Mid-IR supercontinuum generation in a single-mode ZBLAN fiber pumped by a carbon-nanotube-based passively mode-locked erbium-doped femtosecond fiber laser

摘要

The heavy fluoride fiber, ZBLAN (ZrF_4-BaF_2-LaF_3-AlF_3-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.