Partial parabolic amplification in rare-earth-doped optical fiber

Abstract

Nonlinear amplification is a powerful technique for generating ultrashort laser pulses with high peak power in fiber systems. However, the diversity of nonlinear amplification approaches and their inherent complexities present significant challenges to achieving a unified understanding and further scaling of peak power and pulse energy while preserving ultrashort durations. Here, we report the results of a systematic optimization with respect to seed pulse duration that elucidates the dynamics of nonlinear amplification and allows identification of distinct propagation regimes. As part of this analysis, we identify a new regime, termed partial parabolic amplification, which achieves 50-fs pulse duration and yields higher peak power than any other nonlinear amplification regime. An initial experimental demonstration of partial parabolic amplification produces 50-fs and 2.2-uJ pulses with a 25-um-core Yb fiber amplifier, corresponding to a 30-MW peak power. In contrast to other nonlinear amplification techniques, practical energy scaling beyond 10 uJ and 200 MW should be achievable with available gain fibers with larger mode areas, which would fill a gap in existing fiber laser capabilities that would directly impact material processing, nonlinear bio-imaging, and other applications.

Figures (6)

• Figure 1: Optimal nonlinear amplification regimes at different seed durations, assuming frequency-independent gain. (a) Peak power and duration of the dechirped amplified pulse at different seed durations, with optimal nonlinear amplification regimes indicated. (b) Optimization results for negatively-chirped, transform-limited, and positively-chirped seeds. (c) Misfit parameter $M$ corresponding to results in (b). (d) Temporal and spectral profiles of the amplified pulses for the indicated durations of a positively-chirped seed. Spectra are plotted with respect to frequency relative to the center frequency. O: output temporal profile; F: parabola fitted to the output profile.
• Figure 2: Peak power and misfit parameter $M$ of optimal nonlinear amplification regimes with [number-unit-product=-]40 and [number-unit-product=-]60 (full-width at half-maximum) Gaussian gain bandwidth.
• Figure 3: Optimal nonlinear amplification regimes at different seed durations and chirp signs in a [number-unit-product=-]25-core Yb-doped fiber for [number-unit-product=-]1030 and [number-unit-product=-]1060 seeds. Optimization with half the doping concentration is also conducted at short durations for optimal GMNA; resulting peak powers are shown with light blue lines.
• Figure 4: Illustration of gain management for each regime in Yb-doped fiber. (a) Yb gain spectrum with [number-unit-product=-]976 pumping. (b, top and middle rows) Temporal and spectral evolutions of each nonlinear amplification regimes. Evolutions are shown by fields at 0.0, 33.0, 67.0, and 100% propagation distances with blue, red, green, and black lines, respectively. (b, bottom row) Induced frequency shift of the output profiles, from nonlinear chirp generation [Eq. (\ref{['eq:freq_shift']})]. The effect of TOD compensation from gain management occurs in GMNA, GMSSA, and GMPPA, while an effective cubic phase is not produced in SPMA. (c) Leading-edge misfit parameter $M$ of different regimes. To exclude the effect of gain-managed distortion at short wavelengths, $M$ is computed based only on the leading edge of the temporal profile, which corresponds to the long-wavelength part of the spectrum that stays in the flat-gain region. Variation of $M$ in GMNA and GMSSA result from the imperfectly-flat long-wavelength gain, which creates an extended leading temporal edge as shown in (b). Only pulses in the SPMA regime do not undergo effective parabolic shaping.
• Figure 5: Illustration of TOD compensation induced by gain management. (a) Temporal profile (blue) and instantaneous frequency (black), along with (b) spectral profile (blue) and its phase (orange), of a GMN pulse. Phase deviation from a parabola (black dash line) is exaggerated for visualization purposes. (c) Phase of a Treacy dechirper, where $\phi_{\text{Treacy}}=\phi_{\text{quadratic}}+\phi_{\text{cubic}}$. The GMN temporal profile induces a slow leading redshifting and fast trailing blueshifting during pulse propagation, effectively leading to significant temporal stretching of low-frequency components in the leading edge but less stretching for high-frequency parts in the trailing edge. This means that spectral components of lower frequency have larger spectral phases than expected from a parabola, while components of higher frequency have smaller spectral phases, as shown in the orange line and black dash line in (b). Treacy dechirper provides negative group delay dispersion with positive TOD, which cancels the spectral phase of the GMN pulse.