Eye-safe Sensitivity Enhancement of Laser-Doppler-Vibrometry based on Squeezed Light

• Applicants: Professor Dr.-Ing. Christian Rembe; Professor Dr. Roman Schnabel
• Subject Area: Measurement Systems ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431078650

Laser Doppler Vibrometry (LDVy) is a well-established industrial tool for measuring surface vibrations as well as path length changes through transparent media. Application examples are vibration measurements on car engines and machines in industrial fabrication with the purpose of designing better devices with reduced noise emission. Here, LDVy exploits laser beams that are back-reflected from points on vibrating surfaces. Other industrial LDVy applications exploit the change of the refractive index along lines of sight through a transparent vibrating body, a liquid or a gas. Here, LDVy provides the relevant data for improving acoustic resonators of musical instruments or loud spekers, for characterizing the spatial emission of ultra-sonic heads, or for modelling the three-dimensional turbulent stream fields of gases such as air. If the end-user requests an eye-safe operation of the laser Doppler vibrometer (LDV) with the purpose of reducing the operation costs, upper limits are set to the light power and intensity, and thus to the LDV sensitivity and spatial resolution. Due to such requests, commercial LDVs are usually engineered to being photon shot-noise limited and to use the maximal light power being still eye- safe (laser safety class I). Even if an end-user does not request eye-safe LDV operation, upper limits to the light power and intensity are always set, in particular by optical heating and thermal deformation of the measurement object, which produces systematic measurement errors. This project aims at the proof-of-principle of squeezed-light-enhanced laser Doppler vibrometry. We will show that laser light with a strongly-squeezed quantum uncertainty can boost the sensitivity and the vibration-amplitude resolution of LDVs while keeping the light power eye-safe and the light intensity below the threshold above which the measurement object is influenced by the measurement. To achieve this goal, our project combines concepts of quantum optics and engineering sciences. We design and build two continuous-wave squeezed light sources at 1550nm and use them for the improvement of a state-of-the-art heterodyne Mach-Zehnder type LDV and a homodyne polarization Michelson-type LDV. In particular we use them for refractive tomography of an ultra-sonic wave and for the surface scan of a membrane oscillation. Whereas the best commercial LDVs have a sensitivity in the range of 10 fm/√Hz, our project will achieve with the help of squeezed light a sensitivity of better than 1 fm/√Hz.Our project will show that squeezed states of light can help to breach the current physical resolution limit of industrial interferometry. Our result may affect several engineering applications of LDV whenever optical loss can be kept low. Squeezed-light enhanced LDVy may turn out as an example of a quantum technology at the heart of the envisioned second quantum-revolution.

• DFG Programme: Research Grants