Femtosecond fiber CPA system seeded by bandwidth-limited picosecond pulses

Abstract

Summary form only given. Fiber lasers and amplifiers are capable of providing very high average power output and diffraction limited beam quality thanks to intrinsic fiber geometry and confined propagation of radiation. However, the highest pulse peak power is limited by nonlinear interactions in fiber, caused by high irradiances in the core and large interaction length. Thus the design of fiber laser systems generating ultra-short pulses with considerable energies is still a challenge.In order to overcome the peak-power limitation, two main strategies are implemented: increasing of the diameter of the fiber core and chirped pulse amplification (CPA). In the high-energy ultra-short pulse laser system, these two strategies are usually combined and, along with the large-mode-area (LMA) fiber, chirpedpulse amplification is used. In the conventional CPA system, the dispersion between the stretcher and compressor is matched up to the third order and efforts are made to minimize the accumulated nonlinear phase shift [1]. However, it was recently shown that a fiber CPA (FCPA) can operate at high levels of the nonlinear phase shift and still provide high-peak power (quality) pulses [2, 3]. Moreover, when using a fiber as a stretcher and a bulk grating compressor, it is possible to compensate the third-order dispersion mismatch between the stretcher and compressor by exploiting the self-phase modulation (SPM) of asymmetrical “cubicon” pulses [4]. In this contribution we present FCPA system, in which SPM in stretcher fiber and power amplifier is utilized in order to achieve femtosecond pulses. Our fiber laser system was based on the CPA design but seeded by nearly bandwidth-limited picosecond pulses. The whole FCPA setup is depicted in Fig. 1 a. It consisted of a passively mode-locked all-in-fiber picosecond oscillator (1064.7 nm center wavelength), the first pre-amplifier, an acousto-optic down-counter, a polarization-maintaining single-mode fiber stretcher, the second preamplifier, a power amplifier and a bulk grating compressor. In order to achieve femtosecond pulses, we utilized SPM in fiber stretcher to broaden the spectrum and dispersion of the fiber to stretch pulses up to 450 ps. In the final amplification stage, an ytterbium-doped double-clad chirally-coupled core (CCC) fiber was used. During the amplification, the pulses were modified by gain-shaping in an ytterbium-doped amplifier and SPM. Combined influence of gain-shaping and SPM improved pulse compression. Part of the long-wavelength amplified pulse spectrum, which mostly contributed to picosecond pedestal after compression, was blocked in the compressor in order to improve pulse contrast. After amplification, pulses were successfully compressed to 400 fs duration, which is 8 times shorter than the duration of the oscillator initial pulses (Fig 1. b). After compression with spectral filtering, pulse energy of 58 μJ was achieved. Discussed FCPA approach can benefit from a very simple design. The presented laser system is polarization maintaining, it can be easily designed in all-fiber manner by implementing standard splicing techniques (excluding the grating compressor), and should be cost-effective because of the simple master oscillator.