Highly sensitive temperature sensor based on a polymer-infiltrated Mach-Zehnder interferometer created in graded index fiber

Transmissive fluorescent temperature sensor based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber for real-time thermal monitoring of motors using the FIR technique

In this paper, we fabricate a transmissive fluorescent temperature sensor (TFTS) that based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber, which has the advantages of compactness and simplicity, corrosion resistance, high stability and anti-electromagnetic interference. The doping of Mo6+ ions will enhance the up-conversion (UC) fluorescence emission efficiency of Er3+ ions, thus improving the signal-to-noise ratio of TFTS. Using the fluorescence intensity ratio (FIR) technique, the real-time thermal monitoring performance of TFTS is evaluated experimentally. Apart from good stability, its maximum relative sensitivity is 0.01068 K-1 at 274 K in the measured temperature range. In addition, it is successfully used to monitor the temperature variation of the stator core and stator winding of the motor in actual operation. The results show that the maximum error between the FIR-demodulated temperature and the reference temperature is less than 1.2 K, which fully confirms the effectiveness of the TFTS for temperature monitoring. Finally, the FIR-based TFTS in this work is expected to provide a new solution for accurate and real-time thermal monitoring of motors and the like.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Performance-enhanced fiber optic humidity sensors based on SiO2/porous PMMA coatings

Employing functional and structured thin films on fiber optic sensors has tremendously improved capabilities in humidity sensing applications. In this paper, we demonstrate fabrication of a fiber optic evanescent wave humidity sensor based on S i O ₂/porous polymethyl methacrylate (PMMA) thin films. With the exposure to moisture, S i O ₂/porous PMMA thin films absorb water molecules. The refractive index and absorption coefficient of thin films change with ambient humidity, resulting in modulation of the light intensity transmitted in fiber. A good linearity is determined between the logarithm of output light intensity and relative humidity (RH). An optimal average sensitivity of 188.3 lux/%RH is achieved with an increase of 11.7 fold in the RH range of 5% to 95%. The response and recovery times are 8 s and 23 s, respectively. Furthermore, the sensor exhibits low hysteresis, and excellent stability and repeatability.

Stability of silicon resonator temperature sensors with the Pound-Drever-Hall technique

In this paper, we research the temperature stability of silicon-based ring resonator thermometers utilizing the Pound-Drever-Hall (PDH) technique. A slight temperature fluctuation of 12.2 mK in 200 s was experimentally detected by immersing the sensor in the triple point of water (TPW) system with ultrahigh precision. Additionally, factors that affect temperature stability, including fundamental thermal noise, laser frequency drift, and power fluctuation were analyzed and calculated theoretically. This shows high consistency with experimental results. Moreover, it is proved that the laser drift can be suppressed from 11.3 pm to 0.013 pm with the developed experimental system based on the PDH technique. The silicon-based ring resonator as a potential platform for precise temperature monitoring is proved based on this work.

Probe type TFBG-excited SPR fiber sensor for simultaneous measurement of multiple ocean parameters assisted by CFBG

The tilted fiber Bragg grating(TFBG), chirped fiber Bragg grating(CFBG), Vernier effect and metal surface plasmon resonance(SPR) effect are effectively combined to form a probe type fiber sensor for simultaneous measurement of seawater salinity, temperature and depth(STD). The SPR effect excited by the TFBG is achieved by covering a gold layer around the TFBG, which is used to measure the refractive index (RI) of seawater. The core mode of TFBG is used to detect the change of seawater temperature and the measurement of TFBG reflection spectrum is realized by inscribing a CFBG after the TFBG, which makes the sensor have a probe type design and more beneficial to practical applications. The fusion of quartz micro-spheres on the end face of the sensing fiber and the parallel connection of an Fabry Perot(F-P) interference cavity enables the use of Vernier effect to detect the depth of the ocean. Femtosecond laser line-by-line method is used to the inscribing of TFBG, which allows the grating parameters to be changed flexibly depending on the desired spectrum. The experimental results show that the temperature sensitivity is 10.82pm/°C, the salinity sensitivity is 0.122nm/g/Kg, the depth sensitivity is 116.85 pm/m and the depth can be tested to 1000 m or even deeper.

Highly Sensitive Temperature Sensor Based on Cascaded Polymer-Infiltrated Fiber Mach-Zehnder Interferometers Operating near the Dispersion Turning Point

High-accuracy temperature measurement plays a vital role in biomedical, oceanographic, and photovoltaic industries. Here, a highly sensitive temperature sensor is proposed and demonstrated based on cascaded polymer-infiltrated Mach-Zehnder interferometers (MZIs), operating near the dispersion turning point. The MZI was constructed by splicing a half-pitch graded index fiber (GIF) and two sections of single-mode fiber and creating an inner air cavity based on femtosecond laser micromachining. The UV-curable polymer-infiltrated air cavity functioned as one of the interference arms of MZI, and the residual GIF core functioned as the other. Two MZIs with different cavity lengths and infiltrated with the UV-curable polymers, having the refractive indexes on the different sides of the turning point, were created. Moreover, the effects of the length and the bending way of transmission SMF between the first and the second MZI were studied. As a result, the cascaded MZI temperature sensor exhibits a greatly enhanced temperature sensitivity of -24.86 nm/°C based on wavelength differential detection. The aforementioned result makes it promising for high-accuracy temperature measurements in biomedical, oceanographic, and photovoltaic applications.

Enhanced photonic spin Hall effect of reflected light from a doubly linear gradient-refractive-index material

The photonic spin Hall effect (SHE), manifesting itself as spin-dependent splitting of light, holds potential applications in nano-photonic devices and precision metrology. However, the photonic SHE is generally weak, and therefore its enhancement is of great significance. In this paper, we propose a simple method for enhancing the photonic SHE of reflected light by taking advantage of the gradient-refractive-index (GRIN) material. The transverse shifts for a normal (homogeneous) layer and linear GRIN structure with three different types (singly increasing, singly decreasing, and doubly linear ones) are theoretically investigated. We found that the doubly linear GRIN materials exhibit the prominent photonic SHE of reflected light, which is mainly due to the Fabry-Perot resonance. By optimizing the thickness and the lower (higher) refractive index of the doubly linear GRIN layer, the transverse shift for a horizontally polarized incident beam can nearly reach its upper limitation (i.e., half of the beam waist). These findings provide us a potential method to enhance the photonic SHE, and therefore establish a strong foundation for developing spin-based photonic devices in the future.

Efficient coupling between single mode fibers and glass chip waveguides via graded refractive index fiber tips

Coupling characteristics between a single mode fiber (SMF) and a waveguide embedded in a glass chip via a graded index fiber (GIF) tip are investigated at a wavelength of 976 nm. The GIF tips comprise a coreless fiber section and a GIF section. A depressed cladding waveguide in a ZBLAN glass chip with a core diameter of 35 μm is coupled with GIF tips that have a range of coreless fiber and GIF lengths. An experimental coupling efficiency as high as 88% is obtained while a numerical simulation predicts 92.9% for the same GIF tip configuration. Since it is measured in the presence of Fresnel reflection, it can be further improved by anti-reflection coating. Additionally, it is demonstrated that a gap can be introduced between the chip waveguide and the GIF tip while maintaining the high coupling efficiency, thus allowing a thin planar optical component to be inserted. The results presented here will enable miniaturization and simplification of photonic chips with integrated waveguides by replacing bulk coupling lenses with integrated optical fibers.

Temperature and curvature insensitive all-fiber sensor used for human breath monitoring

In this paper, an all-fiber sensor based on hollow core Bragg fiber (HCBF) is proposed and successfully manufactured, which can be used for human breath monitoring. Benefiting from the identical outer diameters of HCBF and single mode fibers (SMFs), the sensor can be directly constructed by sandwiching a segment of HCBF between two SMFs. Based on optical propagation properties of HCBF, the transmission light is sensitive to specific environmental change induced by human breath. Thus, the breath signals can be explicitly recorded by measuring the intensity of the transmitted laser. The sensor presents a rapid response time of ∼0.15 s and recovery time of ∼0.65 s. In addition, the HCBF-based sensor shows good insensitivity to the variation of temperature and curvature, which enables its reliable sensing performance in the dynamic and changeful environment.

Simultaneous measurement of temperature and curvature using ring-core fiber-based Mach-Zehnder interferometer

In this paper, the Mach-Zehnder interferometer (MZI) based on ring-core fiber was proposed and manufactured. Benefiting from the identical diameters of ring-core fiber, no-core fiber, and single-mode fiber, the MZI fiber sensor can be prototyped by sandwiching the ring-core fiber between the no-core fiber and the single-mode fiber (SMF). With the proposed specific structure of the ring-core fiber, the simultaneous measurement of temperature and curvature was achieved with the MZI sensor by means of monitoring the wavelength shift of interference dips. Experimental results have shown that the sensitivity of curvature sensing could reach up to -3.68 nm/m^-1 in the range from 1.3856 m^-1 to 3.6661 m^-1 with high linearity of 0.9959. Meanwhile, the maximum temperature sensitivity is measured to be 72 pm/°C with a fairly good linearity response of 0.9975. In addition, by utilizing the 2×2 matrix algorithm, the dual demodulation of temperature and curvature can be readily realized for the purpose of direct sensing. It is believed that the proposed special structure-based MZI sensor may show great potential applications in the field of fiber-optics sensing and structural health monitoring (SHM).

High-sensitivity special open-cavity Mach-Zehnder structure for salinity measurement based on etched double-side hole fiber

A special open-cavity Mach-Zehnder salinity sensor is presented and verified in this Letter, which has obvious advantages in salinity sensitivity and loss. The open-cavity structure is composed of a short section of etched double-side hole fiber spliced between a pair of multimode fibers and connected in series between a pair of single-mode fibers, which is the SMF-MMF-etched DSHF-MMF-SMF structure proposed in the paper. According to the experiment results, when the cavity length is about 100 µm, the salinity sensitivity of the sensing probe can reach 2 nm/‰, and its refractive index (RI) sensitivity can be more than 10,000 nm/RIU, while having a low loss of ${-}{15}\;{\rm dB}$ and a detection limit of 0.23‰. Based on its characteristics, the sensor is a prospective online monitor of ocean salinity. At the same time, it also provides a low-cost way to construct an open cavity instead of femtosecond inscribing.

Dual Demodulation of Temperature and Refractive Index Using Ring Core Fiber Based Mach-Zehnder Interferometer

We report the ring core fiber spliced with single mode fiber and no core fiber which is used for temperature and refractive index (RI) sensing. The Mach-Zehnder interferometer (MZI) is formed with this kind of sandwich fiber structure and the maximum extinction ratio of the interference spectra reaches 27 dB with the free spectra range of 12 nm. The MZI fiber sensor is applied for temperature sensing with the sensitivity of 69 pm/°C and 0.051 dB/°C. The RI sensitivity reaches 182.07 dB/RIU and -31.44 nm/RIU with the RI ranging from 1.33 to 1.38. The RI value can be directly demodulated with the interference dip intensity which shows insensitivity to temperature. The demodulation of temperature can be achieved by using the linear equations between dip wavelength shift with the variation of temperature and RI.

Highly sensitive hydrogen sensor based on an in-fiber Mach-Zehnder interferometer with polymer infiltration and Pt-loaded WO3 coating

A highly sensitive fiberized hydrogen sensor based upon Mach-Zehnder interference (MZI) is experimentally demonstrated. The hydrogen sensor consists of an MZI realized by creating an air cavity inside the core of a half-pitch graded-index fiber (GIF) by use of femtosecond laser micromachining. Thermosensitive polymer was filled into the air cavity and cured by UV illumination. Subsequently, the external surface of the polymer-filled MZI was coated with Pt-loaded tungsten trioxide (WO₃). The exothermic reaction occurs as Pt-loaded WO₃ contacts the target of the sensing, i.e. hydrogen in the atmosphere, which leads to a significant local temperature rise on the external surface of the coated MZI sensor. The sensor exhibits a maximum sensitivity up to -1948.68 nm/% (vol %), when the hydrogen concentration increases from 0% to 0.8% at room temperature. Moreover, the sensor exhibits a rapid rising response time (hydrogen concentration increasing) of ∼38 s and falling response time (hydrogen concentration decreasing) of ∼15 s, respectively. Thanks to its small size, strong robustness, high accuracy and repeatability, the proposed in-fiber MZI hydrogen sensor will be a promising tool for hydrogen leakage tracing in many areas, such as safety production and hydrogen medical treatment.

High-sensitivity gas pressure sensor based on a multimode interferometer using hollow-core tube lattice fiber

A non-diaphragm fiber gas pressure sensor based on a multimode interferometer (MI) using a hollow-core tube lattice fiber (HC-TLF) as a gas cell is proposed and experimentally demonstrated. The sensor is fabricated by fusion splicing a sandwich structure of a graded-index multimode fiber, HC-TLF, and lead-in/out single mode fiber. Several side-holes are drilled by using a femtosecond laser on the side wall of the HC-TLF to allow gas in and out of the fiber. The positions of side-hole in HC-TLF have been investigated during the experiments, which indicate that the highest gas pressure sensitivity existed as the side-hole located in the gap between adjacent cladding holes of the HC-TLF. The proposed structure exhibits a high sensitivity of 8.1 nm/MPa with the average gas fill time of 2.2 s. This sensor also has low temperature sensitivity and low temperature cross sensitivity of 12.3 pm/°C and 1.5 kPa/°C as the temperature rises to 400°C. In addition, the advantages of the gas pressure sensor, such as small size, rapid response, low temperature cross sensitivity, and simple fabrication process, make it suitable for high-pressure measurement in harsh conditions, e.g., downhole and ocean bottom.

A Review of Coating Materials Used to Improve the Performance of Optical Fiber Sensors

In order to improve the performance of fiber sensors and fully tap the potential of optical fiber sensors, various optical materials have been selectively coated on optical fiber sensors under the background of the rapid development of various optical materials. On the basis of retaining the original characteristics of the optical fiber sensors, the coated sensors are endowed with new characteristics, such as high sensitivity, strong structure, and specific recognition. Many materials with a large thermal optical coefficient and thermal expansion coefficients are applied to optical fibers, and the temperature sensitivities are improved several times after coating. At the same time, fiber sensors have more intelligent sensing capabilities when coated with specific recognition materials. The same/different kinds of materials combined with the same/different fiber structures can produce different measurements, which is interesting. This paper summarizes and compares the fiber sensors treated by different coating materials.

A Low-Cost High-Temperature Sensor Based on Long-Period Fiber/Microfiber Gratings by Local Fictive Temperature Modification

A high temperature-sensitive long-period fiber grating (LPFG) sensor fabricated by the local fictive temperature modification is proposed and demonstrated. High-frequency CO2 laser pulses scan standard single-mode fiber (SMF), and the modification zones extended to the core of SMF. Experimental results demonstrate that the LPFG temperature sensors with 600 μm grating period and 32 period numbers offer the average sensitivity of 0.084 nm/C in the temperature range of room temperature (RM) to 875°C. The LPFGs fabricated here show exponential change in terms of the spectral wavelength shift versus changes in temperature. In addition, the maximum temperature sensitivity of 0.37 nm/C is achieved by employing long-period microfiber grating (LPMFG), fabricated by the microheater brushing technique and the local fictive temperature modification. LPMFG sensor exhibits better temperature characteristics due to a thinner diameter.

Artificial compound eye-tipped optical fiber for wide field illumination

In this Letter, we present a novel, to the best of our knowledge, component with beam delivering and wide field beam homogenizing functions by grafting an artificial compound eye (ACE) micro-structure onto the polymer optical fiber (POF) end face. The 3D ACE mold is fabricated by femtosecond laser-assisted micro machining, and the ACE micro-structure is transferred onto the end face through high accuracy nano-imprinting. The resultant POF end face integrates over 400 spherical micro-lenses, enabling a 40% enhancement in both the acceptance angle and the effective numerical aperture. Meanwhile, the integrated ommatidia array serves as an outstanding beam homogenizer, shaping the output beam into quasi flat-top distribution, which demonstrates promise in wide field homogeneous illumination, by reflection and transmission imaging experiments in both visible and near infrared bands.