Few-cycle all-fiber temporally coherent supercontinuum sources

We present a monolithic fiber optic configuration for generating temporally coherent supercontinuum (SC) pulsed emission with the shortest pulse duration presented to date, to our knowledge, by an all-fiber source. Few-cycle pulses as short as 14.8 fs are obtained, with central emission wavelength of 1060 nm, repetition rate of 75 MHz and average power of 250 mW. The SC generation is obtained by pumping an all-normal dispersion (ANDi) photonic crystal fiber (PCF) with a mode-locked Yb fiber laser. Spectral broadening by self-phase modulation preserves compressible pulses in the temporal domain. Compared to previously reported configurations exploiting ANDi PCFs, all stages of our source are fiber based and fiber coupled between them. Avoidance of free-space propagation between stages confers unequalled robustness, efficiency and cost-effectiveness to this novel configuration. The ANDi PCF was designed and produced to provide a convex, flat-top dispersion curve with group velocity dispersion comprised between -20 and 0 ps/nm/km in the wavelength range from 900 to 1200 nm. A d-scan system was designed and built to compress and characterize the pulses. The spectrum, wider than 150 nm, supports a Fourier limit pulse duration of 13.7 fs, and pulses have been actually compressed down to 14.8 fs, which demonstrates a high level of temporal coherence in the achieved supercontinuum; second- and third-order dispersion of the pulses are measured as low as -145 fs2 and 875 fs3, respectively. The source has been integrated in a twophoton fluorescence and second-harmonic generation microscopy setup, where 3D images of biological samples have been successfully obtained.