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Hertz-Integral-Linewidth Lasers based on Portable Solid-state Microresonators

Xing Jin, Xuanyi Zhang, Fangxing Zhang, Zhenyu Xie, Shui-Jing Tang, Qi-Fan Yang

TL;DR

This work introduces a compact, ambient-condition MgF2 whispering-gallery-mode resonator as a high-performance optical reference for ultrastable lasers. By combining precision fabrication, robust system packaging, and a dual-loop PDH stabilization scheme, the authors achieve a phase noise of $-105\ \mathrm{dBc/Hz}$ at $10\ \mathrm{kHz}$, an integral linewidth of $4\ \mathrm{Hz}$, and a fractional frequency stability of $2.5\times 10^{-14}$ at $10\ \mathrm{ms}$, demonstrated on two independently stabilized lasers. The results surpass many prior compact-resonator platforms and approach laboratory-grade performance in a handheld form factor, enabling portable optical clocks, navigation, and remote sensing. The work also delineates pathways to further improvements, including increased mode volume and advanced packaging, to push toward sub-Hertz linewidths and $10^{-15}$-level fractional stability for field-deployable precision systems.

Abstract

Optical reference resonators serve as a cornerstone in various scientific fields. In recent years, there has been an increasing demand for compact ultrastable reference resonators capable of operating in ambient environments, enabling applications beyond the laboratory, such as navigation, portable optical clocks, and remote sensing. Here, we present a compact ultrastable whispering-gallery-mode \ce{MgF2} reference resonator with a high loaded quality factor of $2.24\times 10^9$. The device is packaged in a compact form of 50$\times$77$\times$90 mm and supports stable optical coupling with polarization-maintaining fiber, which enables robust operation under ambient conditions. Laser stabilization using this resonator yields a phase noise of -105 dBc/Hz at a 10 kHz offset frequency, an integral linewidth of 4 Hz, and a fractional frequency stability of $2.5\times 10^{-14}$ at a 10 ms averaging time. With the high performance and rapid manufacturability, our work offers a promising solution for ultrastable optical frequency references beyond laboratory settings.

Hertz-Integral-Linewidth Lasers based on Portable Solid-state Microresonators

TL;DR

This work introduces a compact, ambient-condition MgF2 whispering-gallery-mode resonator as a high-performance optical reference for ultrastable lasers. By combining precision fabrication, robust system packaging, and a dual-loop PDH stabilization scheme, the authors achieve a phase noise of at , an integral linewidth of , and a fractional frequency stability of at , demonstrated on two independently stabilized lasers. The results surpass many prior compact-resonator platforms and approach laboratory-grade performance in a handheld form factor, enabling portable optical clocks, navigation, and remote sensing. The work also delineates pathways to further improvements, including increased mode volume and advanced packaging, to push toward sub-Hertz linewidths and -level fractional stability for field-deployable precision systems.

Abstract

Optical reference resonators serve as a cornerstone in various scientific fields. In recent years, there has been an increasing demand for compact ultrastable reference resonators capable of operating in ambient environments, enabling applications beyond the laboratory, such as navigation, portable optical clocks, and remote sensing. Here, we present a compact ultrastable whispering-gallery-mode \ce{MgF2} reference resonator with a high loaded quality factor of . The device is packaged in a compact form of 507790 mm and supports stable optical coupling with polarization-maintaining fiber, which enables robust operation under ambient conditions. Laser stabilization using this resonator yields a phase noise of -105 dBc/Hz at a 10 kHz offset frequency, an integral linewidth of 4 Hz, and a fractional frequency stability of at a 10 ms averaging time. With the high performance and rapid manufacturability, our work offers a promising solution for ultrastable optical frequency references beyond laboratory settings.

Paper Structure

This paper contains 7 sections, 1 equation, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Device fabrication and characterization
  3. System package
  4. Laser stabilization and performance characterization
  5. Discussion and conclusion
  6. Appendix
  7. References

Figures (3)

  • Figure 1: Fundamental characteristics of the MgF2 reference resonator. (a), MgF2 resonator fabrication process flow. (b), Photograph of the MgF2 reference resonator (left), microscopic image of its cross section (middle), and the simulated optical mode profile (right). (c), Ringdown measurement of the transmitted optical field when the laser is rapidly swept across the resonance. The exponential decay fitting yields a time constant of 3.9 µ s, corresponding to a loaded quality factor of $2.24\times 10^9$. (d) Transmission spectra over a free spectra range (FSR) of the MgF2 reference resonator.
  • Figure 2: System packaging of the MgF2 reference resonator and experimental setup for laser stabilization and performance characterization. (a), Schematic of the detailed packaging structure of the MgF2 reference resonator. (b), Photograph of the packaged MgF2 reference resonator. (c)Experimental setup for laser stabilization and characterization, AOFS: acousto-optic frequency shifter; EOM: electrical optical modulator; PD: photodetector; PC: polarization controller; LO: local oscillator; VCO: voltage-control oscillator; PNA: phase noise analyzer.
  • Figure 3: Phase noise and frequency stability performance of the MgF2 reference resonator stabilized laser. (a), Phase noise measurement and simulation. Dark blue: laser phase noise measured with a delayed self-heterodyne (DSH) interferometer; light blue: laser phase noise measured from the beat note between two lasers; red dashed line: simulated thermorefractive noise limit of the resonator; gray: experimentally calibrated in-loop noise; brown: experimentally calibrated signal-to-noise ratio (SNR) noise of PDH locking. (b), Integrated phase noise yields an integral linewidth of 4 Hz, corresponding to an integrated phase variance of 1 rad$^2$. (c), Time trace of the beat note between two lasers independently stabilized to two separate MgF2 reference resonators. The inset shows the selected 18-second low-drift data of the time trace. (d) Fractional frequency stability calculated from the full 3-hour dataset (blue) and the 18-second segment (red) shown in (c). The green curve represents the calculated thermorefractive noise-limited fractional frequency stability.