Enhanced temperature sensing with Vernier effect on fiber probe based on multimode Fabry-Perot interferometer

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.

In this paper, we propose and demonstrate a gas pressure fiber probe with high sensitivity magnified by Vernier effect. The probe is composed of two cascaded Fabry–Perot interferometers based on a SMF–SOHST–OFC structure (SMF: single-mode fiber; SOHST: side-opened hollow silica tube; OFC: optical fiber column). The high-frequency CO2 laser drilling method for hollow silica tube can effectively maintain the transient balance of the air pressure inside and outside the cavity without destroying the reflective ends of the optical fibers. Experimental results show that the prepared fiber probe with the SOHST length of 375.2 μ m and column length of 247.3 μ m has high gas pressure sensitivity of 80.3 pm/kPa by demodulating Vernier envelope, and it has relatively low temperature cross-sensitivity of –1.33 kPa/°C. This sensor is highly sensitive and of compact size, which not only can be applied in gas pressure sensing but also has the potential for application in microfluidic detection.

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.

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.

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.

The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.

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.

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 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.

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 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.

High-sensitivity gas pressure sensor with low temperature cross-talk based on Vernier effect of cascaded Fabry-Perot interferometers

This article proposes a novel ultra-sensitive fiber optic temperature sensor based on a compact cascaded Fabry-Perot interferometer (FPI) and Vernier effect (VE). Firstly, a conical ultraviolet (UV) adhesive Fabry-Perot (F-P) cavity is prepared on the end face of a single-mode fiber (SMF). When the F-P cavity based on UV adhesive is subjected to thermal effects, the large thermal effect of the UV adhesive causes a variation of the optical path difference of the light transmitted in the F-P cavity, resulting in spectral drift. Temperature measurement is achieved by demodulating the spectral wavelength drift. Firstly, FPI1 and FPI3 are two UV adhesive based FPIs prepared at the SMF tip using this method. Then, they are inserted separately into two capillary tubes, and forming the compact cascaded FPI structure with the SMF located at the other end of the capillary tube. With the help of the charge coupled device (CCD) imaging system, the length of the air-cavity of two compact cascaded FPI structures is carefully adjusted, forming separately the VE sensor S1 and the harmonic vernier effect (HVE) sensor S2. S1 can further amplify the sensitivity of FPI1, and S2 can further amplify the sensitivity of FPI3. The experimental results show that the temperature sensitivities of FPI1 and FPI3 are 0.4631 nm/°C and −0.1110 nm/°C, respectively, and the temperature sensitivities of S1 and S2 are 3.0099 nm/°C and 3.5080 nm/°C. S1 increased the sensitivity of FPI1 by 6.5 times, and S2 improved the sensitivity of FPI3 by 31.6 times. The proposed sensor boasts a compact structure, simple manufacturing, durability, and high sensitivity. Meanwhile, the small sensing head is particularly useful for temperature detection in narrow spaces.

Parallel-structured Fabry-Perot interferometers gas pressure sensor with ultraviolet glue sensitization based on dual Vernier effect

Fabry-Perot parallel type high sensitivity fiber optic temperature sensor based on vernier effect beyond free spectral range measurement