Abstract-We present a compact Yb-doped fiber chirped pulse amplification system set in a single pass configuration. The system is capable of delivering 34μJ pulses with 200kHz repetition rate. When seeded with a low average power oscillator, the amplifier optical-tooptical efficiency is as high as 37% and the gain reaches 52dB. Temporal pulse degradation due to self-phase modulation is explored. One stage amplification allowed us to limit the set-up complexity, typical of multi-stage CPA systems.Fiber lasers and fiber amplifiers are growing in importance as they can provide both high average output power and good beam quality at the same time [1]. On the other hand, single mode fiber geometry imposes fundamental limits on pulse energy due to limited extractable energy in the fiber, nonlinear pulse distortions and damage issues. In the case of femtosecond pulse amplification fiber nonlinearities are the primary limiting factor because of long propagation paths and small beam diameters.The chirped pulse amplification (CPA) concept is commonly used to overcome this detrimental effect [1][2]. Recently, several modifications have been introduced -pulse shaping [3] or misaligned stretcher and compressor configuration [4] -to push pulse energy limits even further. Most of the setups reported so far utilize two or more amplification stages [5][6][7] to obtain tens of J energy levels with a reduced (<1 MHz) repetition rate. Other works explore the limits of the highest possible pulse energy or average power [3,8] at the expense of significant complication of the amplifier set-ups.Here we describe a simplified system where femtosecond pulses from a low average power modelocked oscillator are amplified in a single pass configuration in a Yb-doped single mode large mode area (LMA) fiber, reaching 50J at 200kHz. To reduce nonlinear effects, the CPA technique is used, but with moderate-size optical elements and thus a modest pulse stretching ratio. We show how the temporal characteristics of the output pulse is affected by fiber nonlinearities.The system consists of a passively mode locked femtosecond oscillator, a grating stretcher-compressor * E-mail: nejbauer@ichf.edu.pl pair, an electro-optical modulator (Pockels cell) pulse picker and an active medium -single mode Yb-doped photonic crystal fiber set in a single pass configuration.The femtosecond oscillator is presented schematically in Fig. 1. The original design is described in [9]. The active medium of the oscillator is 1.2-mm Brewster-cut Yb:KYW crystal pumped with a 980nm single mode fiber coupled laser diode emitting 560mW of average power. Passive mode-locking is obtained by introducing a semiconductor saturable absorber mirror SESAM [10] in the place of a cavity end mirror. The resonator generates pulses of 350fs duration at 94MHz repetition rate. The central wavelength is set to 1030nm with a 4.3nm FWHM bandwidth. The average output power is about 90mW and the beam quality factor is M 2 < 1.1. The schematic of the CPA amplifier is presented in Fig. 2. The seedi...