Air-core fiber or photonic-crystal rod, which is more suitable for energetic femtosecond pulse generation and three-photon microscopy at the 1700-nm window?

Energetic femtosecond pulses at the 1700-nm window are a prerequisite for deep-tissue three-photon microscopy (3PM). Soliton self-frequency shift (SSFS) in photonic-crystal (PC) rod has been the only technique to generate such pulses suitable for 3PM. Here we demonstrate through SSFS in an air-core fiber, we can generate most energetic femtosecond soliton pulses at the 1700-nm window, 5.2 times higher than that from PC rod. However, the air-core soliton pulse width is 5.9 times longer than that of PC rod soliton. Based on comparative 3PM excited with both air-core and PC rod solitons, we propose the more suitable source for 3PM. We further elucidate the challenge of generating shorter soliton pulses from air-core fibers through numerical simulation.