible contamination problems associated with core-glass fabrication before fiber-drawing process.
Table 1. Powder compositions
For fabrication of CDFs, firstly, the powder-filled silica tube was drawn into Cr-doped cylinder rod at approximately 2000 °C by employing the fiber draw-tower (Nextrom OFC20) equipped with preform internal pressure control unit. The silica tube actually acted as a silica crucible for melting. A Cr-doped cylinder rod with a 100-μm core and a 500-μm outer diameter was drawn. The Cr-doped rod was sleeved into a combination of silica tubes with outer diameter of 8-mm and inner diameter of 1-mm to assemble as the preform. Then, the preform was redrawn into a core size of 10-μm with an outer diameter of 125 μm, as shown a cleaved end-face of the CDF in Fig. 2(a) . There were dual layers of UV curable acrylate coating for maintaining pristine surface during take-up and storage. As the drawing speed was around 0.5 to 1 m/min, the CDFs could not obtain crystalline structure immediately from quenching melt in fiber drawing processes. It was reported that the fluorescence of the bulk material of Cr-doped glass ceramics could be enhanced after a heat-treatment process [6]. Therefore, it is motivation to apply the thermal process to fiber-type material for fluorescence enhancement in broadband CDFs. In heat-treatment process, the CDFs were put into a high-temperature furnace for to form the crystalline structure. The temperature was maintained for 4 hours at 525°C for nucleation and at 900°C for growth [6]. Figure 2(b) showed the cleaved end-face of the CDF after heat-treatment process.
Fig. 2 Cleaved end-face of a CDF with a 10-μm core: (a) before heat-treatment, (b) after heat-treatment.
3. Measurements and results
A few hundred meters of the CDFs were drawn by using the drawing-tower with the redrawing method. A Ti-sapphire laser (Tsunami, Spectra-Physics) was employed as the light source for measurement of CDFs fluorescence intensity profiles. The fluorescence spectrum of an 8-cm CDF without heat treatment pumped by 980-nm light source with 0.1 W was measured. There was no apparent peak emission around 1050-nm, as shown in Fig. 3(a) . The fluorescence spectrum of 8-cm CDF with heat treatment exhibits a broadband emission from 900-nm to 1300-nm, as shown in Fig. 3(b). The band of 900-1300 nm was dominated with Cr4+ ions [10, 11] and its power density was 200 pW/nm. The fluorescence intensity of CDFs with heat treatment was a significant improvement compared with the CDFs without heat treatment.
To characterize the composition of the CDFs, an electron probe X-ray micro-analysis (EPMA, JXA-8900R) was employed. Figure 4(a) and 5(a) show the EPMA line-scan of CDFs without and with heat treatment respectively. The relative weight percentage of silica in core of CDFs without and with heat treatment was estimated to be around 86.78% and 87.35%, respectively. Figure 4(b) and 5(b) showed enlarged figures of the picture in Fig. 4(a) and 5(a). It indicates that the relative weight percentage of Al, K, Mg, Ti, and Cr-ions are not restricted within 10-μm-core for the CDFs after heat treatment.
A HRTEM (JEOL JEM-3010) equipped with a LaB6 electron gun operating at 300 kV was utilized to characterize the crystal and microstructure of CDFs. The structure of CDFs core without heat treatment was a pure amorphous structure, as shown in Fig. 6(a) . The core of the CDFs with heat treatment exhibited some nano-structures embedded in SiO2 matrix, as shown in Fig. 6(d). Figures 6(b) and 6(e) show the enlargement of the white squares in Figs. 6(a) and 6(d). The selected-area electron diffraction (SAED) pattern taken from a large area to include enough nano-crystals was verified the identity of the crystal structure, as shown in Fig. 6(f). The ring pattern had four measured interplanar spacings consisted with a cubic unit cell, specifically Cr4+:α-Mg2SiO4 (space group is Pbnm) [11]. The refined cell parameter of crystalline structures, as shown in Fig. 6(e), was about a = 0.449 nm which is verified similar to the crystal olivine structure (α-phase) of forsterite (Mg2SiO4) [12]. Owing to thermal effects during the heat treatment process, the d-spacing would be slight changed. Another interesting observation was that the shapes of the nano-crystals were irregular which was resulted strongly from the temperature on the re-crystallization. In fact, the α-Mg2SiO4 nano-crystalline structures with higher density were not easy to be obtained directly from quenching melt in optical fiber drawing process. However, this issue could be solved by proper balance between powder selection in initial composition and silicate replenishment in drawing process [13].
Fig. 6 HRTEM images showing: (a) core of a CDF without heat-treatment, (b) enlarged area from (a), (c) SAED pattern from (a), (d) core of a CDF with heat-treatment, (e) enlarged area from (d), (f) SAED pattern from (d).
To identify the elements in the core of CDFs, a scanning electron microscope (SEM, JEOL-6330) equipped with energy dispersive X-ray analysis (EDX) was employed. The samples of core of CDFs without and with heat treatment were prepared by the focused ion beam (FIB) technique. Figure 7(a) and 7(b) showed the EDX spectra of CDFs without and with heat treatment. The EDX spectra exhibited that Si, Al, K, Mg, Ti, and Cr ions still co-existed in the core of CDFs after heat treatment. It was well known that Cr3+, Cr4+ and Cr6+ ions could exist in silicate host [10, 14]. However, the crystalline structures in CDFs were hardly to be obtained directly from quenching melt in fiber drawing processes. In this study, the Cr4+:α-Mg2SiO4 nano-crystalline structure was actually observed in the core after heat treatment from HRTEM images, which revealed that the Cr4+-ion substituted with Si4+ at tetragonal sites to form the olivine structure [11]. The structural diagram of olivine structure is shown in Fig. 8 . In order to form more stable tetrahedron structure, the Si4+ ion was replaced with Cr4+ ion to form Cr4+:α-Mg2SiO4 phase as nano-crystalline structure, which significantly increased the Cr4+ emission band of fluorescence spectra in the CDFs after heat-treatment, as shown in Fig. 3(a) and 3(b). These results revealed that the CDFs fabricated by drawing-tower with magnesium silicate powder formulation could exhibit a broadband fluorescence.
Fig. 8 Structural environment of Cr4+ ion in olivine structure that Si4+ ions substituted by Cr4+ ions.
4. Conclusion
In summary, the nano-crystalline CDFs with a 10-μm core and a 125-μm cladding have been successfully fabricated by using the PIT method employing redrawing process and heat treatment. The CDFs fabricated by magnesium silicate powder exhibited an emission from 900 to 1300 nm with power density of 200 pW/nm which was dominated by Cr4+ fluorescence. The fluorescence intensity of the CDFs with heat treatment has a significant improvement compared with those with no heat treatment. EPMA line-scan and EDX spectra indicated that the relative weight percentage of Al, K, Mg, Ti, and Cr ions was restricted around 10-μm-core. The HRTEM images revealed that a Cr4+:α-Mg2SiO4 nano-crystalline structure actually existed in the core after heat treatment. The initial success in fabrication of CDFs promotes the potential for utilizing the CDF as new generation broadband fiber amplifier, fiber laser, and broadband source for ultrahigh resolution OCT. For OCT application, such a broadband emission exhibits an isotropic resolution of 5 µm in air. Although the present spectral density is good for micro-structured thin film characterization, further improvement on power yield to reach tens of microwatts can facilitate its use for biomedical applications. In order to improve the fluorescence efficiency, a refining process toward the formation of uniform nano-crystalline of Cr-doped fibers with high concentration in silica fiber is necessary and currently under investigation.
Acknowledgments
This work was partially supported by the Department of Industrial Technology of MOEA under the Contract 97-EC-17-A-07-S1-025 and 101-EC-17-A-19-S1-209, the National Science Council under the Contract NSC99-2221-E-110-026-MY3.
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