Temperature and humidity measurement is crucial for the growth and storage of crops, as well as for industrial monitoring and control. Compared to electronic sensors, fiber optic sensors offer advantages such as strong anti-interference capability, good stability, excellent electrical insulation, and high sensitivity, making them widely used in various fields. Among them, fiber optic Fabry-Perot interferometric sensors can detect even the smallest changes in physical quantities with high sensitivity, compact size, and light weight. In recent years, with the increasing demand for environmental monitoring, the simultaneous measurement of temperature and humidity has gained increasing attention. However, most fiber optic temperature and humidity sensors either have low sensitivity or cannot measure the both parameters simultaneously. Therefore, developing high-sensitivity fiber optic sensors for both temperature and humidity is of significant practical importance. The parallel fiber Fabry-Perot structure enables multiple sensors to operate simultaneously, allowing the monitoring of different physical quantities. The Vernier effect, through the overlap of signals from two interferometers, amplifies the wavelength shift, which significantly improves the sensitivity of fiber optic sensors.

The sensor consists of a PDMS cavity and a PI cavity formed between two single-mode optical fibers, operating in parallel. By utilizing the similar free spectral range (FSR) of the PDMS and PI cavities, a Vernier effect occurs when the cavities are combined in parallel. This effect, generated through the superposition and interference of the interferometer signals, amplifies even minute changes in wavelength. For monitoring relative humidity and temperature, the PDMS and PI cavities serve as the reference and sensing cavities, respectively, allowing for high-sensitivity measurements of the both parameters. A single Fabry-Perot interferometer measures relative humidity (or temperature) by tracking the shift in its characteristic wavelength in the reflection spectrum as it responds to changes in relative humidity (or temperature). The parallel sensor structure generates a Vernier effect, enabling the monitoring of temperature and humidity by observing the drift of the envelope line in the superimposed reflection spectrum. By leveraging the different sensitivity characteristics of PI and PDMS to temperature and humidity, and using a sensitivity coefficient matrix, the responses of the PI and PDMS cavities can be mathematically processed to achieve simultaneous measurement of both temperature and humidity.

Under experimental conditions at a temperature of 20 ℃, the relative humidity is gradually increased from 30% to 60%, recording data at 10% increments. As relative humidity increases, the characteristic wavelength of the FPI₁ reflection spectrum shifts towards shorter wavelengths, with FPI₁ showing a sensitivity to relative humidity of (-0.409±0.120) nm/%. When FPI₁ is paired with FPI₂ in parallel, and the relative humidity is similarly increased from 30% to 60%, the experimentally determined sensitivity to relative humidity is 1.614 nm/%, approximately four times the sensitivity of a single FP for measuring humidity. With the relative humidity set at 20%, the temperature is incrementally increased from 30 to 34 ℃, and the experimentally determined temperature sensitivity of FPI₂ is (3.194±0.140) nm/℃. After FPI₁ is paired with FPI₂ in parallel, as the temperature increases from 30 to 33 ℃, the temperature sensitivity is (15.611±0.376) nm/℃, about five times the sensitivity of a single FPI₂. By using a sensitivity coefficient matrix to demodulate the accurate measurements of temperature and relative humidity, simultaneous measurement of both temperature and relative is achieved.

In this paper, we present a novel parallel Fabry-Perot fiber optic temperature and humidity sensor based on the Vernier effect. The sensor consists of two air cavities formed by the endfaces of single-mode fibers, filled with PI and PDMS, respectively. These create PI and PDMS cavities, which, when connected in parallel, utilize the Vernier effect to detect and amplify the sensitivity to temperature and humidity. Experimental results show that within a relative humidity range of 30% to 60%, the sensor’s sensitivity to relative humidity is 1.614 nm/%, about four times the sensitivity of a single FPI. Within a temperature range of 30 to 33 ℃, the temperature sensitivity is (15.611±0.376) nm/℃, about five times the sensitivity of a single FPI. By using a sensitivity matrix, the sensor achieves simultaneous measurement of both temperature and humidity. While the operational temperature range of this sensor is relatively limited, it can be flexibly adjusted to meet the application-specific temperature range requirements. This makes the sensor ideal for monitoring temperature and humidity across diverse environments.