A dual-core photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor for measuring refractive index (RI) and temperature sensing and polarization filtering performance, is presented in this paper. The proposed sensor's cladding upper polished surface was coated with a gold film for refractive index sensing (RI) while lower polished surface with a composite film of silver and polydimethylsiloxane (PDMS) for temperature sensing. Proposed sensor was designed and simulated using finite element method (FEM) through commercially available Comsol Multiphysics software. Wavelength sensitivity of 18400 nm/RIU along with resolution of 5.43 x 10(-6) RIU and amplitude sensitivity of 150.74 RIU-1 were obtained for core 1 as RI sensing, while wavelength sensitivity of 12.0 nm degrees C-1 along with resolution of 8.33 x 10(-3) degrees C for core 2 as temperature sensing. It has a maximum loss of 30822 dB m(-1) towards Y-polarization which makes it ideal for polarization filtering, as Y has a higher loss than X-polarization. An extinction ratio (ER) of -92.31 dB at a transferring length of 3 mm was obtained showing its polarization filtering capabilities. Overall results indicate that our sensor is highly sensitive for both RI and temperature sensing and expected to perform better in polarization filtering performances. Its wide detection range in RI combined with temperature sensing increases its potential applications in the medical, chemical, environmental, and biosensor detection industry.

Optical fiber refractive index (RI) sensor with temperature (T) compensation has important application values in biomedicine and environmental monitoring. In order to solve the problem that the detection accuracy of liquid RI is affected by T in practical applications, a T-compensated RI fiber sensor based on cascaded Mach-Zehnder interference (MZI) and multimode interference (IMI) is proposed in this paper. Experimental results showed the proposed sensor achieved a sensitivity of 797.41 nm/RIU for the measurement of RI in the range of 1.3333-1.3640, and a sensitivity of 0.120 nm/degrees C for the measurement of T in the range of 0-60 degrees C. The resolution (R) of RI and T measurements reached 2.5 x 10(-5) RIU and 0.167 degrees C, respectively. The detection limits (DL) of RI and T measurements are 3.135 x 10(-8) RIU and 1.392 degrees C, respectively. A matrix is obtained to demodulate the cross-sensitivity in RI and T measurements. The proposed sensor shows the advantages of simple structure, low hysteresis, and good stability.

Accurate measurement of cryogenic temperature is crucial for various scientific and technological applications. In this paper, we experimentally investigated a fiber optic cryogenic temperature sensor based on a Sagnac interferometer (SI) with a panda polarization-maintaining fiber (PPMF). The PPMF used as the temperature sensing probe in SI enhanced the cryogenic temperature measurement sensitivity by two orders of magnitude compared to most of the published literature. The value of free spectral range (FSR) was used to label the dip wavelength as the movement of the spectrum exceeding an FSR in a large temperature variation scale. Experimental results demonstrated that the proposed sensor achieved an average measurement sensitivity of 1.8 nm/K in a wide temperature range of 170-320 K. Owing to high sensitivity, low hysteresis, and easy fabrication, our proposed sensor emerges as an ideal solution for cryogenic temperature detection.

Panda polarization-maintaining few-mode optical fiber (PPMFMOF) has important research significance in the short distance optical transmission field owing to its advantages of weak nonlinear effects, which is benefit to reduce the use of digital signal processing equipment. Designing a high-performance PPMFMOF quickly and efficiently is expected and yet challenging. In this article, we demonstrated a forward design method for the design of PPMFMOF based on artificial neural network (ANN) to solve the problems of inefficient and time-consuming PPMFMOF design in traditional design method. By studying the influence of different ANN models on the fiber performance, the approximate range of the optimal value was obtained in advance, then the minimum effective refractive index difference (Δneff,min) between adjacent LP modes was used as the optimization object, finally design of PPMFMOF supporting 10 LP modes in C + L band was successfully realized. This method provided low time-consuming, high-efficiency and high-accuracy for the fast design of PPMFMOF and the maximum mean absolute percentage error (MAPE) of the ANN model to predict the effective refractive index (neff) of 10 LP modes is only 3.2211 × 10-7. We believe that the proposed method could also be quickly and accurately applied to other functional optical fiber designs.

Long-period fiber grating (LPFG) is a promising curvature sensor due to its high bending sensitivity. In this article, a fiber ring laser (FRL)-based curvature sensor with an LPFG was proposed and experimentally verified. LPFG acted as both a filter and a curvature sensing element. The maximum measurement sensitivity of curvature reached 7.99 dB/m(-1) in a wide range of 4.89-13.33 m(-1) which was wider than most of previously reported curvature sensors. The bending and filtering characteristics of LPFG were verified by changing its position in FRL. The output wavelength of the FRL could be tuned through the bending of LPFG. The proposed curvature sensor, showing the advantages of a wide measurement range, high resolution, and high sensitivity, has potential applications in monitoring the deformation of surgical instruments, robot arms, and engineering objects.

In this article, vernier effect (VE), which is an effective method to enhance the measurement sensitivity of strain and temperature, was experimentally studied through cascading a Mach-Zehnder interferometer (MZI) with a Sagnac interferometer (SI). The MZI consisted of a tapered single-mode fiber (SMF), while the SI was formed using a panda-shaped polarization-maintained fiber (PMF). Free spectral ranges (FSRs) of the two interferometers were slightly different from each other, and as a result, the VE was successfully excited. The experimental results demonstrated that the measurement sensitivity of strain was improved from 26.7 to -110.92 pm/mu epsilon at the temperature of 24 degrees C, while the measurement sensitivity of temperature was improved from -2978 to 7934 pm/degrees C at the strain of 0 epsilon. The sensitivity was amplified about four times. The cross-sensitivity of temperature and strain measurements is 71.53 mu epsilon/degrees C. The proposed sensor exhibits in advantages of simple in construction, high sensitivity, and multiparameter measurements making it a superior contender for strain and temperature monitoring in engineering

Methane gas leakage can lead to pollution problems, such as rising ambient temperature. In this paper, the Vernier effect of a double D-shaped photonic crystal fiber (PCF) in a Sagnac interferometer (SI) is proposed for the accurate detection of mixed methane gas content in the gas. The optical fiber structure of the effective sensing in the sensing SI loop and the effective sensing in the reference SI loop are the same. Both of them adopt the polarization-maintaining photonic crystal fiber (PM-PCF) designed in this paper. The optical fiber structure of the effective sensing in the sensing SI loop deposited with the methane gas-sensitive film is polished to obtain a double-D structure. This operation makes it easier for methane gas to contact the sensitive film and realize the sensor’s repeated use. The sensing capability of the methane gas sensor was evaluated utilizing the finite element method (FEM). The numerical simulation results show that when the concentration of methane gas in the environment is 0~3.5%, the average sensitivity of two parallel Sagnac loops is 409.43 nm/%. Using Vernier effect cascade SI loops, the sensitivity of the sensor for detecting methane gas increased by four times. Without considering air and humidity, we provide a practical scheme for the development and design of high-sensitivity methane gas sensors.

A novel highly sensitive D-shaped photonic crystal fiber-based surface plasmon resonance (PCF-SPR) sensor for dual parameters of refractive index and temperature detecting is proposed. A PCF cladding polishing provides a D-shape design with a gold (Au) film coating for refractive index (RI) sensing (Core 1) and a composite film of silver (Ag) and polydimethylsiloxane (PDMS) for temperature sensing (Core 2). Comsol Multiphysics 5.5 is used to design and simulate the proposed sensor by the finite element method (FEM). The proposed sensor numerically provides results with maximum wavelength sensitivities (WSs) of 51,200 and 56,700 nm/RIU for Core 1 and 2 as RI sensing while amplitude sensitivities are −98.9 and −147.6 RIU−1 with spectral resolution of 1.95 × 10−6 and 1.76 × 10−6 RIU, respectively. Notably, wavelength sensitivity of 17.4 nm/°C is obtained between −20 and −10 °C with resolution of 5.74 × 10−3 °C for Core 2 as temperature sensing. This sensor can efficiently work in the analyte and temperature ranges of 1.33–1.43 RI and −20–100 °C. Due to its high sensitivity and wide detection ranges, both in T and RI sensing, it is a promising candidate for a variety of applications, including chemical, medical, and environmental detection.

The Vernier effect (VE) in optical interferometers has been used to improve the sensitivity in the measurements of strain, temperature, refractive index, etc. However, as the wavelength shifts beyond a free spectral range (FSR), it is difficult to determine the magnitude of the physical quantity and the measurement range is usually narrow. To overcome this problem, we constructed a fiber optic sensor combining two parallel Sagnac interferometers (PSIs) with a fiber Bragg grating (FBG). The sensitivity of temperature and strain measurements were enhanced about 5 times due to the VE effect in PSIs. FBG spectrum, which showed a monotonic and linear variation trend along with the variation of strain and temperature, was used to estimate the magnitude of the physical quantity beyond the FSR. Finally, an average strain sensitivity of -104.4 pm/mu epsilon was obtained in a wide measuring range of 0-5440 mu epsilon, while an average temperature sensitivity of 6.12 nm/degrees C was obtained in the temperature range of 2-58 degrees C. The measurement range has been extended 6 times and 4 times respectively compared to a single FSR. The integration of PSIs and FBG sensors offers not only high sensitivity, but also a wide measuring range. It can be used in many areas including medical equipment, automotive, and aerospace to make accurate strain and temperature measurements.

Optical fiber-based sensors, which show the merits of high sensitivity, real-time, and rapid detection, have important research value in the detection of volatile organic compounds (VOCs). In this paper, a novel ethanol gas sensor was experimentally investigated based on the cascaded optical fiber-based Sagnac interferometer and Mach-Zehnder interferometer. The Sagnac interferometer was constructed with an anisotropic nematic liquid crystal (NLC) film which acted as the reactor with ethanol gas. A tapered single-mode fiber (SMF) was used to build the Mach-Zehnder interferometer (MZI). The two interferometers were parallelly and tandemly cascaded respectively to achieve the magnification of measurement sensitivity. Experimental results showed that the sensitivities were 8.561 pm/ppm with a magnification factor of 3.76 for the parallel cascaded structure and 7.924 pm/ppm with a magnification factor of 3.48 for the tandemly cascaded structure. The designed optical fiber-based ethanol gas sensor, showing the advantages of compact structure, high sensitivity, small size, and low cost, could be a competitive candidate for the detection of ethanol gas leakage.

In this study, we investigated a novel ethanol gas sensor based on anisotropic nematic liquid crystal (NLC) film-embedded cascade Sagnac interferometers (SIs). NLC film was employed as both the sensing material of ethanol gas and the generator of birefringence in an SI, which acted as the sensing arm. A section of polarization-maintaining fiber (PMF) was utilized to construct another SI, which worked as the reference arm. The sensing arm and reference arm were cascaded together to achieve the amplification of measurement sensitivity in the patterns of parallel and tandem, respectively. Experimental results demonstrated that the parallelly cascade SIs showed a sensitivity of 12.512 pm/ppm with a magnification factor of 5.779, while the tandemly cascade SIs showed a sensitivity of 11.748 pm/ppm with a magnification factor of 5.445. The proposed ethanol gas sensor exhibited the advantages of compact structure, high sensitivity, small size, and low cost could present a secure and advantageous approach for the leakage detection of ethanol gas.

In this paper, a fiber optic glucose concentration sensor based on cascaded Mach-Zehnder interferometers (CMZIs) was proposed and experimentally verified. Two single-mode fibers (SMFs) with slightly different tapering lengths and as well free spectral ranges (FSRs) were obtained through the melt-drawn cones technique and then were cascaded together to achieve the Vernier effect (VE). Experimental results showed that the measurement sensitivity of glucose concentration based on a single tapered SMF was 0.0494 nm (mmol l-1)-1. Furthermore, the measurement sensitivity based on the cascaded tapered SMFs was enhanced by 4.86 times to an ultra-high level of 0.2402 nm (mmol l-l)-1. The resolution reached 0.083 mmol l-1. The refractive index sensitivity reached 9066 nm/RIU. The resolution reached 2.21 x 10-6 RIU. In addition, the proposed glucose concentration sensor based on CMZIs shows good hysteresis. In conclusion, the designed sensor, showing the merits of easy to fabricate, high sensitivity, and high detection resolution, could be used to monitor the tiny variation in glucose concentration.

In this paper, high-order LP modes based Sagnac interference for temperature sensing are proposed and investigated theoretically. Based on the specific high-order LP modes excited through the mode selective couplers (MSCs), we design a stress-induced Panda-type few-mode fiber (FMF) supporting 4 LP modes and construct a Sagnac interferometer to achieve a highly sensitive temperature sensor. The performances of different LP modes (LP01, LP11, LP21, and LP02) are explored under a single Sagnac interferometer and paralleled Sagnac interferometers, respectively. LP21 mode has the highest temperature sensitivity. Compared with fundamental mode (LP01), the temperature sensitivity based on LP21 mode improved by 18.2% at least. In addition, a way to achieve the enhanced optical Vernier effect is proposed. It should be noted that two Sagnac loops are located in two temperature boxes of opposite variation trends, respectively. Both two Sagnac interferometers act as the sensing element, which is different from the traditional optical Vernier effect. The temperature sensitivity of novel enhanced optical Vernier effect is magnified by 8 times, which is larger than 5 times the traditional Vernier effect. The novel approach avoids measurement errors and improves the stability of the sensing system. The focus of this research is on high-order mode interference, which has important guiding significance for the development of highly sensitive Sagnac sensors.

This paper presents a dual-core two-parameter optical fiber sensor based on the surface plasmonic resonance (SPR) effect. It is analyzed by the finite element method. The proposed sensor is a dual-channel structure designed with photonic crystal fiber (PCF) as the base material: one channel is coated with a gold film to measure the refractive index (RI) of the solution to be measured, and the other channel is coated with a gold film and polydimethylsiloxane to measure the temperature of the solution to be measured. The exposed microslot structure on both sides reduces the complexity of sensing measurements. The results show that the maximum RI sensitivity of the sensor is 19,900 nm/RIU, and the maximum sensitivity to temperature is 8.7 nm/℃. This work is conducive to realizing a PCF sensor with high sensitivity, large measurement range, real-time monitoring, and easy preparation. As a result, the sensor is expected to be widely used in fields such as biology and chemicals.

The temperature variation is one of the main error factors in the process of glucose and sodium chloride solution concentrations detection based on optical fiber sensor. In this paper, we proposed a sensor with temperature compensation, which can detect the temperature while measuring the concentrations of glucose or sodium chloride solution. The dual-channel design enables the sensor to obtain accurate measurement results of glucose and sodium chloride concentrations, and also to monitor the environmental temperature simultaneously. The deposition of zinc oxide film not only enhances the sensitivity of the sensor but also prevents oxidation of the silver film. The high stability of polydimethylsiloxane (PDMS) ensures that the two sensing channels of the sensor do not interfere with each other. The proposed dual-channel sensor offers advantages of low cost, a simple structure, and high sensitivity, with potential applications in the fields of food science and biochemistry.

Temperature greatly influences the activity of biological materials; therefore, it is necessary to simultaneously monitor the temperature and concentration of biological materials. Long-range surface plasmon resonance (LRSPR) sensor has high sensitivity and can detect biomolecules. In this article, we proposed a dual-channel LRSPR sensor that can measure both bovine serum albumin (BSA) concentration and ambient temperature. The proposed sensor used a no-core fiber (NCF) with two channels. One channel was used to deposit magnesium fluoride and silver films on NCF for BSA detection, while the other channel was used to deposit three layers of films [magnesium fluoride, silver, and polydimethylsiloxane (PDMS)] on NCF for temperature detection. The sensitivity of the sensor to detect BSA concentration and temperature is 1.02 nm/(mg/ml) and −2.82 nm/°C, respectively. Cross-sensitivity between the two parameters is solved using a matrix. The sensor offers the advantages of simultaneously monitoring two parameters, having a simple structure and possessing high sensitivity, which makes it promising for a broad range of applications in monitoring BSA concentration and temperature.

Utilization of the vernier effect in fiber optic interferometers represents the latest approach to enhance the sensitivity and resolution in fiber optic sensors. In this paper, the Vernier effect in Sagnac interference (SI) with a double D-shaped photonic crystal fiber (PCF) was proposed for the methane gas sensing. The sensing arm in the SI was composed of a double D-shaped PCF which was coated with cryptophane-A film, while the polarization maintaining photonic crystal fiber (PM-PCF) is used as the reference arm. The sensing arm and the reference arm were cascaded in parallel to generate the Vernier effect. The influence of the structural parameters in the PCF on the sensing performance was analyzed using the finite element method. The sensing fiber uses a double-D structure, so methane gas is more easily in contact with the sensitive film. According to the simulation results, the wavelength sensitivity of the parallel Sagnac loops within the methane gas concentration range of 0-3.5% is as high as 170.58 nm/%, which is 3.14 times that of a single Sagnac loop. The proposed sensor exhibits a significant enhancement in sensitivity compared to the other fiber optic-based methane gas sensors, thereby providing a reference sample for the development of high-sensitivity methane gas sensors.

A wide measurement range plasmonic refractive index (RI) sensor based on side-polished photonic crystal fiber (PCF) was proposed and studied in this paper. A composite film containing an Au layer and a TiO 2 layer was coated on the polished surface of PCF to excite and modulate the surface plasmon resonance (SPR) effect. The sensing characteristics were analyzed using the finite element method (FEM). Numerical results showed that the sensing performances were significantly improved through introducing the unique optoelectronic material of TiO 2 film. The designed RI sensor showed an ultra-wide measurement range of 1.20–1.38 with a highest sensitivity of 14600 nm RIU −1 . The best resolution (R) and figure of merit (FOM) reached 6.85 × 10 −6 RIU and 224.62 RIU −1 , respectively. The designed side-polished PCF which is coated with an optimized TiO 2 /Au composite membranes has a promising performance in the field of RI sensing requiring high sensitivity and wide range monitoring.