Interferometric Temperature Sensor Research Articles

Two-mode fiber Mach-Zehnder interferometric temperature sensor in the 50 °C – 650 °C range

In this work a two-mode fiber Mach Zehnder interferometer (TMF-MZI) temperature sensor in the 50 – 650° C range is successfully demonstrated. The sensor design incorporated a tapered fiber with dimensions of 20 mm/20 mm/20 mm for the down-taper, waist-length, and up-taper, respectively, while the waist-diameter was set to 30 μm. Around this taper dimensions, multiple geometric configurations were explored in this research, and it was found that devices with a spectral period around 20 nm held the most favorable spectral characteristics, avoiding the appearance of shorter spectral ripples at high temperatures. The sensor exhibited consistent and repeatable behavior, maintaining its spectral characteristics throughout multiple heating and cooling cycles. The sensor interference spectrum exhibits a consistent wavelength shift through 1350–1700 nm, such shift fits a linear function with an R-square value of ∼0.985 and a sensitivity of 70 pm/°C. The simple fabrication, low cost, and stability of the optical response make this sensor relevant for high-temperature measurements.

Temperature Demodulation for an Interferometric Fiber-Optic Sensor Based on Artificial Bee Colony–Long Short-Term Memory

Demodulation methods play a critical role in achieving high-performance interferometric fiber-optic temperature sensors. However, the conventional passive 3 × 3 coupler demodulation method overlooks certain issues, such as the non-1:1:1 splitting ratio of the coupler, resulting in a non-ideal phase difference in the three output interference signals. These problems significantly impact the measurement results of interferometric temperature sensors. In this paper, we propose a novel arc-tangent method based on a 3 × 3 coupler and a demodulation algorithm combining long short-term memory (LSTM) with an artificial bee colony (ABC). The arc-tangent method is employed to enhance the input phase signal of the ABC-LSTM network model and establish a nonlinear mapping between the phase signal and temperature, effectively preventing the influence of the spectral ratio and phase difference of the 3 × 3 coupler on temperature demodulation. The proposed ABC-LSTM method achieves high-resolution measurements with an interval of 0.10 °C, and the absolute error is below 0.0040 °C within the temperature range of 25.00–25.50 °C. To demonstrate the stability and adaptability of the proposed method under long-term constant temperature conditions, we conducted measurements for approximately three hours in a controlled temperature environment set at 25.00 °C. Experimental results indicate that the maximum error of LSTM-ABC method remains around 0.0040 °C, outperforming the conventional algorithm (0.0095 °C). Furthermore, when comparing the average error values of the conventional passive 3 × 3 coupler method (0.0023 °C), LSTM model (0.0019 °C), and ABC-LSTM model (0.0014 °C), it is evident that the demodulation results of the ABC-LSTM method exhibit the highest level of stability. Therefore, the ABC-LSTM method enhances the accuracy and reliability of interferometric fiber-optic temperature-sensing systems.

Ultra-high sensitivity temperature sensor based on PDMS-assisted-cascaded Mach–Zehnder Interferometer

In this paper, we propose and demonstrate an ultra-high sensitivity temperature sensor based on Polydimethylsiloxane (PDMS) assisted-cascaded Mach–Zehnder Interferometers (PAC-MZI). The sensor is first fabricated by periodically embedding four segments of hollow core fibers (HCF) in a single-mode fiber(SMF). Then, we drill holes into the sidewall of the HCF with a CO2 laser. Finally, the vacuum aspiration technique is used to introduce the PDMS into the HCF. In addition to serving as a beam splitter and combiner, the PDMS-filled HCF also guides multiple modes. The evolution of the transmission spectra during the fabrication process is thoroughly examined. The experimental temperature sensitivity of the sensor surpasses that of conventional interferometric temperature sensors by about 300 times to 9.53 nm/°C. Combining the advantages of easy packaging, compact structure, and high sensitivity, the proposed sensor has a wide range of potential applications in the field that require high-sensitivity temperature measurement.

Ultrahigh-Resolution and Large-Dynamic-Range Temperature Sensor Based on Fiber-Optic EFPI Cavity

A fiber-optic extrinsic Fabry–Perot interferometric (EFPI) temperature sensor with extremely high resolution and large dynamic range is proposed and demonstrated. The sensor uses a thermally expanded core (TEC) fiber as the first reflective surface of the interference cavity to focus the beam to improve the finesse of the interference fringes, which enables extremely high cavity length resolution and large dynamic range with a multivalley averaging method based on dynamic estimation of the fringe order. Using a borosilicate capillary as the temperature-sensitizing material, a cavity length sensitivity of 79.25 nm/°C as well as a good linearity were obtained in the temperature range of 10 °C–100 °C. A temperature resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.5\times 10^{-{5}}\,\,^{\circ }\text{C}$ </tex-math></inline-formula> was demonstrated. The extremely high-temperature resolution with a large dynamic range has an important application value in the monitoring of weak temperature changes in deep sea and underground fluids.

FPI and MZI combined high-sensitivity sensor based on PDMS microcavity

In this paper, an optic-fiber temperature sensor is proposed with an innovative structure design and sensing theory combination. The structure design is consisted of classic SMSMS structure and a polydimethylsiloxane (PDMS) microcavity, which is formed by inserting single mode fibers (SMF) and multimode fibers (MMF) from both ends of a hollow core fiber (HCF) filled with uncured PDMS. By adjusting the length (L) of the cavity into proper value, the reflection temperature sensitivity reaches up to −567.86 pm/℃, and the transmission sensitivity reaches −120.69 pm/℃. Due to the presence of the PDMS microcavity, the reflection spectrum achieved highly sensitive temperature Fabry-Perot interferometer (FPI) sensor. Moreover, a Mach-Zehnder interferometer (MZI) temperature sensor also formed by monitoring the transmission spectrum, and its temperature sensitivity is elevated about 7 and 1.2 times compared with the pure splicing SMSMS and the splicing + PDMS coating structure. By deploying the PDMS microcavity, FPI sensor and MZI sensor were combined inside one device with significant temperature sensing performance, and these two combined sensors can cross-correct within the intersected detection range. With simple manufacture steps, higher temperature sensitivity and bigger sensing range are achieved.

Investigation of Fabry Perot Interferometer Temperature Sensor Based on Hollow Core Anti-Resonance Optical Fiber

A Fabry-Perot interferometer (FPI) temperature sensor was proposed by splicing a self-made hollow core anti-resonance fiber (HC-ARF) between two single mode fibers (SMFs). The HC-ARF was 2 mm long and one SMF was 20 mm in length. The SMF end face was coated with an Au film. When temperature varied from 45 °C to 85 °C, a sensitivity of 81.1 pm/°C was obtained at the waveband of 1550 nm∼1555 nm. When temperature varied from 35 °C to 95 °C, a sensitivity of 138.8 pm/°C was obtained at the waveband of 1571 nm∼1581 nm. Theoretical analysis suggested that the sensitivity of this proposed sensor can be easily adjusted by varying the length of the Au-coated SMF via the Vernier effect, which was confirmed by a subsequent experiment where the SMF length was varied from 20 mm to 170 mm. This work offers a simple sensitivity control method, and the proposed sensor is easy to manufacture, compact in structure, and convenient in performance control, which can be applied for a wide range of biological and chemical applications.

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.

In-Line Interferometric Temperature Sensor Based on Dual-Core Fiber

In this paper, we proposed an in-fiber Mach-Zehnder temperature sensor based on a dual-core fiber (DCF) in which one core, working as the sensing arm, is suspended in an embedded fluidic channel filled with silicone oil, while the other one, working as the reference arm, locates eccentrically in the DCF. Temperature variations would change the refractive index of silicone oil infiltrated as well as the effective index of the guided mode in the suspended core, thus shifting the interference spectra. Both experiments and numerical simulations were carried out to characterize the sensor. The spectrum shifts measured experimentally agreed well with the theoretical results. Experimental sensitivity of the sensor using a DCF infiltrated with &#x007E;20 cm-long silicone oil was found to be as high as &#x2212;1.42 nm/&#x00B0;, comparable to those of the SPR fiber sensors and other interferometric sensors. The measuring range of the sensor was more than 120&#x00B0;. The proposed sensor could be easily fabricated with good robustness and stability, which makes the sensor promising for applications such as environment and architecture monitoring.

Dynamic Characterisation of Fibre-Optic Temperature Sensors for Physiological Monitoring.

Fast, miniature temperature sensors are required for various biomedical applications. Fibre-optics are particularly suited to minimally invasive procedures, and many types of fibre-optic temperature sensors have been demonstrated. In applications where rapidly varying temperatures are present, a fast and well-known response time is important; however, in many cases, the dynamic behaviour of the sensor is not well-known. In this article, we investigate the dynamic response of a polymer-based interferometric temperature sensor, using both an experimental technique employing optical heating with a pulsed laser, and a computational heat transfer model based on the finite element method. Our results show that the sensor has a time constant on the order of milliseconds and a −6 dB bandwidth of up to 178 Hz, indicating its suitability for applications such as flow measurement by thermal techniques, photothermal spectroscopy, and monitoring of thermal treatments.

Complex optical fiber sensor based on the Vernier effect for temperature sensing

The Vernier effect is an effective way of magnifying the sensitivity of the sensor for higher resolution sensing. The Fiber-optic cascaded Fabry-Perot Interferometric (CFPI) temperature sensor has been theoretically studied and experimentally investigated in this paper base on Vernier effect. It has been manufactured by a segment of pure silica hollow-core fiber (HCF) fused between two sections single-mode fibers (SMF), where the optical path length of the HCF is similar to that of the terminal SMF. The combination of a fiber Bragg grating (FBG) and a CFPI structure could be used not only obtain the amount of temperature variation but also know the specific value of the temperature. The temperature sensitivity of the sensor was 706.9 pm/°C, which is 76 times higher than that of single one fiber-optic cavity FP temperature sensor. The sensor has a wide range of potential applications such as high-precision temperature sensing and high-temperature monitoring.

Sagnac Interferometer Temperature Sensor Based on Microstructured Optical Fiber Filled with Glycerin

A Polarization-maintaining photonic crystal fiber (PMPCF) temperature sensor was designed and implemented based on Sagnac interference. The glycerin solution was used as the photosensitive material. According to the finite element method, it was found that the temperature sensor obtained the highest sensitivity when the PMPCF was filled with the glycerin solution with a mass fraction of 70%. In the experiment, with the temperature increasing from -25 ℃ to 85 ℃ with a step of 5 ℃, a linear fit was made between the dip wavelength and the temperature. The results have shown that the highest temperature sensitivity of 1.5005 nm/℃ was obtained for the proposed sensor within the temperature range of -25 ℃ to 85 ℃ which was wider than that of the previous work and implied the good potentiality for temperature measurement application.

All fiber-optic temperature sensor based on cladding etched no-core fiber coated with nanostructured copper oxide-polyvinyl alcohol thin film

The demonstration of an interferometric temperature sensor based on a no-core fiber (NCF) with a thin layer of copper oxide-polyvinyl alcohol (CuO-PVA) coating is presented in this paper. The sensing principle relies on the multimode interference (MMI) effect that occurs in a pure silica multimode fiber (MMF) sandwiched between two single-mode fibers (SMFs), where the MMF section is an NCF. To accomplish high sensitivity, the cladding of the NCF was chemically etched at a rate of ∼2.84 μm min−1 until it reached a diameter of 18 μm. Then a thin film of CuO-PVA was coated onto the etched region to act as the sensitive cladding layer, whose refractive index (RI) varies with the environmental temperature variation. The effective interaction between the CuO-PVA and the strong evanescent field of the etched NCF results in temperature sensing with a maximum resonant wavelength shift of 21.5 nm. The maximum sensor sensitivity obtained was 0.101 nm/°C with a linear coefficient of 99.1 % in the range of 25 °C to 235 °C. To the best of our knowledge, this is the first demonstration of a temperature sensor based on an etched NCF coated with CuO-PVA thin film. This work presents an important step to enhance the performance of an all optical temperature sensor in terms of sensitivity and repeatability.

ZnO composite graphene as sensing coating for interferometric temperature sensor

A simple and reliable temperature sensing method with high sensitivity is proposed. ZnO composite graphene sensitive materials with different shapes are successfully prepared by hydrothermal method and characterized by scanning electron microscpoy. The sensor is fabricated by coating the singlemode-taper multimode-singlemode with ZnO composite graphene. The effects of different shapes of ZnO composite graphene on the sensitivity of temperature sensors are further investigated. The electron migration rate of ZnO composite graphene is changed by temperature change. Furthermore, the surface refractive index of the optical microfiber changes. The results show that the sensor with floriated ZnO composite graphene exhibits a high sensitivity of 282.48 pm °C for temperature ranging from 30 °C to 48 °C, which provides a novel and convenient platform for temperature sensing.

High sensitivity micro-fiber Mach-Zehnder interferometric temperature sensors with a high index ring layer.

The influence of the high index ring layer (HIRL) in a tapered fiber Mach-Zehnder interferometer (MZI) on the interference observed, and thus on its potential applications in temperature sensing, has been investigated. The MZI was comprised of a tapered Ring Core Fiber (RCF), spliced between two single mode fibers (SMF). Since part of core mode from the SMF was converted into cladding modes in the RCF, due to the mismatch in the cores between the RCF and SMF, the residual power enters and then propagates along the center of the RCF (silica). The difference in phase between the radiation travelling along these different paths is separated by the HIRL to generate an interference effect. Compared with fiber interferometers based on core and cladding mode interference, the thin fiber HIRL is capable of separating the high order cladding modes and the silica core mode, under grazing incident conditions. Therefore, the optical path difference (OPD) and the sensitivity are both substantially improved over what is seen in conventional devices, showing their potential for interferometric temperature sensor applications. The optimum temperature sensitivity obtained was 186.6 pm/°C, which is ∼ 11.7 times higher than has been reported previously.

Highly sensitive PDMS-filled Fabry–Perot interferometer temperature sensor based on the Vernier effect

A highly sensitive temperature sensor is demonstrated experimentally, which is fabricated based on a Fabry-Perot interferometer (FPI) filled with polydimethylsiloxane (PDMS). The sensor's sensitivity is -0.653 nm/°C by utilizing the thermal expansion effect of PDMS, which has been greatly improved compared to that of the traditional FPI temperature sensor. Moreover, in order to further improve the sensitivity, a scheme where two parallel FPI structures are used to form the Vernier effect is proposed, which are a sensing FPI and reference FPI, respectively. Such a temperature sensor based on the FPI filled with PDMS and the Vernier effect exhibits a high temperature sensitivity of 17.758 nm/°C. Meanwhile, the proposed sensors show the advantages of high sensitivity, simplicity, and low cost.

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

A novel high-sensitivity Fabry-Perot (FP) interferometer temperature sensor based on liquid crystals (LCs) and the Vernier effect is proposed and demonstrated in this Letter. This sensor is prepared by inserting two cleaved single-mode fibers into a section of a capillary tube to generate an FP cavity. The cavity is filled with LCs, which induces a Vernier effect due to the birefringence of LCs. The refractive indices of the ordinary and extraordinary light have different responses to the temperature changes. As a result, the temperature sensitivity is significantly improved by detecting the peak shifts of a periodic envelope. The experimental results show that the proposed sensor can provide a high-temperature sensitivity of 19.55nm/°C. This sensor offers key features and advantages of the Fabry-Perot interferometer, including low cost and good fringe visibility. Furthermore, such a sensor probe can meet different requirements of temperature sensing in various application areas by using different kinds of LCs.

Investigation on the Thermo-Optic Coefficient of Silica Fiber Within a Wide Temperature Range

In this paper, the thermo-optic coefficient of silica fiber is analyzed and measured within a wide temperature range of 0-1200 °C for the first time. To obtain the thermo-optic coefficient, experiments were carried out by using fiber optic extrinsic Fabry-Perot interferometers (EFPIs) embedded in silica fiber. The optical cavity lengths of the EFPIs were measured with white-light interferometry. The experimental results show that the first and second orders of the thermo-optic coefficient are 1.090 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> /°C and 1.611 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> /°C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The quadratic model for the temperature dependence of the basic optical parameter, the refractive index, is rigorous within a wide temperature range. Furthermore, this paper also provides a temperature calibration method for extrinsic Fabry-Perot interferometric temperature sensors. The temperatures measured by the sensors are in excellent agreement with those measured by the reference thermocouple. Hence, this calibration method is promising for temperature sensors with a wide working temperature range.

Unambiguous Peak Recognition for a Silicon Fabry–Pérot Interferometric Temperature Sensor

Wavelength tracking method is widely used for fiber-optic sensors based on Fabry–Perot interferometers (FPIs). Although wavelength tracking provides high resolution, it encounters the well-known problem of direction ambiguity when the spectrum shifts by more than half of the free-spectral range. In this paper, we propose a new signal processing method that can unambiguously recognize the spectral peaks of a silicon FPI by obtaining their unique order values to overcome the direction ambiguity and achieve measurement over extended range. New elements of the proposed method include: 1) an average peak order calculated from multiple peak pairs is used to reduce variation of the calculated order number, and 2) a modified dispersion model of silicon is involved in the calculation of the peak order to suppress drift of the calculated peak order. The averaging method generally decreases the data processing rate, but does not affect the response time of the sensor. We experimentally demonstrate the feasibility of the proposed method using our recently developed fiber-optic temperature sensor using a 200-μm-thick silicon FPI. Results show that the measurement of temperature over 0 to 100 °C is realized, leading to an increase of more than five times in dynamic range.

Research progress of in-fiber Fabry-Perot interferometric temperature and pressure sensors

Optical fiber sensors based on Fabry-Perot interferometer (FPI) have attracted intensive attention for sensing applications in temperature and pressure measurement, owing to their compact, small size, fast responses, high resolution, high sensitivity, good stability, and resistance to electromagnetic interference. It's known that the in-fiber optic interferometers based on single-mode fibers can exhibit compact structures, easy fabrication and low cost. In this paper, firstly, the basic principle of in-fiber FPIs is introduced. Secondly, we review several kinds of typical in-fiber FPIs formed in single-mode fibers fabricated with different post-processing techniques, such as chemical etching, arc discharge, femtosecond laser micromachining, and polymer coating, etc. Finally, the optical sensors based on in-fiber FPIs, with a capability of simultaneous multi-parameter sensing of temperature and pressure, are summarized and prospected.

Interferometric temperature sensor based on a water-filled suspended-core fiber

Abstract The performance of water-filled suspended-core fibers as key components of interferometric temperature sensors has been analyzed. Use of water is motivated by simplicity of use and by its thermo-optic and thermal expansion coefficients, which provide a good balance between sensitivity and wide temperature range (limited by the free spectral range of the interferometer). Two suspended core fibers have been tested. With one of them, the sensitivity observed is slightly better than the ones of most interferometers based on liquid-filled photonic crystal fibers, with a linear temperature range of 25 °C. With regard to the other one, its most remarkable feature is its temperature range, not limited by any interferometric property but by water melting and boiling temperatures, as its quotient between free spectral range and sensitivity is higher than 100 °C.