Coherence Breakdown Within the Ultra-Stable Regime of Fourier Domain Mode-Locked Lasers Under Increasing Intracavity Power

Fourier Domain Mode-Locked (FDML) lasers are high-speed frequency-swept lasers that offer wide sweep-range, high output power, narrow instantaneous linewidth, and long coherence length [1]. Due to these attributes, they are very useful in bio-imaging applications, particularly in Optical Coherence Tomography (OCT). FDML lasers consist of a fiber cavity that contains a semiconductor optical amplifier (SOA) for roundtrip optical gain, and a tunable Fabry-Perot filter for the intracavity storage of the spectral content of each frequency-chirped pulse. The main drawback of FDML lasers is that their output signal is often distorted by fluctuations known as power-dips (Fig. 1, middle) arising from the interaction of the cavity components, which decreases the coherence length of the output pulses of FDML lasers. Dips of higher strength and longer duration are especially undesired concerning operational convergence, stability, and coherence [2], as they trigger the accumulation of instabilities (phase-offsets) over many roundtrips within the fiber-cavity owing to the nonlinear optical response of the SOA.