Efficient Soliton Spectral Shifting in the 1-km Long Hollow Core Fiber With Nested Capillaries

In this work, large tunable soliton self-frequency shifts (SSFS) from the near to mid-IR are numerically demonstrated for two designs (D1 and D2) of 1-km long air-filled hollow core fiber with nested capillaries. At pumping the fiber D1 with 100-fs pulses and 1.25 µJ energy at 1550 nm, a shift of the soliton spectrum to 2220 nm was obtained with a quantum efficiency of 30%. It is shown that a long-term passage of a soliton with a maximally shifted spectrum near the long-wavelength boundary of the transmission band contributes to a significant decrease in the SSFS rate and stabilization of its power and duration at 518 kW and 494 fs. In the fiber D2, when pumped with 100-fs pulses at 1650 and 1950 nm, SSFS to the region of 3200 nm were obtained with a quantum efficiency of 16 and 32%, respectively. The power (∼2 MW) and duration (∼500 fs) of the soliton at the maximum shift of the spectrum after passing a length of 1 km were practically independent of the pump wavelength. The ratio of the tunable soliton delay time to its duration at the maximum shift reached very large values of ∼3000.