Shanghai Optics and Machinery Institute has made progress in femtosecond fiber lasers based on twisted Sagnac interferometer mode locking

Recently, the research team of the Aerospace Laser Technology and System Department of the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed a torsional Sagnac interferometer and applied it to the fiber laser system, realizing mode locking self starting and pulse shaping. The relevant research achievements were published in the 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 fields such as navigation, sensing, and lasers. The common path structure of Sagnac interferometer has both advantages and disadvantages. One is that precise length control is not required between optical paths, which is crucial for robust interferometric measurements. The second issue is that the transmittance of the Sagnac fiber loop is fixed and cannot be freely tuned. 
Therefore, traditional nine cavity mode-locked fiber lasers based on Sagnac fiber interference loops face the problem of inflexible mode locking.

In this study, researchers proposed a twisted Sagnac interferometer with continuously adjustable phase bias. By introducing 90 ° fusion in the Sagnac loop and utilizing the birefringence of polarization maintaining fibers, linear phase shift differences in clockwise and counterclockwise directions can be introduced and adjusted. When applied in a nine cavity fiber laser system, setting an appropriate transmittance can achieve mode locking self start. The experimental results show that by stretching the fiber to change the linear phase shift difference, the laser can achieve switching between different operating modes. By optimizing the phase shift difference, laser pulses with a spectral bandwidth of 31nm and a pulse duration of 160 fs can be generated at a repetition frequency of 24.5 MHz.

This study achieved real-time continuous adjustment of the transmittance of Sagnac fiber optic interference ring, providing greater flexibility and control for the nine cavity mode-locked laser, and improving its application prospects in optical metrology and sensing fields.
This work was supported by the Youth Innovation Promotion 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 a twisted fiber Sagnac interferometer.

Figure 2: Experimental setup diagram of femtosecond fiber laser based on twisted fiber Sagnac interferometer mode locking.

Figure 3 Spectral tuning and time-domain characteristics under different clockwise and counterclockwise linear phase shift differences and nonlinear phase shift differences.

Source: Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences