We present a compact broadband supercontinuum (SC) generation in a single-mode fluorozirconate ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber pumped by femtosecond pulses from an erbium-doped fiber laser (EDFL). The femtosecond seed pulses are generated through passive mode locking using carbon-nanotubes-based saturable absorber in EDFL ring cavity. An SC spectrum spanning from 1100 to 3200 nm with a spectral flatness of <8 dB has been obtained successfully from a single-mode ZBLAN fiber. A piecewise linear relationship of SC spectrum bandwidth with input pulse power is observed.
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 modelocking 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.
The heavy fluoride fiber, ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) is the commonly used fiber to generate supercontinuum (SC) in mid-IR region due to its transparency, high glass stability and technology maturity. Multi-stage mid-IR SC generation that involves multi-stage SC generation and multi-stage amplification is adopted widely. The system is generally complex, and suffers poor stability. Here, we reduced the complexity and improved stability and compactness of the system by using single-stage SC generation approach. For the first time to our knowledge, we demonstrate a carbon-nanotube (CNT) femtosecond passively mode-locked erbium-doped fiber laser based SC generation in ZBLAN fiber. 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 trend of spectral broadening inside the ZBLAN fiber is studied experimentally at different input power levels. The study of the SC generation inside the ZBLAN fiber at high input powers is limited by the OSA range. We successfully demonstrated an SC generation inside ZBLAN fiber covering a bandwidth of over 2300 nm extending from 900 nm to beyond 3200 nm at an input power of 200 mW. We achieved a smooth spectrum with spectral flatness of less than 8 dB. The relationship between the generated SC bandwidth and input power to the ZBLAN fiber is observed to be almost linear in log scale, with ~110 nm bandwidth increments per unit dBm.
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