Shanghai Institute Of Optical Machinery Makes Progress in Femtosecond Fiber Lasers Based On Mode-locking By Twisted Sagnac Interferometer

Recently, a research team from the Department of Space and Astronautical Laser Technology and Systems, Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences (SIPM), proposed a twisted Sagnac interferometer and applied it to a fiber laser system to realize mode-locked self-starting and pulse shaping, and the related research results were published in the Journal of Lightwave Technology under the title of "Femtosecond fiber laser mode The related research results were published in Journal of Lightwave Technology under the title of "Femtosecond fiber laser mode -locked by a twisted Sagnac interferometer".
Fiber optic Sagnac interferometers have been widely used in the fields of navigation, sensing and lasers, etc. The common path structure of Sagnac interferometers has both advantages and disadvantages. One is that there is no need for precise length control between optical paths, which is essential for robust interferometry. The second is that the transmittance of the Sagnac fiber loop is fixed and cannot be freely tuned, so conventional nine-cavity mode-locked fiber lasers based on Sagnac fiber interferometric loops face the problem that the mode-lockedness cannot be flexibly adjusted.
In this study, the researchers propose a twisted Sagnac interferometer whose phase bias can be continuously adjusted. By introducing a 90° fusion splice in the Sagnac loop and exploiting the birefringence of the bias-preserving fiber, a clockwise and counterclockwise linear phase shift difference can be introduced and adjusted. When applied to a nine-cavity fiber laser system, the self-starting of mode-locking can be achieved by setting an appropriate transmittance. The experimental results demonstrate that by stretching the fiber to change the linear phase shift difference, the laser can realize the switching of different operating modes. By optimizing the phase shift difference, laser pulses with a spectral bandwidth of 31 nm and a pulse duration of 160 fs can be generated at a repetition frequency of 24.5 MHz.
The study realizes the real-time continuous adjustment of the transmittance of the Sagnac fiber interference ring, which provides greater flexibility and control of the nine-cavity mode-locked laser, and enhances its application prospects in fields such as optical metrology and sensing.
This work was supported by the Young Innovators Association of the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the Shanghai Natural Science Foundation.

Figure 1 Schematic diagram of the twisted fiber Sagnac interferometer.

Fig. 2 Diagram of the experimental setup of femtosecond fiber laser based on mode-locking of twisted fiber Sagnac interferometer.

Fig. 3 Spectral modulation and time-domain characteristics for different clockwise and counterclockwise linear phase shift differences and nonlinear phase shift differences.