Large-Mode-Area Optical Fibers Maintain Polarization

An optical fiber has been developed that preserves the coherent properties of light. This tapered fiber could be useful in the construction of high-power pulsed fiber lasers and amplifiers and polarization-sensitive sensors. The fiber enables control over both the distribution of light intensity and the polarization of light. It has an elliptical inner cladding and an extremely large core diameter that increases adiabatically from 8 µm to 70 µm.

Patterns of transverse distribution of optical radiation intensity in the output beam. Courtesy of MIPT Press Office.

The research was conducted by scientists from the Moscow Institute of Physics and Technology (MIPT) and the Kotelnikov Institute of Radio Engineering and Electronics (IRE), in collaboration with researchers from Finland.

To characterize the ability of the fiber to maintain polarization, the researchers used a high-resolution optical frequency domain reflectometry (OFDR) method to investigate the birefringence distribution along the length of the anisotropic long tapered fiber. Researchers were able to measure polarization mode beating within the fiber at a high resolution by directly measuring the distribution of the phase and group birefringence according to OFDR principles.

The team demonstrated the preservation of the linear polarization state of light in long anisotropic tapered fibers with significant changes in the core diameter (from 8 µm to 70 µm) on the entire length (about 10 meters) of a tapered fiber. The adiabatic condition of mode area expansion demonstrated a satisfactory level of accuracy for both types of optical modes — the transverse mode and the polarization mode — in this type of tapered fiber. Investigation of the spatial distribution of birefringence showed a strong correlation with the longitudinal geometric profile of the tapered fibers.

The diameter of the outer cladding (on the left axis) and the core (on the right axis) along the length of fiber samples. The inset in the bottom chart shows a cross section of the anisotropic fiber structure consisting of the core, the elliptical inner cladding, and the outer cladding. Courtesy of MIPT Press Office.

The capability to fabricate long tapered polarization maintaining fibers with large mode area and to characterize them could advance the development of novel high-power fiber lasers and amplifiers with a linear polarization, reduced nonlinearity and single-mode beam quality.

Professor Sergey Nikitov said, “The fiber samples we obtained have demonstrated great results, indicating good prospects for further development of such technological solutions. They will find use not only in laser systems but also in optical fiber sensors, where the change of polarization characteristics is known in advance, since they are determined by external environmental factors, such as temperature, pressure, biological and other impurities. Besides, they have a number of advantages over semiconductor sensors. For example, they need no electrical power and are capable of carrying out distributed sensing, and that is not a complete list.”

The polarization beat length (the violet curve on the left axis) and the outer cladding diameter (the red curve on the right axis) along the length of the fiber samples. Courtesy of MIPT Press Office.

The research was published in Optics Express, a publication of The Optical Society (doi: 10.1364/OE.25.010693).