Open Access
Add to BibSonomy
Abstract
A yttrium aluminosilicate fiber with a Yb doping concentration of 4.53 wt.% was fabricated by powder-in-tube method, in which the preform core was Yb:YAG ceramic nanopowders synthesized by co-precipitation method. Compared with the method of using YAG crystal rod as fiber core, the fiber composition design can be more flexible and a series of crystal rod processing technology can be circumvented. The absorption coefficient of the Yb-doped YAS fibers was measured 28.4 dB/cm at 976 nm. A single-frequency fiber laser was constructed with the YAS fiber length of 1cm. To our knowledge, this is the first time that rare earth (RE) doped YAS fiber prepared by powder-in-tube method was used in fiber laser.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Recently, yttrium aluminosilicate (YAS) glass fibers have attracted much attention due to its unique performance advantages [1–3], such as high RE-doping concentration, high thermal conductivity, low Raman gain, low stimulated Brillouin gain and low photodarkening effect [4,5]. These advantages were beneficial to the realization of high-power single-frequency laser [6]. By replacing the position of Y3+ ions in fiber to dope RE ions, a higher doping level can be achieved. YAS fiber doped with Yb3+ [7], Er3+ [8,9], Tm3+ [10], Nd3+ [11], Ce3+ [12] and Cr3+ [13] ions have been reported at present. Normally, YAS fibers were fabricated by molten-core method from YAG crystal or transparent ceramic rods, and the fibers were composed of YAS glass core and silica cladding [14]. Compared with other multi-component glass fibers, the thermal expansion coefficient of YAS glass can be better matched with the silica cladding, which provides a necessary condition for the fabrication of YAS fibers [15]. During the drawing process, the core is molten and the cladding is softened. The Si in the cladding diffuses into the core, the core components change into YAS glass and the process is completed spontaneously at high temperature(∼2000 °C) [16]. In 2012, YAS fibers with high Yb3+ doping level proved feasible as active fiber in fiber amplifier [17], and the laser performance of YAS fiber has been studied extensively in recent years. To date, over 255 mW single-frequency laser output at 1 µm has been obtained based on Yb:YAS fiber [18], and in another study [19], the output power of laser at 1.06 µm can be up to 6 W by cladding-pump. In these works, all the core materials of preforms were YAG crystal rods, which cannot control the fiber compositions effectively. Therefore, the adjustments of fiber parameters are limited severely. Meanwhile, the high numerical aperture (NA) of the YAS fiber derive from YAG crystal rods was a critical problem affecting optical matching in fiber laser construction. The use of powders as preform core material has demonstrated unique advantages, which allowing complex fiber components design, providing a wide choice of optical material, and improving the optical performances relatively easily. The powder-in-tube method has been proved valuable for the fabrication of all glass fiber and photonic crystal fiber [20,21]. Dutta et al. have made attempts to use the mixture of YAG and silica powder as fiber core material to fabricate fiber, but the transmission loss of fiber was as high as 40 dB/m, which was difficult to be applied in fiber lasers [22].
Single-frequency fiber laser has attracted much attention in many fields due to its low noise and narrow linewidth [23]. Various techniques have been designed to realize the laser in a single-longitudinal-mode operation, such as distributed feedback (DFB) scheme [24], ring cavity configuration [25], and distributed Bragg reflector (DBR) setup [26]. Among them, the structure of DBR was simple relatively and compact, and the generated laser has strong stability. In DBR structure, the longitudinal mode interval can be controlled effectively by shortening the length of the cavity, and the operation of single-longitudinal-mode usually requires several centimeters of cavity length. Higher RE ions doping concentrations can be achieved in fibers with YAS glass core than commercial silica fibers, thus a sufficiently gain can be obtained in shorter fibers. Therefore, RE:YAS fiber has unique advantages in short cavity DBR lasers.
In this work, Yb:YAG ceramic nanopowders with 30 at% Yb3+ doping was fabricated by co-precipitation method, and the Yb:YAS fibers with absorption coefficient of 28.4 dB/cm at 976 nm were drawn using powder-in-tube method. Meanwhile, a single-frequency laser was constructed with linewidth of 230 kHz.
2. Experimental procedure
The 30 at% Yb:YAG ceramic nanopowders were fabricated by co-precipitation method using Al(NO3)3·9H2O, Y(NO3)3·6H2O, Yb(NO3)3·5H2O as raw materials. Nitrate solution with chemical compositions of Yb0.9Y2.1Al5O12 was prepared by dissolving nitrate in distilled water. The prepared nitrate solution was dropped into ammonium bicarbonate (AHC) solution at room temperature in which AHC was used as a precipitant. The solution was stirred constantly during the addition process. And the pH of the AHC solution was monitored continuously, stopping adding when the pH reaches ∼7.5. The resulting precipitate was washed with distilled water and dried at 90 °C for 12 hours to obtain the precursor powder. The pure Yb:YAG ceramic nanopowders were obtained by calcine precursor powder at 1100 °C for 8h.
A flow diagram of the fabrication of YAS fiber is shown in Fig. 1. In order to minimize the effect of air hole in fiber drawing process, the Yb:YAG nanopowders were compacted into a columnar shape with diameter of ∼3mm by cold isostatic pressing to promote it densified initially. In this case, we can obtain YAG rod with a length of several centimeters, which is sufficient for the fabrication of active fiber. The preform was consisted of compacted Yb:YAG nanopowders and cylindrical silica tube. The silica tube was cleaned in advance with ethanol and sealed at one end by oxyhydrogen flame. The Yb:YAG ceramic rod was loaded into a silica tube with inner diameter (ID) of 4 mm and outer diameter (OD) of 10 mm, and then inserted into another silica tube (ID= 11 mm and OD= 20 mm). The preform was manufactured into optical fibers using a standard drawing tower at a high temperature of ∼2050 °C. In this process, the vacuum degree in tube was adjusted to remove air hole and control the size of fiber core. To match the parameters of commercial fiber devices, the diameter of cladding and core was designed to be 125 µm and 6 µm respectively.
In Fig. 2, a single-frequency fiber laser was carried out based on the Yb:YAS fiber, which was constructed with a low-reflection fiber Bragg grating (LR-FBG, reflectivity was 80% at 1062 nm), a high-reflection fiber Bragg grating (HR-FBG, reflectivity was >99% at 1062 nm), and 1 cm home-made Yb:YAS fiber as the gain fiber. A laser diode (LD) of 980 nm was acted as light source, and the wavelength division multiplexer (WDM) of 980/1064 nm was used to inject the pump light and output the laser light. The 3dB bandwidth of LR-FBG and HR-FBG were measured to 0.08 nm and 0.39 nm respectively. The total length of the laser cavity was about 6 cm. The performance of output laser was measured by a power meter and spectrometer.
The micrograph of YAG powder and the distribution of elements in YAS fiber were obtained by SU8020 scanning electron microscopy (SEM) (Hitachi, Japan) and energy-dispersive spectroscopy (EDS). The size distribution of powder particles was measured by Zetasizer Nano ZS90 (Malvern, England). A Bruker D8 advance X-ray diffractometer was used to analyze the X-ray diffraction (XRD) pattern of YAG powder and fiber (Faellanden, Switzerland). The refractive index profiles (RIP) of YAS fiber was conducted using a SHR-1802 optical fiber refractive index analyzer (Aohe photoelectric, China).
3. Results and discussion
3.1. Characterization of Yb:YAG powder
In the process of optical fiber drawing using powder-in-tube method, the air hole between nanopowders was a vital factor that determine the quality of fiber. In high temperature, the fiber will lead to large transmission loss if the air holes are not removed in time. The size and loading density of YAG powder particles play an important role in the whole process. Large air holes may be caused by large particle size. The size distribution of Yb:YAG ceramic nanopowders was shown in Fig. 3(a). The results indicated that the particles size was mostly distributed near ∼100 nm and SEM image of powder also prove it. Some size inhomogeneity was caused by the cluster of powder particles, and the size uniformity may be improved by treating precursor powder with freeze-drying method. The preform core exhibits low viscosities of ∼0.04 Pa·s at the fiber drawing temperature of 2050 °C [27]. We remove air holes as much as possible by vacuuming to reduce defects and improve fiber performance.
Fig. 3. (a) Size distribution of Yb:YAG ceramic powder particles, the inset is SEM image of powder, (b) XRD analysis of the Yb:YAG powder and the optical fiber.
The pure YAG nanopowders without impurity phase was a significant factor to improve the performance of the final fiber. Figure 3(b) shows the XRD of fabricated Yb:YAG nanopowders. It shows pure YAG phase (JCPDS card No.33-0040) with no other impurity phase. The result also shows that Yb3+ ion has replaced Y3+ ion into YAG lattice structure, instead of forming other crystal phases. The measurement of powder composition shows that its purity was higher than 99.7%. The performance of the fiber was ensured by the high purity of powder.
It was allowed to be used as preform core material for fiber drawing due to the above properties of powder.
3.2 Yb-doped YAS fiber
The fabricated optical fibers were ground into powder to analyze its XRD patterns. Compared with the XRD patterns of powder before drawing, no sharp diffraction peaks existed in the fiber as shown in Fig. 3(b). This phenomenon occurred when Si diffuses into the fiber core, which becomes glass instead of remain crystal.
As shown in Fig. 4(a), the image of fiber illustrates that the core was circular in shape with a diameter of ∼6 µm and was demarcated from the cladding clearly. The EDS radial line scanning analysis of the fiber cross section for Si, Al, Y, and Yb was shown in Fig. 4(b). Due to the diffusion and migration of Si in the drawing process, there are large Si in the fiber core, and the distribution of elements shows the characteristics of gradual change. The same conclusion can be drawn from the two-dimensional distribution of elements as shown in Fig. 4(c-f). The phenomenon was consistent with the result that the optical fiber core has the amorphous morphology. The exact amounts of elements of fiber core were shown in Table 1. The Yb2O3 concentration of fiber core was 5.16 wt.%, which can provide high gain for fiber laser as active fiber.
Fig. 4. (a) Image of fiber end face, (b) the EDS radial line scanning analysis curve of element along core diameter of the fiber, (c-f) composition distribution profiles of the fibers cross-section (The red rectangular region in Fig. 4(a)).
Table 1. Elemental composition of Yb:YAS fiber core.
The RIP of the YAS fiber was measured in Fig. 5. The two different directions RIPs of YAS fiber are great overlap, indicating the well refractive index uniformity of the fiber core and the two-dimensional RIP can also prove it. The refractive index of the fiber core was 1.481, which was caused by the diffusion of Si in the cladding into the fiber core. The refractive index of the cladding was 1.457, and the numerical aperture was calculated to be 0.262. Although this NA value was higher than that of silica fiber, it was the lowest level among similar YAS fibers. Some parameters of YAS fiber in other works were shown in Table 2. The lower NA value was benefited by the high freedom of fiber composition design, which allows large amounts of Si diffusion into the core, but still ensure a high RE doping concentration.
Table 2. Comparison of parameters of different YAS fibers.
Fiber attenuation spectrum and absorption spectrum were measured by the cut-back method, and the results were shown in Fig. 6(a) and (b), respectively. The background loss of the Yb:YAS fiber was measured as 5.4 dB/m at 1550 nm. And there is a peak at 1380 nm in the attenuation spectrum, which was due to the high hydroxyl content. The relatively high value of transmission loss was caused by the fact that there are still some porosity defects in the core, although we have eliminated most of them by vacuuming. During the whole fabrication process, we introduce some impurities from the environment, which was another factor contributing to the high loss value. The core material has lower viscosity by increasing the temperature in the fiber drawing process, which is more conducive to the removed of air hole. By drying the material thoroughly and optimizing the fabrication process of preform, the content of hydroxyl and impurities in the fiber can be reduced to make the fiber have lower transmission loss. For the high-gain Yb:YAS fibers, fiber lasers setup was constructed using only a few centimeters generally. The background loss of ∼5.4 dB/m was acceptable. We measured the splicing losses between Yb:YAS fiber and commercial single-mode fiber (SMF), and the losses of two splicing points was 0.15 dB (SMF-YAS fiber) and 0.46 dB (YAS fiber-SMF), respectively. The relatively high value of splicing losses was caused by mode mismatch between the Yb:YAS fiber and SMF. The absorption spectrum of the Yb:YAS fiber shows two typical peaks corresponding to 2F7/2 to 2F5/2 energy level transition of Yb3+ ion. The absorption coefficient was measured to be 28.4 dB/cm at 976 nm and approximately 9.4 dB/cm at 915 nm, respectively. High Yb3+ doped concentration results in strong pump absorption at 976 nm of the Yb:YAS fiber.
Fig. 6. (a) The attenuation spectrum and (b) absorption spectrum of Yb:YAS fiber, the inset is energy level of Yb3+ ions.
3.3 Single frequency laser performance
A DBR single-frequency fiber laser was constructed by short cavity method and its laser performances were measured as shown in Fig. 7. The spectral measurement results of the single-frequency laser were shown in Fig. 7(a), the central wavelength of the output laser was 1062.1nm, which matches the wavelength defined by FBGs. The spectral signal to noise ratio (SNR) was > 60 dB. Figure 7(b) shows output power as a function of pump power. The laser slope efficiency was fitted to 15.3%. The low laser efficiency may be caused by the high background loss of the YAS fiber and the mismatched numerical aperture between the as-draw Yb:YAS fiber and the pigtail of a pair of FBG. The stability of laser output power was monitored for 60min continuously, and the fluctuation of the output power (FOP) was calculated to be 0.68%.
Fig. 7. (a) Output spectrum of the laser, (b) Output power as a function of pump power, the inset is stability of laser over 60 min, (c) Longitudinal modes characteristics of the fiber laser, (d) linewidth of the single-frequency laser.
The single-frequency characteristics of the fiber laser were measured by Scanning Fabry-Perot Interferometers (SFPI) with a free spectral range of 10GHz, as shown in Fig. 7(c), the results of the SFPI show that the laser operated in a single longitudinal mode. The single-frequency linewidth was measured at 40 mW output power by using the delayed self-heterodyne method, and the length delayed fiber was 50 km. The linewidth of 20 dB was measured as 4.6 MHz, and the corresponding linewidth of single-frequency laser was calculated to be 230 kHz. The broad linewidth may be caused by optical mismatches and environmental disturbances, further reduction of line-width can be achieved by optimizing fiber parameters and controlling the environmental factors.
The beam quality of the fiber laser was obtained by M2 measurement system, as shown in Fig. 8. The laser spot can be fitted well by Gaussian function, and the beam quality factors in X axis and Y axis directions were 1.074 and 1.042, respectively. The results show that the laser operates in standard single transverse mode.
4. Conclusion
In conclusion, an optimized method, powder-in-tube method, to fabricate the of RE-doped YAS fiber was developed. A Yb:YAS fiber directly from Yb:YAG ceramic nanopowders which was fabricate by co-precipitation method. Compared with the method of using YAG crystal rod, powder as core material of preform can provide higher freedom in fiber composition design. Fiber composition is no longer limited by the crystal preparation process, such as doping concentration, which allows us to reduce NA while ensuring high doping concentration of the fiber. The lowest NA value of YAS fiber was obtained by using this method, which allows for better optical matching with commercial silica fiber. Moreover, this method is simpler and a series of crystal rod processing can be circumvented. The Yb-doped YAS fibers exhibit high absorption coefficients of 28.4 dB/cm at 976 nm and have acceptable background loss. A single-frequency fiber laser was constructed with the Yb:YAS fiber length of 1 cm and its properties were measured as well. In the future, this optimized method has the potential to achieve a high level of component control and to adjust fiber performance by adding other elements.
Funding
National Natural Science Foundation of China (62035002); National Key Research and Development Program of China (2017YFB0405200).
Acknowledgment
Portions of this work were presented at the Advanced Solid State Lasers in 2021, Fabrication of Yb:YAG-derived silica fiber from powder-in-tube method and its application in single-frequency fiber laser.
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. G. Tang, G. Qian, W. Lin, W. Wang, Z. Shi, Y. Yang, N. Dai, Q. Qian, and Z. Yang, “Broadband 2 mu m amplified spontaneous emission of Ho/Cr/Tm:YAG crystal derived all-glass fibers for mode-locked fiber laser applications,” Opt. Lett. 44(13), 3290–3293 (2019). [CrossRef]
2. Y. Xie, Z. Cong, Z. Zhao, Z. Liu, X. Zhao, Z. Qin, Z. Jia, G. Qin, X. Shao, X. Gao, and X. Zhang, “Linearly polarized single-frequency fiber laser based on the Yb:YAG-crystal derived silica fiber,” Appl. Opt. 59(32), 9931–9936 (2020). [CrossRef]
3. Y. Zhang, W. Wang, J. Li, X. Xiao, Z. Ma, H. Guo, G. Dong, S. Xu, and J. Qiu, “Multi-component yttrium aluminosilicate (YAS) fiber prepared by melt-in-tube method for stable single-frequency laser,” J. Am. Ceram. Soc. 102(5), 2551–2557 (2019). [CrossRef]
4. P. D. Dragic and J. Ballato, “Characterisation of Raman gain spectra in Yb:YAG-derived optical fibres,” Electron. Lett. 49(14), 895–897 (2013). [CrossRef]
5. M. Tuggle, C. Kucera, T. Hawkins, D. Sligh, A. F. J. Runge, A. C. Peacock, P. Dragic, and J. Ballato, “Highly nonlinear yttrium-aluminosilicate optical fiber with a high intrinsic stimulated Brillouin scattering threshold,” Opt. Lett. 42(23), 4849–4852 (2017). [CrossRef]
6. P. D. Dragic, Y.-S. Liu, J. Ballato, T. Hawkins, and P. Foy, “YAG-derived fiber for high-power narrow-linewidth fiber lasers,” in Conference on Fiber Lasers IX - Technology, Systems, and Applications, Proceedings of SPIE2012),
7. Z. Liu, Y. Xie, Z. Cong, Z. Zhao, Z. Jia, C. Li, G. Qin, S. Wang, X. Gao, X. Shao, and X. Zhang, “110 mW single-frequency Yb:YAG crystal-derived silica fiber laser at 1064 nm,” Opt. Lett. 44(17), 4307–4310 (2019). [CrossRef]
8. J. Ballato, T. Hawkins, P. Foy, B. Kokuoz, R. Stolen, C. McMillen, M. Daw, Z. Su, T. M. Tritt, M. Dubinskii, J. Zhang, T. Sanamyan, and M. J. Matthewson, “On the fabrication of all-glass optical fibers from crystals,” J. Appl. Phys. 105(5), 053110 (2009). [CrossRef]
9. Y. Xie, Z. Cong, Z. Zhao, X. Zhang, X. Zhao, W. Zhao, X. Shao, and Z. Liu, “Preparation of Er:YAG crystal-derived all-glass silica fibers for a 1550-nm single-frequency laser,” J. Lightwave Technol. 39(14), 4769–4775 (2021). [CrossRef]
10. G. Qian, W. Wang, G. Tang, X. Guan, W. Lin, Q. Qian, D. Chen, C. Yang, J. Liu, G. Zhou, S. Xu, and Z. Yang, “Tm:YAG ceramic derived multimaterial fiber with high gain per unit length for 2 mu m laser applications,” Opt. Lett. 45(5), 1047–1050 (2020). [CrossRef]
11. S. Zheng, J. Li, C. Yu, Q. Zhou, and D. Chen, “Preparation and characterizations of Nd:YAG ceramic derived silica fibers drawn by post-feeding molten core approach,” Opt. Express 24(21), 24248–24254 (2016). [CrossRef]
12. M. Jia, J. Wen, W. Luo, Y. Dong, F. Pang, Z. Chen, G. Peng, and T. Wang, “Improved scintillating properties in Ce:YAG derived silica fiber with the reduction from Ce4 + to Ce3 + ions,” J. Lumin. 221, 117063 (2020). [CrossRef]
13. Y.-C. Huang, J.-S. Wang, Y.-K. Lu, W.-K. Liu, K.-Y. Huang, S.-L. Huang, and W.-H. Cheng, “Preform fabrication and fiber drawing of 300 nm broadband Cr-doped fibers,” Opt. Express 15(22), 14382–14388 (2007). [CrossRef]
14. J. Ballato and P. Dragic, “Rethinking optical fiber: new demands, old glasses,” J. Am. Ceram. Soc. 96(9), 2675–2692 (2013). [CrossRef]
15. S. Morris and J. Ballato, “Molten-core fabrication of novel optical fibers,” Am. Ceram. Soc. Bull. 92(4), 24–29 (2013).
16. Y. Zhang, Y. Sun, J. Wen, S. Lv, X. Xiao, Z. Ma, L. Yang, G. Bi, H. Guo, and J. Qiu, “Investigation on the formation and regulation of yttrium aluminosilicate fiber driven by spontaneous element migration,” Ceram. Int. 45(15), 19182–19188 (2019). [CrossRef]
17. P. D. Dragic, J. Ballato, T. Hawkins, and P. Foy, “Feasibility study of Yb:YAG-derived silicate fibers with large Yb content as gain media,” Opt. Mater. 34(8), 1294–1298 (2012). [CrossRef]
18. Y. Wan, J. Wen, C. Jiang, F. Tang, J. Wen, S. Huang, F. Pang, and T. Wang, “Over 255 mW single-frequency fiber laser with high slope efficiency and power stability based on an ultrashort Yb-doped crystal-derived silica fiber,” Photonics Res. 9(5), 649–656 (2021). [CrossRef]
19. Y. Xie, Z. Liu, Z. Cong, Z. Qin, S. Wang, Z. Jia, C. Li, G. Qin, X. Gao, and X. Zhang, “All-fiber-integrated Yb:YAG-derived silica fiber laser generating 6 W output power,” Opt. Express 27(3), 3791–3798 (2019). [CrossRef]
20. J.-L. Auguste, G. Humbert, S. Leparmentier, M. Kudinova, P.-O. Martin, G. Delaizir, K. Schuster, and D. Litzkendorf, “Modified powder-in-tube technique based on the consolidation processing of powder materials for fabricating specialty optical fibers,” Materials 7(8), 6045–6063 (2014). [CrossRef]
21. J. Ballato and E. Snitzer, “Fabrication of fibers with high rare-earth concentrations for Faraday isolator applications,” Appl. Opt. 34(30), 6848–6854 (1995). [CrossRef]
22. D. Dutta, A. Dhar, M. Pal, S. Das, and M. C. Paul, and Ieee, “Fabrication of Cr:YAG doped silica optical fiber using modified powder in tube technique,” in3rd Biennial IEEE Workshop on Recent Advances in Photonics (WRAP), 2017.
23. Y. Shen, Y. Qiu, B. Wu, W. Zhao, S. Chen, T. Sun, and K. T. V. Grattan, “Short cavity single frequency fiber laser for in-situ sensing applications over a wide temperature range,” Opt. Express 15(2), 363–370 (2007). [CrossRef]
24. O. V. Butov, A. A. Rybaltovsky, A. P. Bazakutsa, K. M. Golant, M. Y. Vyatkin, S. M. Popov, and Y. K. Chamorovskiy, “1030 nm Yb3 + distributed feedback short cavity silica-based fiber laser,” J. Opt. Soc. Am. B 34(3), A43–A48 (2017). [CrossRef]
25. M. J. Guy, J. R. Taylor, and R. Kashyap, “Single-frequency erbium fiber ring laser with intracavity phase-shifted fiber Bragg grating narrow-band-filter,” Electron. Lett. 31(22), 1924–1925 (1995). [CrossRef]
26. C. Spiegelberg, J. H. Geng, Y. D. Hu, Y. Kaneda, S. B. Jiang, and N. Peyghambarian, “Low-noise narrow-linewidth fiber laser at 1550 nm (June 2003),” J. Lightwave Technol. 22(1), 57–62 (2004). [CrossRef]
27. V. J. Fratello and C. D. Brandle, “Physical-properties of a Y3Al5O12 melt,” J. Cryst. Growth 128(1-4), 1006–1010 (1993). [CrossRef]
28. Y. Wang, Y. Zhang, J. Cao, L. Wang, X. Peng, J. Zhong, C. Yang, S. Xu, Z. Yang, and M. Peng, “915 nm all-fiber laser based on novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber for the pure blue fiber laser,” Opt. Lett. 44(9), 2153–2156 (2019). [CrossRef]
