Sensitivity-enhanced Temperature Sensor Research Articles

Sensitivity-Enhanced Temperature Sensor of Cascaded Lyot Filter and Mach–Zehnder Interferometer Based on the Vernier Effect

Sensitivity-Enhanced Temperature Sensor of Cascaded Lyot Filter and Mach–Zehnder Interferometer Based on the Vernier Effect

Compact Harmonic Vernier Sensor Based on an In-Fiber FPI with Three Reflector System for Simultaneous Gas Pressure and Temperature Measurement.

In this work, we proposed a sensitivity-enhanced temperature sensor, a compact harmonic Vernier sensor based on an in-fiber Fabry-Perot Interferometer (FPI), with three reflective interfaces for the measurement of gas temperature and pressure. FPI consists of air and silica cavities formulated by single-mode optical fiber (SMF) and several short hollow core fiber segments. One of the cavity lengths is deliberately made larger to excite several harmonics of the Vernier effect that have different sensitivity magnifications to the gas pressure and temperature. The spectral curve could be demodulated using a digital bandpass filter to extract the interference spectrum according to the spatial frequencies of resonance cavities. The findings indicate that the material and structural properties of the resonance cavities have an impact on the respective temperature sensitivity and pressure sensitivity. The measured pressure sensitivity and temperature sensitivity of the proposed sensor are 114 nm/MPa and 176 pm/°C, respectively. Therefore, the proposed sensor combines ease of fabrication and high sensitivity, making it great potential for practical sensing measurements.

Sensitivity-enhanced temperature sensor utilizing core offset and hollow core Bragg fiber

A novel sensitivity-enhanced optical fiber temperature sensor (OFTS) utilizing core offset and hollow core Bragg fiber (HCBF) is proposed and experimentally investigated. The OFTS was fabricated by sandwiching the HCBF between two single-mode fibers (SMFs). The operation principle of temperature sensitivity enhancement is to excite inter-modal interference by lateral offset splicing fibers. The four-cladding structure unique to HCBF induces periodic interference envelopes in the transmission spectrum, which can be used to monitor changes in the external environment. The experimental results show that the temperature and strain sensitivities of OFTS is 10.27 pm/°C and −4.03 × 10−4 nm/με, respectively. After introducing different core offsets, the temperature sensitivities are improved to 16.29 pm/°C, 18.29 pm/°C, and 30.03 pm/°C, respectively. The strain sensitivities are −1.98 × 10−4 nm/με, −4.33 × 10−4 nm/με, and −4.96 × 10−4 nm/με, respectively. Moreover, due to the high-temperature sensitivity, strain insensitivity, and compact structure, the proposed sensor has broad application prospects in complex environments such as aerospace.

Sensitivity-enhanced temperature sensor by cascaded configuration of polarization mode interferometer and Lyot filter based on Vernier effect

We proposed and experimentally demonstrated a novel high-sensitivity optical fiber temperature sensor (OFTS) which is composed of the Lyot filter cascading a polarization mode interferometer (PMI). The Lyot filter is fabricated by inserting a polarization-maintaining fiber (PMF) between two linear fiber polarizers at two 45-degree angles. By simply cleaving the end fiber of the PMI, the free spectral range (FSR) can be maximized to approach the FSR of the Lyot filter, which introduces the Vernier effect. Experimental results show that the cascaded sensor can provide high-temperature sensitivity of 10.666 nm/°C and 26.562 nm/°C, which are 6.71 and 12.168 times that of single PMI, respectively. The method is demonstrated to achieve a stable and controlled Vernier effect obtaining ultra-high temperature sensitivity, which agrees well with the theoretical simulation results. The proposed sensor has the advantages of high sensitivity, good stability, and easy fabrication.

Sensitivity-Enhanced Temperature Sensor by Cascaded Configuration of Polarization Mode Interferometer and Lyot Filter Based on Vernier Effect

Sensitivity-Enhanced Temperature Sensor by Cascaded Configuration of Polarization Mode Interferometer and Lyot Filter Based on Vernier Effect

Sensitivity-enhanced temperature sensor based on Mach-Zehnder interferometer coated with thermal-sensitive material.

In this paper, a Mach-Zehnder interferometer temperature sensor with a single-mode fiber-thin-core fiber-single-mode fiber cascaded is designed. When the light is transmitted from the single-mode fiber to the thin-core fiber, different modes will be excited because of the mode-field mismatch. Optical power from the input fiber can be partly coupled to the cladding modes of the thin-core fiber, and the cladding modes of the thin-core fiber then re-coupled to the lead-out fiber, which constitutes the Mach-Zehnder interferometer in the output single-mode fiber. To improve the temperature sensitivity of the sensor, we coated the thermal-sensitive material (such as polymethoxane or ultraviolet glue) on the surface of the thin-core fiber. The experimental results show that the temperature sensitivity of the sensor coated with a polydimethylsiloxane film was increased from 32.0 to 90.0 pm/°C, about three times, compared to the uncoated thin-core fiber sensor. When the surface of the thin-core fiber is coated with an ultraviolet glue film, the temperature sensitivity of the sensor was increased from 32.0 to 166.8 pm/°C, about six times. The designed sensor, with compact structure, simple production, strong robustness, and high sensitivity, has a wide application prospect in industrial production and national defense technology.

Sensitivity enhanced temperature sensor with cascaded Sagnac loops based on harmonic Vernier effect

A sensitivity enhanced temperature sensor with cascaded Sagnac loops based on harmonic Vernier effect has been proposed and verified by experiments. The harmonic Vernier effect can be achieved, through controlling the polarization-maintaining fiber (PMF) length of one Sagnac loop to be an integral multiple of the other with a small deviation. The experimental results show that in temperature range of 30–70 °C, the temperature measurement sensitivity of the proposed sensor device based on the harmonic Vernier effect is 3.66 nm/°C, which is 22.5 times that of the single Sagnac loop (0.163 nm/°C), and 1.7 times that of the sensor based on the basic Vernier effect (2.12 nm/°C). Thanks to the advantages of the proposed sensing structure’s high sensitivity and full fiber configuration, it can be used in various specific fields, such as power grids and mines.

A sensitivity-enhanced temperature sensor with end-coated PDMS in few mode fiber based on vernier effect

In this paper, a few mode fiber temperature sensor based on vernier effect is proposed. The sensor consists of two cascaded microcavities and realizes temperature sensing based on vernier effect. The single mode fiber(SMF) and few mode fiber(FMF) are fixed by a capillary glass tube to form an air reference microcavity. The end of the FMF is dipped with an appropriate amount of polydimethylsiloxane(PDMS) to form a sensing microcavity with a certain thickness, which is an air microcavity-FMF–PDMS structure. Due to the different refractive index of each medium, the light is reflected on the four critical surfaces, and the four reflected light beams interfere, the vernier effect is formed by adjusting the microcavity length to a suitable length. When the external temperature changes, the microcavity length and refractive index of PDMS change, which leads to the change of interference spectrum. Therefore, the high sensitivity measurement of temperature can be realized by the change of reflection spectrum. The temperature characteristics of the sensor are studied experimentally. The results show that the temperature sensitivity is significantly improved compared with the traditional single microcavity sensor. In the temperature range of 35°C∼45°C, the temperature sensitivity can reach 4.7nm/°C, and the linearity is very good. The sensor has the advantages of compact structure, simple fabrication and high sensitivity, so it has wide application prospects in the field of temperature sensing.

Sensitivity-enhanced temperature sensor based on encapsulated S-taper fiber Modal interferometer

We demonstrated a simple technique for a highly sensitive temperature sensor based on an “S”-shape fiber taper Mach-Zehnder interferometer (MZI) with no need of special fibers or micromachining technique using high power lasers. To enhance the sensitivity, an S-shape single mode fiber (SMF) taper was encapsulated in a capillary filled with high thermos-optic coefficient liquid. We experimentally investigated the tuning parameters for the sensor, such as the taper shape, the material of the capillary, and the role of the filling liquid. Our results show a maximum sensitivity of −15.66 nm/°C in a measurement range of 28–32 °C, and −11.88 nm/°C from 25.1 °C to 35 °C for isopropanol sealed in Teflon capillary. The multi-interference model simulation shows that a critical cladding mode is responsible for the high sensitivity. Simple configuration, ease of fabrication, robustness and low cost make it suitable for high sensitivity temperature sensing applications.

Sensitivity enhanced temperature sensor with serial tapered two-mode fibers based on the Vernier effect.

A sensitivity enhanced temperature sensor with cascaded tapered two-mode fibers (TTMFs) based on the Vernier effect is proposed and experimentally demonstrated. It is confirmed that series connection exhibits higher extinction ratio than parallel one both by theory and experiments, which provides guidance for related experiments. In experiments, two TTMFs have the same single-mode fiber-TTMF-single-mode fiber configuration, while the free spectral ranges (FSRs) are chosen with slightly difference by modifying the parameters in the tapering process. Experimental results show that the proposed temperature sensor possesses sensitivity of -3.348 nm/°C in temperature measurement range from 25 °C to 60°C, 11.3 times sensitivity enhancement in comparison with single TTMF. Benefiting from advantages of high temperature sensitivity, simplicity of manufacture and long distance sensing, this novel sensitivity enhanced temperature sensor can be applied to various particular fields, such as oil wells, coal mines and so on.

Sensitivity enhanced temperature sensor: one-port 2D surface phononic crystal resonator based on AlN/sapphire

According to the structure of a one-port surface acoustic wave (SAW) resonator, a one-port surface phononic crystal (SPC) resonator is designed. Both one-port SAW resonator and one-port SPC resonator are fabricated on AlN/Sapphire. Frequency responses and quality factors are studied. The one-port SAW resonator has a fundamental resonance mode at 693.07 MHz. Unlike the one-port SAW resonator, the one-port SPC resonator has two distinct resonance modes at 680.28 MHz and 698.91 MHz, respectively. From the simulation results by finite element method (FEM), these two resonance modes may come from the two cavities of the one-port SPC resonator. Frequency-versus-temperature behaviors of these two resonators are investigated. The results show that the frequencies of resonance modes of these two resonators are decreased linearly with the increased temperature. A new kind of temperature sensor is proposed by using frequency difference between these two resonance modes of the one-port SPC resonator. The eventual temperature coefficient of frequency (TCF) value of the temperature sensor is ppm °C−1 which is twice more than that of the one-port SAW resonator. All results prove that temperature sensitivity is enhanced by using two mode characters of one-port SPC resonator based on AlN/Sapphire.

Sensitivity-enhanced temperature sensor by hybrid cascaded configuration of a Sagnac loop and a F-P cavity

A hybrid cascaded configuration consisting of a fiber Sagnac interferometer (FSI) and a Fabry-Perot interferometer (FPI) was proposed and experimentally demonstrated to enhance the temperature intensity by the Vernier-effect. The FSI, which consists of a certain length of Panda fiber, is for temperature sensing, while the FPI acts as a filter due to its temperature insensitivity. The two interferometers have almost the same free spectral range, with the spectral envelope of the cascaded sensor shifting much more than the single FSI. Experimental results show that the temperature sensitivity is enhanced from −1.4 nm/°C (single FSI) to −29.0 (cascaded configuration). The enhancement factor is 20.7, which is basically consistent with theoretical analysis (19.9).

Sensitivity-enhanced temperature sensor with fiber optic Fabry-Perot interferometer based on vernier effect

Fiber-optic temperature sensors have gained much attention owing to their intrinsic features of light weight, immunity to electromagnetic interference, and capability for distributed measurement. Especially, temperature sensors based on Fabry-Perot interferometers (FPIs) are attractive owing to their advantages of compact size and convenient reflection measurement. However, due to the low thermal expansion or/and thermo-optic coefficient of fiber, the temperature sensitivities of these sensors are normally low (~10 pm/℃ or even lower). In order to improve the temperature sensitivity, a device with dual cascaded FPIs is proposed and demonstrated in this paper, which works on vernier effect and exhibits a much higher temperature sensitivity. The device is fabricated by splicing a short segment of large mode area (LMA) fiber to a short segment of capillary tube fused with a section of single-mode fiber to form an extrinsic Fabry-Perot interferometer with a glass cavity cascaded to an intrinsic FPI with a narrow air cavity. By setting the lengths of capillary tube and LMA fiber to allow similar free spectral ranges to be obtained, and superimposing of the reflection spectra of the two FPIs, the vernier effect can be generated. Firstly, the principle of temperature sensing based on vernier effect of this device is analyzed and simulated theoretically, and it is found that the temperature sensitivity can be improved significantly by using vernier effect compared with that of a single FPI with an air-cavity or glass cavity by directly tracing resonant dips/peaks. Then, the temperature responses of the FPI with single air-cavity and dual cascaded cavities are measured, respectively. Experimental results match well with the theoretical analysis carried out. The temperature sensitivity of the proposed sensor is improved greatly from 0.71 pm/℃ for a single FPI sensor with an air-cavity to 179.30 pm/℃ by employing the vernier effect. Additionally, the sensor exhibits good repeatability in a temperature range of 100-500℃. The proposed sensor has the advantages of compact size (1 mm in dimension) and high sensitivity, which makes it promising for temperature sensing in a variety of industries, such as food inspection, pharmacy, oil/gas exploration, environment, and high-voltage power systems.

Sensitivity-enhanced temperature sensor based on PDMS-coated long period fiber grating

A sensitivity-enhanced temperature sensor based on a poly-dimethylsiloxane (PDMS)-coated long period fiber grating (LPFG) has been proposed and experimentally investigated. By embedding the LPFG in a temperature-sensitive elastomeric polymer, the temperature sensitivity of the proposed sensor could be effectively improved by 4 times higher than those of the conventional bare LPFG sensors due to the high thermo-optic coefficient (TOC) of PDMS. It can be found that the temperature sensitivities of higher-order modes are higher than those of lower-order modes by analyzing transmission spectra characteristics of the sensor. Because of LPFG is sensitive to surrounding refractive index (RI), the PDMS-coated LPFG will have a high temperature sensitivities of 255.4pm/°C in the range of 20–80°C. Due to the high measurement resolution of 0.078°C, the sensor is promising to be applied to the fields that high-precision temperature measurement is required.

Sensitivity-enhanced temperature sensor with cascaded fiber optic Sagnac interferometers based on Vernier-effect

A novel fiber optic temperature sensor has been proposed and experimentally demonstrated with ~9 times sensitivity enhancement by using two cascaded Sagnac interferometers. These two Sagnac interferometers consist of the same type of polarization maintaining fibers with slightly different lengths. The working principle is analogous to a Vernier scale. One interferometer acts as filter, while the other is for temperature sensing. The envelope of the cascaded sensor shifts much more than single one with a certain enhancement factor, which related to the free space range difference between the filter and sensor interferometers. Experimental results show that the temperature sensitivity is enhanced from −1.46nm/°C based on single Sagnac configuration to −13.36nm/°C.