Femtosecond fiber laser mode-locked with a twisted Sagnac interferometer

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.