The fabrication and characteristics of Ce/Cr-doped crystal fibers employing drawing tower technique are reported. The fluorescence spectrum of the Ce/Cr fibers at the core diameter ranging from 10 to 21 µm exhibited a 200-nm near-Gaussian broadband emission which enabled to provide an axial resolution of 1.8-μm and a power density of 79.1 nW/nm. The proposed broadband Ce/Cr-doped crystal fibers may be provided as a high-resolution light source for the use in optical coherence tomography system as well as industrial inspection and biomedical imaging applications.
© 2015 Optical Society of America
Recently, optical coherence tomography (OCT) has has rapidly advanced in noninvasive biomedicine imaging technology. For fiber-based OCT applications, several types of broadband light sources with a high axial resolution have been used, such as super-luminescent diodes (SLDs), photonic crystal fiber, and Ti: sapphire laser [1–3]. However, the characteristics of these light sources did not exhibit Gaussian spectrum. The resolution of fiber-based OCT system became poor owing to the pixel crosstalk resulting from the high side-lobe noise of non-Gaussian spectrum . In contrast to non-Gaussian spectrum, an amplified spontaneous emission (ASE) can exhibit both broadband and near-Gaussian spectrum. A fiber laser was employed to collect near-Gaussian ASE light in order to improve the power intensity of a light source for a fiber-based OCT system. A Ce:YAG (yttrium aluminum garnet, YAG) material was used as the source rod to fabricate the cerium-doped fibers (CeDFs) which was developed as a broadband light source of ultra-high resolution OCT systems by using laser heated pedestal growth (LHPG) techniques  and drawing-tower with rod-in-tube (RIT) technique . However, a smaller core diameter (< 20 μm) with better uniformity of the core diameter for the CeDFs were difficult to reach during LHPG. In addition, the fluorescent intensity of the CeDFs fabricated by drawing tower could be reduced owing to cerium oxide was highly volatile and Ce:YAG was highly reactive to SiO2, resulting in the inter-diffusion between core and cladding. Recently, the chromium-doped fibers (CrDFs) have been successfully fabricated by a drawing tower with powder-in-tube (PIT) technique . The fluorescence intensity of CrDFs was a notable improvement about a hundred times better compared with the RIT method. However, the transmission loss and the non-circularity of CrDFs were still important issues needed to be conquered . Therefore, a novel redrawing technique which modifies the PIT method by incorporating the RIT process to reduce the transmission loss and a new formula of powders for improving the fluorescence intensity of Ce/Cr-doped fibers are essential.
In this Letter, the formation of crystal powders with X-ray diffraction (XRD) examination verified by the forsterite (Mg2SiO4) and leucite (KAlSi2O6) crystalline structures are used to fabricate the CeDFs and CrDFs. To characterize the CeDFs and CrDFs, the fluorescent spectrum is measured. The result shows the redrawing PIT technique can substantially improve the power level of emission up to 79.1 nW/nm and generate near-Gaussian broadband emissions mainly spanning the visible and near infrared regions (600-950 nm and 950-1300 nm). The wavelength regime is advantageous to reach to a maximum penetration depth in biological tissue . According to point-spread function, it is known that the near-Gaussian spectrum is much helpful to obtain high-quality images. A fiber-based OCT system with both the CeDF and CrDF light sources are used to achieve 200-nm broadband light source with a highest axial resolution of 1.8 µm. In comparison with the previous works on CeDFs having a bandwidth of 90 nm, an axial resolution of 1.4 µm, and a power density of 13.9 nW/nm , this work significantly improve the performance and is more promising for commercial applications. This indicates that the CeDFs and CrDFs fabricated by a fiber drawing-tower may be one step forward to being utilized as a near-Gaussian broadband light source for high resolution OCT system as well as for industrial inspection and biomedical imaging applications. For fiber amplifier application, such broadband fluorescence in the CeDFs and CrDFs are also important to provider a broadband amplifier for the use in next-generation optical communication systems.
2. Crystal powder and fiber fabrication
All reagents were commercial products purchased from SHOWA. The compositions of forsterite (Mg2SiO4) and leucite (KAlSi2O6) powder in weight percentage were prepared. Stoichiometric amounts of the required cation sources were combined and ground together with a small amount of ethanol by an alumina crucible and pestle until the ethanol and deionized water evaporated and mixtures were almost dry. Then, the mixed powder was poured into another alumina crucible and dried at 120°C. The forsterite powder was heated up to 1300°C and soaked for 2 hours. The leucite powder was heated up to 1650°C and soaked for 2 hours. Then, relatively slow cooling rate (3°C/min) was adopted to prevent quenching during the synthesis process. Final, the well-mixed crystal powder was poured into the center of the silica tube with outer diameter of 20 mm and inner diameter of 8 mm to assemble the powder/silica preform. The inter-diffusion between core and cladding is an inevitable issue under a high temperature operation during fiber-drawing process. The SiO2 in cladding may diffuse into the core region and become one of the new compositions in the core. Therefore, the silicon oxide is chosen as the core of crystal fiber. The influence of diffusion of silica tube into the core may be negligible to affect the forsterite and leucite crystals during the drawing process. Furthermore, this process helps eliminate the crucible contamination problems associated with core-glass fabrication before fiber-drawing process.
In fabrication of crystal fibers, the heating temperature of the furnace was raised to 1650°C for two hours of fabrication. The silica tube with bubbles eliminated can actually act as a silica crucible in the powder melting process. Then, the powder-filled silica tube was drawn into crystal fiber rod at approximately 1900°C by employing a fiber draw-tower (Nextrom OFC20) equipped with a preformed internal pressure control unit. Figure 1 shows the microphotographs for powder/silica preforms at different core diameters after fiber drawing. A crystal fiber rod with a 0.6 mm core and a 1.8 mm outer diameter was drawn. The crystal fiber rod was shrunk into a combination of silica tubes with an outer diameter of 8-mm and an inner diameter of 2 mm, to be assembled as the preform. The drawing speed was around 0.1 m/min. Then, the capillary filled with crystal rod was drawn into crystal fibers. Finally, there were dual layers of UV curable acrylate coating for maintaining pristine surface during take-up and storage steps. Tens of meters for three kinds of crystal fibers have been drawn by using drawing-tower method. The first kind of CeDF was composed of Ce-doped forsterite powder as the material for a 16-μm core, as shown in Fig. 2(a). The second kind of CeDF’s was Ce-doped leucite powder for a 21-μm core, as shown in Fig. 2(b). The third kind of CrDF’s was Cr-doped forsterite powder for a 10-μm core, as shown in Fig. 2(c). Because the core diameter and the refractive index difference between core and cladding of the Ce/Cr-doped crystal fiber are large. The Ce/Cr-doped crystal fibers are multimode.
3. Measurements and results
An XRD with Bruker D8 diffractometer was used to determine the crystallographic phase of core of CeDFs and CrDFs. The XRD pattern of Ce and Cr-doped forsterite at fiber-core are shown in Fig. 3(a) and 3(c), respectively. The data matched well with the standard diffraction of Mg2SiO4 and MgSiO3 (JCPDS Card No. 34-0189 and No. 11-0273) . Each of the strong diffraction peaks were indexed to the Mg2SiO4 lattice parameters a = 5.981 Å, b = 10.19 Å, and c = 4.755 Å. The XRD pattern of Ce-doped leucite at fiber-core is shown in Fig. 3(b). The data matched well with the standard diffraction of KAlSi2O6 (JCPDS Card No. 38-1423) . Each of the strong diffraction peaks were indexed to the Mg2SiO4 lattice parameters a = 13.065 Å, b = 13.065 Å, and c = 13.755 Å. The characteristic peaks in the spectrum corresponded to each individual crystal planes. The diffraction peaks suggested that the poly-crystallized Mg2SiO4 and KAlSi2O6 were high in quality as well as quantity in each core of Ce/Cr-doped crystal fibers.
To characterize the fluorescence of CeDFs and CrDFs, the fluorescence were measured from a optical spectrum analyzer (OSA) with a 200 mW laser at the wavelength of 446-nm and 633-nm as the excitation source. Figure 4 shows the fluorescence spectrum of the Ce:Mg2SiO4 crystal fiber pumped with 446-nm laser. It exhibited a broadband emission from 600 to 800 nm with a power density of 79.1 nW/nm at the central wavelength of 670 nm and the full width at half maximum (FWHM) was 80nm. In comparison with the previous work on fabrication of crystal fiber using RIT method , this work truly reflects a significant improvement in the power density of the crystal fiber using redrawing PIT method. The fluorescence spectrum of a 3-cm Ce:KAlSi2O6 crystal fiber pumped by 446-nm laser was measured and exhibited a broadband emission from 700 to 950-nm, as shown in Fig. 4. The central wavelength was peaked at 806-nm with a FWHM of 200 nm and its power density was reached to 63.8 nW/nm. We employed a 633-nm laser as the light source to measure the fluorescence spectrum of Cr:Mg2SiO4 crystal fiber. The fluorescence spectrum shows a broadband emission from 950 to 1300 nm and its power density is 27.3 nW/nm, as indicated in Fig. 4. Compared with Ce:Mg2SiO4 crystal fiber, the weak fluorescence leads to the large noise with curve in Cr:Mg2SiO4 crystal fiber. The broadband emission peaked at1050-nm wavelength was dominated with Cr4+ ion and its FWHM is 160 nm. It is well known that Ce3+ and Cr4+ ions would exist in Mg2SiO4 host. This reveals that the Ce3+ and Cr4+ substituted for Mg2+ at octahedral sites to form the olivine structure and then generate broadband emissions starting from visible light all the way to near infrared wavelength regime. Each of the broadband spectra can be used as a light source for OCT system applications.
Table 1 lists a number of clinical applications by possible employing Ce/Cr-doped fiber for the use in OCT system, such as ophthalmology, dermatology, dentistry, cardiology, and industrial inspection [12–16]. In OCT imaging systems, the optical bandwidth of the light source is inversely proportional to the axial resolution. To supply high axial resolution the fabricated Ce/Cr-doped fiber can provide a near-Gaussian broadband light source extending to the ideal bandwidth with the best penetration depth in biotissues for high resolution OCT imaging applications.
A designed fiber-based OCT system with laser pumping source and a 2 x 2 3-dB coupler is shown in Fig. 5. A 446-nm and 633-nm laser with 200mW was respectively focused by L1 to pump the crystal fiber. L2, L3, and fiber collimators were aligned to compensate for the optical path length difference between the reference and signal arms, especially when the bandwidth was very broad. The coherence signals and function were analyzed by the oscilloscope and computer. According to Fig. 6, the Ce:Mg2SiO4, Ce:KAlSi2O6, and Cr:Mg2SiO4 fibers has a measured axial resolution of 1.9 µm, 1.8 µm, and 3.0 µm in the air, respectively. According to near-infrared window in biological tissue, the proposed broadband fluorescence light source is enabling to provide suitable wavelength range, simple configuration, and high-contrast imaging quality.
In conclusion, the CeDFs and CrDFs have been successfully fabricated by drawing tower with the modified redrawing PIT method. The CeDFs showed an emission from 600 to 800 nm and 700 to 950 nm with a power density of 79.1 nW/nm and 63.8 nW/nm, respectively. The CrDFs exhibited an emission from 975 to 1300 nm with a power density of 27.3 nW/nm which was dominated by Cr4+ near-Gaussian ASE spectrum. For fiber-based OCT system, the measured broadband emission of CeDFs and CrDFs showed three different high axial resolutions of 1.8, 1.9, and 3-µm in air. Furthermore, fiber-based OCT system with the broadband fluorescence sources spanning the visible and near infrared regions could provide the deepest penetration in biomedical specimens. In this study, the initial success in the development of the CeDFs and CrDFs indicates that the fabricated crystal fibers may be widely applicable as a new generation broadband source for the use in various spectroscopic OCT applications. Therefore, the proposed broadband and high axial-resolution light source has a great potential for future OCT source applications, such as biological research and industrial inspection.
This work was partially supported by the Department of Industrial Technology of MOEA under the Contract 101-EC-17-A-19-S1-209, and the Ministry of Science and Technology under the Contract MOST 103-2221-E-110-003.
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