The dispersion turning point (DTP) in microfibers has proven to be an effective exceptional phenomenon for sensitivity improvement. In order to break the adiabaticity condition and excite higher-order modes effectively to satisfy the condition of DTP, the transition segments of tapered optical microfiber should be abrupt.

Recently, a research group led by Prof. WU Yihui from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) proposed a zigzag-shaped optical microfiber (Z-OMF) interferometer working at the DTP. This study is published in Optical Express.

The Z-OMF was fabricated in a controllable manner by applying a slight tilt angle to the optical fiber. For a Z-OMF, the asymmetric taper structure favored the excitation of the HE₁₁ and HE₂₁ modes. The beating between HE₁₁ and HE₂₁ modes occured when they traveled along the Z-OMF optical path. When the bending angle of the Z-OMF reached 1.61°, high contrast interference fringes were formed between the HE₁₁ and the HE₂₁ modes.

Fig 1. Schematic diagram of the Z-tapered optical microfiber sensor (Image by WEI)

By optimizing the diameter of the OMF, the group effective refractive index (RI) difference between HE₁₁ and HE₂₁ mode equaled zero, satisfying the condition of dispersion turning point. At the optical fiber diameter of 2.3μm, the DTP was located at near 1300nm, and the RI sensitivity was1.46×10⁵ ± 0.09×10⁵ nm/RIU.

Compared with the tapered optical microfiber, straight conical fiber, the proposed Z-OMF is easier to obtain the condition of the DTP. The ultra-high RI sensitivity indicated that Z-OMF is a promising platform for chemical and biological sensing applications.