Post-processing of antiresonant hollow-core fibers and its applications

• Main Author: Xiong, Daiqi
• Other Authors: Wonkeun Chang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/164501
• Institution: Nanyang Technological University

Hollow-core fiber (HCF) is one of the most important inventions in the field of fiber optics over the past few decades. On one side, as a bendable hollow optical fiber, the propagation property of light in HCF is similar to that in free space, which breaks the absorption limit of solid medium and creates an ideal waveguide with tunable dispersion and nonlinearity, low latency. On the other hand, as a platform for the interaction between light and matter, the laser pulses can be guided over long distances within the hollow fiber core of micrometer magnitude, breaking the limitation of diffraction limit in free-space transmission and greatly enhancing the interaction between light and matter. As an optical platform with dual advantages of fiber optics and free-space optics, HCF can play its unique advantages in optical applications. This work is based on a particular type of HCF - antiresonant hollow-core fiber (AR-HCF). It offers broadband guidance, low confinement loss, high damage threshold, and a tunable dispersion landscape by filling different gas species and pressure. The post-processing methods, tapering and wet-etching of AR-HCF, further facilitate the convenient tailoring of optical parameters. Firstly, a new method of the fiber-based in-line bandpass filter is realized is demonstrated by tapering a piece of AR-HCF to half of its original size. This exploits the periodical loss property of the AR-HCF - shorter wavelength is suppressed by the continuously varying tube-wall thickness of the tapered AR-HCF while the long cutoff wavelength is induced by its high confinement loss in a limited core size. The proposed bandpass filter can preserve all the benefits of the HCF and integrate into a fiber system seamlessly. Following is the waist part of the tapered AR-HCF as a small-mode-area HCF (SMA-HCF). By tightly focusing the light into a small core with a diameter of only 10 $\mu$m, the highly nonlinear region in the gas medium is accessible. This allows us to explore gas-based ultrafast nonlinear optics with much lower single-pulse energy required by traditional methods. The onset of the deep UV in the gas-filled SMA-HCF with only hundred-nanojoule-level pump is realized. At last, the UV generation in gas-filled AR-HCF is enhanced through theoretical and experimental studies of two factors - reducing the resonant bands and increase the self-steepening effect. The wet-etching technique achieves a 250 nm reduction of the core-wall thickness of the AR-HCF which enhances the conversion efficiency from 2% to 6%. Besides, compressing the pump pulses and/or pumping with longer wavelength can increase the self-steepening effect to further enhance the bluish spectral broadening, resulting an enhanced UV generation.