Fundamental thermal noise in antiresonant hollow-core fibers

Fluctuations of the optical length induced by fundamental thermal noise are known to set the ultimate phase resolution of fiber-based interferometers. Although this noise has been studied in detail for optical fibers made of solid glass material, its impact on the performance of hollow-core optical fibers has not yet been assessed. In such fibers, the guided light interacts only weakly with the glass material whose thermal and thermo-optic properties normally determine the thermal noise level, suggesting that a difference in performance should be expected. Based on the comparison of several interferometers optimized for phase sensitivity, we present measurements of thermal noise in the 20 to 200 kHz range in hollow-core nested antiresonant nodeless fibers (NANF) with their core filled with air at different pressures. In this frequency range, our measurements are in good agreement with the adapted thermoconductive noise model we introduce, suggesting that the thermo-optic contribution from the gas that fills the core is generally dominant, regardless of the exact hollow-core fiber design. While we show that an antiresonant hollow-core fiber filled with air at atmospheric pressure is noisier at 1550 nm than a silica fiber of equal optical length and mode-field area, we also demonstrate the lowest thermal noise power per unit optical length ever measured in a fiber (approximate to 1.3 x 10(-17) (rad(2)/Hz)/m at 30 kHz) using a large-mode- area NANF evacuated and sealed at 0.15 atm. In addition to lowering the internal pressure, we predict that the noise density in this spectral range can be reduced by filling the core with a low-polarizability noble gas. Our results indicate that low-loss antiresonant hollow-core fibers can compete with ultrastable cavities for the purpose of laser frequency stabilization; when evacuated, such fibers should constitute the best option to significantly decrease the fundamental noise floor in interferometric applications currently based on conventional solid-core fibers.