Adaptive optical head for industrial vibrometry applications

Abstract

A self-mixing laser interferometer has been combined with a compact and robust optical system including an adaptive optical element in the form of a voltage controllable liquid lens. The use of the liquid lens enables the self-mixing interferometer to adjust the optical focus position and the beam spot diameter on the target surface, and subsequently the feedback level within the laser cavity. The optical system has been designed to focus the beam at distances from a few centimetres from the front facet of the laser diode to infinity. With such a simple arrangement, it is possible to modify and control the intensity of the back reflected light from the target surface into the laser cavity, by simply changing the voltage applied to the lens to modify the focus condition on the target surface. The final effect obtained is full active control of the feedback level of the self-mixing effect taking place. This has allowed keeping the feedback level of the interferometer in the desired regime for measurements along very long distances and for different measurement situations, so extending the capabilities of a classical self-mixing interferometer. The advantage of the proposed adaptive optical head is thus its combination of precise metrology capabilities plus a great potential in automated feedback control and operator-free industrial applications. Signal reconstruction of the target vibration amplitude presents a maximum error of λ/16 as compared with a commercial capacitive sensor in the whole focusable range for displacement measurements. An improved working range of 6.5 cm to 280 cm staying in the same feedback regime has been experimentally demonstrated.