Programmable frequency-swept ultrafast fiber laser and its real-time dynamic analysis

Background Wavelength-swept sources for swept-source optical coherence tomography (SS-OCT) and high-speed coherent detection must simultaneously provide rapid scanning, a wide tuning span, stable coherence, and flexible sweep trajectories. However, maintaining coherent mode locking during fast, user-defined wavelength sweeping remains challenging because the intracavity spectral filter, gain dynamics, and soliton spectral confinement evolve on different time scales.

Purpose This work demonstrates an all-fiber, passively mode-locked ultrafast ring oscillator whose wavelength-sweep trajectory is programmed directly in the RF domain.

Methods An intracavity polarization-selective acousto-optic grating (AOG) is employed as a dynamically tunable spectral-control element. The AOG forms a translating low-loss transmission window, with its center wavelength uniquely determined by the RF drive frequency, enabling deterministic wavelength scanning through arbitrary waveform modulation. To resolve the transient spectral evolution during swept operation, round-trip-resolved single-shot spectra are recorded using dispersive Fourier transform (DFT). The sweep dynamics and spectral reproducibility are examined at sweep rates of 2000 and 10000 nm/s.

Results At both sweep rates, the spectral envelope drifts smoothly along the programmed trajectory, while the Kelly sidebands remain distinct throughout the sweep. The adjacent-shot spectral cosine similarity remains high, confirming that coherent mode locking is sustained during rapid wavelength scanning. The round-trip-resolved measurements further indicate sub-microsecond updates of the output center wavelength, far faster than the AOG passband build-up time. This behavior suggests that the observed wavelength evolution arises from the combined effect of intracavity gain–loss dynamics and spectral confinement of the mode-locked pulse.

Conclusions These measurements provide direct real-time evidence for the dynamical mechanism of trajectory-programmable high-speed swept ultrafast fiber sources and verify that RF-domain programming can sustain coherent swept mode locking under high-speed wavelength scanning.