1022 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 9, MAY 1, 2016 Yb-Doped Pedestal Silica Fiber Through Vapor Phase Doping for Pulsed Laser Applications Maitreyee Saha, Sourav Das Chowdhury, Nishant Kumar Shekhar, Atasi Pal, Mrinmay Pal, Chandan Guha, and Ranjan Sen Abstract— This letter describes successful fabrication and detail characterization of ytterbium (Yb)-doped pedestal aluminosilicate fibers through vapor phase doping technique using modified chemical vapor deposition system (MCVD). Fab- ricated preforms have uniform step-index profiles devoid of any profile ripples, central dip, and/or core-clad interface defects, which are very common to the preforms made by a solution doping method. Fibers with a pedestal design exhibit good optical properties, low photodarkening-induced losses, high SNR values, and higher efficiencies making them suitable for high-power pulsed laser applications as compared with normal Yb-doped fibers. One of the pedestal fibers has demonstrated the output energy of 186 μJ with the 1.86-W average power. The pulse has a width of 100 ns at a 10-kHz repetition rate, which provides a peak power of 1.86 kW. Index Terms— Fiber lasers, optical fiber fabrication, rare earth compounds, vapor deposition, ytterbium. I. I NTRODUCTION H IGH power fiber lasers lead to dramatic advances in industrial, strategic, scientific and material processing applications which require pulsed lasers with high energy and high peak power. The key part of such laser module is the rare earth (RE) doped active fiber. The ytterbium doped fiber (YDF) laser has the potential to generate pulses as short as 4.5 fs with peak power of 100 kW at ∼1 μm wavelength [1]. Q-switched lasers with pulse duration from few ns to few hundreds ns along with pulse energy as high as 100 mJ are now commercially available. In 2014, Eidam et al. have shown record peak power of 3.8 GW for a 500 fs pulse from a chirped pulsed amplifier system using YDF [2]. Combination of multi-millijoule pulse energies, high peak power and com- pact cavity arrangement without need of active cooling system, made pulsed fiber lasers superior than their solid–state counter part in the field of marking, engraving, nonlinear frequency conversion, range finding, remote sensing [3] etc. Manuscript received September 19, 2015; revised December 30, 2015; accepted January 25, 2016. Date of publication February 2, 2016; date of current version March 17, 2016. This work was supported in part by the GLASSFIB Project of Council of Scientific and Industrial Research, India, and in part by the Department of Electronics and Information Technology, India. M. Saha, S. D. Chowdhury, N. K. Shekhar, A. Pal, M. Pal, and R. Sen are with the Fiber Optics and Photonics Division, Central Glass and Ceramic Research Institute, Kolkata 700032, India (e-mail: maitreyee.cgcri@gmail.com; sdaschoudhury6@gmail.com; nishantkumarshekhar@gmail.com; atasi@cgcri.res.in; mpal@cgcri.res.in; rsen@cgcri.res.in). C. Guha is with the Chemical Engineering Department, Jadavpur University, Kolkata 700032, India (e-mail: cguha2003@yahoo.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2016.2524040 YDF lasers are capable of providing diffraction–limited power nearly two orders of magnitude higher than the solid–state laser systems due to their excellent beam quality, high optical efficiency of ≥80%, long term power stability and wide pump absorption band [4]–[6]. But designing a high-energy/high-peak-power fiber laser is difficult enough as it is prone to nonlinear effects such as stimulated Raman scattering (SRS) [6], stimulated Brillouin scattering (SBS) which hinder in reliable system operation. To increase the energy storage and reduce fiber nonlinearities, it is necessary to enhance Yb content as well as the core diameter which decreases the mode confinement and permits the use of shorter fiber lengths without compromising the absorption efficiency. However, this leads to multi-mode operation and compromised beam quality, unless special fiber designs are used [3]. For getting single mode operation and good beam quality from a large core fiber, numerical aperture (NA) of the fiber should be low which compromises co-dopants concentration in the core. However, lower co-dopant/Yb ratio causes excess photodark- ening induced losses (PDIL) [7] which deteriorates the output power of fiber lasers. A fiber with pedestal design offers many advantages to address these critical complications and can be realized to overcome fabrication related challenges. In pedestal structure, an increased high-index core is surrounded by raised inner cladding layer without RE ions and thus, lowering the effective NA of the core. Additionally, high refractive index (RI) of the core allows higher co-dopant/Yb ratio which suppresses PDIL. But fabrication of pedestal Yb–doped fiber (PED–YDF) using conventional solution doping (SD) method [8] is very difficult due to phase separation, bubble formation, RI profile ripples, non–circular core formation, core–clad interface defect generation [9], [10] etc. Moreover, SD process suffers from poor repeatability, low dopant uniformity along the preform length and reached the limit regarding large core size [11], [12]. The vapor phase doping (VPD) technique, based on high temperature sublima- tion of RE-chelate compounds [13], provides a reliable alterna- tive and obviates the process technology related drawbacks of SD method [14]. This letter presents successful fabrication of large core PED–YDFs through VPD technique by optimizing the process parameters which can be reliably used for high power pulsed laser applications. II. EXPERIMENTS A. Preform Fabrication Through Vapor Phase Doping A set of normal Yb-doped preforms and pedestal Yb–doped preforms were fabricated through VPD tech- nique in conjunction with MCVD process. The inner clad of the preforms was formed by sintered silica–alumina 1041-1135 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.