Double harmonic mode-locking in soliton fiber ring laser acquired through the resonant optoacoustic coupling

High repetition rates of ultrafast fiber lasers are crucial for their various applications, such as optical fiber telecommunications, microwave photonics, frequency metrology, and the high-speed optical processing. Passive harmonic mode-locking (HML) of a soliton fiber laser to optoacoustic resonance (OAR) allows for high pulse stability and low timing jitter. However, the pulse repetition rate (PRR) of such lasers is limited to ~1 GHz for standard fibers and ~2.5 GHz for photonic crystal fibers due to the available acoustic modes. Here, we address these limitations by demonstrating a soliton fiber laser built from standard fiber components and subjected to double harmonic mode-locking (DHML). Initially, the laser is adjusted to operate at 15-th harmonic of its cavity, enabling OAR at ~200 MHz, but is then driven to operate at a high harmonic of the OAR frequency, reaching ~12 GHz. This breakthrough is made possible through controllable optoacoustic interactions in a short, 50 cm segment of unjacketed cavity fiber. We propose that the precise alignment of the laser cavity harmonic and fiber acoustic modes leads to a long-lived narrow-band acoustic vibration. This vibration sets the pace for pulses circulating in the cavity by suppressing modes that do not conform to the Vernier principle. The surviving modes, equally spaced by the OAR frequency, in cooperation with the gain depletion and recovery mechanism, facilitate the formation of stable high-frequency pulse sequences, enabling DHML. In this process, the OAR rather than the laser cavity defines the elementary step for laser PRR tuning. Throughout the entire PRR tuning range, the soliton fiber laser exhibits enhanced stability, demonstrating a better than ~40 dB supermode suppression levels and picosecond pulse timing jitter.