Femtosecond fiber laser mode-locked with a twisted Sagnac interferometer

Fiber-based Sagnac interferometers have been widely used in applications such as navigation, sensing and laser. The common-path structure of a Sagnac interferometer cuts both ways. On one hand, precise length control between optical paths is not required, which is critical for robust interferometric measurements. On the other hand, the transmittance of a Sagnac fiber loop is fixed, which lose the freedom of tuning and is especially unfavorable for laser development. Here, we propose a polarization axis cross-aligned Sagnac interferometer whose phase bias can be continuously tuned. By inviting a 90° splicing in the Sagnac loop and utilizing the birefringence of polarization maintaining fiber, a linear phase bias can be induced and adjusted. When it is applied in a fiber laser system and set with a proper bias of the transmittance, mode-locked operation can be self-started. In our demonstration, the laser can generate pulses with a spectral bandwidth of 31 nm and a pulse duration of 160 fs under 24.5 MHz repetition rate. The real-time continuous adjustment of transmittance provides greater flexibility and control over the laser systems, and enables potential applications in fields like optical metrology and sensing.