Large mode-field area multi-element silica glass fibers for gigahertz ultrafast lasers.

Multi-gigahertz ultrafast fiber lasers are critical for many significant applications, including bioimaging, optical communications, and laser frequency combs. The gain fiber which is expected to simultaneously satisfy large mode-field area, highly gain coefficient and resistance to photodarkening, will effectively protect mode-locked materials/devices that generally possesses low damage threshold (<10 mJ/cm) and enhance stability in the centimeter-scale fiber lasers. However, the gain fiber still remains a significant challenge. In this study, multi-element Er-Yb: silica glass fibers with large mode-field area are fabricated. Benefiting from the multi-element design, normalized frequency V-parameter of the silica glass fiber with a core diameter of around 10 µm is <2.405. Using the large mode-field area fibers, ultrafast fiber lasers with 1.6 GHz fundamental repetition rate are proposed and demonstrated. The signal-to-noise rate of the radio-frequency signal reaching up to 90 dB and the long-term stability are realized. The results indicated the fabricated large mode-field area fibers are demonstrated to be ultrafast fiber lasers with short resonant cavities, which could be extended to other rare-earth glass fiber device for exploration of high-power amplification systems.