Abstract:We report the generation of a 12.6W average power, 50MHz repetition rate pulse train, compressible to high fidelity 170fs pulses, from an ultrafast ytterbium-doped fiber laser system via adaptive amplitude and phase pre-shaping.
IntroductionYtterbium-doped fibre laser systems, coupled with the chirped-pulse amplification (CPA) technique, have allowed the realization of compact ultrafast laser systems producing high average power at various repetition rates [1]. Power scaling in fibre CPA systems is non-trivial, because a number of factors can degrade the pulse fidelity at the output, such as uncompensated material dispersion, nonlinearity, and a non-uniform spectral gain profile with finite width. Due to the optical confinement and long interaction length of the fiber geometry, nonlinear effects, most notably self-phase modulation (SPM), are a critical consideration in constructing fiber CPA laser systems. However, conventional approaches in mitigating SPM have limitations; temporal stretching is physically limited by the finite size of the grating compressor, while the scaling of large-mode-area (LMA) fibers will eventually undermine the advantages of fiber geometry. Hence, novel approaches in mitigating SPM are necessary for further power scaling in fiber CPA laser systems while maintaining pulse fidelity.Adaptive pulse shaping allows for the generation of high-fidelity pulses by controlling the spectral phase profile [2]. However, due to damage limitations, pulse shaping often needs to be implemented prior to high power amplification in CPA systems [3]. In fiber CPA systems, amplitude-only shaping has recently been demonstrated to control the nonlinear-phase modulation induced by SPM at low energy [4], but it cannot compensate for higherorder spectral phase due to the material dispersion. Our group recently demonstrated a phase-only shaping to produce high-quality pulses in a high-energy fiber laser system [5].In this paper, we report our latest experimental result in generating high-fidelity femtosecond pulses in a fiber CPA system by adaptive amplitude and phase pre-shaping [6]. A pulse shaper based on a dual-layer liquid crystal spatial light modulator (LC-SLM) was implemented in the fiber CPA system for amplitude and phase shaping prior to amplification. The LC-SLM was controlled using a differential evolution (DE) algorithm, to maximize a two-photon absorption (TPA) detector signal produced by the compressed fiber CPA output pulses. We show that our approach compensates for both accumulated phase from higher-order material dispersion and nonlinear phase modulation. A train of pulses with an average power of 12.6 W at a 50 MHz repetition rate was produced from the fiber CPA system, compressible to high fidelity pulses with a full-width at half-maximum (FWHM) duration of 170 fs.
We theoretically and numerically investigate indirect mid-infrared pulse shaping via parametric transfer, specifically difference-frequency generation. We define a quantitative measure for the fidelity of parametric transfer, and investigate the effect of material dispersion and process nonlinearity on the parametric transfer. We show that a good fidelity transfer of a broadbandwidth pulse can be efficiently achieved with a reasonable wavelength tunability, by careful design of the experimental configuration.
A derivation of approximate analytical expressions for band edges ω± of the first band gap of a multilayer periodic structure is presented for both TE and TM waves at arbitrary angles of incidence. It is found that the approximate expressions give an excellent agreement with the numerical results as verified by the band edge variation with respect to the filling fraction [Formula: see text] and refractive index contrast Δn. The analytical expressions for the band edges are further employed to derive a semi numerical optimization of the relative gap width [Formula: see text] with respect to the filling fraction ν. The result is again shown to be in good agreement with the numerical result.
We demonstrate increased peak power from an Yb fiber CPA system operating with strong self-phase modulation by shaping the spectral-phase of the input pulses. An adaptive control loop used feedback from the output autocorrelation. We investigated pre-compensation of both SPM phase distortion at high energies, and residual dispersion from mismatched stretcher/compressor technologies at low energies. Phase shaping resulted in improved pulse quality. When using a bulk grating stretcher, shaping increased the autocorrelation peak by a factor of 2.9, and with a fiber stretcher, shaping increased the autocorrelation peak by a factor of 3.4. High-quality 800 fs, 65 microJ recompressed pulses were produced. This technique could benefit a wide variety of fiber amplifier systems and is self-optimising for operation at both low and high pulse energies.
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