μJ-level, kHz-repetition rate femtosecond fiber-CPA system at 1555 nm Grzegorz Sobon n , Pawel Kaczmarek, Aleksander Gluszek, Jaroslaw Sotor, Krzysztof M. Abramski Laser and Fiber Electronics Group, Wroclaw University of Technology, 50-370 Wroclaw, Poland article info Article history: Received 23 December 2014 Received in revised form 3 February 2015 Accepted 25 February 2015 Available online 26 February 2015 Keywords: Chirped pulse amplification Fiber amplifiers Mode-locked lasers Femtosecond lasers abstract In this work, we demonstrate a high-power, fiber-based chirped pulse amplification (CPA) setup utilizing Er- and Er/Yb-doped fibers, operating at 1555 nm central wavelength. The integrated all-fiber pulse- picker allows to reduce the repetition frequency down to the kHz-range, which enables generation of sub-picosecond pulses with energies above 2 μJ and pulse peak power exceeding 1 MW. The system utilizes an Er/Yb co-doped large mode area fiber in the final amplification stage. Thanks to the used mode-field adapters and fiber-based components, the setup is almost fully fiberized, except the bulk grating pulse compressor. In order to provide compactness and simplicity, the compressor was designed using dense 1100 lines per millimeter gratings, that allow to keep the small grating separation. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Chirped pulse amplification is a key technique which enables generation of ultrashort optical pulses with high peak power and large energy. The technique was introduced over 30 years ago by Strickland and Mourou [1] and is still the most common approach to achieve high-power laser pulses. The main advantage of CPA in comparison to other techniques enabling high-intensity pulse generation (e.g. optical parametric oscillators, OPO), is the possi- bility of designing the system based on robust, cost effective and flexible optical fiber technology [2,3]. For example, for pulse stretching, proper dispersive fiber might be used, instead of clas- sical grating-based stretchers (e.g. Martinez-type stretcher [4]). For example, Adel et al. used an all-fiber stretcher based on a telecom-grade module, which introduced a very large dispersion of 89 ps 2 in a double-pass configuration [5]. As gain media, rare- earth-doped fibers, especially double-clad (DC) or large-mode area (LMA) might be used. The radiation from the pumping lasers might be introduced to the active fibers through high-power multimode beam combiners, instead of utilizing complex bulk systems with aspheric lenses and dichroic mirrors. In recent years, the CPA technique was extensively developed at the 1 μm wave- length (with the use of Yb-doped fibers) and led to many im- pressive results. As an example, Eidam et al. [6] demonstrated a CPA system based on Yb-doped photonic crystal fiber (PCF) emitting 830 W of average output power with 680 fs pulses at a relatively large 78 MHz repetition frequency. Ultrashort, femtosecond pulses with high peak power and large energy are required by many applications, e.g. laser micromachining [7], surgery [8], terahertz-wave generation [9], optical imaging [10] or supercontinuum generation [11]. However, all those applications require robust, compact and environmentally stable sources. All those features can be provided by the all-fiber technology. In contrast to the great popularity of the CPA technique com- bined with Yb-doped fibers operating at 1 μm, the 1.55 μm wa- velength seems to be somehow neglected by the high-power laser community, since there are only few reports on 1.55 μm CPAs, and only one (to the best of our knowledge) commercially available high-energy (100 μJ) femtosecond system designed for materials processing applications, e.g. corneal surgery [12]. Recently, mostly high-repetition rate systems (without pulse-pickers) were devel- oped, mainly single-mode [13,14], as well as using LMA fibers [15,16]. For example, Pavlov et al. [14] reported amplification of high-repetition rate 450 fs pulses in single-mode fibers up to 2.5 W at 150 MHz repetition rate. Sub-300 fs pulses at a repetition rate of 22 MHz and with an average output power of 1.5 W were generated from a CPA system based on Er/Yb co-doped LMA fiber, as demonstrated by Roy et al. [16]. Earlier, some research was also carried out on low-repetition rate CPAs (with the use of pulse- pickers) for high pulse energy generation. As an example, Minelly et al. [17] demonstrated a CPA system based on multi-mode Er- and Er/Yb-doped fibers emitting 700 fs pulses at a 5 kHz repetition rate and 100 μJ pulse energy. However, the setup involved a complex bulk Martinez-type pulse stretcher, a free-space coupled pulse picker and required free-space beam coupling. Another setup based on LMA Er/Yb-doped fiber was demonstrated by Morin et al. [18], delivering 605 fs pulses with 1.5 μJ pulse energy. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2015.02.051 0030-4018/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ48 713203189. E-mail address: grzegorz.sobon@pwr.edu.pl (G. Sobon). Optics Communications 347 (2015) 8–12