Contents lists available at ScienceDirect Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte Broadband low-dispersion low-nonlinearity photonic crystal fiber dedicated to near-infrared high-power femtosecond pulse delivery Van Thuy Hoang b , Bartłomiej Siwicki a, ⁎ , Marcin Franczyk a , Grzegorz Stępniewski a,b , Hieu Le Van a,c , Van Cao Long c , Mariusz Klimczak a , Ryszard Buczyński a,b a Institute of Electronic Materials Technology, Glass Department, Wólczyńska 133, 01-919 Warsaw, Poland b University of Warsaw, Faculty of Physics, Pasteura 7, 02-093 Warsaw, Poland c University of Zielona Góra, Institute of Physics, Prof. Szafrana 4a, 65-516 Zielona Góra, Poland ARTICLE INFO Keywords: Silica photonic crystal fiber Low chromatic dispersion High-power signal delivery Femtosecond signal delivery Low nonlinearity ABSTRACT A low-dispersion and low-nonlinearity silica photonic crystal fiber is designed and developed. The investigated fiber is effectively single-mode and has low dispersion -20 to 40 ps/nm/km in the 1–1.7 μm wavelength range. The silica PCF can withstand a 1017 nm QCW laser beam with a maximum tested power of 9.1 W. The in- vestigated PCF with NA = 0.15 is suggested as a promising medium for a high-power femtosecond undistorted pulse delivery in the near-infrared region. 1. Introduction Femtosecond laser technology requires very stringent control of linear and nonlinear responses of the optical setup. Robustness and low cost of devices delivering femtosecond laser pulses motivates fiber- based femtosecond laser development. Photonic crystal fibers (PCFs) can play a very significant role in this, because of the flexibility of engineering of their dispersion profiles, as well as nonlinear and modal properties through effective mode area design [1]. Furthermore, PCFs can be made of various types of glasses, supplemented with quantum dots, liquids or gases, thus the change of the effective fiber dispersion can be easily performed [2]. From among different glasses, which en- able drawing of structured fibers, silica is the most versatile material, because it assures broad transmission window with low losses from the near UV to the near-IR range, exceptional glass purity (provided by use of chemical vapor deposition techniques [3]) and very high laser da- mage threshold. The maximum power that can be delivered by a single-mode fiber strongly depends on the fiber geometry, as well as on wavelength and duration of the laser pulse at input [4]. However, increase in the peak power of the laser pulse is accompanied by an increase in the nonlinear response strength of the fiber. Nonlinear effects, such as stimulated Raman scattering (SRS) [5], which usually dominates broadband, low- repetition signals and self-phase modulation (SPM), which causes an instantaneous phase shift [6], distort the pulse in spectral and temporal domains. The effect of fiber nonlinearity can be partly overcame by use of large mode area (LMA) fibers, since the size of the effective mode area A eff (λ) of the fiber determines fiber nonlinearity, which is ex- pressed with nonlinear coefficient γ = γ π λ n λ cA λ 2 () · () , eff 2 (1) where n 2 is nonlinear refractive index and c is the speed of light in the vacuum. On the other hand, the effect of fiber dispersion distorts the pulse in time domain [5]. The dispersion D of the fiber is expressed as the second order derivative of n eff =− D λ c d n dλ Re( ) , eff 2 2 (2) where Re(n eff ) is the real part of the complex effective refractive index of the propagating mode. Low chromatic dispersion is necessary for short-pulse transmission in optical fibers. Currently, the progress in femtosecond fiber laser technology is driven by a breadth of applications, including material processing [7,8], medicine [9] and even telecommunications [10,11]. Each of these areas imposes specific demands on the set of parameters of components used in development of a particular laser. One of the key challenges in fiber- based femtosecond systems is controlling the pulse parameters, i.e. structure and width, both in the time and spectral domains, over pro- pagation along optical fibers comprising the laser setup. Usually for ultra-short pulse delivery, hollow-core photonic bandgap https://doi.org/10.1016/j.yofte.2018.03.003 Received 24 October 2017; Received in revised form 27 February 2018; Accepted 6 March 2018 ⁎ Corresponding author. E-mail address: bartlomiej.siwicki@itme.edu.pl (B. Siwicki). Optical Fiber Technology 42 (2018) 119–125 1068-5200/ © 2018 Elsevier Inc. All rights reserved. T