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Abstract
We describe the fabrication and characterization of single-mode Ti:sapphire crystalline fibers, cladded by dip coating and high-temperature sintering at 1750 oC. Solid-state single crystal growth was observed at the perimeter of the crystalline core, and the growth speed along ($1\bar{1}0$) is about 2.7 times faster than that of (001). The grown cladding was single-crystalline as evidenced by electron backscattered diffraction and scanning electron microscopy examinations. From optical transmission measurement at 1550 nm, the far-field distribution of the transmitted light matches well with that of the fundamental mode. With a core size of 30 µm, the refractive index difference between core and clad was measured to be 1.0 × 10−4. From fluorescence mapping on the fiber end face, a very limited amount of the Ti3+ ions in core were diffused into the grown crystalline cladding during the solid-state growth.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fibers with crystalline core and clad have high potential for power applications. When compared with the photonic crystal fibers with glass core [1–4], the crystalline fiber has, in general, a higher melting point and larger dielectric breakdown field, that make it advantageous for high power light source applications. However, most fibers use glass as a cladding [3,5], which are still affected by the fringe field. Air-cladding can avoid this damage, but it induces high losses [6–7]. For standing the multi-ten-kilowatt level laser pumping and avoiding the stimulated Brillouin scattering (SBS) due to high SBS gain coefficient of the silica-glass. Using ceramic or crystal cladding, laser power can be scaled up without damaging the fiber while keeping a low transmission loss [8]. For single-mode structures, the laser intensity and the amplified spontaneous emission can be maintained at high level, whereas for optical amplification, it is essential to use single-mode waveguiding to achieve high signal gain.
In cladding the crystal core with glass, it is hard to find the appropriate material ensuring a small refractive index difference between core and cladding necessary to achieve single mode. Because of the large difference of glass and crystal dispersion curves, the window for single-mode propagation is narrow [9–12]. Thus, it is preferable to use clad and core materials similar in composition and structure. Since the refractive index of the dopant, Ti2O3, is bigger than that of the host, Al2O3, light doping with Ti ions can slightly increase the refractive index of the core, making it larger than the clad. As the dispersion curves of core and cladding are similar, the single-mode wavelength range is wide.
In addition, it is essential to match crystal orientation of core and sapphire cladding, and this will ease achieving single mode operation with a large core diameter of 30 µm. For sapphire crystalline core (i.e. Al2O3 molecular formula), the birefringence makes it not feasible of using ceramic cladding for single-mode operation. The refractive indice of sapphire o-ray (no) and e-ray (ne) are 1.746 and 1.738 at 1550 nm [13]. Figure 1 shows the relation between the core-clad refractive index difference and core diameter for single-mode operation of large-core fibers. To fabricate single-mode Ti:sapphire fibers with a core diameter of 30 µm, it is necessary to keep the refractive index difference smaller than 1.14 × 10−4. However, the difference of ceramic cladding index (i.e. average of the o-ray and e-ray indices and Ti doped a-cut crystalline core index (no) is 4.0 × 10−3, which is too large in o-ray propagation for single-mode operation. For e-ray propagation, the refractive index of core (ne) is smaller than that of ceramic cladding ($\frac{{{{\boldsymbol n}_{\boldsymbol o}} + {{\boldsymbol n}_{\boldsymbol e}}}}{2}$), and it cannot be a guided propagation mode. It should be noted that the index-anti-guided ceramic cladded sapphire fiber could still be served as a single-mode waveguide provided that the gain guiding effect is strong enough [14].
2. Single-mode crystal fiber fabrication
The Ti:sapphire crystal fiber core was fabricated by the laser-heated pedestal growth (LHPG) process, using a 25-watt, continuous-wave CO2 laser as the heating source [15]. The feed rod was an a-cut bulk sapphire crystal with a cross-section of 500 × 500 µm2 and the growth chamber was open to the atmosphere. The seed rod was an a-cut 30-µm-diameter Ti:sapphire single crystalline fiber. Ti:sapphire crystalline core cross section was elliptical with an axial length ratio of approximately 5:4 [15].
To fabricate the fiber clad, dip-coating process with nanoparticles solution was adopted. The solution was composed of 70 wt.% pure alumina powder (AKP, Sumitomo Chemical), 29.9 wt.% de-ionized water, and 0.1 wt.% SiO2 as a sintering aid [16–17]. The length of the Ti:sapphire single-crystal core was 5 cm. After each dip-coating process, the fibers were baked with dry air at 200 °C. To increase the thickness of the cladding, 20 to 30 times of dip-coating process were applied on the crystal core. Finally, they were sintered in air for 2, 6, 12, and 24 hours at 1750 °C or 24 hours at 1650 °C, as shown in Fig. 2. Since 1750 °C is significantly lower than the melting point of sapphire crystal, i.e. 2050 °C, no change on the core size, dopant concentration, and crystal orientation were observed.
Fig. 2. Dip-coating process for the sapphire cladding formation. Steps 1–3 were repeated for 20 to 30 times.
3. Results and discussion
Using a scanning electron microscope (SEM, JSM-7800F Prime, JEOL) with the ImageJ software, we analyzed the pore and grain distributions, and the thickness of solid-state growth of the ceramic cladding layer. The crystallographic orientation of the core and the cladding, their interface, and the regions of the multi-domain ceramic cladding were determined by electron backscatter diffraction (EBSD, JSM-7800F Prime, JEOL). The EBSD patterns were mapped with Kikuchi lines to determine the orientation from core to cladding. Finally, a l550-nm excitation light was coupled into the cladded fiber to observe the mode pattern. With the near field and far field profiles, we calculated the M2 value and checked for the single-mode operation.
3.1 Transmission mode characterization
The dip coating followed by high temperature processes generated a double-cladding structure, where the inner is crystalline and the outer is ceramic. The solid-state growth occurred in the inner cladding with an orientation dictated by the crystalline core. The refractive index difference is only determined by the small concentration of Ti ions in the core. A reflectance confocal system was used to measure the refractive index differences. Figure 3 shows the measured refractive index profile across the fiber diameter. The refractive index fluctuation of the ceramic layer is mainly due to the pores, different orientations of grains, and the grain boundary scattering. At a core diameter of 30 µm, the measured refractive difference of 1.0 x10−4 made it a single-mode waveguide.
To observe the near- and far-field patterns, we used an InGaAs CCD camera with a 1550-nm laser as the illuminator. The near-field pattern was obtained by imaging the light intensity profile at the output facet of the crystal fiber using a 10x lens; while the far-field pattern was measured by placing the CCD 3 cm away from the output facet of the fiber. Single-mode patterns were obtained from both measurements, and typical results are shown in Figs. 4(a)-(b). The ImageJ software was used to fit the far-field pattern in Fig. 4(b). The normalized intensity of the far-field pattern and Gaussian fitting are shown in Figs. 4(c)-(d).
Fig. 4. Mode patterns of the ceramic cladded fiber at a wavelength of 1550 nm. (a) Near field measurement at the crystal fiber end face, (b) far field measurement at 3 cm from the end face. The normalized intensity of far field distribution along the (c) ($1\bar{1}0$) and (d) (001) directions.
The OptiFiber software was adopted to calculate the anisotropic beam waists. At the fiber end face, they were respectively 30 µm and 24 µm along the ($1\bar{1}0$) and (001) directions, while the beam radii at 3 cm from the fiber end face were 0.549 mm and 0.673 mm along the ($1\bar{1}0$) and (001) directions. The single-mode beam propagation in the far field is
where z is the distance from the fiber end face, and ${z_R}\; $ is the Rayleigh length, ${\frac{\pi }{\lambda }\omega_0}^2$. Based on the single-mode beam propagation of Eq. (1), $\omega(z )$ was 0.494 mm and 0.617 mm along the ($1\bar{1}0$) and (001) directions. The corresponding beam divergence angles were 0.0173 rad and 0.0216 rad; while the numerical aperture (NA) were 0.0173 and 0.0216.The ${M^2}$ factor was calculated with divergence angle, beam radius, and propagation wavelength. The ${M^2}$ factor is
where ${\theta _0}$ is the beam divergence angle. The ${M^2}$ factor we calculated are 1.06 and 1.06 along ($1\bar{1}0$) and (001) directions. Thus, the ceramic cladded fibers are single mode fibers.3.2 Cladding structure characterization
When ceramic-cladded fibers are sintered at high temperature, solid-state single crystal growth (SSCG) can be induced [18]. The grain size grows and the interspace between the grains is reduced, while some pores are generated [19–20]. As shown in Fig. 5, the central oval shapes mark the single crystal core, where pores along the core edges can be observed. The migration of the atoms was governed by a driving force resulting from difference in free energy of different states. The pores originated at the interspace between ceramic cladding grains, and were reduced in size by annealing. SSCG occurred when the driving force for the growth of a particular grain is bigger than the critical driving force. The atoms attached to the grain surface entered a high-energy state and they were able to break the link with the surrounding structures. The sintering aid, SiO2, helped atomic diffusion and densification of ceramics. As a result, some pores were removed [21]. The average residual pores of fibers at a sintering temperature of 1750 °C for 24 hours were 0.5 µm, which cause scattering and propagation losses. The measured transmission loss of fibers was 4.7 dB/cm. To eliminate the pores, smaller powders may be needed and under a vacuum environment.
Fig. 5. SEM end-face images of crystal and ceramic cladded fibers. Red dashed lines mark the crystalline core. (a) Sintering at 1650 °C for 24 hours, at 1750 °C for (b) 2 hours, (c) 12 hours, and (d) 24 hours.
From Figs. 5(a) to (d), the grain size and the thickness of crystal cladding increases with longer high-temperature sintering, because the atoms had a higher energy to break their bounds to the surrounding structures. With long sintering time, the pore size decreased, as shown in Figs. 5(b) and (c). The thickness along the (001) and ($1\bar{1}0$) directions were estimated to be 110 nm and 40 nm, respectively.
As shown in Fig. 6, the anisotropic SSCG is plotted with respect to the sintering time. The diameter of crystal cladding along ($1\bar{1}0$) is larger than that along (001). It indicates that the activation energy for atom diffusion across the grain-grain boundary and the driving force for atom diffusion are smaller along ($1\bar{1}0$) [18,22–25].
Fig. 6. Thickness of crystal cladding in the (001) and ($1\bar{1}0$) directions at a sintering temperature of 1750 °C. The markers denote the experimental results and the solid lines are fitting curves.
The crystallographic orientations of the core and the composite clads are depicted in Fig. 7. The inverse pole figure [26] was employed to quantize the crystal orientation of the core, crystal cladding, and ceramic multi-domain. As expected, the orientations of the core and the grown inner cladding are the same due to SSCG. With the high temperature sintering, the binding energy of ceramic grains was broken, and grains were re-arranged to the same crystal orientation of the core single crystal. The ambient ceramic domains adapted a similar orientation as the temperature rises, as shown in Fig. 7(b). Outside the ceramic region, these domains need longer sintering times or higher temperatures to stimulate the re-arrangement. Because of the same orientation of core and inner cladding, refractive index difference is small enough to achieve single-mode operation at a core diameter of 30 µm, as shown in Fig. 1.
Fig. 7. (a) EBSD end-face image of ceramic crystal cladded fiber, which was sintered for 2 hours at 1750 °C, and (b) crystal orientation map displayed in inverse pole figure coloring.
3.3 Fluorescence mapping of Ti3+ distribution
A confocal microscopic system (Zeiss LSM 880) was used to obtain the reflective fluorescence mapping with a 30-mW, 514-nm excitation light, and 525-nm long-wavelength-pass filter. When the crystalline fibers were as grown, it had Ti4+ clusters. They were sintering at a temperature of 1600 °C in a hydrogen atmosphere before the dip-coating process. The Ti4+ were reduced to Ti3+ ions. However, after the dip-coating process, the fibers were sintered in air. The Ti3+ ions were oxidized to Ti4+. As shown in Fig. 8, the fluorescence intensity is very weak in the core, and the slight fluorescence from the cladding is due to the native impurities. The weak fluorescence is attributed to the low Ti3+ concentration, and plenty of Ti4+-Ti4+ pairs and other non-fluorescent Ti ions [14].
Fig. 8. Reflective fluorescence (a) mapping and (b) intensity profile. The yellow line in (a) marks the line scan location in (b).
The Ti4+ clusters were reduced to Ti3+ ions in the core after sintering at a temperature of 1600 °C in an argon/hydrogen chamber as evidenced from the enhanced core fluorescence intensity. As shown in Figs. 9, the fluorescence intensity of the core is 6.2 times stronger than that of cladding. This also indicates that there was a very limited out-diffusion during the sintering and hydrogen annealing. For a sintering temperature below 1750 °C, Ti ions did not diffuse out of the core, and the ambient ceramic cladding can be converted into a single-crystal cladding layer.
Fig. 9. Reflective fluorescence (a) mapping and (b) intensity profile with fibers for 2 hours of sintering at 1750 °C, then followed by hydrogen annealing. The yellow line in (a) marks the line scan location in (b). The inset in (b) shows the fluorescence mapping around the crystalline core.
4. Conclusions
Single-mode crystalline fibers have been successfully fabricated with the solution based dip coating process at a sintering temperature of 1750 °C. Evidence of solid-state growth on the perimeter of the crystalline core was observed. The grain size and the thickness of solid-state growth increased with high-temperature sintering, and the growth speed along ($1\bar{1}0$) was faster than that along (001) due to the smaller driving force among atoms. The solid-state growth produced an inner cladding with the same orientation direction as the crystalline core. A 1550-nm laser was coupled into the fiber to observe single-mode output patterns at both the near and far fields. During the sintering process, the Ti3+ dopant did not diffuse out to the inner cladding, showing a good dopant confinement. The realization of a fully crystalline single-mode fiber could be useful for applications where high power and/or high brightness light sources are needed.
Funding
Ministry of Science and Technology, Taiwan (MOST 107-2634-F-002-017).
Acknowledgments
This work was partially supported by the Ministry of Science and Technology, Taiwan under grant # MOST 107-2634-F-002-017. The authors would like to thank Prof. C. S. Lin and Ms. Y. T. Lee of Instrumentation Center, National Taiwan University for supporting the FEG-SEM measurements.
Disclosures
The authors declare no conflicts of interest.
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