Open Access
Add to BibSonomy
Abstract
In this paper it is demonstrated that laser sintering technology combined with fiber fabrication by the stack and draw method can be used to realize efficient fiber lasers. More precisely, a ytterbium-doped large-mode-area photonic crystal fiber with a core obtained by laser sintering technology is studied. Electron probe micro-analysis (EPMA) mapping reveals that the elements Yb, Al and Si are distributed throughout the fiber core with an excellent homogeneity. The laser performance demonstrates a high laser slope efficiency of 81.03 % for a laser emission at 1035 nm and a low power threshold of 3.04 W within only 1 m of fiber. The optical and spectroscopic properties of the fiber are studied, together with the sensitivity of the fiber to photodarkening (PD). The results make laser sintering technology interesting for the realization of fibers with large core and complex designs toward high-power fiber lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past few decades fiber lasers achieved a tremendous development owing to their unparalleled advantages such as outstanding beam quality, wider tunable wavelength range, lower pumping threshold, higher conversion efficiency, better heat dissipating, higher coupling efficiency and more compact size, compared with other conventional laser systems [1–3]. Material processing, science research, medical surgery, military and other industrial areas benefit greatly from the use that double-cladding fiber geometry can make of powerful laser diode semiconductor sources to obtain a Gaussian-like powerful beam. However, the power scaling of fiber lasers has been mainly constrained by the impact of nonlinear effect and thermal effect, which are imposed by fiber materials and structures.
One way to solve these problems is to increase the fiber core size and decrease the fiber length [4]. Thus, it’s good to design and fabricate large-mode-area (LMA) fibers, whose geometry size makes it possible to decrease the optical power density in the core and increase the nonlinear effect threshold [5,6]. But the large size of the fiber core will lead to multi-mode light transmission, which will decrease the fiber beam quality. In such LMA fibers it is not a simple task to maintain the single mode characteristic, unless the numerical aperture (NA) in the fiber core can be minimized by accurately controlling the core/cladding structure in LMA geometry. But with the existing fabricating technology, up to now, it is a little hard to meet the demands [7].
A photonic crystal fiber (PCF) structure can be made up by utilizing the well-designed capillaries around a solid active silica rod. The effective low NA in PCF can be obtained through adjusting the diameter of air holes and the hole-to-hole spacing of capillaries. This method can not only increase the fiber core size, but also keep the fiber in an endless single mode characteristic. As a result, the fiber can achieve the high-power scaling and an outstanding beam quality at the same time [8]. Nevertheless, it is not easy to prepare such a well-designed LMA PCF, especially a large-diameter active rod as the fiber core [9,10].
Owing to the well-known limitations of low doping concentration, geometry, and homogeneity [11], it is difficult to further enlarge the fiber core size by the Modified Chemical Vapor Deposition (MCVD) technique. Recently efforts have been devoted by some scientific research groups to explore new and improved ways. For example, researchers in Jena University and Heraeus company have developed a new powder sinter technology [12]. This non-CVD technique can fabricate the large-size Yb3+-doped silica rod with high doping homogeneity. Other methods to deal with these issues include SOL-GEL method [13], the glass phase-separation technology [14] and the direct sand vitrification method [15]. In this paper, we offer an alternative method called laser sintering technology (LST) based on CO2 lasers for the fabrication of large-size active rod for fiber core [16]. As a result of the fact that the silica glasses can strongly absorb the high energy of CO2 laser operated at 10.6 um wavelength, the CO2 laser could act as the ideal heat source to melt the active silica glass and it was also reported as an intermediate step to fabricate silica optical fibers [17]. And by this technique the large size active silica rod doped with Yb3+ could be easily obtained. Compared with other traditional technique, LST can easily ensure pure oxidizing atmosphere during the fabricating process without equipment damage or special demands for the accessories [16]. Furthermore, the technology is well suited for the preparation of the fibers with large active core and complex designs such as the LMA PCF and multicore PCF.
In the first part of the paper we will report on the realization of an Yb-doped LMA PCF with an active core fabricated by the laser sintered technology. Second, the optical properties of the fiber will be presented. Finally, the laser performance of the fiber will be investigated together with results about photodarkening behavior. In our work, it is the first time that the laser sintered technology is used to successfully fabricate a PCF with so large core of 54 um, and then the PCF is applied in the fiber laser. The experimental results well indicate that it is an effective technique to prepare the fibers with large core and complex designs toward high-power fiber lasers.
2. Fiber preparation and parameters
The PCF consists of two kinds of silica glass: the active core is totally from the laser sintered rod doped with Yb3+ and Al3+. The inner and outer claddings consist of silica glass from the Heraeus Quarzglas glass company. The inner cladding of the PCF is made up of a hexagonal lattice of air holes with a diameter, d, of about 11 um and a hole-to-hole spacing Λ, equal to 11 um (d/Λ=0.1). Such a geometry ensures a single-mode behavior around the wavelength of 1 um. To form the LMA core, during the stacking process 19 capillaries in the center were replaced by the Yb3+-doped rod. The Yb3+-doped active core is prepared by the laser sintering technology [16] and its fabrication process is schematic illustrated in Fig. 1(a). The CO2 lasers are the heat source. The temperature of melting zone could be controlled to be about 2000°C in a short period of time by adjusting the laser power. The mixed powder of the SiO2, Yb2O3, and Al2O3 was delivered by the carrier gas of O2 to the melting zone where the CO2 laser can radiate. With the rotation and falling of the base rod, the melted liquid glass moved below the melting zone and the final Yb3+-doped glass rod could be prepared. Through the stack-and-draw technique, the pure silica capillaries were stacked around the Yb3+-doped rod and the well-designed preform was drawn into Yb3+/Al3+co-doped LMA PCF by the fiber drawing tower. Fig. 1(b) is the cross-section of drawn Yb3+-doped LMA PCF. The basic optical parameter and sizes of this fiber were listed in the Table 1. The refractive index of the Yb3+-doped core glass was measured by the Abbe refractometer and the measurement accuracy is ±0.0003.
Fig. 1 (a) The Schematic diagram of the laser sintering process. (b) Cross-section of the drawn Yb3+-doped LMA PCF.
Table 1. The basic optical parameters and sizes of LMA PCF
To assess the homogeneity and concentration fluctuation of the active core of the fiber, an EPMA system (EPMA-1600, Shimadzu, Kyoto, Japan) was used to characterize the distribution of the elements Yb, Al and Si. Fig. 2 is the EPMA mapping of the fiber cross section for elements Yb, Al and Si distribution, respectively. The scanned points dispersedly distributed in the whole scanning cross section, where the red dots indicate the element concentrations are high and the blue ones indicate the element concentrations are low. According to the Al, Yb and Si distributions in the core, it indicates the elements are uniformly doped in the fiber. Moreover, as shown in the Fig. 3, the EPMA radial line scanning presents the content and fluctuation of the doping elements. The concentrations of Yb, Al and Si in the fiber core are about 0.57 wt% , 2.46 wt% and 71 wt% , corresponding to 0.08 at% , 2.14 at% and 59.62 at%, respectively. There is no significant radial fluctuation of the doping levels along the scanning line, especially for Yb, well indicating a uniform distribution of elements in the fiber core.
Fig. 2 EPMA mapping of fiber cross section for elements silicon (left), aluminium (middle) and ytterbium (right).
Fig. 3 EPMA radial line scanning analysis of fiber cross section for elements silicon, aluminium and ytterbium.
3. Optical properties
The absorption spectrum was measured using the light source (ZHW-3000, QSPEC Solutions) operated in ultraviolet and visible band, and the emission properties were measured by the excitation of the laser diode (LD) operated at 976 nm. Both the spectrums were recorded with an optical spectrum analyzer (Maya 2000-Pro, Ocean Optics) operated from 200 to 1100 nm. Background loss of the fiber was measured by the cutback technique using a supercontinuum source (SuperK COMPACT, NKT Photonics) and optical spectrum analyzer (Maya 2000-Pro and NIRQuest 256, Ocean Optics). The fluorescence decay curve was measured via a Tektronix TDS 3012c Digital Phosphor Oscilloscope with pulsed 980 nm LD. The experimental configuration of luminescence decay lifetime measurements is schematic illustrated in the Fig. 4. All the measurements are performed at the room temperature.
The measured absorption and fluorescence spectra scaled to unity at the maximum are presented in Fig. 5. This fiber exhibits very typical absorption and stimulated emission cross section of Yb3+ with two absorption peaks at 915 nm/976 nm and two strong luminescence emission peaks at 976 nm/1025 nm. At the 976 nm wavelength, the absorption cross-section, deduced from the Yb concentration and mode/dopant overlap integral, is estimated to be 1.527 pm2. Then the emission cross section could be calculated using the Reciprocity Method described by McCumber [18] from the obtained absorption cross-section and it is 0.592 pm2 at 1025 nm. These values of cross sections are very similar to what has been reported in Yb3+-doped silicate glasses [19], Yb-doped and Yb/Al-codoped high silica glasses [20]. The background attenuation values of the Yb3+-doped LMA PCF are 0.099 dB/m, 0.250 dB/m and 0.254 dB/m at 633 nm, 1200 nm and 1550 nm, respectively. The luminescence lifetime is measured for a 5 mm Yb3+ core rod used for fiber fabrication. It is seen in Fig. 6 that the measured luminescence decay is an exponential. The luminescence lifetime at 1025 nm deduced from the exponential fit is 1.346 ms, which is larger than what is commonly reported in Yb-doped germanium-silicate fibers [21], Yb-doped aluminosilicate fibers and pure silica Yb-doped fibers obtained by the SOL-GEL route [22,23]. The higher lifetime is favorable to achieve laser action.
Fig. 6 Luminescence decay measured on the Yb3+-doped LMA PCF. The decay is recorded at 1025 nm whereas the excitation wavelength is 980 nm.
4. Laser properties and photodarkening
In order to demonstrate the laser performance of the PCF, the laser experimental schematic is adopted as Fig. 7. The fiber laser was made of Fabry-Perot cavity with 1 m PCF as the gain medium by spatial light coupling method. The PCF was end-pumped by fiber-coupled laser diode emitting at 976 nm. The 976 nm pump beam was coupled into the PCF by collimator lens F1 (f=20 mm) and focusing lens F2 (f=12.7 mm). At last, the PCF was cut into 3 cm and the coupling efficiency was measured to be 46.57 %. The resonator was formed by the end face of PCF with a 4 % Fresnel reflection and a total reflecting mirror HR for the lasing wavelength. DM (THORLABS DMSP1000) is a 45° dichroic mirror with high transmittance at the pump wavelength and high reflectivity at the laser wavelength, used to separate the laser beam from the pump light. The filter (THORLABS FELH1000) serves to eliminate the residual 976 nm pump light to ensure the reliability of the laser output. The laser output power is recorded by a power meter (THORLABS S314C). In addition, the output laser beam was collected by a laser beam profiler (THORLABS BP104-UV) to investigate the beam property.
Fig. 7 Experimental setup of fiber laser. HR is a high reflecting mirror around 1035 nm, DM is a dichroic mirror (HT@λ = 976 nm and HR@λ=1035 nm for i = 45°), Filter serves to eliminate the residual 976 nm pump light.
Fig. 8(a) shows the output laser power as a function of the launched pump power. Laser threshold is about 3.04 W. When the injected pump power is 14.25 W, the output power is 9.18 W with a slope efficiency of 81.03 %. The maximum output power is currently limited by the available pumped power. The output laser spectrum is shown in Fig. 8(b). The pump source is 976 nm laser diode. The center wavelength of lasing is located at 1035 nm and the full width at half maximum is less than 10 nm. The inset of Fig. 8(b) shows the near-field 2D-graphic power intensity distribution of the 1035 nm output laser beam. The shapes of power distribution resemble Gaussian distribution. Thus, the laser beam is similar to a nearly fundamental mode Gaussian beam. The pretty good efficiencies reported here underline the good laser performance of PCF made by laser sintering technique.
Fig. 8 (a) Laser output power dependence on the launched pump power. (b) Laser spectrum of the Yb3+-doped PCF laser. Inset: Near field laser beam profile.
To investigate the structural changes induced by pump power irradiation, we tested the photodarkening (PD) induced loss of this PCF. The experimental setup is illustrated in Fig. 9 [24]. The 9.7 cm-long Yb-doped fiber was photo-darkened by the 976 nm pumping light during long time. In counter propagation we injected a 633 nm probe light every 5 minutes and measured the transmission as a function of pumping time. It’s worth noting that a 2 m-long single mode passive fiber was fused with the 9.7 cm-long Yb-doped PCF to eliminate the photobleaching effect of Yb-doped PCF caused by 633 nm probe light. The power of 976 nm light injected into the PCF is 6.15 W and it provides 46.8 % population inversion in the PD test. DM (THORLABS DMLP900) is a dichroic mirror with high transmittance at 976 nm and high reflecting at 633 nm for i=45°. Both of the two filters (THORLABS FESH0750) were employed to filter out the residual 976 nm pump light to ensure the reliability of the 633 nm laser power. The main limitation on this set-up is from the 633 nm LD probe power variation over time. Every 5 minutes, the 633 nm LD stability and 633 nm outpower were recorded by computer-controlled USB-type power meters 1 (THORLABS S121C) and power meters 2 (THORLABS S142C), respectively.
Fig. 9 Experiment setup for photodarkening at 633 nm. DM is a dichroic mirror (HT@λ =976 nm and HR@λ =633 nm for i=45°). Filter1 and Filter2 serve to eliminate the residual 976 nm pump light.
From the time-dependent transmission T(t) = P(t)/P0 (P0 and P(t) are the 633 nm probe transmission powers before and during PD, separately), we calculated the time-dependence of the PD excess loss α(t) =−10log(T(t))/L in units of dB/m (L is the fiber length). The formula used to fit measured PD curve is the standard stretched exponential function [25] as shown in
This stretched exponential function is commonly used for the description of PD kinetic. α(t) represents the loss induced at a time t after the pump is switched on. αeq is the equilibrium loss representing the evolution of PD loss at saturation. τ−1 is the rate constant manifesting the PD evolution rate, and β is the stretching parameter [25].
The temporal evolution of PD induced loss deduced from the experiment is presented in Fig. 10. From the fitting result in the bottom right corner we can see the obtained PD equilibrium loss αeq value is about 1.07 dB/m at 633 nm, exhibiting a relatively low PD. The stretching parameter β is about 0.68, similar to previous report. Besides, the inset in the top left corner of Fig. 10 is the stability of 633 nm LD, which further proves PD induced loss at 633 nm or the decrease of 633 nm output power has nothing to do with the stability of 633 nm LD.
5. Conclusion
In conclusion in this paper we have demonstrated LST combined with fiber fabrication by the stack and draw method can be used to realize an efficient Yb-doped LMA fiber laser. As a practical application, an Yb-doped LMA PCF with a core diameter of 54 um was successfully prepared and the distribution of ions showed an excellent material homogeneity in the fiber core. Optical properties and spectroscopic properties, together with the sensitivity of the fiber to PD have been presented. The laser experiment demonstrates a high laser slope efficiency of 81.03 % for a laser emission at 1035 nm and a low power threshold of 3.04 W within a short fiber length of 1 m. The results indicate the LST is an effective technique to prepare the fibers with large diameter and complex structures toward high-power fiber lasers.
Funding
National Natural Science Foundation of China, Grants 61575066, 61735005 and 61527822, GDUPS 5(2017), Guangdong Natural Science Foundation Grant No. 2017A030313333, Science and Technology Program of Guangzhou Grant No. 201707010133, Science and Technology Planning Project of Guangdong Province Grant No.2017KZ010101.
Acknowledgments
The authors wish to thank Dr Wei Zhang of Shenzhen Institute of Information Technology for his assistance in the experiments.
References
1. A. Carter and E. Li, “Recent progress in high-power fiber lasers for high-power and high-quality material processing applications,” Proc. SPIE 6344, 63440F (2006). [CrossRef]
2. C. Carlson, P. Dragic, B. Graf, R. Price, J. Coleman, and G. Swenson, “High power Yb-doped fiber laser-based LIDAR for space weather,” Proc. SPIE 6873, 68730K (2008). [CrossRef]
3. J. Nilsson and D. N. Payne, “Physics. High-Power Fiber Lasers,” Science 332(6032), 921–922 (2011). [CrossRef] [PubMed]
4. T. Liu, Z. M. Yang, and S. H. Xu, “3-Dimensional heat analysis in short-length Er3+ /Yb3+ co-doped phosphate fiber laser with upconversion,” Opt. Express 17(1), 235–247 (2009). [CrossRef] [PubMed]
5. H. Kubota and M. Nakazawa, “Numerical analyses of ultrashort fiber laser with optical nonlinear effect,” Technical Report of IEICE. 100, 61–66 (2000).
6. A. Shirakawa, J. Ota, M. Musha, K. Nakagawa, K. Ueda, J. R. Folkenberg, and J. Broeng, “Large-mode-area erbium-ytterbium-doped photonic-crystal fiber amplifier for high-energy femtosecond pulses at 1.55 um,” Opt. Express 13(4), 1221–1227 (2005). [CrossRef] [PubMed]
7. W. Xu, Z. Lin, M. Wang, S. Feng, L. Zhang, Q. Zhou, D. Chen, L. Zhang, S. Wang, C. Yu, and L. Hu, “50 um core diameter Yb 3+ Al 3+ /F − codoped silica fiber with M 2<1.1 beam quality,” Opt. Lett. 41(3), 504–507 (2016). [CrossRef] [PubMed]
8. P. Russell, “Photonic crystal fibers,” Science 299(5605), 358–362 (2003). [CrossRef] [PubMed]
9. N. A. Mortensen, “Effective area of photonic crystal fibers,” Opt. Express 10(7), 341–348 (2002). [CrossRef] [PubMed]
10. J. Limpert, T. Schreiber, S. Nolte, H. Zellmer, T. Tunnermann, R. Iliew, F. Lederer, J. Broeng, G. Vienne, A. Petersson, and C. Jakobsen, “High-power air-clad large-mode-area photonic crystal fiber laser,” Opt. Express 11(7), 818–823 (2003). [CrossRef] [PubMed]
11. A. Langner, M. Such, G. Schotz, V. Reichel, S. Grimm, F. Just, M. Leich, J. Kirchhof, B. Wedel, G. Kohler, O. Strauch, O. Mehl, V. Krause, and G. Rehmann, “Development, manufacturing and lasing behavior of Yb-doped ultra large mode area fibers based on Yb-doped fused bulk silica,” Fiber Lasers Vii: Technology, Systems, And Applications 7580, 75802X (2010).
12. W. He, M. Leich, S. Grimm, J. Kobelke, Y. Zhu, H. Bartelt, and M. Jöger, “Very large mode area ytterbium fiber amplifier with aluminum-doped pump cladding made by powder sinter technology,” Laser Phys. Lett. 12(1), 015103 (2015) [CrossRef]
13. S. K. Wang, Z. L. Li, C. L. Yu, M. Wang, S. Y. Feng, Q. L. Zhou, D. P. Chen, and L. L. Hu, “Fabrication and laser behaviors of Yb3+ doped silica large mode area photonic crystal fiber prepared by sol-gel method,” Opt. Mater. 35(9), 1752–1755 (2013). [CrossRef]
14. Y. Chu, Y. Yang, X. Hu, Z. Chen, Y. Ma, Y. Liu, Y. Wang, L. Liao, Y. Xing, H. Li, J. Peng, N. Dai, J. Li, and L. Yang, “Yb3+ heavily doped photonic crystal fiber lasers prepared by the glass phase-separation technology,” Opt Express 25(20), 24061–24067 (2017). [CrossRef] [PubMed]
15. M. Devautour, P. Roy, S. Février, C. Pedrido, F. Sandoz, and V. Romano, “Nonchemical-vapor-deposition process for fabrication of highly efficient Yb-doped large core fibers,” Appl. Opt. 48(31), G139–G142 (2009). [CrossRef] [PubMed]
16. W. Zhang, J. T. Liu, G. Y. Zhou, C. M. Xia, S. J. Liang, Y. Chen, and Z. Y. Hou, “Optical properties of the Yb/Er co-doped silica glass prepared by laser sintering technology,” Opt. Mater. Express 7(5), 1708–1715 (2017). [CrossRef]
17. J. Scheuner, A. M. Heidt, S. Pilz, P. Raisin, A. E. Sayed, H. Najafi, M. Ryser, T. Feurer, and V. Romano, “Advances in optical fibers fabricated with granulated silica, ” Optical Fiber Communications Conference and Exhibition (OFC), 1–3 (2017)
18. D. E. McCumber, “Einstein Relations Connecting Broadband Emission and Absorption Spectra,” Physical Review 136(4A), A954–A957 (1964). [CrossRef]
19. N. Dai, L. Hu, J. Yang, S. Dai, and A. Lin, “Spectroscopic properties of Yb3+-doped silicate glasses, ” Journal of Alloys and Compounds 363(1), 1–5 (2004). [CrossRef]
20. Y. Qiao, L. Wen, B. Wu, J. Ren, D. Chen, and J. Qiu, “Preparation and spectroscopic properties of Yb-doped and Yb-Al-codoped high silica glasses, ” Materials Chemistry and Physics 107(2), 488–491 (2008). [CrossRef]
21. R. Paschotta, J. Nilsson, P. R. Barber, J. E. Caplen, A. C. Tropper, and D. C. Hanna, “Lifetime quenching in Yb-doped fibres,” Opt. Commun 136(5–6), 375–378 (1997). [CrossRef]
22. U. Pedrazza, V. Romano, and W. Lüthy, “Yb3+:Al3+:sol-gel silica glass fiber laser,” Optical Materials 29(7), 905–907 (2007). [CrossRef]
23. A. Baz, H. El Hamzaoui, I. Fsaifes, G. Bouwmans, M. Bouazaoui, and L. Bigot, “A pure silica ytterbium-doped sol-gel-based fiber laser,” Laser Phys. Lett. 10(5), 055106 (2013). [CrossRef]
24. Manek-Hönninger, J. Boullet, T. Cardinal, F. Guillen, S. Ermeneux, M. Podgorski, R. Bello Doua, and F. Salin, “Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber,” Opt. Express 15(4), 1606–1611 (2007). [CrossRef] [PubMed]
25. S. Jetschke and U. Röpke, “Power-law dependence of the photodarkening rate constant on the inversion in Yb doped fibers,” Opt. Lett. 34(1), 109–111 (2009). [CrossRef] [PubMed]
