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Hänsch-Couillaud locking of a large Sagnac interferometer: advancing below the flicker floor

Jannik Zenner, Karl Ulrich Schreiber, Simon Stellmer

TL;DR

The paper tackles the flicker-noise limitation of passive laser gyroscopes based on the Sagnac effect by implementing Hänsch-Couillaud locking, traditionally RAM-free compared with PDH locking. A lock-in enhancement modulates the AOM to suppress flicker noise, achieving a shot-noise-limited performance with a minimum rotation-rate sensitivity of $7.7\times 10^{-5}\ \Omega_E$ (≈$3.1\ \mathrm{nrad/s}$). The result demonstrates that HC locking, when augmented with lock-in detection, can reach performance comparable to PDH-locked devices in large ring lasers and offers a simple, cost-effective path to RAM-free stabilization. Long-term limitations are attributed to environmental drifts affecting polarization optics, suggesting that better environmental control could substantially improve sensitivity and stability.

Abstract

Large Sagnac interferometers in the form of active ring lasers have emerged as unique rotation sensors in the geosciences, where their sensitivity allows to detect geodetic and seismological signals. The passive laser gyroscope variant, however, is still at a stage of development, and thus far, only the Pound-Drever-Hall frequency stabilization technique has been explored, a method limited by residual amplitude modulation. Here, as an alternative method, we present the first Hänsch-Couillaud locked passive laser gyroscope. We find that this method is limited by flicker noise, and we introduce a cost-effective lock-in scheme to overcome this limitation. We achieve a sensitivity of 3.1 nrad/s, corresponding to a fraction of $7.7\cdot 10^{-5}$ in the Earth's rotation rate.

Hänsch-Couillaud locking of a large Sagnac interferometer: advancing below the flicker floor

TL;DR

The paper tackles the flicker-noise limitation of passive laser gyroscopes based on the Sagnac effect by implementing Hänsch-Couillaud locking, traditionally RAM-free compared with PDH locking. A lock-in enhancement modulates the AOM to suppress flicker noise, achieving a shot-noise-limited performance with a minimum rotation-rate sensitivity of (≈). The result demonstrates that HC locking, when augmented with lock-in detection, can reach performance comparable to PDH-locked devices in large ring lasers and offers a simple, cost-effective path to RAM-free stabilization. Long-term limitations are attributed to environmental drifts affecting polarization optics, suggesting that better environmental control could substantially improve sensitivity and stability.

Abstract

Large Sagnac interferometers in the form of active ring lasers have emerged as unique rotation sensors in the geosciences, where their sensitivity allows to detect geodetic and seismological signals. The passive laser gyroscope variant, however, is still at a stage of development, and thus far, only the Pound-Drever-Hall frequency stabilization technique has been explored, a method limited by residual amplitude modulation. Here, as an alternative method, we present the first Hänsch-Couillaud locked passive laser gyroscope. We find that this method is limited by flicker noise, and we introduce a cost-effective lock-in scheme to overcome this limitation. We achieve a sensitivity of 3.1 nrad/s, corresponding to a fraction of in the Earth's rotation rate.

Paper Structure

This paper contains 6 sections, 2 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Setup
  3. Hänsch-Couillaud locking method
  4. Lock-in method for sub-flicker floor locking
  5. Results
  6. Conclusion

Figures (3)

  • Figure 1: Schematic view of the lock-in Hänsch-Couillaud setup used to stabilize each laser direction onto the ring cavity. Abbreviations: AOM acousto-optic modulator, AWG arbitrary waveform generator, GTP Glan-Taylor prism, HWP half-wave plate, LPF low-pass filter, PID proportional-integral-derivative controller, QWP quarter-wave plate, RFG radio frequency generator, TIA transimpedance amplifier, VCA voltage controlled attenuator, WP Wollaston prism.
  • Figure 2: Power spectral densities of the measured Sagnac frequency. The standard HC lock (orange) clearly is flicker noise dominated in the low frequency regime up until about 2. For the lock-in method (blue), noise contributions in this regime are purely shot-noise like.
  • Figure 3: Classic Allan deviations of the measured Sagnac frequency. The lock-in method (blue) beats the standard HC lock (orange) at averaging times between 0.3s and 200s as it is not flicker noise limited. It reaches a minimum at about 6s of 7.7e-5$\,\Omega_{\text{E}}$ or 24m Hz.