New Technique Enables Absolute Length Sensing in Long-Baseline Optical Cavities for Seismic Monitoring
Physicists have demonstrated a method for continuously measuring the absolute length of a long, high-finesse optical cavity using the relative phase between two lasers in transmission. One laser is locked to cavity resonance while a second is phase-locked to it at a frequency offset equal to an integer multiple of the cavity's free spectral range; length changes shift the second laser's phase, providing a calibrated length readout. The technique transforms the 123-meter cavity into a strainmeter sensitive enough to detect anthropogenic noise, distant earthquakes, and Earth tides.
A team of researchers has developed and demonstrated an absolute length sensing technique for a 123-meter, high-finesse optical cavity, achieving sub-micron precision in continuous measurements. The method relies on two lasers: the first is held on resonance with the cavity, while the second is phase-locked to the first with a frequency separation equal to an integer multiple of the cavity's initial free spectral range. When the cavity length changes, the free spectral range shifts, causing the second laser to detune slightly from resonance and accumulate a measurable phase offset in transmission; this phase offset is then calibrated to yield absolute length changes. The system achieves an effective strain sensitivity of 10⁻¹⁰ to 10⁻⁹ m/m for transient seismic events. The researchers validated the instrument by successfully recording length changes associated with anthropogenic (human-generated) noise sources, a distant earthquake, and both diurnal and semidiurnal Earth tidal deformations. The work, submitted to arXiv in June 2026, represents a significant step toward using precision optical cavities as practical geophysical sensors. It has not yet undergone formal peer review.
What's missing
As a preprint, this work has not yet undergone formal peer review. Key open questions include: how the technique performs compared to existing absolute length sensing or seismometry methods in terms of noise floor and bandwidth; whether the calibration remains stable over long timescales or varying environmental conditions; and the scalability of the approach to other cavity geometries or lengths.
What different sources said
- arXiv physicsCenter
Demonstration of length control for a filter cavity with coherent control sidebands
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