Full length article Design and analysis of high-power segmented-core trench-assisted Yb-free erbium doped ﬁber ampliﬁer Ankita Gaur ⇑ , Vipul Rastogi Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India article info Article history: Received 1 December 2016 Received in revised form 1 March 2017 Accepted 18 April 2017 Keywords: Erbium doped ﬁber ampliﬁer High power ampliﬁer Leaky ﬁber Segmented-core ﬁber Trench-assisted ﬁber Large-mode-area ﬁber Selective single mode ampliﬁcation abstract Limited power handling capacity of single mode ﬁber compels to design effective-single mode large-core ﬁber for high power ampliﬁers. This article proposes a 0.15 NA, large-mode-area, bend-insensitive, Yb- free EDFA for the selective ampliﬁcation of fundamental mode. The ﬁber uses a leaky design to ensure fundamental-mode ampliﬁcation by higher-order mode discrimination. The segmented-core design in the ﬁber helps in achieving large-mode-area. The annular segments and low index trench in the ﬁber con- trol the leakage losses and gains of the modes. We show an EDFA design with 811 mm 2 mode-area, 0.014 dB bending loss for 10 mm diameter loop at 1530 nm wavelength and highly selective single- mode output. Our calculations also show a linear increase in the output signal power with pump power with a slope efﬁciency of 52.8%. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction High power ﬁber lasers and ampliﬁers are useful in industrial, medical, research and defense areas [1,2]. Two main requirements of high power lasers and ampliﬁers are beam quality of ﬁber that depends on the single-mode operation and the power handling capacity of ﬁber which depends on mode area [3,4]. Although beam quality of a conventional single mode ﬁber is good but its power handling capacity is low. The simplest approach to increase the damage threshold of ﬁber is single transverse mode ampliﬁca- tion in large core multi-mode ﬁber [5]. The increase in mode area due to increase in core size enhances the power handling capacity of ﬁber. However, the modes of multimode ﬁber could exhibit mode coupling. Therefore, it is necessary to design a large mode area (LMA) ﬁber with single mode operation [6]. The high discrim- ination between the propagation loss of the fundamental mode and the higher-order modes is essential to achieve effective single mode operation in a large core ﬁber. Bend ﬁbers, double clad ﬁbers, photonic band gap ﬁbers, photonic crystal ﬁbers and leaky ﬁbers are some of the approaches to realize large-mode-area ﬁbers. Sup- pression of higher order modes (HOMs) in a large core ﬁber could be achieved by using low-NA bend ﬁber [7]. A helical-core ﬁber with 40 mm core and a 0.10 numerical aperture was also studied for scaling mode area while maintaining single-mode operation [8]. A coiled Yb-doped, double-clad multimode ﬁber ampliﬁer with induced-bend loss in HOMs has been investigated to achieve single-transverse-mode operation [9]. Gain ﬁltering of higher order modes using selective doping proﬁle was also studied [10]. A large- core (40 mm), ytterbium-doped photonic crystal ﬁber ampliﬁer with single transverse mode of mode area 1000 mm 2 has been reported [11]. A tunable single-frequency, single-mode Er-Yb co- doped ﬁber ampliﬁer has been studied experimentally using clad- ding pumping with output power of 151 W at 1563 nm [12]. Power scaling is one of the challenges in designing high power ampliﬁers. In recent years signiﬁcant progress has been made in the power scaling of Yb-free Er 3+ ﬁber ampliﬁer for eye-safe appli- cations. A commercially available ﬁber Er60-20/125DC has been used to study 3.5 W diffraction-limited, single-frequency output [13]. An output power of 28.5 W has been reported using Yb-free Er-doped LMA ﬁber with resonant cladding pumping [14]. An out- put power of 67 W and the differential efﬁciency of 30% with respect to absorber pump power has been demonstrated experi- mentally using commercially available multimode EDFA Er60- 40/140DC with adequate cooling technique [15]. An Er-doped pho- tonics crystal ﬁber has been studied experimentally to achieve 70 W of output power and 18.5% of efﬁciency at 1556 nm [16]. 75 W output power and 40% slope efﬁciency with respect to pump power has been reported using a single-mode low-NA erbium doped ﬁber of core diameter 34 mm [17]. http://dx.doi.org/10.1016/j.optlastec.2017.04.013 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: ankitagaur.phy@gmail.com (A. Gaur). Optics and Laser Technology 95 (2017) 46–50 Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt