Demonstration of $\bf3.5\times10^{-13}$ laser frequency stability at 1000 s using an iodine-filled hollow-core fiber photonic microcell

TL;DR

This work tackles achieving high-stability laser references using a gas-filled hollow-core PMC, integrated into a compact all-fiber system. The authors identify parasitic interferences as the dominant noise source and develop suppression strategies based on amplitude reduction, phase stabilization via phase-lock loops, and optical isolation with enclosure. They demonstrate a fractional frequency stability of $3.5\times10^{-13}$ at $1000\ \mathrm{s}$ integration time, the best reported for iodine-filled PMCs, approaching shot-noise limits in the short term. This advances all-fiber-based frequency references with potential for space applications due to reduced size and weight, and outlines pathways for further improvements through temperature control and improved PMC end-caps.

Abstract

We present a laser frequency stabilization system based on an iodine-filled hollow-core photonic microcell (PMC), which is a sealed version of a hollow-core photonic crystal fiber (HC-PCF). A 532 nm laser is locked to the a1 component of the R(56) 32-0 transition of molecular iodine in the fiber cell, and its frequency stability is compared to that of the same component in a free-space iodine cell. Noise analysis reveals that the system is limited by parasitic beams that interfere with the beam of interest and degrade the error signal. We have identified and characterized three types of parasitic interference and designed suppression methods for each. After applying these suppression methods, the frequency stability improved by more than an order of magnitude. The system achieves fractional frequency stability of $3.5\times10^{-13}$ for integration times around 1000 s. To our knowledge, this represents the best frequency stability achieved using a gas-filled hollow-core photonic crystal fiber frequency reference.

Figures (9)

• Figure 1: Schematic diagram of the experimental setup (see text for details), including the optical system layout, laser modulation and demodulation, and feedback loops. PBS: polarizing beam splitter. BS: beam splitter. QWP: quarter-wave plate. AOM: acousto-optic modulator. EOM: electro-optical modulator. FI: Faraday isolator. PZT: piezo-electric transducer. PD: photodetector.
• Figure 2: Hyperfine structure of iodine R(56) 32-0 transition measured in the commercial free-space system and the PMC system. The linewidths of the a1 components are 2.4 MHz and 37 MHz in the free-space cell and PMC cell, respectively.
• Figure 3: Parasitic interference effect in error signal. a) Error signal response to the phase of parasitic beam ($\phi_\epsilon$). b) Measured error signal of the system with parasitic interference effect.
• Figure 4: Illustration of examples for three types of parasitic beams, some components are omitted for simplicity. a) Higher-order mode of the PMC. b) Pump beam back reflection at the pump-PMC interface. c) Circulating pump beam (dash-dotted line hides the additional pump path).
• Figure 5: Frequency offset of the locked laser versus position of the PMC tip with the collapsing region. $\nu_0$ is 40 MHz away from the a1 components of R(56)32-0 of iodine.