Shanghai Institute of Optics and Mechanics introduced Mobius ring structure into fiber laser research

Recently, the research team of the High Power Fiber Laser Technology Laboratory of the Shanghai Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences has studied the effect of fiber twist on ring fiber lasers, and conducted a detailed study of the polarization and longitudinal mode characteristics of two kinds of Mobius ring fiber lasers. The relevant research results were published in Optica with the title of "Polarization and longitudinal modes of Mobius fiber ring lasers", and were selected as the cover.

Mobius ring, due to its unique topological characteristics, has received extensive attention in optical research. When the Mobius ring structure is introduced into the optical cavity, the phase and polarization characteristics different from the ordinary optical cavity will be observed. There are reports on optical microcavity of Mobius ring in the literature, but the preparation of microcavity structure requires high-precision technology, and it is very convenient to build Mobius ring cavity or even laser using fiber. After twisting the polarization axes at both ends of the polarization-maintaining fiber and fusing them together, the Mobius ring cavity can be formed. Among them, 180 ° and 90 ° torsion is the case of two kinds of Mobius rings. In the actual optical fiber fusion, it is difficult to distinguish between 0 ° and 180 ° fusion, so the 180 ° torsion may be formed unintentionally, so the difference of output characteristics under 0 ° and 180 ° torsion is a problem worth studying. The 90 ° torsion has been used to solve the problem of temperature sensitivity in fiber optic gyroscopes, but there is no research on such fiber lasers.

Researchers have established a theoretical model based on Jones matrix, and solved the intrinsic polarization state and longitudinal mode of polarization-maintaining fiber ring cavity with arbitrary torsion angle. The theoretical analysis shows that the torsion of the fiber axis will lead to the hybridization of the eigen polarization state and the geometric phase shift, and further cause the frequency shift of the longitudinal mode. Two kinds of Mobius rings (180 ° and 90 ° torsion) are analyzed in detail. The 90 ° torsion of the fiber polarization axis will bring unique polarization and longitudinal mode characteristics. In the experiment, a 90 ° twisted Mobius ring fiber laser was built, and the polarization and longitudinal mode beat spectrum of the laser output were measured, which verified the theoretical prediction.

This work has carried out the research of Mobius ring fiber laser for the first time, providing a convenient platform for the research of Mobius ring structure laser, and opening up a new direction for fiber laser and its application. It is also the first time to introduce the concepts of geometric phase and topology into the study of fiber lasers.

The research was supported by the National Natural Science Foundation.

Figure 1 (a) Schematic diagram of torsionally polarized fiber ring cavity. (b) The experimental device diagram of Mobius ring fiber laser.

Fig. 2 Polarization characteristics of Mobius ring fiber laser. (a) Degree of polarization at different pump power. (b) In the single polarization state, the typical polarization state described by Poncare sphere.

Fig. 3 Longitudinal mode beat spectrum measured by (a) direct output at different pump power through (b) fast axis and (c) slow axis of polarization beam splitter.

Source: Shanghai Institute of Optics and Precision Machinery, Chinese Academy of Sciences