Fiber optic sensors (FOS)
We are designing fibre optic sensors (FOS) based on as an extrinsic fibre Fabry-Perot interferometer used as a metrological tool for remote long-term displacement measurements with high sensitivity and resolution, low power consumption and wide dynamic range.
Targeted applications are in sub-wavelength detection and when the measurand evolves very slowly in field applications beyond the laboratory.
We have designed a 3 km long FOS with a dual modulation scheme to the laser current to induce a virtual reference displacement signal for continuous quasi-static measurements with directional determination. Real-time algorithms permit automatic quadrature phase error correction which can occur during large displacements, compensation of source intensity variations and noise reduction. An innovative sensor probe configuration relaxes the alignment tolerance to ~1°.
During calibration procedures carried out without any temperature control, the FOS exhibits a resolution of 70 pm and a precision of 2 nm over a frequency range of 10-3 – 500 Hz, together with a displacement dynamic of at least 106 (10 nm – >1 mm).
Critical applications to geophysics
The main application is Earth strain detection in geophysics. Based on our FOS, 3 novel instruments have been designed:
- A seismometer
- A borehole tiltmeter
- A long baseline hydrostatic levelling sensor
They are deployed to an underground facility in Vaucluse and are remotely operating continuously since March 2012. The instruments' sensitivity permit to detect Earth tides as well as earthquake events worldwide.
Detection of Earth tide: amplitude is varying of 130 nm in 12hrs
Seismometer (SISMO LINES)
Our seismometer has demonstrated similar frequency bandwidth and background noise compared to the Streckeisen STS2 Braodband while offering the advantage of not being sensitive to electromagnetic interferences.
SISMO LINES fiber optic seismometer
SISMO LINES detection of 11 April 2012 M8.7 Sumatra earthquake. Top: measurements from reference Streckeisen STS2 seismometer; Bottom: measurements from SISMO LINES
Borehole Tiltmeter (IF LINES)
Our borehole tiltmeter compared to the electrolytic LILY from Jewell, Inc, exhibits better precision and less system drift while concurrently being insensitive to environmental conditions.
IF LINES fiber optic borehole tiltmeter before installation in borehole
IF LINES detection of 5 March 2012 M6.1 Argentina earthquake. Left: tri-axial detection of movement (displacements); Right: 2-D surface reconstruction of movement provoked by quake event
Long baseline hydrostatic levelling sensor (ILB LINES)
For our long baseline hydrostatic levelling sensor, compared to the capacitive device from FOGALE, noise levels are similar at the very low frequency range (~10-6 – 10-5 Hz) while towards the higher frequency region (~ >10-2 Hz), our instrument is at least 100 times better in noise performance. The detection limit of our sensor is ~10-11 rad for a 150 m baseline together with a long-term drift of < 40 nrad / month, including possible hydrostatic loading/unloading effects.
ILB LINES fiber optic hydrostatic long baseline inclinometer
Comparison of noise performance with reference LILY tiltmeter and commercial Fogale HLS system
ILB LINES detection of 11 April 2012 M8.7 Sumatra earthquake. Left: noise level of 10-11 radians; Right: Sumatra quake event with 330 x 10-9 radian tilt for magnitude 8.7 earthquake