Temperature sensor based on an isopropanol-sealed photonic crystal fiber in-line interferometer with enhanced refractive index sensitivity

High anti-interference dual-parameter sensor using an EIT-like effect photonic crystal cavity coupled system

We propose a sensor with high anti-interference ability using a photonic crystal cavity coupled system for simultaneous sensing of the refractive index (RI) and temperature (T) based on an electromagnetically induced transparency-like effect. A transparent window is achieved in the transmission spectrum through destructive interference between the air mode resonance and dielectric mode resonance in two one-dimensional photonic crystal structures. The T-sensitive material (SU-8) is used in the coupled system, promoting sensitivity and anti-interference ability. The capability of the system to simultaneously detect a small range of RI and T is demonstrated using three-dimensional finite-difference time-domain simulations and the fitting process. The RI sensitivities for the air and dielectric modes were 215 nm/refractive index unit (RIU) and 0 nm/RIU, respectively. The T sensitivities for the air and dielectric modes were 19 pm/K and -83pm/K, respectively. The sensor resists external interference, enabling it to resist the error caused by readings. The footprint of the sensor is 29×1.8µm² (length×width), contributing to future optical on-chip integration sensor design.

Highly sensitive temperature sensing probes based on liquid cladding elliptical micro/nanofibers

A highly sensitive temperature probe based on a liquid cladding elliptical micro/nanofiber is proposed, which exploits a fiber loop mirror with an output port probe for remote and highly-sensitive measurements based on evanescent field coupling. The thermo-optical effective liquid cladding avoids the influence of other environmental parameters (except for temperature), while protecting the micro/nanofibers from external disturbance and contamination. This renders the sensing probe only sensitive to temperature changes, making it suitable for real-world temperature measurements. An isopropanol cladding elliptical microfiber with a diameter of 3.4 µm demonstrated a sensitivity of -16.38 nm/°C for a remote temperature measurement.

Surface plasmon resonance temperature sensor based on a photonic crystal fiber filled with silver nanowires

A surface plasmon resonance (SPR) temperature sensor based on a photonic crystal fiber (PCF) filled with silver nanowires is proposed in this paper. We inject ethanol solution filled with silver nanowires into the grapefruit PCF to realize temperature sensing. The sensitivity of the sensor can reach -433pm/^∘C by numerical simulation, and the experimental result is -160pm/^∘C. Simulations and experiments show that the wavelength of the resonance peak will blueshift with the invalidity of silver nanowires, and the resonance effect of the sensor will weaken. It can provide reference for the realization and application of other SPR sensors based on PCF.

A Temperature Plasmonic Sensor Based on a Side Opening Hollow Fiber Filled with High Refractive Index Sensing Medium

A surface plasmon resonance temperature sensor based on a side opening hollow-core microstructured optical fiber is proposed in this paper. This design employs a gold nanowire to excite the plasmon mode, and can be easily filled with the sensing medium through the side opening of the fiber, which not only simplifies the fabrication of the sensor but can also use the high refractive index sensing medium. The coupling characteristics, sensing performance and fabrication tolerance of the sensor are analyzed by using the finite element method. The simulation results indicate that the maximum sensitivity is 3.21 nm/°C for the x-polarized core mode in the temperature range of 13.27–50.99 °C, and 4.98 nm/°C for the y-polarized core mode in the temperature range of 14.55–51.19 °C, when benzene is used as the sensing medium. The sensor also shows a good stability in the range of ±10% fabrication tolerance.

Infiltrated Photonic Crystal Fibers for Sensing Applications

Photonic crystal fibers (PCFs) are a special class of optical fibers with a periodic arrangement of microstructured holes located in the fiber's cladding. Light confinement is achieved by means of either index-guiding, or the photonic bandgap effect in a low-index core. Ever since PCFs were first demonstrated in 1995, their special characteristics, such as potentially high birefringence, very small or high nonlinearity, low propagation losses, and controllable dispersion parameters, have rendered them unique for many applications, such as sensors, high-power pulse transmission, and biomedical studies. When the holes of PCFs are filled with solids, liquids or gases, unprecedented opportunities for applications emerge. These include, but are not limited in, supercontinuum generation, propulsion of atoms through a hollow fiber core, fiber-loaded Bose⁻Einstein condensates, as well as enhanced sensing and measurement devices. For this reason, infiltrated PCF have been the focus of intensive research in recent years. In this review, the fundamentals and fabrication of PCF infiltrated with different materials are discussed. In addition, potential applications of infiltrated PCF sensors are reviewed, identifying the challenges and limitations to scale up and commercialize this novel technology.

Ultrasensitive and Multifunction Plasmonic Temperature Sensor with Ethanol-Sealed Asymmetric Ellipse Resonators

In order to improve the low temperature sensitivity of conventional sensors, a plasmonic multifunction temperature sensor with high sensitivity is proposed and investigated systematically in this paper. The sensor consists of two metal layers and two ethanol-sealed elliptical resonators connected to a straight waveguide by two rectangular tubes. We numerically analyzed the transmission characteristics of the Nano-device to assess its performance with the finite element method and achieved great optical properties. The results show that an obvious blue shift of the transmission spectrum appears by varying temperatures, exhibiting a great sensing effect. Sensitivity of the sensor reaches -3.64 nm/°C, far greater than conventional temperature sensors. Our research also demonstrates that the transmission spectrum could be modulated efficiently by the ratio of semi-short axis to semi-major axis of the ellipse resonators and the width of two same rectangular tubes. Furthermore, the Nano-device has a filtering characteristic. The transmittances of pass-band and stop-band are 96.1% and 0.1%, respectively. The results of this study can pave the way for low-cost sensing application in high-density photonic circuits and biosensors.

Real-time compensation of errors in refractive index shift measurements of microring sensors using thermo-optic coefficients

We report a method for compensation of errors caused by temperature fluctuations in refractive index measurements using Silicon photonic microring sensors. The method involves determination of resonance wavelength shifts caused by thermal fluctuations using real-time measurement of on-chip temperature variations and thermo-optic coefficient (TOC) of analyte liquids. Resistive metal lines patterned around Silicon microrings are used to track temperature variations and TOC of analyte is calculated by measuring wavelength shifts caused by controlled increments in device temperature. The TOC of de-ionized water is determined to be -1.12 × 10^-4/°C, with an accuracy of ±8.26 × 10^-6/°C. In our system, chip-surface temperature variations were measured with an instrument limited precision of 0.004 °C yielding a factor of 16 enhancement in tracking accuracy compared to conventional, bottom-of-chip temperature measurement. We show that refractive index detection limit of the microring sensor is also improved by the same factor.

Thermo-optic tuning of a packaged whispering gallery mode resonator filled with nematic liquid crystal

Thermo-optic tuning of whispering gallery modes (WGMs) in a nematic liquid crystal-filled thin-walled capillary tube resonator is reported. WGMs were excited by the evanescent field from a tapered optical fiber. Tapered optical fiber fabrication and reduction of wall thickness of the capillary tube was carried out by a ceramic micro-heater brushing technique. A simple and robust packaging technique is demonstrated to ensure stable and repeatable operation of the device. Tunability of WGMs with temperature was demonstrated with a sensitivity of 267.5 ± 2.5 pm/°C. The demonstrated thermo-optic method for WGMs tuning is potentially useful for many tunable photonic devices and sensors.

Design and fabrication of a heterostructured cladding solid-core photonic bandgap fiber for construction of Mach-Zehnder interferometer and high sensitive curvature sensor

A heterostructured cladding solid-core photonic bandgap fiber (HCSC-PBGF) is designed and fabricated which supports strong core mode and cladding mode transmission in a wide bandgap. An in-line Mach-Zehnder interferometer (MZI) curvature sensor is constructed by splicing single mode fibers at both ends of a HCSC-PBGF. Theoretical analysis of this heterostructured cladding design has been implemented, and the simulation results are consistent with experiment results. Benefiting from the heterostructured cladding design, an enhanced curvature sensing sensitivity of 24.3 nm/m^-1 in the range of 0-1.75 m^-1 and a high quality interference spectrum with 20 dB fringe visibility are achieved. In order to eliminate the interference of longitudinal strain and transverse torsion on the result of the curvature sensing experiment, we measure the longitudinal strain and transverse torsion sensing properties of HCSC-PBGF, and the results show that the impact is negligible. It is obvious that this high-sensitivity and cost-effective all fiber sensor with a compact structure will have a promising application in fiber sensing.

Highly sensitive temperature sensor based on an ultra-compact Mach-Zehnder interferometer with side-opened channels

We demonstrated an ultra-compact and highly sensitive temperature sensor by using an in-fiber Mach-Zehnder interferometer (MZI) with side-opened channels. The MZI was constructed by off-center splicing a thin piece of microstructured optical fiber (MOF) between two single-mode fibers. Then, two side-opened channels were drilled through the MOF along the transverse direction by using a femtosecond laser to let the liquid with a high thermo-optic coefficient enter into the air cavity of the MZI. Due to the large effective refractive index (RI) difference between the core mode and the cavity mode excited by the offset splicing point, the scale of the MOF-based MZI can be reduced to several hundred micrometers. Experimental results show that the MOF-based MZI infiltrated with isopropanol has a temperature sensitivity up to 3.8 nm/°C with small strain cross-sensitivity of 0.0001 °C/μϵ in the temperature range of 22°C-34°C, which makes it a competitive fiber sensor in highly sensitive temperature sensing applications, including healthcare and consumer electronics.

Analysis of Hollow Fiber Temperature Sensor Filled with Graphene-Ag Composite Nanowire and Liquid

A hollow fiber temperature sensor filled with graphene-Ag composite nanowire and liquid is presented and numerically characterized. The coupling properties and sensing performances are analyzed by finite element method (FEM) using both wavelength and amplitude interrogations. Due to the asymmetrical surface plasmon resonance sensing (SPR) region, the designed sensor exhibits strong birefringence, supporting two separate resonance peaks in orthogonal polarizations. Results show that x-polarized resonance peak can provide much better signal to noise ratio (SNR), wavelength and amplitude sensitivities than y-polarized, which is more suitable for tempertature detecting. The graphene-Ag composite nanowire filled into the hollow fiber core can not only solve the oxidation problem but also avoid the metal coating. A wide temperature range from 22 ∘C to 47 ∘C with steps of 5 ∘C is calculated and the temperature sensitivities we obtained are 9.44 nm/ ∘C for x-polarized and 5.33 nm/ ∘C for y-polarized, much higher than other sensors of the same type.

Photonic crystal fiber modal interferometer based on thin-core-fiber mode exciter

A thin-core-fiber excited photonic crystal fiber modal interferometer has been proposed and experimentally demonstrated. By employing a thin-core fiber as the mode exciter, both of the core and cladding modes propagate in the photonic crystal fiber and interfere with each other. The experimental results show that the transmission dips corresponding to different-order modes have various strain responses with opposite shift directions. The strain sensitivity could be improved to 58.57  pm/με for the applied strain from 0 to 491 με by utilizing the wavelength interval between the dips with opposite shift directions. Moreover, due to the pure silica property of the employed photonic crystal fiber, the proposed fiber modal interferometer exhibits a low-temperature sensitivity of about 0.56  pm/°C within a temperature range from 26.4°C (room temperature) to 70°C. Additionally, the proposed fiber modal interferometer has several advantages, such as good stability, compact structure, and simple fabrication. Therefore, it is more applicable for strain measurement with reducing temperature cross-sensitivity.

Nano-displacement sensor based on photonic crystal fiber modal interferometer

A stable nano-displacement sensor based on large mode area photonic crystal fiber (PCF) modal interferometer is presented. The compact setup requires simple splicing of a small piece of PCF with a single mode fiber (SMF). The excitation and recombination of modes is carried out in a single splice. The use of a reflecting target creates an extra cavity that discretizes the interference pattern of the mode interferometer, boosting the displacement resolution to nanometer level. The proposed modal interferometric based displacement sensor is highly stable and shows sensitivity of 32  pm/nm.

Surface plasmon resonance temperature sensor based on photonic crystal fibers randomly filled with silver nanowires

We propose a temperature sensor design based on surface plasmon resonances (SPRs) supported by filling the holes of a six-hole photonic crystal fiber (PCF) with a silver nanowire. A liquid mixture (ethanol and chloroform) with a large thermo-optic coefficient is filled into the PCF holes as sensing medium. The filled silver nanowires can support resonance peaks and the peak will shift when temperature variations induce changes in the refractive indices of the mixture. By measuring the peak shift, the temperature change can be detected. The resonance peak is extremely sensitive to temperature because the refractive index of the filled mixture is close to that of the PCF material. Our numerical results indicate that a temperature sensitivity as high as 4 nm/K can be achieved and that the most sensitive range of the sensor can be tuned by changing the volume ratios of ethanol and chloroform. Moreover, the maximal sensitivity is relatively stable with random filled nanowires, which will be very convenient for the sensor fabrication.

Liquid seal for temperature sensing with fiber-optic refractometers

Liquid sealing is an effective method to convert a fiber-optic refractometer into a simple and highly sensitive temperature sensor. A refractometer based on the thin-core fiber modal interferometer is sealed in a capillary tube filled with Cargille oil. Due to the thermo-optic effect of the sealing liquid, the high refractive-index sensitivity refractometer is subsequently sensitive to the ambient temperature. It is found that the liquid-sealed sensor produces a highest sensitivity of −2.30 nm/°C, which is over 250 times higher than its intrinsic sensitivity before sealing and significantly higher than that of a grating-based fiber sensors. The sensing mechanisms, including the incidental temperature-induced strain effect, are analyzed in detail both theoretically and experimentally. The liquid sealing technique is easy and low cost, and makes the sensor robust and insensitive to the surrounding refractive index. It can be applied to other fiber-optic refractometers for temperature sensing.

Semi-open cavity in-fiber Mach-Zehnder interferometer for temperature measurement with ultra-high sensitivity

We demonstrate a semi-open cavity in-fiber Mach-Zehnder interferometer based on optical fiber tube (OFT) for temperature measurement with high sensitivity. The interferometer is composed of an OFT sandwiched between two multimode fibers, with lateral offset. The air hole of the OFT was not completely sealed and liquid is poured into the air hole through the unsealed gap. Light from the multimode fiber is split into two beams: one beam transmits directly through the silica tube while the other travels along the liquid-filled cavity. The device has ultra-high temperature sensitivity due to the much larger thermo-optic coefficient of the liquid compared with that of silica. Experimental results show that the temperature sensitivity is 6.35 nm/°C for an ethanol-filled structure.

Intermodal interferometer based on a fluid-filled two-mode photonic crystal fiber for sensing applications

A fluid-filled two-mode photonic crystal fiber (PCF)-based intermodal interferometer and its sensing characteristics are demonstrated and investigated. The interferometer works from the interference between LP(01) and LP(11) core modes of the fluid-filled PCF. Solutions to enhance the temperature sensitivity of the interferometer are also discussed. Via choosing a higher fluid-filled length ratio of PCF, a sensitivity of more than -340 pm/°C at 1480 nm is achieved, which is the highest value for a PCF intermodal interferometer-based sensor, to our best knowledge. Furthermore, there exist significant differences in temperature and strain sensitivity for two different interference dips, thus the interferometer can be used as a dual-parameter sensor with a compact structure through matrix demodulation.

Ultrasensitive temperature sensor based on an isopropanol-sealed optical microfiber taper

We demonstrate an ultrasensitive temperature sensor based on an isopropanol-sealed optical microfiber taper (OMT) in a capillary. The OMT is highly sensitive to ambient refractive index (RI) with a maximum sensitivity of 18989 nm/RI unit in the range of 1.3955-1.4008. The thermo-optic effect of isopropanol and the thermal expansions of the sealant and sealed liquid turn the OMT into an ultrasensitive temperature sensor with the maximum sensitivity of -3.88 nm/°C in the range of 20°C-50°C. The temperature sensitivity contributions from different mechanisms are also investigated theoretically and experimentally.

Temperature sensing using the bandgap-like effect in a selectively liquid-filled photonic crystal fiber

A compact temperature sensor based on a selectively liquid-filled photonic crystal fiber (PCF) is proposed using controlled hole collapse in PCF post-processing. The first ring around the core is filled with liquid of higher refractive index than the matrix, while the outer rings of holes are filled with air. The bandgap (BG)-like effect of the high refractive index ring is analyzed. Absorption loss spectra of the fiber are found to be quite sensitive to the refractive index of liquid when the liquid is lossy. Using the BG-like effect, a fiber temperature sensor is fabricated by selectively injecting a mixture of dimethyl sulfoxide and aqueous gold colloids with a high thermo-optic coefficient to the PCF. Temperature sensitivity up to -5.5 nm/°C is experimentally confirmed.

Phenomenon in an alcohol not full-filled temperature sensor based on an optical fiber Sagnac interferometer

An alcohol not full-filled high-birefringence photonic crystal fiber (HiBi-PCF) temperature sensor based on an optical fiber Sagnac interferometer (OFSI) is demonstrated and investigated in detail. A new phenomenon that the resonant dip wavelengths of the temperature sensor blueshift with temperature increasing is observed, which is contrary to that of the previously reported alcohol filled HiBi-PCF OFSI temperature sensor. By considering the influences of the group birefringence and the thermo expansion of alcohol, this phenomenon is explained very well. The temperature sensitivity of the proposed sensor is about -1.17 nm/°C and is only one-sixth of that of the alcohol full-filled HiBi-PCF OSFI.

Temperature sensor based on surface plasmon resonance within selectively coated photonic crystal fiber

We demonstrate a temperature sensor based on surface plasmon resonances supported by photonic crystal fibers (PCFs). Within the PCF, to enhance the sensitivity of the sensor, the air holes of the second layer are filled with a large thermo-optic coefficient liquid and some of those air holes are selectively coated with metal. Temperature variations will induce changes of coupling efficiencies between the fundamental core mode and the plasmonic mode, thus leading to different loss spectra that will be recorded. In this paper, variations of the dielectric constants of all components, including the metal, the filled liquid, and the fused silica, are considered. We conduct numerical calculations to analyze the mode profile and evaluate the power loss, demonstrating a temperature sensitivity as high as 720 pm/°C.