We report suppression of photodarkening in Yb-doped nano-crystalline fibers in silica host. The photodarkening induced loss reduced by 20 times compared to Yb-doped aluminosilicate fibers. The laser efficiency of the nano-crystalline fiber was 79%

Bhadra, S.K.

Boyland, A.J.

Das, S.

Kalita, M.P.

Pal, M.

Paul, M.C.

Standish, R.J.

Webb, A.

Yoo, S.

English

Southampton (e-Prints Soton)

Ytterbium Doped Nano-crystalline Optical Fiber for Reduced Photodarkening  S. Yoo, M. P. Kalita, A. J. Boyland, A. Webb, R. J. Standish, and J. K. Sahu  Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom Email: sey@orc.soton.ac.uk  M. C. Paul , S . Das , S . K . Bhadra and M . Pal Fiber Optics Laboratory, Central Glass and Ceramic Research Institute, CSIR, Kolkata 70032, India   Abstract: We report suppression of photodarkening in Yb-doped nano-crystalline fibers in silica host.  The  photodarkening  induced  loss  reduced  by  20  times  compared  to  Yb-doped aluminosilicate fibers. The laser efficiency of the nano-crystalline fiber was 79%.  Ó2010 Optical Society of America OCIS codes: (160.2290) Fiber materials; (060.2320)   Fiber optics amplifiers and oscillators 1. Introduction Ytterbium-doped silica fiber lasers has been a front runner in the race for higher powers due to its excellent conversion  efficiency.  However,  pump  induced  photodarkening  in  Yb-doped  fibers  has  been  recognized  as  a bottleneck for power scaling in many applications. The photodarkening process is host material dependent. The induced  loss  is  much  more  pronounced  when  the  Yb3+  ions  are  incorporated  in  aluminosilicate  host  than  in phosphosilicate host [1,2]. Thus, the photodarkening appears to be suppressed by modifying the host composition. Here,  we  investigate  Yb  doped  Y2O3  (or  Y3Al6O12)  nano-crystalline  in  a  silica  rich  matrix  for  reduced photodarkening. Y2O3 is an attractive material for laser applications and efficient Yb3+:Y2O3 ceramic lasers have been reported in [3,4]. It is expected that Y2O3 nanoparticles will provide different surroundings for the Yb3+ ions which helps to reduce the photodarkening even in high Yb3+ concentration. 2. Experiments and results The Yb-doped Y2O3 (or Y3Al6O12) nanoparticles were attained from SiO2-Al2O3-P2O5-Li2O-BaO core glass of an optical    fiber  preform  through  a  standard  MCVD-solution  doping  technique.  The  presence  of  crystalline nanoparticles in a heat treated preform were confirmed under transmission electron microscope (TEM) measurement [5]. The D-shaped fibers were pulled in double clad structure from the preforms and high power operation was demonstrated with good lasing efficiency [5]. Figure 1 represents refractive index profile of the nano-crystalline fibers with cross-sectional view in the inset. The core diameter was 18 mm with 0.136 of core NA. The background loss was ~ 70 dB/km at 1285 nm measured by high resolution OTDR. The Yb concentration was ~6000 ppm wt%.                                                             Time (sec)0 3000 6000 9000 12000log(T) at 633 nm-2.0-1.5-1.0-0.50.0Yb doped nanocrystalline fiberYb doped aluminosilicate fiber Fig. 1. Refractive index profile of Yb3+-doped nano-crystalline fiber. Inset: cross-sectional view of the fiber Fig. 2. Temporal characteristics of transmitted power at 633 nm for Yb-doped nano-crystalline fiber and aluminosilicate fiber    The photodarkening of the Yb-doped nano-crystalline fibers was evaluated by monitoring the transmitted output power through the fiber under 965 nm irradiation. We used fiber-coupled single mode laser diode as a pump source. The output end of the pump fiber was spliced to the fiber under test. A He-Ne laser at 633 nm was used as a Radius (um)-60 -40 -20 0 20 40 60Refractive Index difference0.0060.0080.0100.0120.014       a2027_1.pdf   OSA / CLEO/QELS 2010       JWA98.pdf  978-1-55752-890-2/10/$26.00 ©2010 IEEEprobe beam which coupled to the nano-crystalline fiber and propagated the same direction as the pump beam.  The probe beam was chopped and the output was detected by photodiode and lock-in amplifier. We used 1 cm of the fibers to suppress the amplified spontaneous emission. We measured photodarkening induced loss of a Yb-doped aluminosilicate fiber fabricated in-house for comparison. The pump input power was maintained to provide ~35% of population inversion of Yb3+. The temporal characteristics of the transmitted probe power are represented in Fig. 2. The photodarkening induced loss is significantly reduced in Yb-doped nano-crystalline fiber. When we fitted the measured results with stretched  exponential form, we  found that the  saturated induced loss in Yb-doped nano-crystalline fiber reduced by 10-20 times compared to the aluminosilicate counterpart, depending on the host material compositions.    The laser efficiency of the fiber was tested under 976 nm end pumping configuration. A linear 4%- 4% reflecting cavity was formed by perpendicularly cleaved end facets. A dichroic mirror at the pump throughput end separated the pump and signal wavelength. The laser performance is shown in Fig. 3(a). The slope efficiency was 79% with respect to the launched pump power. Figure 3(b) shows the laser spectrum which is quite broad covering from 1036-1072 nm. The measurement resolution was 1 nm.  Launched pump power (W)0 5 10 15 20 25Signal power (W)0510152025MeasuredLinear fit Wavelength (nm)1000 1050 1100 1150Power (a.u. in dBm)-80-60-40-200 Fig. 3. Laser performance of Yb-doped nano-crystalline fiber (a) and laser operating wavelength (b)  3. Conclusion In conclusion, we reported Yb-doped Y2O3 nano-crystalline fibers fabricated by a conventional MCVD-solution doping technique. The fiber exhibited low photodarkening induced loss compared to the Yb-doped aluminosilicate fiber, with good laser efficiency. This class of fibers will keep the advantage of the mechanical properties of silica glass, whereas the surrounding of RE ions can be engineered by varying the nanoparticle compositions.  Acknowledgement: The authors would like to acknowledge CSIR and Royal Society,UK for  provision of  financial support under CSIR-Royal Society Joint Collaborative Research Work.  4.  References  [1] S. Jetschke, S. Unger, A. Schwuchow, M. Leich, and J. Kirchhof, “Efficient Yb laser fibers with low photodarkening by optimizing of the core composition,” Opt. Express 16, 15540-15545 (2008).   [2] J. K. Sahu, S. Yoo, A. Boyland, C. Basu, M. Kalita, A. Webb, C. L. Sones, J. Nilsson, and D. N. Payne, “488 nm irradiation induced photodarkening  study  of  Yb-doped  aluminosilicate  and  phosphosilicate  fibers,”  in  Conference  on  Lasers  and  Electro-Optics/Quantum Electronics and Laser Science Conference  and  Photonic Applications Systems  Technologies  2008 Technical Digest (Optical Society  of America, Washington, DC, 2008), JTuA27.   [3] A. Shirakawa, K. Takaichi, H. Yagi, J-F. Bisson, J. Lu, M. Musha, and K. Ueda, “Diode pumped mode-locked Yb3+:Y2O3 ceramic laser,’’ Opt. Express 11, 2911-2916 (2003).  [4] J. Kong, D. Y. Tang, C. C. Chan, J. Lu, K. Ueda, H. Yagi, and T. Yanagitani, “High-efficiency 1040 and 1078 nm laser emission of a Yb:Y2O3 ceramic laser with 976 nm diode pumping,” Opt. Lett. 32, 247-249 (2007).  [5] J. K. Sahu, M. C. Paul, M. P. Kalita, A. Boyland, C. Codemard, S. Yoo, A. Webb, R. J. Standish, J. Nilsson, S. Das, S. K. Bhadra, M. Pal, A. Dhar, and R.Sen, “Ytterbium Doped Nanostructured Optical Fibers For High Power Fiber Lasers,” in European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference (Institute of Electrical and Electronics Engineers, New York, 2009), CJ2.1 WED.        a2027_1.pdf   OSA / CLEO/QELS 2010       JWA98.pdf  

488 nm irradiation induced photodarkening study of Yb-doped aluminosilicate and phosphosilicate fibers,”

and R.Sen, “Ytterbium Doped Nanostructured Optical Fibers For High Power Fiber Lasers,”

Efficient Yb laser fibers with low photodarkening by optimizing of the core composition,” Opt.

High-efficiency 1040 and 1078 nm laser emission of a Yb:Y2O3 ceramic laser with 976 nm diode pumping,”

Ytterbium doped nano-crystalline optical fiber for reduced photodarkening

Ytterbium doped nano-crystalline optical fiber for reduced photodarkening

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