Flat broadband supercontinuum generation in a short length of highly nonlinear fiber pumped by a femtosecond carbon-nanotube based passively mode-locked erbium-doped fiber laser

摘要

Highly nonlinear fibers (HNLF) is one of the most preferred optical nonlinear mediums for efficient supercontinuum (SC) generation due to its property of good laser beam confinement in a small core area and high nonlinearity. Till date, SC generation in HNLF is explored using very long length fibers, typically sub-kilometre. In this work, for the first time to our knowledge, the physical length of the HNLF used to generate broad SC is reduced by considering the nonlinearity length. This is also first-time demonstration of a very flat and broad SC generation in a short-length of HNLF using carbon-nanotube (CNT) based passively mode-locked erbium-doped femtosecond fiber laser. The CNT based mode-locking has achieved a pulse width of 620 fs with a repetition rate of 18 MHz at a centre wavelength of 1565 nm. A single stage amplification is employed on the mode-locked pulses and the spectral evolution of pulse inside the HNLF at various input power levels is studied. A flat and broad SC covering a bandwidth of ~1500 nm ranging from 1000 nm to 2500 nm has been demonstrated successfully. The spectrum throughout the ~1500 nm band is smooth with a flatness of <5 dB. The effect of input pulse polarization on SC spectrum smoothness has been studied and maintained proper polarization state to achieve spectral flatness of <5 dB.