Abstract

We investigate the waist diameter of Tapered Optical Fiber (TOF) using the mode evolution during the tapering process of single mode optical fiber (SMF28) through Short Time Fourier Transform (STFT) analysis. During the fiber pulling process, some of modes exited due to waveguide modification of TOF, hence, interfere with each other, resulting an up-chirp signal. Analyzing with STFT, reveals spatial beat frequencies between different modes. Determining the cut-off value for spatial frequencies and the knowledge of theoretical cut-off diameter for different modes, the final diameter of waist region with precision of 4 nm can be estimated. Finally, the cut-off diameter was calibrated with conventional scanning electron microscopy (SEM) and then different spectra for different TOF structure are analyzed. The calculated and imaging data is in a good agreement with each other. We propose this method for characterization and optimizing of TOF fabrication process such as control the waist radius, the number of modes and their propagation constants.

© 2018 The Author(s)