Compression of μJ-level fs pulses from a monolithic Yb-fiber amplifier at 1 μm wavelength in a hollow-core photonic bandgap fiber

Summary form only given. Yb-doped femtosecond fiber chirped pulse amplifiers (FCPAs) are extremely attractive to replace their solid-state counterparts in when it comes to robustness and ease of operation. However, in most Yb-doped FCPAs delivering high-energy output pulses the advantages of robustness and ease of operation are forfeited since free-space optical components are used to stretch and compress the pulses. The main bottleneck to obtain fiber based stretching and compression is to match the dispersion of the stretcher and compressor. Recent advances have yielded good results with fiber-based stretchers and free space compressors [1-3]. However, the free space compressors require rather large distances between the compressor elements, which reduces the robustness of the system. On the other hand, while anomalous dispersion solid-core fibers have been available, e.g. photonic crystal fibers [4] or higher-order mode fibers [5], such fibers are not suitable as compressor fibers for pulse energies exceeding several nJ, because of the high nonlinearities experienced in the solid core. The use of a hollow-core (HC) photonic bandgap fiber (PBF) can alleviate this problem, an approach that was demonstrated in Er-doped fiber lasers [6]. Short lengths of HC PBF have been used to compress pulses from Yb-fiber oscillators [7], but not yet in Yb-FCPA. Here, we present a μJ-level femtosecond Yb-FCPA that uses a dispersion-matched dispersion compensating fiber (DCF) stretcher [8] and a HC PBF compressor.The Yb-fiber laser system consists of an all normal dispersion Yb-fiber oscillator operating at 40 MHz repetition rate, two PM single mode fiber (SMF) preamplifiers and a PM large mode area (LMA) fiber amplifier. The pulses from the oscillator are stretched in a hybrid fiber-pulse stretcher that consists of 8 m of ClearLite 980 fiber followed by 5 m of DCF. After amplification, the pulses are compressed in 25 m of HC PBF. The measured group delay dispersion of the- compressor at 1060 nm is about -2 ps2 resulting in a stretched pulse duration of ~30 ps. Because the effective area of the DCF is rather small (~6.7 μm2), the seed energy was limited to 0.2 nJ and the pulses were pre-stretched in the ClearLite 980 fiber in order to minimize the nonlinearities in the DCF stretcher. After the first amplification stage a fiber pigtailed acousto-optic modulator (AOM) was used to reduce the repetition rate to 2 MHz. In preliminary test experiments, we used a large mode area (LMA) power amplifier as a final amplification stage. The LMA has a core diameter of 30 μm, corresponding to a mode field area of ~625 μm2. Free space coupling and mode conversion was used to couple the amplified pulses into the HC PBF. The output pulses from the compressor were characterized by second harmonic frequency resolved optical gating (SH FROG). The shortest compressed pulse duration was achieved by optimizing the fiber length of the pre-stretcher. Figure 1 shows the retrieved pulse with a duration of 220 fs and its spectral phase for a pulse energy of 250 nJ. At the moment experiments using a power amplification stage utilizing a single mode fiber with 10 μm core diameter are underway. This will allow us to directly couple (by fusion-splicing) the light from the power amplifier to the HC PBF. The results from these experiments will be presented at the conference. We thank K. Jespersen and L. Grüner-Nielsen from OFS Denmark for their support, providing the fiber stretcher and measuring the stretcher and compressor dispersion.