Optical feedback interferometry is a well known technique that can be used to build non-contact, cost effective, high resolution sensors. In the case of displacement measurement, different research groups have shown interest in increasing the resolution of the sensors based on this type of interferometry. Such efforts have shown that it is possible to reach better resolutions by introducing external elements such as electro-optic modulators, or by using complex signal processing algorithms. Even though the resolution of the technique has been increased, it is still not possible to characterize displacements with total amplitudes under λ/2. In this work, we propose a technique capable of measuring true nanometre amplitude displacements based on optical feedback interferometry. The system is composed by two laser diodes which are calibrated within the moderate feedback regime. Both lasers are subjected to a vibration reference and only one of them is aimed to the measurement target. The optical output power signals obtained from the lasers are spatially compared and the displacement information is retrieved. The theory and simulations described further on show that sub-nanometre resolution may be reached for displacements with amplitudes lower than λ/2. Expected limitations due to the measurement environment will also be discussed in this paper.
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.
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