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Abstract
Highly Yb-doped silica glass with low refractive index for fabrication of Yb-doped large-mode-area photonic crystal fiber (LMA PCF) is in favor of decreasing fiber length and thus increasing the threshold of nonlinear effects in pulse laser amplification. Accordingly, fluorine incorporation in highly Yb-doped silica glass is vitally important to compensate the sharp increase in refractive index caused by ytterbium and aluminum ions. In this work, the fluorine doping concentration in Yb/Al/P/F co-doped silica glass was significantly improved by a modified sol-gel method combined with high temperature sintering. The effects of fluorine doping on glass structure have been investigated in details by Raman spectra, nuclear magnetic resonance (NMR) and advanced pulse electron paramagnetic resonance (EPR) measurements. The incorporation of fluorine yields Si-F bonds and Yb-F bonds formation and leads to the mild change in spectroscopic properties. An optimized silica core glass rod with high Yb (0.77 wt.%) and fluorine (0.8 wt.%) doping concentration, low refractive index and acceptable optical quality was prepared. Based on the highly fluorine and ytterbium doped silica core glass, a polarization maintaining (PM) photonic crystal fiber with 40 µm core was prepared and the pump absorption coefficient at 976 nm was ∼6.5 dB/m. An average amplified power of 103 W was achieved from a 2-m-long PCF with the bend diameter of 23 cm. The slope efficiency (with respect to pump power) was 52% with laser beam quality factor M2 of 1.46.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Pulsed laser amplification has significantly benefited from the development of gain materials and Yb-doped silica fiber is one of the most popular gain mediums [1–5]. Good beam quality is maintained over a long length due to the confinement of laser by fiber structure. However, the confined high peak power laser in the small fiber core results in detrimental nonlinear effects, such as stimulated Raman scattering (SRS) and self-phase modulation (SPM). These parasitic nonlinear effects degrade the output power and spectra. Generally, the threshold of non-linearity in a fiber scales with mode field area and is inversely proportional to fiber length. Therefore, increasing the fundamental mode area and shortening the fiber length are two effective ways to suppress nonlinear effects. Unfortunately, there is a dilemma in conventional double-cladding fiber that the large core diameter and good beam quality not being met at the same time. With the increase in core diameter, the beam quality is degraded with multimode operation, which is unacceptable in pulsed laser amplification. Consequently, various fiber structure designs were proposed to maintain good beam quality with large core diameter, such as rod-type photonic crystal fiber [6,7], photonic bandgap fiber [8,9], leakage channel fiber [10,11], large pitch fiber [12,13], chirally-coupled-core fiber (CCCF) [14] and other microstructure fiber [15,16]. Among them, Yb-doped PCF is the most classic architecture with an ordered array of microscopic air-holes [17]. These microscopic air-holes favor for the convenient regulation of effective refractive index of cladding. Nevertheless, it still remains a grand challenge to manipulate the refractive index of Yb-doped core glass close to that of pure silica glass cladding and maintain good uniformity. In contrast to the commercial method of modified chemical vapor deposition (MCVD) combined with solution doping, sol-gel technique is a well-known method to provide centimeter-size long glassy silica rod [18]. Our group has reported a modified method combining sol-gel process with high temperature powder sintering to fabricate Yb-doped silica glass [19]. Sol-gel process ensures dopants mixing in solution and consequent high doping uniformity, and high temperature powder sintering allows preparing large-size bulk glass. We previously reported a 50 µm core LMA PCF with low ytterbium and fluorine doping level in fiber core [20]. Thus, a 6.5-m-long fiber needed to be used to get enough gain due to the low pump absorption coefficient. The long fiber length is unfavorable to the suppression of nonlinear effects in picosecond pulse amplification.
In this work, the doping levels of ytterbium and fluorine in Yb/Al/P/F co-doped silica glass are significantly increased by optimization in fabrication process. Aluminum is added into the silica glass network to increase the solubility of ytterbium ions and low doping concentration of phosphorus is performed to suppress the formation of Yb2+ [20]. Fluorine incorporation during sol-gel process is used to compensate the increased refractive index caused by ytterbium and aluminum co-doping [21]. The effects of fluorine doping on spectroscopic properties and glass structure were investigated by absorption and emission spectra, Raman spectra, NMR and advanced pulse EPR measurements. At last, core glass composition was optimized with ytterbium concentration of 0.77 wt.% and refractive index was manipulated to be lower than that of pure silica glass. The ytterbium doping concentration is increased to two times as compared with our previous report. Using the optimized core glass rods as core and boron doped silica glass as stress area, a PM PCF with 40 µm core diameter was fabricated by the stack-and-draw technique. The cladding absorption coefficients were improved to ∼1.9 dB/m at 915 nm and ∼6.5 dB/m at 976 nm, respectively. The picosecond pulse amplification performance was presented with a 2-m-long PM PCF.
2. Experimental details
Silica glasses co-doped with Yb/Al/P (denoted Yb-AP) and Yb/Al/P/F (denoted Yb-APF) were prepared by modified sol-gel method combined with high temperature sintering. High purity tetraethoxysilane (TEOS, MOS, Kermel), AlCl3·6H2O (99.9995%, Alfa), YbCl3·6H2O (99.99%, Alfa), (NH4)2SiF6 (99.999%, Alfa) and H3PO4 (85 wt.%, Alfa) were used as precursors to form a homogeneous and transparent doped sol. The doping of fluorine was performed through directly dissolving (NH4)2SiF6 in solution of water and ethanol during the sol-gel process. The detailed sample preparation steps were described in our previous work [20]. Yb-AP and Yb-APF1 glass samples with the same Yb concentration (0.5 wt.%) were prepared to study the spectroscopic and structural properties (Table 1). The doping level of ytterbium, aluminum and phosphorus in these two samples are identical and the fluorine doping level in Yb-APF1 is as high as 2 wt.%. (see Table.1 and Fig. 1). The high fluorine content in Yb-APF1 was designed to benefit the detection of fluorine related signal in Raman, NMR and EPR measurements. These two glass samples were cut and polished to 2-mm thick slices for spectroscopic measurements. The Yb-APF2 glass with higher Yb concentration (0.77 wt.%) and relative lower content of fluorine (0.8 wt.%) compared to Yb-APF1 glass was prepared to act as core glass of PCF. Its refractive index profile was measured with an optical fiber analyzer (IFA-100) at 633 nm. A polarization maintaining (PM) ytterbium doped PCF was fabricated by the stack-and-draw technique at ∼2000 °C.
Table 1. Tested compositions in glass samples.
The Al, P, and Yb concentration of the prepared glass samples were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, radial-view Thermo iCAP 6300) after complete dissolution of the samples in aqueous HF. The F concentration was determined by a commercial scanning electronic microscopy-electron probe microscopy analysis (SEM-EPMA, JXA8230) with the test error being about 10%. A F-doped silica glass rod from Heraeus Quarzglas with F concentration of 7 wt.% was used as a reference. Table 1 shows the tested compositions of glass samples and Fig. 1 shows the F concentration profiles in glass samples, determined by EPMA line scan analysis. The F concentrations in Yb-APF1 and Yb-APF2 are about 2 wt.% and 0.8 wt.%, respectively. Absorption spectra were recorded in the range of 850−1050 nm using a Lambda 950 UV-VIS-NIR spectrophotometer. Emission spectra (excitation at 896 nm by a Xe lamp) and decay curves of Yb3+ (pumped with a pulsed 975-nm diode laser) were measured on a high-resolution spectrofluorometer (Edinburgh Instruments, FLS 920). For both absorption and emission spectra, scanning was performed in 1-nm steps. All tests were performed at room temperature (∼298 K). To check the distribution of dopants (Yb, Al, P, F, B) concentration in the PCF, an elements line scanning analysis was conducted using SEM-EPMA. The cladding absorption coefficients of PCF at 915 nm and 976 nm were measured by the cutback method with a white light source (Thorlabs, SLA201L/M).
Raman spectra were recorded in the range of 250–1450 cm−1 (a 633-nm laser was used for excitation) using a Renishawinvia Raman microscope. All of the solid-state NMR spectra were recorded on a Bruker Avance III HD 500 mol/L spectrometer (11.7 T, Bruker, Billerica, MA, USA). 19F magic angle spinning (MAS) NMR spectra were carried out at 470.54 MHz, using a 2.5 mm probe operated at a spinning rate of 25.0 kHz. 90° pulses of 2.35 µs length were used with a relaxation delay of 32 s, ensuring full signal recovery. Chemical shifts were referenced to CFCl3 by using AlF3 analysis (−172.5 × 10–6) as a secondary reference. Line shape analysis and deconvolutions were performed using DMFIT software [22].
To study the local environments of Yb3+ in Yb/Al/P/F co-doped glasses, the pulsed solid-state EPR experiments were conducted on a BRUKER ELEXSYS X-band EPR spectrometer (E580). The microwave frequency was 9.85 GHz. The two-pulsed (π/2-τ-π-τ-echo) echo-detected field-swept (EDFS) EPR spectra were recorded at 4 K with a time delay τ = 136 ns. Two-dimensional hyperfine sublevel correlation (2D-HYSCORE) EPR spectra were recorded at 4 K, using a π/2-τ-π/2-t1-π-t2-π/2-τ echo pulse sequence. The π/2 and π pulse lengths were 6 and 12 ns, respectively. Time delays of τ = 124 ns and τ = 104 ns were set for two static magnetic fields of 500 mT and 600 mT, respectively.
Laser amplification experiment of ytterbium doped PCF will be presented later.
3. Results and discussion
3.1 Effect of fluorine on spectroscopic properties
To investigate the influence of fluorine doping on glass spectroscopic properties, two contrastive samples (Yb-AP and Yb-APF1) were prepared. Figure 2(a) shows absorption spectra of Yb-AP and Yb-APF1 glass samples from 850 to 1050 nm. The peak position of the sharp peak keeps constant at ∼975 nm with incorporation of fluorine (Yb-APF1). Whereas, the peak of broad shoulder at ∼915 nm has a red shift. Figure 2(b) shows the emission spectra of Yb-AP and Yb-APF1 glass samples from 910−1150 nm. The emission peak position has an obvious blue shift from Yb-AP (∼1025 nm) to Yb-APF1 (∼1020 nm). As is known, the spectroscopic properties are closely associated with the ligand field and consequent energy-level scheme of dopants [23].
Fig. 2. (a) Absorption and (b) normalized emission spectra of Yb-AP and Yb-APF1 samples. (c) Schematic energy-level diagram of Yb3+ ion derived from the Lorenz fitting of the absorption and emission spectra of Yb3+. (d) Fluorescence decay curves of the 2F5∕2 energy level.
To study the Stark splitting energies of Yb3+ ions, each spectrum is resolved by the Lorentz fitting and the schematic energy-level diagram of 2F7/2 and 2F5/2 manifolds is illustrated in Fig. 2(c). As was reported, there was no obvious Stark splitting difference of Yb3+ in glass matrix between low temperature and room temperature [24]. Thus, the room-temperature absorption and emission spectra were used for the analysis of Stark levels splitting. As shown in Fig. 2(c), co-doping with fluorine leads to the red shift of (1→6,7) absorption transitions and blue shift of (5→2,3,4) emission transitions, which are fully consistent with our previous results in Yb/Al/F co-doped silica glass [20,25]. The lifetime of 2F5/2 energy level in Yb-APF1 (1.28 ± 0.02 ms) is a little longer than that of Yb-AP (1.21 ± 0.02 ms), as shown in Fig. 2(d). These changes in spectroscopic properties indicate the modification of local environment around Yb3+ by co-doping of fluorine.
3.2 Effect of fluorine on structure of Yb/Al/P/F co-doped glass
Figure 3 shows the Raman spectra of Yb-AP, Yb-APF1, F-doped silica glass and pure silica glass. The spectra of pure silica glass composed of a strong broad band around 420 cm−1 (labeled ω1) and weaker bands at 800 cm−1 (labeled ω3), high frequency bands at 1065 cm−1 (labeled ω4(TO)) and 1200 cm−1 (labeled ω4(LO)), assigned to [SiO4/2] tetrahedron network vibrations [26]. The sharp peaks at 490 and 600 cm−1 were assigned to four-membered (labeled D1) and three-membered rings (labeled D2) structure, respectively [27]. The asymmetric band at 800 cm−1 has been used to normalize the spectra. The characteristic peaks at 935 cm−1 in the spectra of F-doped silica glass and Yb-APF1 glass are attributed to the stretching vibration of Si-F bonds [28], which also indicates the high doping concentration of fluorine in Yb-APF1 glass. It is worth to note that the intensities of D1 and D2 decrease as the F content increases. Compared with pure silica glass, the broad band at 1000–1250 cm−1 with maxima at ∼1140 cm−1 in Yb-APF1 and Yb-AP glass is attributed to [AlPO4] join [29,30] and confirms the formation of [AlPO4] join.
Figures 4(a) and (b) show the 19F MAS NMR spectra of reference F-doped silica glass and Yb-APF1 glass, respectively. These spectra exhibit a broad asymmetric line shape, indicating multiple contributions. The spectra were deconvoluted into two distinct Gaussian components, with chemical shifts of −138 and −146 ppm, respectively. In agreement with data reported in the study of fluorine-doped amorphous silica [31], these two chemical shifts are related to two different structural environments. The first fluorine peak at −146 ppm is related to the SiO3/2F group, where a fluorine atom has replaced one of the bridging oxygen atoms bonded to a tetrahedral silicon atom. The second fluorine peak at −138 ppm is related to SiO4/2F group, where a fluorine atom bonded to a silicon atom with four bridging oxygen atoms. Table 2 shows the chemical shifts, line widths and relative areas of two individual fluorine species determined from the deconvolution of 19F MAS NMR spectra (Fig. 4). Obvious differences of line widths and area fractions between F-doped silica glass and Yb-APF1 glass can be observed. Compared with F-doped silica glass, Yb-APF1 glass contains more content of SiO4/2F group and less SiO3/2F group. These results are probably related to co-doping of Yb3+ ions in glass matrix, which influence the bonding of fluorine atoms.
Fig. 4. Experimental 19F MAS NMR spectra of (a) F-doped silica glass and (b) Yb-APF1 glass. Green curves denote the deconvolutions into two Gaussian components. Black curves denote experimental data.
Table 2. 19F NMR spectral deconvolution of F-doped silica glass and Yb-APF1 glass.
Figure 5(a) shows the EDFS EPR spectra of Yb-AP and Yb-APF1 samples. Similar types of broad spectra can be observed from 50 mT to 1100 mT, while the maximum of EDFS EPR is located at ∼460 mT. The line shape of EDFS EPR is sensitive to the local environment of Yb3+ [32]. Compared with Yb-AP sample, it can be found that there is a small shift of peaks to higher magnetic field in Yb-APF1 sample. This observation further indicates the mild modification of local environment around Yb3+ ions with the incorporation of fluorine [33,34].
Fig. 5. (a) EDFS EPR spectra recorded at 4 K for Yb-AP and Yb-APF1 samples. (b) 2D-HYSCORE spectrum recorded at a magnetic field of 500 mT for Yb-AP sample. (c) and (d) 2D-HYSCORE spectra recorded at two magnetic fields of 500 mT and 600 mT for Yb-APF1 sample.
To probe local environment of Yb3+ ions directly, an advanced pulsed-EPR experiment (2D-HYSCORE EPR) was conducted at 4 K. This type of experiment is based on the hyperfine interaction between the electronic spin and its surrounding nuclear spin and allows to probe the local environment of paramagnetic Yb3+ centers via spin echo modulation at the Larmor frequency of the neighbor nuclei (I≠0) [32,35]. The Larmor frequency $\mathrm{\omega }$ of the nuclei can be determined by Eq. (1), where $\mathrm{\gamma }$ is the gyromagnetic ratio and B is the magnitude of the applied magnetic field.
Figure 5(b) shows the 2D-HYSCORE spectrum of Yb-AP samples at a magnetic field of 500 mT. The spectrum reveals three diagonal peaks at 4.2 MHz, 5.6, MHz and 8.6 MHz, corresponding to the Larmor frequencies of 29Si (I = 1/2, natural abundance = 4.68%), 27Al (I = 5/2, 100%), and 31P (I = 1/2, 100%) at a magnetic field of 500 mT. All of the resonances are observed in the (+, +) quadrant of the spectra. This result reveals that Si, Al, and P atoms are located in the vicinity of Yb3+ in Yb-AP sample. The pattern at 20.0 MHz corresponding to the Larmor frequency of 19F at a magnetic field of 500 mT is not observed in the Yb-AP sample. Figures 5(c) and 5(d) show the 2D-HYSCORE spectra at 500 mT and 600 mT for Yb-APF1 sample, respectively. According to Eq. (1), the Larmor frequency of nuclei is proportional to magnetic field and thus the Larmor frequencies of 29Si, 27Al and 31P at a magnetic field of 600 mT are 5.1 MHz, 6.7 MHz, 10.4 MHz, respectively. The three diagonal peaks related to 29Si, 27Al and 31P are visible in HYSCORE spectra for Yb-APF1 sample. Furthermore, the diagonal peaks at 20.0 MHz and 24.0 MHz are related to 19F signals at 500 mT and 600 mT, respectively. Importantly, the 19F hyperfine splitting with a pair of off-diagonal resonances that are symmetrically displaced from the diagonal position corresponding to the 19F Zeeman frequency is observed. These weak correlation peaks give direct evidence of 19F species strongly interacting with the unpaired electron in Yb3+ and are attributed to direct Yb-F bonds in Yb-APF1 samples [34,36].
Based on above structural results from Raman, NMR and EPR measurements, a schematic possible coordination environment around Yb3+ in Yb/Al/P/F co-doped glass is shown in Fig. 6. F atoms is mainly bonded to Si atoms and a small amount of F atoms forms Yb-F bonds with Al and P atoms yielding [AlPO4] join. This mild modification of coordination environment around Yb3+ with incorporation of fluorine leads to the change in spectroscopic properties.
3.3 Laser performance of PM PCF
Yb-doped silica glass rod with high Yb3+ doping concentration, low refractive index close to pure silica glass, high optical quality and good spectroscopic properties is essential for the fabrication of LMA PCF. Therefore, Yb-APF2 was used as core glass of PCF. A glass rod (sized $\varnothing5\,{\rm mm}\times90\,{\rm mm}$) with refractive index slightly lower than that of pure silica glass was successfully prepared and the surface was polished, as shown in Fig. 7(a). Compared with Yb-APF1, the ytterbium doping concentration was increased to 0.77 wt.% and the fluorine doping concentration was reduced to 0.8 wt.% in Yb-APF2 glass (see Table 1). The ytterbium doping concentration is two times higher as compared with our previous report [20]. Figure 7(b) shows the picture of core glass rod illuminated by a red laser pointer (633 nm, < 10 mW). Few bubbles and scattering points in core-glass was found, which helps to reduce the background attenuation of PCF. It is worth emphasizing that much effort needs to be taken to eliminate air bubbles in core glass and get large transparent glassy silica rod due to the high evaporation of fluorine during high-temperature process. The core glass rod jacketed by pure silica glass was drawn to fiber with 400 µm diameter for the measurement of refractive index profile and the result is shown in Fig. 7(c). The average refractive index of Yb-doped area at 633 nm is comparable to the one of pure silica glass. Importantly, the radial refractive index fluctuations were significantly decreased by a high-temperature flame heating method [37].
Fig. 7. (a) Core glass rod sized $\varnothing5\,{\rm mm}\times90\,{\rm mm}$. (b) Pictures of glass rod illuminated by a red laser pointer. (c) Refractive index profile of Yb-APF2 fiber.
Using Yb-APF2 glass rods as core and boron-doped silica glass rods as stress area, a preform was prepared by stack and capillary method and then a PM PCF with ∼40 µm core diameter was drawn at ∼2000 °C under precisely controlled pressure. The PM PCF was coated with low refractive index acrylate as outer cladding and the NA of pump cladding is 0.46. The core NA was measured to be ∼0.03. The microscope image of the PM PCF cross section is illustrated in Fig. 8(a). The diameters of Yb-doped core and inner cladding are ∼40 µm and ∼260 µm, respectively. The diameter of the air holes is ∼2 µm, whereas the pitch is ∼16 µm. There are some light spots in the stress area due to the residual pure silica cladding around the boron-doped silica glass rods. Figure 8(b) shows the EPMA radial line scan analysis of B, F, P, Yb and Al elements in the fiber cross section and the scan path is illustrated in Fig. 8(a).
The overall cladding absorption coefficients (including background loss) at 915 nm and 976 nm are ∼1.9 dB/m and ∼6.5 dB/m, respectively. The polarization extinction ratio is measured to be 13 dB. To investigate the laser pulse amplification performance, a master-oscillator power-amplifier (MOPA) architecture was built, as shown in Fig. 9(a). A 976-nm laser diode (LD) pumping source was used with a 220/240 µm output fiber. The beam quality profile was measured by High Power-Laser Quality Monitor (HP-LQM, Prime). A seed source with an average power of 2.2 W (∼1030 nm wavelength with 30 MHz repetition rates and 30 ps pulse duration) was coupled into a 2-m-long PM PCF. The PM PCF with 40 µm core diameter still keeps soft characteristics, which benefits for compact laser system. The fiber was coiled on an aluminum heatsink without water-cooling and the bend diameter was 23 cm, as shown in Fig. 9(b). The measured average amplified power versus pump power is shown in Fig. 9(c). A maximum average output power of ∼103 W was achieved under 201 W pump power. The slope efficiency (with respect to the pump power) was ∼52%. The mode field diameter is ∼30 µm. The inset in Fig. 9(c) shows the output laser beam profile in the far field and the beam quality factor M2 with 103 W output power is 1.46. Figure 9(d) shows the output laser spectrum at central wavelength of 1030.8 nm. The optical signal-to-noise ratio (OSNR) of amplified signal is larger than 40 dB. No stimulated Raman scattering was observed up to 103 W average output power.
Fig. 9. (a) Experimental setup of a master oscillator power amplifier system. (b) Picture of PCF on an aluminum heatsink. (c) Measured amplified output power as a function of pump power. Inset: laser beam profile in the far field. (d) Output laser spectrum with output power of 103 W.
4. Conclusion
In this work, Yb/Al/P/F co-doped silica glass samples with high fluorine and ytterbium contents were prepared by a modified sol-gel method combined with high-temperature sintering. The effects of fluorine doping on spectroscopic properties and glass structure were systematically studied by absorption and emission spectra, Raman spectra, NMR and EPR measurements. The co-doping of fluorine yields Si-F bonds and Yb-F bonds formation, which leads to the changes in spectroscopic properties of Yb3+ ions. A silica glass rod with high Yb doping concentration (0.77 wt.%), low refractive index and acceptable optical quality was prepared and a 40 µm core PM PCF was fabricated by the stack-and-draw technique. The polarization extinction ratio was measured to be 13 dB. An average amplified power of 103 W with slope efficiency (with respect to the pump power) of 52% was achieved from a 2-m-long PCF. The beam quality factor M2 is 1.46. We demonstrate that highly incorporation of fluorine in Yb-doped silica glass by sol-gel process is beneficial for fabrication of LMA PCF. Further work is focused to improve the laser slope efficiency of PCF.
Funding
National Natural Science Foundation of China (61775224, 61875216).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. I. Fsaifes, L. Daniault, S. Bellanger, M. Veinhard, J. Bourderionnet, C. Larat, E. Lallier, E. Durand, A. Brignon, and J.-C. Chanteloup, “Coherent beam combining of 61 femtosecond fiber amplifiers,” Opt. Express 28(14), 20152–20161 (2020). [CrossRef]
2. Z. Zhang, J. Tian, C. Xu, R. Xu, Y. Cui, B. Zhuang, and Y. Song, “Noise-like pulse with a 690 fs pedestal generated from a nonlinear Yb-doped fiber amplification system,” Chin. Opt. Lett. 18(12), 121403 (2020). [CrossRef]
3. R. Lü, H. Teng, J. Zhu, Y. Yu, W. Liu, G. Chang, and Z. Wei, “High power Yb-fiber laser amplifier based on nonlinear chirped-pulse amplification at a repetition rate of 1 MHz,” Chin. Opt. Lett. 19(9), 091401 (2021). [CrossRef]
4. H. Chang, Z. Cheng, R. Sun, Z. Peng, M. Yu, Y. You, M. Wang, and P. Wang, “172-fs, 27-µJ, Yb-doped all-fiber-integrated chirped pulse amplification system based on parabolic evolution by passive spectral amplitude shaping,” Opt. Express 27(23), 34103–34112 (2019). [CrossRef]
5. Y. Xu, Z. Peng, Z. Chen, Y. Shi, B. Wang, and P. Wang, “Research progress of ytterbium-doped fiber-solid high-power ultrashort pulse amplification,” Chin. J. Lasers 48(5), 0501003 (2021). [CrossRef]
6. J. Limpert, N. Deguil-Robin, I. Manek-Hnninger, F. Salin, and C. Jakobsen, “High-power rod-type photonic crystal fiber laser,” Opt. Express 13(4), 1055–1058 (2005). [CrossRef]
7. O. Schmidt, J. Rothhardt, T. Eidam, F. Röser, J. Limpert, A. Tünnermann, K. P. Hansen, C. Jakobsen, and J. Broeng, “Single-polarization ultra-large-mode-area Yb-doped photonic crystal fiber,” Opt. Express 16(6), 3918–3923 (2008). [CrossRef]
8. G. Gu, F. Kong, T. Hawkins, J. Parsons, M. Jones, C. Dunn, M. T. Kalichevsky-Dong, K. Saitoh, and L. Dong, “Ytterbium-doped large-mode-area all-solid photonic bandgap fiber lasers,” Opt. Express 22(11), 13962–13968 (2014). [CrossRef]
9. C. A. Robin, I. Hartl, F. Kong, G. Gu, T. W. Hawkins, M. Jones, J. Parsons, M. T. Kalichevsky-Dong, B. Pulford, and I. Dajani, “∼1 kilowatt Ytterbium-doped all-solid photonic bandgap fiber laser,” Proc. SPIE 10083, 1008311 (2017). [CrossRef]
10. W. S. Wong, X. Peng, J. M. Mclaughlin, and L. Dong, “Breaking the limit of maximum effective area for robust single-mode propagation in optical fibers,” Opt. Lett. 30(21), 2855–2857 (2005). [CrossRef]
11. D. Liang, H. A. Mckay, L. Fu, M. Ohta, A. Marcinkevicius, S. Suzuki, and M. E. Fermann, “Ytterbium-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding,” Opt. Express 17(11), 8962–8969 (2009). [CrossRef]
12. F. Jansen, F. Stutzki, H. J. Otto, M. Baumgartl, and A. Tünnermann, “The influence of index-depressions in core-pumped Yb-doped large pitch fibers,” Opt. Express 18(26), 26834–26842 (2010). [CrossRef]
13. J. Limpert, F. Stutzki, F. Jansen, H. J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann, “Yb-doped large-pitch fibres: Effective single-mode operation based on higher-order mode delocalisation,” Light: Sci. Appl. 1(4), e8 (2012). [CrossRef]
14. C. Liu, G. Chang, N. Litchinitser, A. Galvanauskas, and K. Tankala, “Effectively single-mode chirally-coupled core fiber,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (2007), paper ME2.
15. D. Jain, C. Baskiotis, T. C. May-Smith, J. Kim, and J. K. Sahu, “Large mode area multi-trench fiber with delocalization of higher order modes,” IEEE J. Sel. Top. Quantum Electron. 20(5), 242–250 (2014). [CrossRef]
16. M. Laurila, M. M. Jørgensen, K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Lægsgaard, “Distributed mode filtering rod fiber amplifier delivering 292W with improved mode stability,” Opt. Express 20(5), 5742–5753 (2012). [CrossRef]
17. W. J. Wadsworth, J. C. Knight, and P. S. J. Russell, “Large mode area photonic crystal fibre laser,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (2001), paper CWC1.
18. H. E. Hamzaoui, M. Bouazaoui, and B. Capoen, “Sol–gel materials for optical fibers,” in Sol-Gel Derived Optical and Photonic Materials, chap. 14, A. Martucci, L. Santos, R. Estefanía Rojas Hernández, and R. Almeida, eds. (Woodhead Publishing, 2020), pp. 315–346.
19. S. Liu, H. Li, Y. Tang, and L. Hu, “Fabrication and spectroscopic properties of Yb3+-doped silica glasses using the sol-gel method,” Chin. Opt. Lett. 10(8), 081601 (2012). [CrossRef]
20. F. Wang, L. Hu, W. Xu, M. Wang, S. Wang, J. Ren, L. Zhang, D. Chen, N. Ollier, G. Gao, C. Yu, and S. Wang, “Manipulating refractive index, homogeneity and spectroscopy of Yb3+-doped silica-core glass towards high-power large mode area photonic crystal fiber lasers,” Opt. Express 25(21), 25960–25969 (2017). [CrossRef]
21. M. Leich, A. Kalide, T. Eschrich, M. Lorenz, A. Lorenz, K. Wondraczek, D. Schönfeld, A. Langner, G. Schötz, and M. Jäger, “2 MW peak power generation in fluorine co-doped Yb fiber prepared by powder-sinter technology,” Opt. Lett. 45(16), 4404–4407 (2020). [CrossRef]
22. D. Massiot, F. Fayon, M. Capron, I. King, S. Calvé, B. Alonso, J. O. Durand, B. Bujoli, Z. Gan, and G. Hoatson, “Modelling one- and two-dimensional solid-state NMR spectra,” Magn. Reson. Chem. 40(1), 70–76 (2002). [CrossRef]
23. S. Zhou, Q. Guo, H. Inoue, Q. Ye, A. Masuno, B. Zheng, Y. Yu, and J. Qiu, “Topological engineering of glass for modulating chemical state of dopants,” Adv. Mater. 26(47), 7966–7972 (2014). [CrossRef]
24. B. Yang, X. Liu, X. Wang, J. Zhang, L. Hu, and L. Zhang, “Compositional dependence of room-temperature Stark splitting of Yb3+ in several popular glass systems,” Opt. Lett. 39(7), 1772–1774 (2014). [CrossRef]
25. W. Xu, C. Yu, S. Wang, F. Lou, S. Feng, M. Wang, Q. Zhou, D. Chen, L. Hu, M. Guzik, and G. Boulon, “Effects of F− on the optical and spectroscopic properties of Yb3+/Al3+-co-doped silica glass,” Opt. Mater. 42, 245–250 (2015). [CrossRef]
26. F. L. Galeener, “Planar Rings in Vitreous Silica,” J. Non-Cryst. Solids 49(1-3), 53–62 (1982). [CrossRef]
27. A. Pasquarello and R. Ca R, “Identification of Raman defect lines as signatures of ring structures in vitreous silica,” Phys. Rev. Lett. 80(23), 5145–5147 (1998). [CrossRef]
28. N. Shimodaira, K. Saito, and A. J. Ikushima, “Raman spectra of fluorine-doped silica glasses with various fictive temperatures,” J. Appl. Phys. 91(6), 3522–3525 (2002). [CrossRef]
29. S. G. Kosinski, D. M. Krol, T. M. Duncan, D. Douglas, J. B. Macchesney, and J. R. Simpson, “Raman and NMR spectroscopy of SiO2 glasses co-doped with Al2O3 and P2O5,” J. Non-Cryst. Solids 105(1-2), 45–52 (1988). [CrossRef]
30. F. Wang, C. Shao, C. Yu, S. Wang, L. Zhang, G. Gao, and L. Hu, “Effect of AlPO4 join concentration on optical properties and radiation hardening performance of Yb-doped Al2O3-P2O5-SiO2 glass,” J. Appl. Phys. 125(17), 173104 (2019). [CrossRef]
31. R. E. Youngman and S. Sen, “The nature of fluorine in amorphous silica,” J. Non-Cryst. Solids 337(2), 182–186 (2004). [CrossRef]
32. T. Deschamps, N. Ollier, H. Vezin, and C. Gonnet, “Clusters dissolution of Yb3+ in codoped SiO2-Al2O3-P2O5 glass fiber and its relevance to photodarkening,” J. Chem. Phys. 136(1), 014503 (2012). [CrossRef]
33. R. Zhang, M. D. Oliveira, Z. Wang, R. G. Fernandes, A. D. Camargo, J. Ren, L. Zhang, and H. Eckert, “Structural studies of fluoroborate laser glasses by solid state NMR and EPR spectroscopies,” J. Phys. Chem. C 121(1), 741–752 (2017). [CrossRef]
34. M. Oliveira, T. Uesbeck, T. Goncalves, C. J. Magon, P. S. Pizani, A. D. Camargo, and H. Eckert, “Network structure and rare-earth ion local environments in fluoride phosphate photonic glasses studied by solid-state NMR and electron paramagnetic resonance spectroscopies,” J. Phys. Chem. C 119(43), 24574–24587 (2015). [CrossRef]
35. T. Deschamps, H. Vezin, C. Gonnet, and N. Ollier, “Evidence of AlOHC responsible for the radiation-induced darkening in Yb doped fiber,” Opt. Express 21(7), 8382–8392 (2013). [CrossRef]
36. M. de Oliveira, T. S. Gonçalves, C. Ferrari, C. J. Magon, P. S. Pizani, A. S. S. de Camargo, and H. Eckert, “Structure–property relations in fluorophosphate glasses: an integrated spectroscopic strategy,” J. Phys. Chem. C 121(5), 2968–2986 (2017). [CrossRef]
37. M. Wang, F. Wang, S. Feng, C. Yu, S. Wang, Q. Zhou, L. Zhang, F. Lou, D. Chen, and L. Hu, “272 W quasi single-mode picosecond pulse laser of ytterbium doped large mode area photonic crystal fiber,” Chin. Opt. Lett. 17(7), 071401 (2019). [CrossRef]
