Reconfigurable highly-sensitive interferometric sensor with Vernier effect and a virtually tunable reference cell

Abstract

The optical Vernier effect is a powerful tool for increasing the sensitivity of an interferometer greatly. However, when the measurement range is large, the optical path difference between the sensing and reference cells becomes large too, which will cause a significant decrease in the sensitivity. To overcome this problem, we propose a novel method, which introduces a virtual reference cell instead of a real one in the measurement system, with a variable cavity length adapting to the measurement range. Both theoretical analysis and experiment measurement verified that by tuning the cavity length of the reference interferometer, LR, the Vernier effect based sensitivity can be maintained at a high level, although the cavity length of the sensing interferometer, LS, changes in a large range. A new formula presented in this paper also reveals that the Vernier effect induced sensitivity magnification factor M depends only on the ratio between the cavity lengths LS/LR in a more general way (denoted as factor g), instead of the absolute or difference between the two cavity lengths described in the previous literature. The experimental results of a large displacement measurement show that when the displacement changes from 0 to 100μm, g changes from 1.01 to 1.35, which results in M dropping from 110 to 4 as LR is fixed. However, M can be recovered back if LR is adjusted to make g equals 1.01 again when LS is changed. The proposed method provides a new approach to establish a highly sensitive interferometric sensor based on the Vernier effect, consisting of a single real sensing cell and a virtually variable reference cell, which can maintain the sensitivity at a high level even though the measurement range is large.