High sensitivity temperature sensor based on fiber air-microbubble Fabry-Perot interferometer with PDMS-filled hollow-core fiber

Ultra-sensitive temperature sensor based on PDMS filled Fabry-Perot cavity and air-bubble Fabry-Perot cavity in parallel

A simple to fabricate, cost-effective, robust, and highly accurate fiber optic temperature sensor is proposed and experimentally demonstrated. The sensor head consists of an in-fiber Fabry-Perot interferometer (FFPI) embedded in a mold made of a polyester resin. FFPI, which in general present good strain sensitivity and low-temperature sensitivity, was assembled by splicing a microsegment of capillary fiber of 14.66 μm between two single-mode fibers. The reflected spectrum of the FFPI exhibited a single interference dip within a span of 80 nm. The dimensions of the resin mold can be modified by changing the ambient temperature, which in turn changes the length of the FFPI air-cavity. The dip of the reflected spectrum is shifted toward shorter (larger) wavelengths when external temperature is decreased (increased). The large free spectral range (FSR) allowed to monitor the temperature changes by either a wavelength or an intensity interrogation scheme. Using a laser and an optical power meter it was possible to demonstrate a real-time temperature sensor with a sensitivity of -60.79 nW/°C and a resolution of 0.5 °C, in the range of 10-30 °C. We believe that the practicality of this real-time sensor can be very attractive for environmental sensing application.

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.

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.

A high-sensitivity temperature sensor is demonstrated by coating a layer of polydimethylsiloxane (PDMS) on the surface of a silica toroidal microresonator on a silicon chip. Combining both the advantages of the WGM microcavity (with ultrahigh Q factor) and PDMS (with large thermal effect), the PDMS-coated microresonator is highly sensitive to the temperature change of the surroundings. We find that, when the PDMS layer becomes thicker, the fundamental WGM experiences a transition from red- to blue-shift with temperature increasing due to the negative thermal-optic coefficient of PDMS. The measured sensitivity (0.151 nm/K) is one order of magnitude higher than pure silica microcavity sensors. The ultra-high resolution of the temperature sensor is also analyzed to reach 10^-4 K. With both high sensitivity and resolution, the thermal sensor can be employed to monitor a slight temperature variation which cannot be realized with conventional temperature sensor. Its on-chip feature can also fulfill the demand for integration and miniaturization in optics.

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.

Simultaneous sensing of axial strain and temperature based on a sensitivity-selectable all-fiber Lyot-like filter

The optical fiber Fabry-Perot interferometer (FPI) has been widely investigated as a potential temperature sensor. To function as a temperature sensor, the cavity of the FPI is typically constructed from either silica fibers or polymers. The silica cavity FPIs can function at temperatures exceeding 1 000 °C. However, their temperature sensitivity is constrained by their relatively low thermal optical coefficients and thermal expansion of silica materials. Although the polymer cavity FPI exhibits high temperature sensitivity, its cavity is susceptible to deterioration in high-temperature environments. Here, to overcome this challenge and achieve high-sensitivity temperature sensing in a high-temperature environment, we propose a new type of temperature FPI sensor by inserting and sealing a section of Cr20Ni80 metal microwire inside a section of a silica hollow core fiber (HCF) spliced to a standard single-mode fiber (SMF). The FPIs exhibit a high degree of temperature sensitivity due to the high thermal expansion of the Cr20Ni80 metal microwire. Since the Cr20Ni80 metal has a high melting temperature of 1 400 °C, such FPIs can function in high-temperature environments. Moreover, the temperature sensitivity of this FPI can be modified without affecting its reflection spectrum by changing the length of the metallic microwire within the hollow core fiber. The experimental results indicate that the proposed FPIs exhibit temperature sensitivity of greater than −0.35 nm/°C within the temperature range of 50 °C to 440 °C. Our proposed metal microwire-based FPIs are economical, robust, simple to fabricate, and capable of functioning in high-temperature environments, rendering them appealing options for practical applications.

Channel dropping waveguide filters based on single and multiple resonators in silicon-on-insulator (SOI) technology are of great interest due to their compactness and high wavelength selectivity, which is a desirable feature for photonic modulators, detectors, and other optically integrated components in telecommunication systems, in particular for wavelength division multiplexing (WDM) systems. Particular advantage of these filters is that they are capable of producing relatively large free spectral range (FSR) as well as narrow 3-dB bandwidth of the filter resonances. Herein we report experimental results and discuss the possibility of designing mono-mode and (nearly) polarization independent SOI ring and racetrack resonators with the FSR in excess of 30 nm.

In this paper, a miniature Fabry-Perot temperature probe was designed by using polydimethylsiloxane (PDMS) to encapsulate a microfiber in one cut of hollow core fiber (HCF). The microfiber tip and a common single mode fiber (SMF) end were used as the two reflectors of the Fabry-Perot interferometer. The temperature sensing performance was experimentally demonstrated with a sensitivity of 11.86 nm/°C and an excellent linear fitting in the range of 43–50 °C. This high sensitivity depends on the large thermal-expansion coefficient of PDMS. This temperature sensor can operate no higher than 200 °C limiting by the physicochemical properties of PDMS. The low cost, fast fabrication process, compact structure and outstanding resolution of less than 10−4 °C enable it being as a promising candidate for exploring the temperature monitor or controller with ultra-high sensitivity and precision.

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.

We demonstrate a MEMS tunable silicon photonic ring resonator with a 20 µm radius and a 5 nm free spectral range (FSR) for wavelength selection in reconfigurable optical networks. The device shows a loaded Q of 12000, and 300 pm tuning range at a wavelength of 1544 nm.

Whispering gallery (WG) mode resonators are experiencing a surge in the application of biosensing, attributing to high speed, flexibility, low cost, multi-analyte detection and compatibility with extra optical elements. The decoration of localized surface plasmon nanostructures to a dielectric WG mode resonator offers unprecedented sensitivity compared with a pure dielectric resonator. However, at the moment, the experimental quality (Q) factors of dielectric micro rings always achieve the value of 104 to 106, whereas the only reported hybrid counterpart possesses the Q factor of 300 and most studies are focused on the theoretical design. The resulting experimental detection limits (DLs) are in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU, which is one order of magnitude lower than the best value of hybrid resonators. The hybrid resonator should possess the experimental Q factor of 5000 to 10000 so as to match their purely dielectric rivals. We demonstrated a novel hybrid plasmonic WG mode resonator working on the multimode resonance mechanism. Under the condition of the multimode resonance, predominant field was distributed on the waveguide surface resulting in an improved bulk sensitivity. The multimode resonance promoted satisfied resonance frequency reduced and thus a high Q factor and a low value of DL could be achieved in the experiment. Afterwards, the resonator was made via micro-and nanofabrication. It was shown that the experimental Q factor of 5685 and the free spectral range larger than 35 nm, which were superior to the previously reported value to our best knowledge. Hypoglycemia and Hyperglycemia will produce noticeable organ damage over time. Early detection of glucose concentration is crucial for clinical diagnosis. The proposed resonator was used for the detection of glucose solution. The experimental sensitivity was 428 nm/RIU and the DL was 1.31 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> RIU. The device with a footprint of 1500 μm × 12.7 μm, is able to integrate with other optical elements and developed as a commodity in the future.

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 novel photonic channelizer based on a series-coupled asymmetric double-ring resonator (SADRR) filter is proposed. Due to the vernier effect in series-coupled double-ring resonator, the novel filter has a small shape factor and relatively large free spectral range (FSR), which can contribute to obtaining high-precision signal. In the proposed channelizer, the RF signal is multicasted by an optical frequency comb (OFC) spectrally sliced by the SADRR filter and finally channelized by an optical wavelength-division multiplexer (WDM). A 5-channel channelizer centered at 1~9 GHz with a 2-GHz step is achieved by numerical simulation.