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Abstract
Cr4+:Y2SiO5 crystal fiber was grown by the laser-heated pedestal growth method. Severe spinodal decomposition domains were observed at low growth speed. At a growth rate of 30 mm/min, the spinodal decomposition area ratio was reduced to 0.4%. Using borosilicate as the cladding, 0.85 mW of broadband emission with a 3-dB bandwidth of 246 nm centered at 1257 nm was generated. The near-infrared light source is eminently suitable for optical coherence tomography with reduced scattering for deep tissue penetration. The estimated axial resolution is 2.8 µm in free space.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Cr4+:Y2SiO5 (YSO) crystal could emit around 1.25 µm with broad bandwidth and a nearly Gaussian spectrum, which makes it suitable for high-axial-resolution optical coherence tomography (OCT) [1–3]. The near-infrared emission is also advantageous for deep tissue penetration [4]. Warshaw et al. synthesized YSO with a chemical method in 1961 [5]. YSO has excellent high-temperature stability and low oxygen permeability [6]. The thermal expansion coefficient of YSO matches well with silicon-based materials. Methods for growing YSO include the Czochralski method [7–9], the hydrothermal technique [10], and the flux method [11]. High-quality YSO is hard to grow because defects develop during the growth process. The common defects are dislocations by stress [7], voids, and growth striation [9], which is generated due to the fluctuation of growth parameters such as rotation rate and growth rate during the Czochralski growth. In this paper, the laser-heated pedestal growth (LHPG) method [12] was used to grow the YSO single crystal fiber. The defect reduction and spectral properties of the Cr4+:YSO crystal fiber are discussed. Compared with the Cr4+ doped crystals of YAG [13], GSGG [14], and forsterite [15,16], the 1.25-µm central wavelength of Cr4+:YSO has better tissue penetration. When grown into the fiber shape, the high area-to-volume ratio of the fibers enables a high heat dissipation efficiency [17].
2. Fabrication of glass-clad Cr4+:YSO crystal fibers
A bulk Cr4+:YSO crystal was cut into rectangular rods with a 500×500 µm2 cross-section to serve as the source material. The rectangular rod was grown into a crystal fiber with a 120-µm diameter by the LHPG method at the growth rates of 10, 20, and 30 mm/min. To clad the single crystalline Cr4+:YSO fiber, the borosilicate capillary made of Corning 7740 was adopted.
It is widely known as the Pyrex borosilicate glass. The borosilicate capillary was heated by the co-drawing LHPG method to produce the glass-clad [12].
For the LHPG method, the crystallographic orientation of the grown crystal fiber depends on the orientation of the seed rod. Since the point group of YSO is C2h, a high symmetry direction ($0\bar{1}1$) was selected as the source to prepare the LHPG seed. To determine the crystallographic axis of the Cr4+:YSO bulk, the focused ion beam (FIB, Versa 3D DualBeam) was used to prepare the transmission electron microscope (TEM) samples. Figure 1(a) shows the TEM (Philips Tecnai F30 field emission gun) image as well as the selected area electron diffraction (SAED) pattern. From the TEM image, the atomic arrangement of a single crystal can be clearly overserved. From the SAED image, the crystallographic axis of the long-axis of the Cr4+:YSO bulk was determined to be ($0\bar{1}1$) by the angles and lengths of the crystal reciprocal parameters of the diffraction pattern [18]. The CaRIne Crystallography software was used to construct the atomic structure of a Cr4+:YSO, as shown in Fig. 1(b). Moreover, the crystal orientation ($0\bar{1}1$) was inserted into the CaRIne Crystallography subroutine to ascertain the reciprocal lattice as shown in Fig. 1(c), which matches well with the SAED pattern of Fig. 1(a).
Fig. 1. (a) TEM image of the Cr4+:YSO bulk. The inset shows the SAED pattern. (b) The structure model of Cr4+:YSO by the CaRIne Crystallography software. (x, y, z) shows the laboratory coordinate, while (a, b, c) depicts the crystallographic coordinate. (c) The SAED simulation of ($0\bar{1}1$) oriented Cr4+:YSO by the CaRIne Crystallography software.
3. Crystal and defect properties
The end-face images of the Cr4+:YSO single-crystal fiber were observed by scanning electron microscope (SEM, Hitachi TM-3000). As shown in Fig. 2, irregular patterns and parallel lines were observed around the fiber center. The irregular patterns could result from the stress striations in the Cr4+:YAG during the LHPG process [19]. The stress striation in YAG with (111) growth direction is due to the fluctuation of the growth parameters, and it represents the history of crystal growth and the shape and change of the solid-liquid interface [20]. It causes stress-induced dislocation and forms stress striation. Other origins of the striations may be the fluctuations of temperature caused by the heating source and impurity inclusions [21]. As the growth rate increased, the irregular domains decreased in size, as shown from Fig. 2(a) to Fig. 2(c). The parallel lines could be explained by spinodal decompositions. Due to the high-temperature annealing and laser source fluctuation, the Y2SiO5 decomposed into Y2O3-rich and SiO2-rich regions. It was predicted that spinodal decomposition [22,23] might occur in Y2O3-SiO2 system [24–26], whose phase diagram has a miscibility gap, such as ZrO2-SiO2 [27] and La2O3-SiO2 [28]. Because the two-phase compositions produced in the core also caused stresses in the interface of the two phases. Thus, stress striations were created.
Fig. 2. The SEM image of Cr4+:YSO single crystal fibers with different growth rates (a) 10, (b) 20, and (c) 30 mm/min. The center region of (a) shows irregular shape; while parallel lines exhibit in (c).
At the highest growth rate (i.e., 30 mm/min), the irregular shapes disappeared and left only very thin defects barely visible. To further study the irregular patterns and parallel lines, the white parts in Fig. 2 are designated as group A, while group B is single-crystal Cr:YSO.
ImageJ software was used to calculate the areas of group A and group B in Fig. 2. The area ratios of groups A and B at different growth rates are shown in Fig. 3. Apparently, the areas of group A decrease linearly when the growth rate increases. This could be explained by the fact that the heating temperature fluctuation became smaller, and less residual stress remained in the core at a higher growth rate [29]. The reduction of irregular shapes and parallel lines improved the qualities of the single crystal fibers.
Fig. 3. The area ratio of Cr4+:YSO crystal fibers at different growth rates. The red line shows a linear fit.
To characterize the concentration profile of yttrium oxide (Y2O3), silicon dioxide (SiO2), and chromium oxide (CrO2), the electron probe X-Ray microanalyzer (EPMA, JEOL JXA-8200) was utilized to scan across the parallel lines. From Fig. 4(a), the concentrations of CrO2 and Y2O3 are higher in group A than those in group B. However, the concentrations of SiO2 were lower than that in group B. When temperature cooled down during the growth process, part of Y2O3 separated out from YSO [30] and entered into the irregular patterns and parallel lines [31]. Thus, SiO2 and Y2O3 have little incongruence, which caused spinodal decompositions in group A regions [24].
Fig. 4. (a) The SEM image of a Cr4+:YSO single-crystal fiber at the growth rate of 30 mm/min. (b) Concentration profiles of Y2O3, SiO2, and CrO2 across the crystal fiber core measured by EPMA. The red arrows mark the CrO2-rich regions.
To investigate the atomic structures of groups A and B near their interfaces, the FIB was used to cut the interface between groups A and B for TEM sample preparation. In Fig. 5, the TEM images of group B are sorted out and clearly show the atomic arrangement. By individually investigating groups A and B, there existed single crystal and amorphous structures simultaneously in group A as shown in Figs. 5(b) and 6(a). According to Fig. 4(b), Y2O3 and CrO2 were rich in group A, while SiO2 was deficient in group A. The initial single-crystal became amorphous and full of defects simultaneously [32], while the spinodal decomposition showed the residual strain. During the fiber polishing process, it was aware that the core was more easily broken due to residual stress. The crystals produced by the Czochralski methods also had dislocations and stress [30,33]. When the growth rate increased, it reduced the CrO2 diffusion, which tremendously alleviated the spinodal decomposition. In Fig. 6(b), it is clear that the SAED and TEM images are consistent with those of bulk YSO shown in Fig. 1(a).
Fig. 5. The TEM images of interface between group A and group B with a growth rate of 10 mm/min at (a) x10000 and (b) x500000 magnifications.
Fig. 6. The TEM images of (a) group A and (b) group B. The inset in (b) shows the SAED pattern of group B.
4. Broadband emission characteristics of Cr4+:YSO crystal fibers
To study the emission characteristics, Cr4+:YSO single-cladding crystal fibers (SCFs) with a core diameter of 125 µm and a growth rate of 30 mm/min were used to compare with a Cr4+:YSO bulk crystal. The SCFs were cut into lengths of 6, 12, 24, and 36 mm so that the pump absorption, signal emission, and propagation characteristics could be evaluated. A 1064-nm Yb-doped fiber laser was used as the pump source, and it was followed by an aspheric lens (5720-C, New Focus, f = 15.4 mm, NA = 0.16) to focus light into the Cr4+:YSO SCF. An aspheric lens (5720-B, New Focus, f = 2.8 mm, NA = 0.65) was used to collect the amplified spontaneous emission (ASE). An 1100-nm long-wavelength-pass filter was utilized to block the residual pump light. As shown in Fig. 7(a), the emission peak of the Cr4+:YSO bulk crystal was located at 1246 nm, and the 3 dB bandwidth was 147 nm, which was in good agreement with that in literature [34]. For the ASE measurements, the respective emission peaks of Cr4+:YSO SCFs with lengths of 6, 12, 24, and 36 mm were 1257, 1265, 1288, and 1294 nm, and the 3-dB bandwidth were 246, 237, 235, and 230 nm, as shown in Fig. 7(b).
The redshift of the emission peaks could be explained by the quasi-three level configuration of Cr4+:YSO, similar to that in the Er3+ doped silica fibers [35,36]. The emission of the front section of the Cr4+:YSO SCF excites the back section of the SCF and emits at a longer wavelength. The length-dependent red shift makes Cr4+:YSO SCF flexible to meet the need of various applications to have the broader testing range or deeper device/tissue penetration.
To numerically analyze the emission dynamics of the Cr4+:YSO SCF, a distributed model was adopted. As shown in Fig. 8, Cr4+:YSO is a quasi-three level laser material with pump and signal excited-state absorptions (ESAs) [37]. The energy levels which can interact with the pump or the fluorescence are numbered as levels 0 to 5. Since the lifetimes τ of levels 3, 4, and 5 are very short compared with the lifetime (∼0.6 µs), we assume N3 ≈ N4 ≈ N5 ≈ 0. Therefore, the total electron density (NT) can be approximated as NT = Ng + N2, where Ng and N2 are the ground state and excited state population density, respectively.
Fig. 8. Simplified Cr4+:YSO quasi-three level energy diagram. Solid arrows: radiative transitions. Dotted arrows: non-radiative transitions. σa is absorption cross-section, ${\lambda _p}$ is wavelength of the pump, σe is emission cross-section, ${\lambda _s}$ is the wavelength of the signal, $\mathrm{\sigma }_{esa}^{\; p}$ is pump excited state absorption, $\mathrm{\sigma }_{esa}^{\; s}$ is signal excited state absorption, τr is radiative lifetime, and τnr is a non-radiative lifetime.
Based on Fig. 8, the rate equation describing the dynamics of the excited state population density can be expressed as
By equating (1) to zero, the steady-state equation is derived as
The pump light propagating in the Cr4+:YSO crystal fiber encounters ground state absorption (${\sigma _a}$), ESA, and propagation loss. The equation describing the evolution of the pump power Pp in the crystal fiber at the axial position z is:
The finite-difference time-domain method was employed to simulate the laser characteristics. As shown in Fig. 9, the simulated ASE fits well with that of the experimental data. At present, the low optical conversion efficiency is mainly attributed to the high propagation loss. The 4.9-dB/cm propagation loss of Cr4+:YSO was large because the stress and strain at interface between group A and B caused small cracks and scattering due to spinodal decomposition [38–40]. To decrease the propagation loss, a high growth speed is necessary to eliminate the spinodal decomposition. As a result, the optimized length of Cr4+:YSO SCF was 6.5 mm, as shown in Fig. 10. The simulated ASE and residual pump were 0.776 mW and 1684 mW at a pump power of 5.75 W. The optical fitting parameters of Cr4+:YSO crystal fibers are revised from those of Cr4+:YSO bulk [41] and those of Cr:YAG crystal fibers [42]. The parameters of Cr4+:YSO crystal fibers are summarized in Table 1. The absorption cross-sections of Cr4+:YSO crystal fiber and bulk are 7 × 10−19 cm-2 and 1.3 × 10−19 cm-2, and ESAs are 4.1 × 10−19 cm-2 and 0.65 × 10−19 cm-2, respectively. Emission cross-sections are 4.02 × 10−19 cm-2 and 1.0 × 10−19 cm-2, respectively.
Fig. 9. (a) ASE (b) residual pumps of Cr4+:YSO SCF with different pump powers. The solid lines are measurement and the dash lines are simulation.
Table 1. Summary of optical parameters of glass-clad Cr4+:YSO and Cr4+:YAG crystal fibers.
Table 1 also shows a comparison of the optical parameters between Cr4+:YSO and Cr4+:YAG crystal fibers. The emission cross-section of Cr4+:YSO is higher than that of Cr4+:YAG, and both pump and signal ESAs of Cr4+:YSO are lower than those of Cr4+:YAG. However, the propagation loss of Cr4+:YSO is higher than that of Cr4+:YAG, and the lifetime of Cr4+:YSO is shorter than that of Cr4+:YAG. Both result in the low ASE power of Cr4+:YSO. The lifetime of Cr4+:YSO at high temperatures is very short [44]. Heat accumulation would reduce the ASE power. The crystal fiber structure has good heat dissipation, so the ASE power is enhanced by a cooler, as shown in Fig. 11. The maximum ASE power with thermal cooling was 0.85 mW at a pump power of 5.75 W.
For biomedical imaging using OCT, near-infrared wavelengths are preferred to have deep penetration [45–48]. The large emission bandwidth of Cr4+:YSO SCF is advantageous to have a high axial resolution. Using the emission spectrum of Cr4+:YSO SCF for OCT applications, the calculated point spread function (PSF) is shown in Fig. 12(a). The axial resolution was 2.7 µm in free space. The first adjacent pixel cross-talk was -25.4 dB, the second adjacent pixel cross-talk was -32.1 dB, and the third adjacent pixel cross-talk was -34.5 dB, shown in Fig. 12(b).
Fig. 12. (a) Calculated point spread function (dashed green line) of Cr4+:YSO SCF. (b) A Nyquist sampling of the point spread function for axial pixel cross-talk estimation.
5. Conclusion
Cr4+:YSO SCF was fabricated with the LHPG technology, and the spinodal decomposition in YSO was analyzed with SEM, TEM, and EPMA. During the LHPG process with a high growth rate, the impact of the CO2 laser power fluctuation was reduced, and the molten zone became stable. At the highest growth rate of 30 mm/min, the unintended domain area can be down to 0.4%, the irregular shape defect can be eliminated, and the parallel lines can be much thinned. As a broadband light source, 0.85 mW of ASE was generated from the glass-clad Cr4+:YSO crystal fiber. The emission peak was at 1257 nm, and the 3-dB bandwidth was 246 nm with a near Gaussian distribution. For OCT applications, it can achieve an axial resolution of 2.7 µm for cellular-resolution imaging in deep tissue.
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 Ms. S.-J. Ji of Ministry of Science and Technology (National Taiwan University) for the assistance of FIB experiments, and Mr. H. R. Chen of Ministry of Science and Technology (National Taiwan University) for the assistance of TEM experiments.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. J. M. Schmitt, A. Knuttel, M. Yadlowsky, and M. A. Eckhaus, “Optical-coherence tomography of a dense tissue: statistics of attenuation and backscattering,” Phys. Med. Biol. 39(10), 1705–1720 (1994). [CrossRef]
2. A. Dubois, G. Moneron, and C. Boccara, “Thermal-light full-field optical coherence tomography in the 1.2 µm wavelength region,” Opt. Commun. 266(2), 738–743 (2006). [CrossRef]
3. N. D. Gladkova, G. A. Petrova, N. K. Nikulin, S. G. Radenska-Lopovok, L. B. Snopova, Y. P. Chumakov, V. A. Nasonova, V. M. Gelikonov, G. V. Gelikonov, R. V. Kuranov, A. M. Sergeev, and F. I. Feldchtein, “In vivo optical coherence tomography imaging of human skin: norm and pathology,” Skin Res. Technol. 6(1), 6–16 (2000). [CrossRef]
4. R. R. Anderson and A. P. John, “The optics of human skin,” J. Invest. Dermatol. 77(1), 13–19 (1981). [CrossRef]
5. I. Warshaw and R. Roy, “Polymorphism of the rare earth sesquioxides,” J. Phys. Chem. 65(11), 2048–2051 (1961). [CrossRef]
6. G. Chen and J. Zhang, “Investigation on optical temperature sensing behaviour for Y4.67Si3O13:Tm3+,Yb3+ phosphors based on upconversion luminescence,” Opt. Mater. Express 8(7), 1841–1849 (2018). [CrossRef]
7. Y. K. Kuo, J. Y. Chang, C. C. Lin, and H. M. Chen, “Tunable Cr:YSO Q-switched Cr:BeAl2O4 laser: numerical study on laser performance along three principal axis of the Q switch,” Jpn. J. Appl. Phys. 39(7A), 4002–4005 (2000). [CrossRef]
8. C. D. Brandle, A. J. Valentino, and G. W. Berkstresser, “Czochralski growth of rare-earth orthosilicates (Ln2SiO5),” J. Cryst. Growth 79(1-3), 308–315 (1986). [CrossRef]
9. Z. Shoudu, W. Siting, S. Xingda, W. Haobing, Z. Heyu, Z. Shunxing, and X. Jun, “Czochralski growth of rare-earth orthosilicates-Y2SiO5 single crystals,” J. Cryst. Growth 197(4), 901–904 (1999). [CrossRef]
10. H. Pang, G. Zhao, M. Jie, J. Xu, and X. He, “Study on the growth, etch morphology and spectra of Y2SiO5 crystal,” Mater. Lett. 59(28), 3539–3542 (2005). [CrossRef]
11. B. A. Maksimov, Y. A. Kharitonov, V. V. Ilyukhin, and N. V. Belov, “Crystal structure of the Y-Oxysilicate Y[SiO4]O,” Sov. Phys. Dokl. 13, 1188–1190 (1969).
12. K. Y. Hsu, M. H. Yang, D. Y. Jheng, C. C. Lai, S. L. Huang, K. Mennemann, and V. Dietrich, “Cladding YAG crystal fibers with high-index glasses for reducing the number of guided modes,” Opt. Mater. Express 3(6), 813–820 (2013). [CrossRef]
13. Y. C. Huang, Y. K. Lu, J. C. Chen, Y. C. Hsu, Y. M. Huang, S. L. Huang, and W. H. Cheng, “Broadband emission from Cr-doped fibers fabricated by drawing tower,” Opt. Express 14(19), 8492–8497 (2006). [CrossRef]
14. W. Chen, K. Spariosu, R. Stultz, Y. K. Kuo, M. Birnbaum, and A. V. Shestakov, “Cr4+:GSGG saturable absorber Q-switch for the ruby laser,” Opt. Commun. 104(1-3), 71–74 (1993). [CrossRef]
15. N. V. Kuleshov, V. G. Shcherbitsky, V. P. Mikhailov, S. Hartung, T. Danger, S. Kuck, K. Petermann, and G. Huber, “Excited-state absorption and stimulated emission measurements in Cr4+:forsterite,” J. Lumin. 75(4), 319–325 (1997). [CrossRef]
16. K. Cerqua-Richardson, B. Peng, and T. Izumitani, “Spectroscopic investigation of Cr4+-doped glasses,” OSA Proceedings on Advanced Solid-State Lasers 13, 52–55 (1992).
17. S. C. Wang, C. Y. Hsu, T. T. Yang, D. Y. Jheng, T. I. Yang, T. S. Ho, and S. L. Huang, “Laser-diode pumped glass-clad Ti:sapphire crystal fiber laser,” Opt. Lett. 41(14), 3217–3220 (2016). [CrossRef]
18. H. K. Bhadeshia, Geometry of Crystals, Polycrystals, and Phase Transformations (CRC Press, 2017).
19. J. C. Chen, C. Y. Lo, K. Y. Huang, F. J. Kao, S. Y. Tu, and S. L. Huang, “Fluorescence mapping of oxidation states of Cr ions in YAG crystal fibers,” J. Cryst. Growth 274(3-4), 522–529 (2005). [CrossRef]
20. Y. Peizhi, D. Peizhen, Y. Zhiwen, and T. Yulian, “The growth defects in Czochralski-grown Yb:YAG crystal,” J. Cryst. Growth 218(1), 87–92 (2000). [CrossRef]
21. D. Jun, D. Peizhen, and X. Jun, “The growth of Cr4+,Yb3+:yttrium aluminum garnet (YAG) crystal and its absorption spectra properties,” J. Cryst. Growth 203(1-2), 163–167 (1999). [CrossRef]
22. A. Cartalade, A. Younsi, É Régnier, and S. Schuller, “Simulations of phase-field models for crystal growth and phase separation,” Procedia Mater. Sci. 7, 72–78 (2014). [CrossRef]
23. J. Ballato, C. McMillen, T. Hawkins, P. Foy, R. Stolen, R. Rice, L. Zhu, and O. Stafsudd, “Reactive molten core fabrication of glass-clad amorphous and crystalline oxide optical fibers,” Opt. Mater. Express 2(2), 153–160 (2012). [CrossRef]
24. N. A. Toropov and I. A. Bondar, “Silicates of the rare earth elements,” Bull. Acad. Sci. USSR, Div. Chem. Sci. 10(4), 502–508 (1961). [CrossRef]
25. J. J. Chambers and G. N. Parsons, “Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon,” J. Appl. Phys. 90(2), 918–933 (2001). [CrossRef]
26. U. Kolitsch, H. J. Seifert, T. Ludwig, and F. Aldinger, “Phase equilibria and crystal chemistry in the Y2O3-Al2O3 -SiO2 system,” J. Mater. Res. 14(2), 447–455 (1999). [CrossRef]
27. H. Kim and P. C. McIntyre, “Spinodal decomposition in amorphous metal-silicate thin films: phase diagram analysis and interface effects on kinetics,” J. Appl. Phys. 92(9), 5094–5102 (2002). [CrossRef]
28. J. Liu, X. Wu, W. N. Lennard, and D. Landheer, “Surface-directed spinodal decomposition in hafnium silicate thin films,” Phys. Rev. B 80(4), 041403 (2009). [CrossRef]
29. H. E. Cook, “Brownian motion in spinodal decomposition,” Acta Mater. 18(3), 297–306 (1970). [CrossRef]
30. N. I. Leonyuk, E. L. Belokoneva, G. Bocelli, L. Righi, E. V. Shvanskii, R. V. Henrykhson, N. V. Kulman, and D. K. Kozhbakhteeva, “High-temperature crystallization and X-ray characterization of Y2SiO5, Y2Si2O7 and LaBSiO5,” J. Cryst. Growth 205(3), 361–367 (1999). [CrossRef]
31. D. A. Porter and K. E. Easterling, Phase Transformations in Metals and Alloys (revised reprint) (CRC Press, 2009).
32. J. Androulakis, C. H. Lin, H. J. Kong, C. Uher, C. I. Wu, T. Hogan, B. A. Cook, T. Caillat, K. M. Paraskevopoulos, and M. G. Kanatzidis, “Spinodal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectrics: enhanced performance in Pb1-xSnxTe-PbS,” J. Am. Chem. Soc. 129(31), 9780–9788 (2007). [CrossRef]
33. G. J. Zhao, X. H. Zeng, S. M. Zhou, J. Xu, Y. Tian, and W. X. Huang, “Growth defects in Czochralski-grown Ce:YAlO3 scintillation crystals,” Phys. Stat. Sol. (a) 199(2), 186–191 (2003). [CrossRef]
34. B. H. T. Chai, Y. Shimony, C. Deka, X. X. Zhang, E. Munin, and M. Bass, “Laser performance of Cr4+:Y2SiO5 at liquid nitrogen temperature,” OSA Proceedings on Advanced Solid-State Lasers (1992).
35. M. Yamada, O. N. O. Hirotaka, M. O. R. I. Atushi, T. Kanamori, S. U. D. O. Shoichi, and Y. Ohishi, “Ultra-broadband and gain-flattened EDFAs for WDM signals,” OSA on Optical Amplifiers and Their Applications (1997).
36. S. Yamashita and M. Nishihara, “L-band erbium-doped fiber amplifier incorporating an inline fiber grating laser,” IEEE J. Sel. Top. Quantum Electron. 7(1), 44–48 (2001). [CrossRef]
37. N. I. Zhavoronkov, V. P. Mikhailov, N. V. E. Kuleshov, B. I. Minkov, and A. S. Avtukh, “Absorption saturation in Cr4+-activated silicate crystals and use of these crystals in passive laser switches,” Quantum Electron. 25(1), 31–33 (1995). [CrossRef]
38. M. E. Lines, “Scattering losses in optic fiber materials. II. numerical estimates,” J. Appl. Phys. 55(11), 4058–4063 (1984). [CrossRef]
39. S. Morris, T. Hawkins, P. Foy, C. McMillen, J. Fan, L. Zhu, R. Stolen, R. Rice, and J. Ballato, “Reactive molten core fabrication of silicon optical fiber,” Opt. Mater. Express 1(6), 1141–1149 (2011). [CrossRef]
40. M. Lancry, J. Canning, K. Cook, M. Heili, D. R. Neuville, and B. Poumellec, “Nanoscale femtosecond laser milling and control of nanoporosity in the normal and anomalous regimes of GeO2-SiO2 glasses,” Opt. Mater. Express 6(2), 321–330 (2016). [CrossRef]
41. W. M. Yen and W. Jia, “Advances in the spectroscopy of Cr4+-doped laser materials,” J. Appl. Spectrosc. 62(5), 966–978 (1995). [CrossRef]
42. D. Y. Jheng, K. Y. Hsu, Y. C. Liang, and S. L. Huang, “Broadly tunable and low-threshold Cr4+:YAG crystal fiber laser,” IEEE J. Sel. Top. Quantum Electron. 21(1), 16–23 (2015). [CrossRef]
43. C. C. Lai, C. P. Ke, S. K. Liu, D. Y. Jheng, D. J. Wang, M. Y. Chen, Y. S. Li, P. S. Yeh, and S. L. Huang, “Efficient and low-threshold Cr4+:YAG double-clad crystal fiber laser,” Opt. Lett. 36(6), 784–786 (2011). [CrossRef]
44. W. Jia, “Near infrared luminescence properties of the laser material Cr:Y2SiO5,” J. Lumin. 55(5-6), 293–301 (1993). [CrossRef]
45. G. Zonios, A. Dimou, I. Bassukas, D. Galaris, A. Tsolakidis, and E. Kaxiras, “Melanin absorption spectroscopy: new method for noninvasive skin investigation and melanoma detection,” J. Biomed. Opt. 13(1), 014017 (2008). [CrossRef]
46. I. V. Meglinski and S. J. Matcher, “Quantitative assessment of skin layers absorption and skin reflectance spectra simulation in the visible and near-infrared spectral regions,” Physiol. Meas. 23(4), 741–753 (2002). [CrossRef]
47. A. N. Bashkatov, E. A. Genina, V. I. Kochubey, V. S. Rubtsov, E. A. Kolesnikova, and V. V. Tuchin, “Optical properties of human colon tissues in the 350-2500 nm spectral range,” Quantum Electron. 44(8), 779–784 (2014). [CrossRef]
48. K. Y. Hsu, D. Y. Jheng, M. H. Yang, Y. S. Lin, K. Y. Huang, Y. H. Liao, and S. L. Huang, “Bidirectionally pumped Cr4+:YAG crystal fiber light source for optical coherence tomography,” J. Opt. Soc. Am. B 28(2), 288–292 (2011). [CrossRef]
