7 nJ High-fidelity 60 fs pulses at 1035 nm from an integrated ytterbium fiber oscillator with a higher-order-mode fiber

Summary form only given. All integrated mode-locked Yb-doped fiber oscillators delivering high fidelity pulses are very attractive as seed sources for Yb-fiber amplifier systems [1] as well as Yb solid state lasers [2] delivering sub-200 fs pulses. The main demands for seed sources of such systems are good pulse quality and compressibility, and enough seed energy. High fidelity pulses are also beneficial to minimize collateral damage in applications like micromachining and microsurgery. The requirement for pulse energy and pulse fidelity becomes even more demanding for phase stabilized amplifier systems [3] requiring sub-100 fs pulses with pulse energy of several nJ. The robustness and stability of fiber oscillators offer an interesting alternative to solid state oscillators.In order to achieve high quality, short, high-energy pulses from a fiber oscillator, intracavity dispersion control is of utmost importance. Generally dispersion control is ensured with free-space components like gratings. However, intracavity gratings compromise the robustness of the fiber oscillator, and compensation of higher-order dispersion cannot be achieved. Recently, an anomalous dispersion fiber with designable dispersion that can be routinely integrated was demonstrated [4]. Using such a higher-order-mode (HOM) fiber in an oscillator with net anomalous intracavity dispersion, 0.5 nJ, 57 fs pulses were realized [5]. Since we operate our oscillator with net normal intracavity dispersion, we can obtain higher pulse energy from the oscillator. Preliminary analytic calculations show that for intermediate output energies, the best pulse fidelity can be obtained after compression with a simple (e.g. grating) compressor (see the left panel of Fig. 1). For higher output energies, optimal compression can only be obtained using adaptive compression schemes. By reducing the intracavity dispersion, pulses with a broader spectral bandwidth can be generated, which can be compressed to shorter pulse durations. More detailed numerical simulations describing our oscillator are underway. Here, we present a 20 MHz fiber ring oscillator that works in the net normal dispersion regime and uses a HOM fiber for dispersion compensation. The fiber length of the HOM module is chosen such that it matches the dispersion of most of the single mode fiber in the oscillator. As a result, only a limited amount of group delay dispersion and third order dispersion are accumulated per cavity roundtrip. Modelocked operation in the oscillator is based on nonlinear polarization rotation. The oscillator has two output ports: The first serves for pulse cleaning, and is, together with the non-linear polarization rotation and the spectral filter, responsible for maintaining the modelocked operation. The output ratio at the second (main) output port is controlled with a half wave plate. The pulses are subsequently compressed using a transmission grating pair. We have performed a complete characterization of the oscillator output pulses. The results of an SHG-FROG measurement after compression with a transmission grating pair are presented in Fig. 1 (d) and (e). In stable single pulse operation the oscillator delivers up to 7.5 nJ pulses that can be dechirped to 62 fs. The measured spectral bandwidth corresponds to a Fourier-transform limited (FL) duration of 61 fs. In Fig. 1 (e) we compare the FL and measured pulse profile, indicating the excellent pulse quality delivered from our oscillator. In conclusion, we present a fiber oscillator with all-fiber dispersion compensation with record performance delivering pulse parameters comparable to solid state oscillators and 14 times the energy of a previous oscillator using an HOM fiber delivering pulses with comparable duration [5].