Optical fiber sensing technology has attracted attention because of its high sensitivity, corrosion resistance, immunity to electromagnetic interference, small size, and wide measurement bandwidth. Temperature plays a significant role in daily life and various applications; particularly, the controllability and accuracy of temperature sensitivity in biological protein activity and medical experiments are essential. Among them, the method of increasing temperature sensitivity through the vernier effect is widely used in optical fiber sensors. The optical vernier effect can be achieved using a sensing interferometer and a reference interferometer. The reference interferometer, which is not sensitive to temperature, provides a 'calibration master ruler' to the sensing interferometer based on the optical vernier effect, thereby amplifying the temperature sensitivity, as opposed to using a single sensing interferometer. However, the instability of the interferometer can result in measurement errors. Therefore, this study proposes using a virtual reference interferometer for achieving a sensitivity-tunable optical vernier effect to increase the temperature sensitivity. This method is applied to the Fabry-Perot (FP) temperature sensor based on UV glue, and the sensitization effect of its temperature sensitivity is verified through experiments. Subsequently, the feasibility of realizing the increase of temperature sensitivity is demonstrated.
Theoretically, the principle of the vernier effect sensitization is derived from the interference formula. Therefore, the amplification of the vernier envelope is simulated by changing the cavity length of the sensing interferometer or that of the reference interferometer. It is verified that the increase in sensitivity can be achieved using the virtual vernier interferometer, and this sensitivity can be regulated by changing the cavity length. Experimentally, the FP temperature sensor is fabricated by inserting a tapered optical fiber into a capillary tube with UV glue. The UV glue expands and contracts at different temperatures, resulting in a change in the cavity length and a drift in the superimposed spectrum. In the experiment, various temperature sensitivities are controlled by changing the cavity length of the reference interferometer. The feasibility of the scheme is verified.
Based on the principle of increased sensitivity based on the vernier effect, when the cavity length of the sensing interferometer or reference interferometer is changed, the superimposed vernier interference envelope spectrum achieves a certain multiple of sensitivity amplification, compared to that achieved using single FP sensing interferometer. Subsequently, by the spectral simulation of the double cavity length, it is deduced that when the free spectral range of the sensing interferometer is larger than that of the reference interferometer, the cavity length of the sensing interferometer decreases with increasing temperature, and the spectrum moves to the left drift, whereas the interference spectral envelope drifts to the left. When the free spectral range of the sensing interferometer is smaller than that of the reference interferometer, the cavity length of the sensing interferometer decreases with increasing temperature and the spectrum shifts to the left, whereas the interference spectral envelope shifts to the right (Figs. 3 and 4). Therefore, the temperature sensitivity can be controlled by introducing reference interferometers with different cavity lengths without changing the sensing interferometer. In the experiment, according to the theoretical formula, the cavity lengths of the relevant reference interferometers are set as 180, 160, 150, 144, and 140 μm, respectively, and their temperature sensitivities are 4.018, 6.021, 8.009, 10.033, and 12.096. It is inferred through experiments that, compared with the single sensing interferometer with a temperature sensitivity of 2.015 nm/℃, the FP temperature sensor based on the virtual vernier achieves 2-6 times temperature sensitivities, which is consistent with the results obtained by the FP temperature sensor theoretically (Figs. 9-13).
In this study, the temperature sensitivity regulation of the FP sensor is realized using the sensitization method of the optical vernier based on a virtual reference interferometer. The envelope drift simulations of the superimposed spectra with different double cavity lengths are conducted according to the amplification principle of the vernier envelope obtained from the sensing formula. In the experiment, when the cavity length of the virtual reference interferometer is changed, the control of different temperature sensitivities ranging from 2-6 times is realized, thereby verifying that the sensitization scheme of the optical vernier is feasible. This virtual simulation method not only avoids the influence of the external environment on the reference interferometer during the measurement process but also effectively increases the temperature sensitivity. This method does not require additional modification of the interferometer and can be applied to other interferometric fiber optic sensors; it has significant potential for the enhancement of existing fiber optic sensor networks.
