Monolithic gain-managed nonlinear fiber amplifier delivering 2.7-μJ ultrashort pulse with broad spectrum seeding

Abstract

Nonlinear fiber amplification of ultrashort laser pulse has been recognized as an efficient way of significantly extending the spectrum coverage and realizing sub-100 fs de-chirped duration along with the energy scaling. However, conventional approaches typically rely on narrow-spectrum, low-energy (nJ-level) seed lasers, severely limiting the achievable pulse energy even with gains exceeding 30 dB. Here, we demonstrate an unconventional nonlinear fiber amplifier that overcomes this limitation by leveraging a broadband seed pulse with a 20 dB spectral bandwidth of 74 nm and a pulse energy 52.5 nJ, which are respectively scaled to 83 nm and 3 μJ by a backwardly pumped Yb-doped photonic crystal fiber with core/cladding diameter of 40/200 μm, corresponding to an optical gain of only 17.6 dB. The pulse duration is de-chirped to 76 fs with an energy of 2.7 μJ, which to the best of our knowledge is the highest that obtained from any nonlinear fiber amplifiers. In the meantime, a decent performance in terms of output beam quality (M²<1.12), polarization extinction ratio (∼17 dB), and long-term stability is verified at the maximum operation energy. According to simulations, the laser pulse experienced a nonlinear amplification with the spectrum asymmetrically extending to the short wavelength side, mostly driven by the self-phase modulation and high excitation of the gain fiber at the output end. Such a pulse evolution process could be further leveraged to realize higher energy ultrashort lasers by using gain fiber with larger core diameters, and greatly expand the application potential of nonlinear fiber amplifiers.