Metrology for the Detection of Acoustic Waves – MEDUSE

Optical fibers allow obtaining an important sensitivity to the optical phase perturbations induced by sound waves. Moreover, they are relatively easy to treat and to optically align. In our project, the optical fibers are coupled with liquid crystal light elements that permit to improve the sensitivity of the fiber detectors thanks to the techniques of dynamic holography. At the exit of the optical fibers, the phase perturbations induced on the light by the acoustic waves are amplified and revealed thanks to the slow-light effects that are associated to the adaptive hologram created in the liquid crystals. The information is converted in intensity modulation and directly recorded with photodiodes.

At half way of the project, the most important results are: 1) the demonstration of the improvement of the acoustic wave detection achieved with the hybrid method based on the association of optical fibers with adaptive holography, with respect to the techniques based only on optical fibers, 2) the realization and validation of a protocol for the localization of acoustic waves.

The final perspectives of the project are the realization of detectors based on optical fibers/adaptive holography working in underwater media, offering a high level of sensitivity and a good easiness of deployment. The emplacement of networks of detectors is also aimed to, both for civilian security and defense applications.

U. Bortolozzo, S. Residori, J.P. Huignard, «Adaptive Holography in Liquid Crystal Light-Valves », Materials 2012, Special Issue «Advanced Materials for Modern Holographic Applications», 1477-1486 (2012).

The aim of the project is to develop a novel type of optical fiber sensor based on multimode mode fibers and slow-light enhancement of the sound detection through adaptive holography in liquid crystal light-valves.
Detecting acoustic waves by using optical sensors is one of the most attractive challenge in current photonics, with applications ranging from underwater detection, imaging through scattering media and precision metrology. In this project we aim at bringing together our competencies on slow-light, liquid crystal light-valves and adaptive holography, on the one hand (INLN), with the competences on optical fiber sensors, active and passive Bragg gratings and array detection systems, on the other hand (TRT), in order to provide a novel approach to the problem of optical metrology and this with especially the aim of detecting small displacements and acoustic waves in noisy / fluctuating environments. Theoretically we will provide a validation of the method for different proposed fields (e.g hydrophones, gravimeters, accelerometers, photo-acoustic cells,..) and an estimate of the potential ultimate performances of such systems using the proposed interrogation scheme based on slow-light adaptive holography. Experimentally we will focus on the realization of a sensitive optical sensor able to operate in noisy and fluctuating environment, such as underwater, and with capability of detecting acoustic waves as phase modulations arising in optical complex fields.
Underwater detection of sound waves is of fundamental importance for coastal tracking and includes military applications (ex. detection/identification of submarines and surface ships, detections of divers or of unmanned underwater vehicles, …) as well as civilian applications (ex. bioacoustics, monitoring underwater seismic activity, detecting environmental noise, …). At this purpose, optical fiber sensors are extensively used for both their sensitivity and for their capability to realize small diameter (typ. 5-10 mm) hydrophone arrays. They are mainly based on optical fiber Bragg gratings coupled to a mechanical structure which transform the radial pressure of the acoustic wave into a longitudinal strain, this strain resulting in a phase change for a laser beam reflected from the grating. To detect the phase change precise interferometric measurements have to be performed, which are often a limitation to the practical application of the systems, especially when the detection has to be performed in fluctuating environments. In this project we will use the high sensitivity to phase change provided by slow-light and adaptive holography in liquid crystal light-valves to realize a high sensitivity, and simplified, interrogation system for optical fiber sensors. By implementing wave-mixing in the light-valve we have already demonstrated a large phase sensitivity in a Mach-Zehnder interferometer as well as a highly sensitive accelerometer based on the Sagnac effect. Most important for the purposes of the project, by using the adaptive character of the wave-mixing process in the light-valve, we have demonstrated the efficient detection of fractions of picometer displacements for objects vibrating at typical sound frequencies (from 10 Hz to 100kHz). The capability of the adaptive holography in light-valve to self-maintain the linear detection condition allows performing phase detection in complex speckle fields, a property that will allows to use multimode fibers, instead of the usually employed monomode fibers, to implement the optical sensor. Multimode fibers are much easier to use (less fragile, more easy to align and to couple with the input beam, less expensive) while they provides a higher sensitivity to sound waves due to the coupling of many modes along the fiber. As compared to single mode fiber, it will thus results in a simplification of the acousto-strain amplification packaging, while the adaptive holography detection will provide a very efficient detection.