A method to measure sub nanometric amplitude displacements based on optical feedback interferometry

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.