Chip-scale modulation-free laser stabilization using vacuum-gap micro-Fabry-Pérot cavity

Abstract

Narrow-linewidth lasers are vital for a broad range of scientific and technological applications, including atomic clocks and precision sensing. Achieving high frequency stability is often as critical as ensuring scalability, portability, and cost-effectiveness in the development of low noise laser systems. Conventional electro-optic stabilization techniques, such as Pound-Drever-Hall locking to ultra-high-finesse resonators held in a vacuum chamber, provide excellent performance but remain challenging to scale. Here, we propose and experimentally demonstrate a cavity-coupled interferometric laser stabilization technique implemented on a silicon photonic chip and integrated with a compact, scalable micro-Fabry-Pérot cavity. The vacuum-gap optical cavity operates in air, achieving a quality factor of approximately $2.0\times 10^9$ and a fractional frequency instability of $1.45\times 10^{-12}$ at one-second averaging time. Integration of the proposed technique with the compact cavity yields more than 38-fold reduction in the laser's integrated linewidth and nearly three orders of magnitude suppression of frequency noise at 10 Hz offset frequency. The hybrid-integration of the proposed photonic chip with the micro-Fabry-Pérot cavity establishes a scalable and portable route toward chip-integrated ultra-stable lasers, paving the way for precision optical systems deployable beyond laboratory environments.

Figures (4)

• Figure 1: Cavity-coupled MZI (CCMZI) laser stabilization.a, Block diagram of the CCMZI photonic chip. b, Photograph of the vacuum-gap micro-Fabry–Pérot ($\mu$FP) cavity. The ultra-low-expansion (ULE) spacers have a thickness of 3 mm, corresponding to a free spectral range of 50 GHz. The diced cavity dimensions are 9 mm $\times$ 9 mm $\times$ 7 mm. c, Microphotograph of the implemented CCMZI photonic integrated circuit (PIC), fabricated using the Advanced Micro Foundry (AMF) standard silicon-on-insulator (SOI) photonic process. d, Packaged PIC with a single-mode fiber at its input and a gradient-index (GRIN) lens at the output. e, The packaged PIC is mounted on a temperature-controlled aluminum submount, with output light coupled to the cavity through free-space. The $\mu$FP cavity is enclosed in a custom-designed, 3D-printed airtight housing for enhanced acoustic shielding. Details of PIC and cavity packaging are discussed in Methods. MMI multi-mode interferometer, BPD balanced photodetector, TC tunable coupler, PBRC polarization beam rotator-combiner, PBS polarization beam splitter, PMI poor man’s isolator.
• Figure 2: Open-loop characterization.a, Measurement setup for CCMZI open-loop response and $\mu$FP cavity characterization. b, Measured transmission of the $\mu$FP cavity, showing a resonance linewidth of 95 kHz, corresponding to a Q-factor of $2.0\times 10^9$ at wavelength of 1550.218 nm. c, Measured error signal of the CCMZI circuit. PC, polarization controller; VCO, voltage-controlled oscillator; OSA, optical spectrum analyzer; PD, photodetector; TIA, transimpedance amplifier.
• Figure 3: The closed-loop operation.a, Measurement setup for laser frequency stabilization and phase noise measurement. b, Power spectral density (PSD) of the free-running and stabilized laser. The integrated linewidth, $\Delta\nu_{\beta}$, is obtained by averaging the frequency noise PSD from 1 Hz up to the frequency at which the PSD intersects the $\beta$-separation line di2010simple. c, Stabilized laser beat-note fluctuations measured over 10 minutes. d, Modified Allan deviation of the fractional frequency instability when the laser is locked to the $\mu$FP cavity. Further details are provided in Methods.
• Figure 4: Effect of mechanical vibration on laser phase noise.a, Effect of mechanical instability in CCMZI architecture. b, Measured PSD of laser frequency noise locked to the $\mu$FP with and without the aluminum brackets.