[Paper Review] First Results of GINGERino, a deep underground ringlaser

Jacopo Belfi, N. Beverini+12 more|arXiv (Cornell University)|Jan 12, 2016

Geophysics and Sensor Technology13 references3 citations

TL;DR

GINGERino, a 3.6 m square ring-laser installed deep underground at the Gran Sasso National Laboratory (LNGS), demonstrates sub-10⁻¹⁰ rad/s sensitivity over short integration times, validating the site’s low-noise potential for future tests of the Lense–Thirring effect. Despite challenges from air pressure-induced low-frequency noise, the instrument confirms the feasibility of long-term, high-stability rotation measurements in an underground environment.

ABSTRACT

Large ring-laser gyroscopes are capable of measuring angular rotations with a precision well below fractions of $prad/s$, not far from $10^{-14}$ $rad/s$, the accuracy required for General Relativity tests, this is what the GINGER (Gyroscope-IN-GEneral-Relativity) experiment is aiming for. These features do not guarantee the possibility of measuring the General Relativity Lense--Thirring effect, that manifests itself as a tiny ($\approx 10^{-9} imes Ω_E$) perturbation of the Earth rotation rate. An underground location being in principle less affected by external local disturbances represents a good candidate for housing such a challenging experiment. GINGERino is a test apparatus to investigate the residual local disturbances in the most inner part of the underground international laboratory of the GranSasso (LNGS). It consists of a square ring laser with a $3.6$ m side. The instrument has been tailored to be the larger allowed by the particular location inside the laboratory. Its main objective is to measure the very low frequency rotational motions, in order to prove that LNGS is a suitable location for very low noise measurements and, possibly, General Relativity tests. Aside this main goal, GINGERino will provide unique data for geodesy and geophysics. Its installation has been completed during 2015. Since then, several long set of data have been collected, and the apparatus has been continuously running unattended for more than one week. The typical power spectrum sensitivity was a few $ 10^{-10} rad/s/\sqrt(Hz)$, with integration time not longer than tens of seconds. Improvements of the apparatus are ongoing in order to improve the integration time.

Motivation & Objective

• To assess the suitability of the Gran Sasso underground laboratory (LNGS) for hosting the high-precision GINGER experiment targeting the Lense–Thirring effect.
• To measure very low-frequency rotational motions of the Earth using a compact ring-laser gyroscope in a controlled underground environment.
• To identify and characterize local environmental disturbances—especially pressure and seismic noise—that could limit long-term stability and sensitivity.
• To validate the technical feasibility of achieving the required 10⁻¹⁴ rad/s sensitivity for testing general relativity via ring-laser gyroscopy.
• To provide unique geodetic and geophysical data through continuous, unattended operation of a high-sensitivity ring-laser.

Proposed method

• Deployment of a 3.6 m square ring-laser cavity in the deepest part of the LNGS underground laboratory to minimize surface disturbances.
• Use of a ring-laser based on the Sagnac effect, where frequency difference between co-propagating beams measures rotation rate relative to inertial frame.
• Simultaneous data acquisition from co-located seismometers and environmental monitors (pressure, temperature) to correlate noise sources.
• Analysis of power spectra and polarization of seismic noise to identify directional noise sources, particularly along the tunnel axis.
• Application of time-frequency (TF) analysis to detect correlations between pressure fluctuations and low-frequency seismic noise.
• Implementation of mechanical improvements to increase cavity ring-down time and mirror quality to enhance sensitivity.

Experimental results

Research questions

• RQ1 Can the Gran Sasso underground site support the ultra-low noise environment required for detecting the Lense–Thirring effect?
• RQ2 To what extent do atmospheric pressure variations induce low-frequency noise in ring-laser and seismometer measurements?
• RQ3 Is the observed horizontal seismic noise directional and correlated with air motion along the tunnel, indicating a pressure-driven origin?
• RQ4 Can the ring-laser achieve sufficient integration time and stability to measure Earth's rotation rate at the 10⁻¹⁴ rad/s level?
• RQ5 How do local environmental disturbances such as barometric pressure, temperature, and hydrological changes affect the long-term stability of ring-laser measurements?

Key findings

• GINGERino achieved a typical power spectrum sensitivity of a few ×10⁻¹⁰ rad/s/√Hz over integration times not exceeding tens of seconds.
• The ring-down time of the cavity improved from ~240 μs to ~150 μs after mechanical upgrades, indicating progress in cavity stability.
• A directional excess of low-frequency noise was observed in the horizontal components of the seismometers, aligned with the tunnel axis, suggesting air motion as a source.
• Time-frequency analysis revealed a strong correlation between pressure fluctuations (with day/night and weekday/weekend cycles) and horizontal seismic noise.
• Pressure variations inside the GINGERino room showed clear diurnal and weekly modulations, with amplitude up to ~1 kPa/100, correlating with seismic noise peaks.
• The data suggest that air pressure fluctuations are a dominant source of low-frequency noise, and that hermetic isolation of the cavity room may be necessary to suppress this effect.