Passively Mode-Locked Figure-Eight Fiber Laser Using a Compact Power-Imbalanced Nonlinear Optical-Loop Mirror

We present an experimental demonstration of a passively mode-locked figure-eight fiber laser (F8L) based on a power-imbalanced nonlinear optical loop-mirror (PI-NOLM) comprising a section of 25 m long high-nonlinear fiber (HNLF) and a tunable attenuator (ATT). The asymmetric power, which results in different nonlinear phase shifts on the clockwise (CW) and counter-clockwise (CCW) directions, is achieved using an ATT. We greatly reduce the length of the nonlinear cavity, which can effectively diminish the interference from external environment. A balanced optical coupler (OC) and an ATT act as a tunable OC. So, a stable mode-locked pulse train with full width at half maximum (FWHM) of ∼571 fs and signal-to-noise ratio (SNR) of ∼60 dB is obtained.