Displacement and Vibration Measurements with Interferometers: Benchmark Configurations, Performance, and R&D Opportunities

Many different approaches have been proposed or published in recent years on the measurement of small vibrations and/or large‐amplitude displacements by interferometers, either traditional or self‐mixing type. Most of them start from procuring a pair of orthogonal signals cos 2 k Δ s and sin 2 k Δ s , where k = 2π/ λ and Δ s is the variation of target distance, from which to proceed to calculate the measurand Δ s with the atan of the ratio followed by an unfolding operation. These attempts are commented on, and it is shown that it is not really necessary to add phase modulators or other extra optical components nor to use elaborate signal processing to obtain the orthogonal pair, because simple hardware solutions readily supply, without the cosine and sine pair, a resolution close to the quantum limit together with excellent dynamic range, precision, linearity, and bandwidth. The examples reported in this article establish a benchmark of performances that must be referred to in developing new approaches that make sense, also considering that such performances have been offered by commercial instruments for several decades.