High-sensitivity Fabry-Perot interferometer temperature sensor probe based on liquid crystal and the Vernier effect.

The Vernier effect is often utilized to boost the sensing ability of optical fiber sensors. In this paper, theoretical model of cascaded Fabry-Perot interferometer (FPI) with thin film based on Vernier effect is established. The sensitivities of the envelope spectra, thin film cavity and mixed cavity of air-thin film are analyzed qualitatively. According to the theoretical analysis, although sensitivity from mixed cavity of air-thin film is amplified, the value is equal to the sensitivity of sing thin film cavity. Experimental verification is carried out by an example of thin film named polydimethylsiloxane (PDMS) polymer. Herein, a new FPI constructed by air cavity from a hollow-core fiber, PDMS cavity, and air-PDMS mixed cavity is proposed and demonstrated. In order to facilitate the generation of the Vernier effect, the length of the PDMS cavity is intentionally designed shorter than the air cavity, making the free spectral range of the air-PDMS cavity and air cavity is approximately equal. The temperature change makes the refractive index and thermal expansion of PDMS change, while gas pressure change results in elastic deformation of PDMS. The Vernier envelope wavelength shifts with the temperature and gas pressure change. The proposed FPI features high temperature and gas pressure sensitivities of 3.07 nm/℃, and 23.07 nm/MPa, and a high magnification factor of 17 when the lengths of HCF and PDMS are 82.5 and 3.7 μm, respectively. The experimental results show that the temperature and pressure sensitivities of the cascaded FPI’s envelope spectra are equal to the sensitivity of a single thin film microcavity, and the theoretical calculation is in good agreement with the experimental verification. The theoretical model is also applicable to thin film prepared by other polymer materials. Additionally, the proposed FPI has good stability, reversibility, and repeatability, which is a good choice in the field of optical fiber sensing.

A highly-sensitive temperature sensor with controllable sensitivity based on Vernier effect by cascading a tunable extrinsic Fabry-Perot interferometer (FPI) and a fixed reflective Lyot filter (RLF) is theoretically investigated and experimentally demonstrated. The temperature sensitivity can be tuned by modulating the cavity length of the extrinsic FPI and online monitoring the envelope of superimposed spectrum with optical spectrum analyzer (OSA). The FPI works as the reference arm to tune the temperature sensitivity of the sensing system, while the RLF with 1-meter polarization maintaining fiber (PMF) acts as the sensing probe. Experimental results prove that by changing the cavity length of the FPI, the sensitivities of −3.82 nm/°C, −8.33 nm/°C and −14.63 nm/°C can be achieved. Compared with the single sensing element, the sensitivities are magnified by 3.78, 8.25 and 14.49 times. The proposed temperature sensor is feasible to be applied practically in scenarios which require different temperature sensitivities in demanded temperature detection ranges.

In this work, we present a high-sensitivity fiber-optic temperature sensor based on the enhanced Vernier effect. The sensor consists of a cascade of three Fabry–Perot interferometers (FPIs), where FPI1 contains a polydimethylsiloxane (PDMS) cavity, FPI2 contains an air microcavity, and FPI3 contains a PDMS–air hybrid cavity. Since the effective lengths of the PDMS cavity and the PDMS–air hybrid cavity are close to each other, the superposition of the reflection spectra of FPI1 and FPI3 produces the traditional Vernier effect. Due to the thermo-optic effect and thermal expansion effect of PDMS and air, the reflection spectra of FPI1 and FPI3 will vary in the opposite direction with the temperature change, to achieve the enhanced Vernier effect. A temperature sensitivity of −35.45 nm/°C much higher than that of the fiber FPI sensor based on conventional Vernier effect is achieved, in good agreement with the theoretical analysis. The proposed sensor has the advantages high stability, mechanical strength, small size, and low cost, which has a great potential for marine environment detection and bioscience.

A high-sensitivity all-fiber Fabry-Perot interferometer (FPI) gas refractive index (RI) sensor based on lateral offset splicing and Vernier Effect is proposed. The sensing head consist of a reference FPI and a sensing FPI. A reference FPI and the sensing FPI is connected by two circulators. The sensing FPI is an open cavity made by lateral offset splicing of single mode fibers (SMFs), and the reference FPI is spliced by SMF and fiber tube. The Cascaded double-cavity produces a Vernier Effect and improves the sensitivity of gas RI. Experimental results show that the sensor has a high gas RI sensitivity of 16335.96 nm/RIU. The sensor has broad application prospects in gas composition detection, environmental monitoring, biosensors and other fields.

A high-sensitivity fiber optic temperature sensor based on the enhanced harmonic Vernier effect (HVE) is proposed, which consists of two Fabry–Perot interferometers (FPI) that are sensitive to temperature and connected in parallel. FPI1 is a polydimethylsiloxane (PDMS) cavity formed by filling a ceramic ferrule with PDMS, and FPI2 is an air-cavity formed by inserting a single-mode fiber into a ceramic ferrule coated with PDMS film on the end face. FPI1 and FPI2 have opposite temperature responses and an approximate 2-fold free spectral range (FSR) relationship. As the temperature rises, the interference spectrum of FPI1 gradually red-shifts, while the interference spectrum of FPI2 gradually blue-shifts, resulting in an enhanced HVE. Its temperature sensitivity is much higher than that of a single FPI, and the amplification rate is significantly higher than that of ordinary Vernier effect. Two enhanced HVE sensors S1 and S2 are developed using this method, but there is a certain difference in their FSR detuning. The experimental results reveal that within the temperature range of 30–35 °C, the temperature sensitivity of S1 and S2 reach − 44.39 nm/°C and − 23.14 nm/°C, respectively. Both S1 and S2 have extremely high temperature sensitivity, but FSR detuning has a significant impact on sensitivity amplification. Additionally, the proposed enhanced HVE sensor has good repeatability and stability in measuring temperature.

Sensing at small dimensions in biological and medical environments requires miniaturized sensors with high sensitivity and measurement resolution. In this work a small optical fiber probe was developed to apply the Vernier effect, allowing for enhanced temperature sensing. Such effect is an effective way of magnifying the sensitivity of a sensor or measurement system in order to reach higher resolutions. The device is a multimode silica Fabry-Perot interferometer structured at the edge of a tapered multimode fiber by focused ion beam milling. The Vernier effect is generated from the interference between different modes in the Fabry-Perot interferometer. The sensor was characterized in temperature, achieving a sensitivity of -654 pm/°C in a temperature range from 30°C to 120°C. The Vernier effect provided a temperature sensitivity over 60-fold higher than the sensitivity of a normal silica Fabry-Perot interferometer without the effect. The temperature resolution obtained was 0.14°C, however this value was limited by the resolution of the OSA and can be improved further to less than 0.015°C.

Purpose The purpose of this paper is to design a high-sensitivity temperature sensor based on the vernier effect (VE) of dual-parallel Fabry-Perot interferometers (FPIs). Design/methodology/approach Initially, ultraviolet (UV) glue is coated on the end-face of a smooth single-mode fiber (SMF) and then tapered using a charged coupled device (CCD) precision displacement platform. Once the UV glue solidified, it is cut flat with a cutting tool to create FPI1. Subsequently, an SMF with a smooth end-face is inserted into a ceramic ferrule, and the other end-face of the ceramic ferrule is coated with polydimethylsiloxane (PDMS) to form FPI2 and FPI3. By adjusting the distance between the end-face of the SMF and the end-face of the ceramic ferrule, FPI2 and FPI3 are made to have different cavity lengths and free-spectral ranges (FSRs). In addition, the FSR of FPI2 is approximately equal to that of FPI1, while the FSR of FPI3 is roughly twice that of FPI1. When the temperature increases, the interference spectral lines of FPI1 gradually red-shift, whereas those of FPI2 and FPI3 gradually blue-shift. The parallel combination of FPI1 and FPI2 results in an enhanced traditional vernier effect (TVE), and the parallel combination of FPI1 and FPI3 leads to an enhanced harmonic vernier effect (HVE). Findings Experiments demonstrate that the temperature sensitivity of FPI1 is 0.17 nm/°C. The temperature sensitivities of FPI2 and FPI3 are −0.46 nm/°C and −1.90 nm/°C, respectively. Within the 30–37 °C temperature range, the enhanced traditional vernier effect sensor S1, formed by the parallel connection of FPI1 and FPI2, attains a temperature sensitivity of 9.42 nm/°C. The enhanced harmonic vernier effect sensor S2, constructed by the parallel combination of FPI1 and FPI3, reaches a temperature sensitivity of −29.59 nm/°C, respectively. These sensitivities represent a significant improvement over those of a single FPI. Originality/value These experimental results validate that the enhanced VE and the enhanced HVE represent an effective approach for enhancing sensor sensitivity. Moreover, compared with the enhanced VE, the enhanced HVE exhibits a more pronounced impact.

Acoustooptic effects of nematic liquid crystals are investigated by means of an elastic wave propagating in a glass plate. A nematic liquid crystal cell is mounted on the central region of a glass plate for elastic wave propagation. The elastic wave propagating in the glass plate has a wide frequency range. Two types of periodical textures are observed in the nematic liquid crystals under the existence of the elastic wave. One exists in both homogeneously and homeotropically aligned cells. This texture depends not on the kind of liquid crystal but on the carrier frequency of the elastic wave. The other is recognized only in homogeneously aligned cell. This texture depends not on the carrier frequency of the elastic wave but on the layer thickness and the kind of liquid crystal. Both periodical textures are induced by the elastic wave propagating in the glass plate.

In this study, a highly sensitive gas pressure sensor based on two Fabry–Perot interferometers (FPIs) in parallel with the Vernier effect is designed and demonstrated. In the sensor, FPI1 and FPI2 were used as the detection and reference units, respectively. FPI1 and FPI2 have similar structures, both comprising a single-mode fiber and capillary splicing. However, the microaperture is processed on the capillary of the FPI1 with a femtosecond laser, enabling the external gas to enter the FPI1 cavity through the microaperture channel to realize the gas pressure measurement. The Vernier effect is achieved by superimposing the reflective spectra of FPI1 and FPI2, which have a small cavity length. The gas pressure response, repeatability, and stability of the sensor structures were investigated. The sensitivity of the sensor was improved owing to the Vernier effect. Experimental results indicate that the sensor sensitivity is up to 47.76 nm MPa−1, which is 11.9 times that of a single FPI1 sensor without the Vernier effect. The measured temperature cross-sensitivity of the sensor is approximately 5.1 kPa/ °C. In addition, the sensor has the advantages of simple manufacturing, robust structure, and stable operation. Finally, the manufacturing method of FPI and the parallel connection mode of FPIs could also be used in other sensing fields, providing an alternative design scheme for high-sensitivity sensors.

A PDMS-optical fiber Fabry-Pérot interferometric (FPI) temperature sensor based on Vernier effect is proposed. The sensing probe is formed by cascading two single-mode fibers (SMFs) with a PDMS cavity, while the reference probe consists of two SMFs spliced to a hollow-core fiber (HCF). The two cavities have lengths of 146.8 and 218.4 μm, respectively. The structure is compact and easy to be fabricated. The air-filled cavity provides a highly stable reference interference signal, while the PDMS cavity, which is sensitive to temperature, undergoes significant cavity-length variation under temperature changes. Due to the slight mismatch between the free spectral ranges (FSRs) of the two cavities, a distinct Vernier envelope is generated in the reflection spectrum, effectively amplifying the temperature-induced spectral shift and thus greatly improving the sensing sensitivity. Experimental results show that the parallel FPI structure achieves a temperature sensitivity of approximately 21.54 nm/℃ in the range of 25-30 ℃, with a linearity better than 99.975%, which is significantly higher than that of conventional PDMS-based FPI sensors. The sensor also exhibits good temperature reversibility, demonstrating great potential for applications in industrial, medical, environmental, and aerospace fields.

We have proposed and experimentally demonstrated an ultrasensitive fiber-optic temperature sensor based on two cascaded Fabry-Perot interferometers (FPIs). Vernier effect that significantly improves the sensitivity is generated due to the slight cavity length difference of the sensing and reference FPI. The sensing FPI is composed of a cleaved fiber end-face and UV-cured adhesive while the reference FPI is fabricated by splicing SMF with hollow core fiber. Temperature sensitivity of the sensing FPI is much higher than the reference FPI, which means that the reference FPI need not to be thermally isolated. By curve fitting method, three different temperature sensitivities of 33.07, -58.60, and 67.35 nm/°C have been experimentally demonstrated with different cavity lengths ratio of the sensing and reference FPI, which can be flexibly adjusted to meet different application demands. The proposed probe-type ultrahigh sensitivity temperature sensor is compact and cost effective, which can be applied to special fields, such as biochemical engineering, medical treatment, and nuclear test.

A point-wise and quasi-distributed optical sensing technique with the Vernier effect is proposed and achieved by multiplexing Fabry-Perot interferometers (FPIs). The FPIs are fabricated by LiNbO3 (LN) crystals of varying lengths to enable simultaneous measurement of electric field (E-field) and temperature. Compared to the traditional bulk-type optical E-field sensors, this innovative sensor enables E-field measurement without being limited by half-wave voltage and effectively avoids the influence of natural birefringence. By an algorithm based on the Fourier transform, sub-FPIs are addressable, and their interference spectra can be demodulated independently, where any two FPIs are paired to generate the fundamental Vernier effect (FVE) or harmonic Vernier effect (HVE). Thus, the measurement sensitivities of FPIs can be significantly improved by monitoring the spectral shift of the envelope in the superimposed spectra. The quasi-distributed sensing experiment is conducted using three cascaded FPIs, yielding E-field sensitivities of 2.84 nm/E (FVE) and 3.37 nm/E (HVE), with a standard deviation (STD) of spectrum variation after temperature compensation below 3.19 × 10-3. The proposed multiplexing method is significant for practical applications of the E-field quasi-distributed measurement.

Low extinction ratio (ER) and high temperature cross-sensitivity are serious but common problems for most strain sensors based on Vernier effect. In our work, we propose a strain sensor based on Vernier effect. It consists of hybrid cascaded Sagnac loop, one part is a microfiber coupler Sagnac loop (MFC–SL), and the other is a Sagnac loop connected to a Section of polarization-maintaining photonic crystal fiber (PMPCF–SL). Different from the traditional Vernier effect, in the proposed sensor, the spectra of two interferometers shift to opposite directions when the strain changes, which belongs to the enhanced Vernier effect. When ambient temperature varies, the spectra of the two interferometers shift to the same direction, which belongs to the reduced Vernier effect. Thus, its strain sensitivity is improved and its temperature cross sensitivity is reduced. In addition, the MFC–SL and PMPCF–SL exhibit a high fringe ER due to their good transmission of two orthogonal modes. Experimental show that the sensitivity of strain is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 124.17\,\,\text {pm}/\mu \varepsilon $ </tex-math></inline-formula> , while the temperature sensitivity is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.13~\text {pm}/^{\circ }\text{C}$ </tex-math></inline-formula> , and the ER of envelope is about 9 dB. Its robustness can be enhanced by the method of packaging with silicone adhesive. It will have a potential application in the field of strain sensing.

A novel highly sensitive airflow sensor based on Fabry–Perot interferometer (FPI) and Vernier effect is proposed and demonstrated. The sensor is fabricated by splicing a section of hollow-core fiber (HCF) between a lead-in single mode fiber (SMF) and a section of SMF, which is almost one-fifteenth the length of the HCF. Airflow changes the cavity length of the FPI, which leads to the shift of the reflection spectrum. The reflection beam from the last end of the SMF is utilized to generate the Vernier effect, which enlarges the shift of the spectrum for 9.57 times than that of a single FPI. According to the experimental results, when the airflow ranges from ${\text{0 to 7 m/s}}$ , the spectrum of the sensor shifts as high as 7.9 nm which is almost eight times more than that of the self-heating sensor based on silver-coated FBG. And the highest airflow velocity sensitivity of the sensor can reach ${\text{1.541 nm/(m/s)}}$ in the region of ${\text{3m/s} \sim {7m/s}}$ . Besides, the sensor has a series of advantages such as fast response, low cost, compact size, and reflection measurement.

A novel sensitivity-enhanced intrinsic fiber Fabry-Perot interferometer (IFFPI) high temperature sensor based on a hollow-core photonic crystal fiber (HC-PCF) and modified Vernier effect is proposed and experimentally demonstrated. The compact all fiber IFFPIs are easily constructed by splicing one end of the HC-PCF to a leading single mode fiber (SMF) and applying an arc at the other end of the HC-PCF to form a pure silica tip. The Vernier effect is formed by three beams of lights reflected from the SMF-PCF splicing joint, and the two air/glass interfaces on the ends of the collapsed HC-PCF tip, respectively. This Vernier effect based fiber sensor can stand with high temperature up to 1200°C, in this work, and experimental results exhibit good linearity, stability and repeatability. The temperature sensitivity can be further enhanced by reducing length of pure silica tip—that is, simply optimizing the discharge time and intensity. The fabrication of the sensitivity-enhanced IFFPI is straightforward, reproducible and low cost, which shows a potential for practical applications in various industrial processes.