Bragg Reflection Wavelength Research Articles

Smoke Detection Using rGO-Coated eFBG Sensor for Early Warning of Coal Fire in Mines

This experimental work demonstrates the process of smoke detection in coal mines using reduced graphene oxide (rGO) coated on etched fiber Bragg grating (eFBG)-based sensors. The lab experiments were conducted using the test fire of combustible coals collected from the different geographic regions of coal mines in Jharkhand and West Bengal in India. The experimental results showed that the coal samples higher in carbon elements interacted with rGO on fiber more strongly. Obtained results show that the increment in the shift of reflected Bragg wavelength with respect to air was observed by exposing the sensor head to the smoke particles inside the smoke chamber. The percent enhancement in the Bragg wavelength shifts are found to be 18.82%, 30%, and 34.74% in Ramnagar, Jharia, and Bokaro, and the sensitivity has been achieved by bare eFBG is ~104.32 pm/sample and by the rGO-coated eFBG sensor is ~134.33 pm/sample. Other parameters like gross calorific values (GCVs) of different coals are also measured using proximate analysis and also directly through a bomb calorimeter. The reported sensor can be installed in coal mines for early fire warning detection.

A refractive index sensor based on micro-nano fiber with chirped fiber Bragg grating embedded for a microfluidic chip

A refractive index (RI) sensor based on micro-nano fiber (MN-fiber) with chirped fiber Bragg grating (CFBG) Fabry–Perot cavity (FP-cavity) for a microfluidic chip has been proposed. A single-mode fiber is drawn by hydrogen flame heading come into MN-fiber. Two CFBGs are written into this MN-fiber by the ultraviolet (UV) laser mask exposure method. One is at the tapered region, another is at the micro-nano region. Then a micro-nano fiber with chirped fiber Bragg grating (MN-CFBGs) FP-cavity sensor is formed. The Bragg reflection wavelengths of two CFBGs are 1620 nm, 3-dB bandwidth are above 50 nm. The reflectance of two CFBGs are 70% and 99%, respectively. The effects of reflectivity and bandwidth of the CFBGs FP-cavity, diameter and length of MN-fiber with this sensor’s optical properties are analysed is and discussed. This sensor is embedded in a microfluidic chip and the MN-fiber region is immersion microfluid in different channels. The experimental results show that refractive index sensitivity of the sensor is –986 nm/refractive index unit (RIU), and the signal of the sensor has little noise. The CFBG-FP sensor not only has high sensitivity and lager measurement range, but also high contrast resonance signal and stability.

Optical sensing of organic vapor using blue phase liquid crystals

ABSTRACT This study presents the investigation of the use of blue phase liquid crystals (BPLCs) for the sensing of toxic gas vapours of toluene, phenol and 1,2 dichloropropane. The vapours of these three toxic solvents were obtained by evaporating at different temperatures within the different times, and BPLC was separately exposed to these gas vapours. The diffusion and adsorption of these vapours on the BPLCs were investigated depending on the dimensions and polarity of the molecules. The optical response of BPLC exposed to gas vapours of toluene, phenol and 1,2 dichloropropane was determined with Bragg reflection wavelength as a function of temperature and time. We found that BPLC produced a remarkable change to the red side of the spectrum due to diffusion of toluene gas vapour by the BPLC sample and itsBragg reflection wavelength change within the time showed a good linearity correlation. Moreover, the diffusion coefficient of the toluene gas vapour in BPLC sample was calculated, and it was determined as 8.224 × 10−12 (cm2/s) by using a least-squares method.

Gain‐clamping in L‐band zirconium–erbium co‐doped fiber amplifier with FBG based lasing control

AbstractIn this study, gain clamping performance in an L‐band zirconium‐erbium co‐doped fiber amplifier (Zr‐EDFA) with a very short length of heavily doped fiber and FBG based lasing control structure was presented. A simple gain clamping design based on a symmetrical narrow band C‐band fiber Bragg grating (FBG) pair addition was used in the L‐band Zr‐EDFA with bidirectionally pumping scheme. An L‐band gain of higher than 25 dB was obtained with a maximum noise figure (NF) of 5 dB in the conventional Zr‐EDFA design which was higher than the previous results obtained with forward pumping scheme. When a couple of narrow band FBGs at Bragg reflection wavelength of 1542 nm are used as the lasing mechanism for gain clamping in L‐band operation, small signal gain of Zr‐EDFA decreases, but on the other hand, the gain is clamped at almost 15.5 dB with a flattened gain spectrum along a dynamic range of 25 dB for the signal input powers between −30 and −5 dBm with the expense of 1.5 dB NF penalty.

Polymer-stabilized blue phase liquid crystal sensor for sensitive and selective detection of organic vapors

ABSTRACT Polymer-stabilised blue-phase liquid crystal (PS-BPLC) is a soft artificial three-dimensional photonic crystal (PC) functional material for optics and optoelectronics because of its superior optical properties. In this work, we demonstrated that PS-BPLC film could sense and selectively detect the xylene, heptane, cyclohexane, dichloromethane and ethyl alcohol vapours rapidly and accurately. When the PS-BPLC film was alternately exposed to volatile vapours and air, the Bragg reflection wavelength of the PS-BPLC could reversibly and significantly shift because of the adsorption and desorption of the volatile vapours. Different organic vapours could be well discriminated by the linearity and the critical concentration at which the Bragg reflection disappeared. This organic vapour sensor also had a good sensitivity, high repeatability, humidity independence and long-term stability, which promote a widespread use in environmental monitoring, point-of-care biomedical analysis and optical security coatings.

Label-free single-substrate quantitative protein assay based on optical characteristics of cholesteric liquid crystals

Cholesteric liquid crystal (CLC) is characterized by the selective reflection in the Bragg condition for constructive interference. Instead of assembling a LC cell with a pair of glass substrates, only a single glass substrate modified with vertical alignment reagent and layered with a CLC film of 3.4 ± 0.2 μm in thickness was utilized in this study for the detection of bovine serum albumin (BSA). By fine-tuning the ratio of the thickness d to the helical pitch P of the CLC film, both qualitative and quantitative biosensing capabilities were demonstrated with an E7/R811 CLC (d/P close to but smaller than unity) and an E7/R5011 CLC (d/P > > 1) film, respectively. The E7/R811 film was in the unwound homeotropic state in the absence of BSA, but transitioned to the fingerprint state at 10−9 to 10−6-g/ml BSA and to the planar state between 10−5 and 10−2-g/ml BSA. Quantitative protein assay based on E7/R5011 CLCs was established through transmission spectrometric analysis, and detection sensitivity was optimized by adjusting the Bragg reflection wavelength. A limit of detection of 5.9 × 10−9 g/ml was achieved for the CLC film consisting of 2.0-wt% R5011 in the nematic host E7 and exhibiting a Bragg reflection wavelength of 700 nm. As a majority of clinical assays are performed on a single solid substrate, the CLC-based, single-substrate biosensing platform allows for better integration with current technologies. Besides, the protein array employed in this work can be further miniaturized and converted to a protein microarray format to facilitate high-throughput analysis.

A Study on MoS₂ Nanolayer Coated Etched Fiber Bragg Grating Strain Sensor

In this paper, we report on the comprehensive study on Molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanolayer coated etched Fiber Bragg Grating (eFBG) strain sensor. MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanolayer is coated using Physical Vapor Deposition (PVD) of Molybdenum (Mo) on eFBGs followed by sulfurization of the same in an inert atmosphere at 450° C. Such coating technique provides a direct control over the coating thickness of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , thereby enabling a study based on the effect of nanolayer coating thickness on the intrinsic strain sensitivity as well as the power of the back reflected Bragg wavelength of eFBG in the 0.78eV spectral region. High uniformity of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanolayer coating ensures consistent, repeatable and highly linear FBG strain sensors with a correlation coefficient of 0.988 in the range of 0 to 2500 με. A maximum intrinsic strain sensitivity of ~6.65 pm/με with a resolution of ~150 nε have been achieved with optimized MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coated eFBG sensors. This kind of consistent, highly sensitive and linear strain sensors when incorporated with proper packaging schemes can be particularly useful for applications demanding high sensitivity of FBG sensors such detection of seismic vibrations, underwater acoustic signals, low amplitude accelerations, etc.

Design of a Sensor System Using Fiber Bragg Grating for Liquid Level and Liquid Density Measurement

In this work, a simple but versatile sensing system for very accurate sensing of liquid level and liquid density is presented. The sensor works based on basic strain sensitivity of Fiber Bragg Grating (FBG) and principle of liquid obeying Archimedes’ law of buoyancy. In this system, a cylindrical shaped mass suspended from a Fiber Bragg Grating and partially immersed in the liquid to be sensed. If the liquid level in the container or liquid density varies, that change the up thrust on the suspended mass and load on the Fiber will be changed accordingly. The change in the load on Fiber changes strain on the FBG and the reflected Bragg wavelength also changes. The proposed device with proper calibration should be able to carry out real time and nonstop liquid level and liquid density measurements. A mathematical analysis of the system considering liquid properties and geometrical structure of the suspended mass is presented here. Sensitivity of the system for liquid level monitoring is also reported. Achieved results shows the path for the utilization of the proposed sensor system for precise liquid density measurement and liquid level sensing in very large storage tanks used for commercial/residential applications.

Influence of Bragg reflection of chirped tilted fiber Bragg grating on Raman suppression in high-power tandem pumping fiber amplifiers.

The key remaining technological challenge to the realization of further power scaling for high-power fiber laser systems is overcoming the stimulated Raman scattering (SRS) effect. In past years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to be a simple and effective way to suppress SRS in high-power fiber amplifiers. However, the weak reflection at the Bragg wavelength could be strongly amplified, which not only limits the power and efficiency but also degrades the beam quality. We report here, for the first time to the best of our knowledge, the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg reflection wavelengths are fabricated and used for the comparison experiments. Test results show that the CTFBGs of longer Bragg wavelengths have a better suppression effect, in particular, at a higher power level. By further moving the Bragg wavelength of a CTFBG out of the Raman gain spectral range, a better suppression effect and a promotion in laser efficiency could be achieved, which is very useful for further power scaling.

Detection of copper nanoparticles templated by DNA using etched Fibre Bragg Grating sensor

In recent years, DNA-templated copper nanoparticles have been used to detect several important analytes, primarily through fluorescence spectrometry. However, the fluorescence of these nanoparticles decays within a few minutes making the quantitative estimation of analytes difficult. Despite the loss of fluorescence, the nanoparticles remain intact as evidenced by microscopy techniques like TEM and AFM. Here, etched fibre Bragg Grating (eFBG) sensors are presented as a detection/characterization tool for copper nanoparticles templated by double stranded DNA. The proposed sensor has a sensitivity of 1.5395 nm/RIU and a resolution of 0.00612 RIU. The formation of the copper nanoparticles on the DNA template attached to the eFBG sensor leads to a significant shift of ≈ 58 pm in the reflected Bragg wavelength signal. Further, the application of the proposed eFBG sensors for analyte detection utilizing the interruption in the synthesis of copper nanoparticles is demonstrated here with the example of melamine detection. The Bragg wavelength shift varies linearly with the log concentration of melamine ( ${R}^{2}=0.9482$ ) and the sensor can detect up to 0.1 ppm of melamine.

Effect of Temperature and Gamma-Ray Irradiation on Optical Characteristics of Fiber Bragg Grating Inscribed Radiation-Resistant Optical Fiber

A new radiation-hard germano-silicate glass optical fiber with a pure silica glass buffer and a boron-doped silica glass inner cladding was fabricated for temperature sensor application based on the fiber Bragg grating (FBG) under γ-ray irradiation environment. The temperature dependences of optical attenuation at 1550.5 nm and Bragg reflection wavelength shift from 18 °C to 40 °C before the γ-ray irradiation were about 4.57×10−4 dB/ °C and 5.48 pm/ °C, respectively. The radiation-induced optical attenuation at 1550.5 nm and the radiation-induced Bragg reflection wavelength shift under the γ-ray irradiation with the total dose of 22.85kGy at 35 °C were about 0.03dB/m and 0.12nm, respectively, with the γ-ray irradiation sensitivity of 5.25×10−3 pm/Gy. The temperature and the γ-ray irradiation dependence of optical attenuation at 1550.5nm in the FBG written fiber with boron-doped silica glass inner cladding were about 6 times and 4 times lower than that in the FBG written fiber without boron-doped silica glass inner cladding under a temperature change from 18 °C to 40 °C and the γ-ray irradiation with the total dose of 22.85 kGy at 35 °C, respectively. Furthermore, the effect of temperature increase on the Bragg reflection wavelength of the FBG written fiber with boron-doped silica inner cladding was much larger about 1000 times than that of the γ-ray irradiation. However, no influence on the reflection power of the Bragg wavelengths and the full width at half maximum (FWHM) bandwidth under temperature and the γ-ray irradiation change was found. Also, after the γ-ray irradiation with the dose of 22.85kGy, no significant change in the refractive index was found but the residual stresses developed in the fiber were slightly relaxed or retained.

Small and Robust All-Polymer Fiber Bragg Grating Based pH Sensor

The smallest all-polymer optical fiber Bragg grating based transducer element for pH sensing is presented. We show that, considering it's size and robustness, it out-performs similar state-of-the-art fiber Bragg grating based pH sensors regarding both sensitivity and response time. A 5-10 μm thick pH sensitive hydrogel coating is placed on a PMMA-based microstructured polymer optical fiber Bragg grating. The hydrogel expands or contracts depending on the pH and thus changes in pH are monitored by following the fiber strain induced changes in the reflected Bragg wavelength λB. Prior to applying the hydrogel coating the mPOF is etched from 150 to 80 μm to enhance sensitivity and surface crazing is introduced with a 50/50 vol% solution of acetone and methanol to enhance spreading of the hydrogel during the application and adhesion after cure. With this design, we achieved a sensitivity of Δλ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> = 73 pm/pH ± 2 pm/pH and response times below 4.5 mins. for pH 5-7 and 4-7, respectively, and a thermal cross sensitivity of 31.4 pm/°C ± 0.4 pm/°C.

Photonic titanium dioxide film obtained from hard template with chiral nematic structure for environmental application

In the present work, mesoporous TiO2 with a photonic structure was elaborated using cellulose nanocrystals (CNCs) as a biotemplate by two-step hard template methods. This strategy enables to replicate the chiral nematic (CN) structure of the photonic films (biotemplate) in TiO2 films. A series of iridescent CNCs films with different weight ratios of silica/CNCs composite photonic films were prepared via evaporation induced self-assembly (EISA) method. The films showed iridescent color and tuneable Bragg reflection wavelengths by solely changing the ratio between the silica and the CNCs biotemplate. Polarized optical microscopy (POM) performed on hydride SiO2/CNCs films showed a birefringence and typical fingerprint of chiral nematic structure. This birefringence was also observed for TiO2 films obtained using SiO2 films as a hard template, which suggested the transfer of the chiral nematic structure in TiO2 materials. Afterwards, their optical, morphological and electronic properties were studied by scanning electron microscope (SEM), POM, energy-dispersive X-ray spectroscope (EDX) and time resolved microwave conductivity (TRMC). The photocatalytic activities were evaluated by following the phenol degradation using high performance liquid chromatography (HPLC). The results showed that the structuration of the TiO2 film using a chiral nematic SiO2 film as hard template enhances the photocatalytic performance compared to non-structured mesoporous TiO2.

Magnetic Optical FBG Sensors Using Optical Frequency-Domain Reflectometry

The optical frequency-domain reflectometry (OFDR) sensing system and signal processing method, which is based on the short-time Fourier transform, enable us to interrogate reflected wavelengths of fiber Bragg gratings (FBGs) and their relative position along the optical fiber with high spatial resolution. An FBG is an optical fiber with an appropriately inscribed periodic refractive index modulation. When a broadband optical signal is launched into the FBG, narrow wavelength bandwidth is reflected. Due to an attached transducer, such as Terfenol-D, the reflected wavelength changes with the applied magnetic field. The proposed magnetic FBG sensor consists of a small rod of magnetostrictive alloy TbDyFe, also known as Terfenol-D, which has large magnetostriction. The FBG is attached to the Terfenol-D rod with UV glue. Consequently, the external magnetic field is applied to the sensor and as a result, the Terfenol-D rod length increases. The reflected wavelength shift is observed and measured by the OFDR sensing system. Therefore, we determine the external magnetic field along the optical fiber from the reflected Bragg wavelength shift. The measurement results show that the proposed magnetic field sensors based on Terfenol-D using OFDR can be successfully used for measurement of the external magnetic field along an optical fiber.

Polymer waveguide WDM channel selector operating over the entire C and L bands.

A tunable channel selector operating over both the C and L bands of wavelength-division-multiplexing optical network is proposed based on a thermo-optic tunable Bragg grating device in a polymer waveguide. A tilted Bragg grating cascaded with an asymmetric Y-branch waveguide effectively implements a small Bragg-reflection wavelength filter that does not require an external circulator. To increase the operating wavelength span of the channel selector, two Bragg gratings with different periods are fabricated on a single substrate for covering C and L bands, respectively. A wide tuning range over 80 nm along with a narrow bandwidth is demonstrated. Moreover, the polarization dependence of the reflection spectrum is observed to be less than 0.1 nm.

A smart “shear sensing” bolt based on FBG sensors

This paper develops a new type of smart bolt that can simultaneously measure the axial and shear forces through the use of embedded fiber Bragg gratings (FBG) sensors. Based on the linear relationship between the equivalent bending moment and the shear force, the equation to calculate the shear force using the Bragg wavelength variation is derived. A method to design a smart bolt is presented in this paper. To enable the experimental study, a smart bolt was fabricated, calibrated, and tested. Calibration of the smart bolt reveals a linear relationship between the reflected Bragg wavelength and the applied shear forces and axial forces. The variation of the wavelengths of the embedding FBGs in different positions in the bolt was analyzed, and a model was developed to relate the FBG wavelength to axial and shear forces experienced by the bolt. Experimental results demonstrate that this smart bolt is suitable for bolt axial forces monitoring and shear forces monitoring.

Optical Characterization of Transmitted Visible Photoluminescence of Bragg‐resonanced Porous Silicon

The investigation and optical characterization of Bragg resonance luminescence porous silicon (BRL PS) exhibiting both optical reflectivity and sharp photoluminescence (PL) at the Bragg resonance wavelength were reported. Two types of Bragg reflector porous silicon (BR PS) samples which exhibit strong reflection in visible or near infrared region and broad red PL were fabricated. When the Bragg reflection wavelength places in the range of emission wavelength, BR PS starts to exhibit narrowing of PL behavior. The optimization of Bragg resonance phenomenon of PL to generate sharp Bragg‐resonated PL by changing the number of repeats of Bragg‐stack layers. BR PS showing the reflection wavelength of 640 nm with 50 repeats exhibited sharp PL at the Bragg resonance wavelength with broad weak PL. BRL PS having 70% porosity with 1.45 refractive index displayed the second‐order reflectivity peak at 640 nm, thus sharp PL peak at 640 nm was the second‐order transmitted and amplified PL peak at the Bragg resonance wavelength. The optical properties of free‐standing BR PS film were investigated. The front side surface of BR PS shows both reflectivity and PL, however the back side surface of free‐standing BR PS shows only reflectivity without PL.

Design of vibration sensor based on fiber Bragg grating

Fiber grating is a kind of new type of fiber optic light source device which has been rapidly changing in the refractive index of the core in recent years. Especially, it can realize the high precision of the external parameters by means of the special structure design and the encapsulation technology [1, 2]. In this paper, a fiber grating vibration sensor which is suitable for vibration monitoring in key areas is designed based on the technical background of vibration monitoring system. The sensor uses a single beam structure and pastes the fiber Bragg grating (FBG) to measure the vibration wavelength on the surface. When the vibration is simply harmonic vibration, the Bragg reflection wavelength will change periodically, and the periodic variation of the wavelength curve can be measured by the fiber grating demodulator, then the correctness of the experimental results is verified. In this paper, through the analysis of the data measured by the demodulator, the MATLAB software is used to verify the data, and the different frequency domains, the modes, and the phase frequency curves are obtained. The measurement range is 0 Hz–100 Hz, and the natural frequency is 90.6 Hz.

Label-free protein sensing by employing blue phase liquid crystal.

Blue phases (BPs) are mesophases existing between the isotropic and chiral nematic phases of liquid crystals (LCs). In recent years, blue phase LCs (BPLCs) have been extensively studied in the field of LC science and display technology. However, the application of BPLCs in biosensing has not been explored. In this study, a BPLC-based biosensing technology was developed for the detection and quantitation of bovine serum albumin (BSA). The sensing platform was constructed by assembling an empty cell with two glass slides coated with homeotropic alignment layers and with immobilized BSA atop. The LC cells were heated to isotropic phase and then allowed to cool down to and maintained at distinct BP temperatures for spectral measurements and texture observations. At BSA concentrations below 10-6 g/ml, we observed that the Bragg reflection wavelength blue-shifted with increasing concentration of BSA, suggesting that the BP is a potentially sensitive medium in the detection and quantitation of biomolecules. By using the BPLC at 37 °C and the same polymorphic material in the smectic A phase at 20 °C, two linear correlations were established for logarithmic BSA concentrations ranging from 10-9 to 10-6 g/ml and from 10-6 to 10-3 g/ml. Our results demonstrate the potential of BPLCs in biosensing and quantitative analysis of biomolecules.

Fast Interrogation of Fiber Bragg Gratings with Electro-Optical Dual Optical Frequency Combs.

Optical frequency combs (OFC) generated by electro-optic modulation of continuous-wave lasers provide broadband coherent sources with high power per line and independent control of line spacing and the number of lines. In addition to their application in spectroscopy, they offer flexible and optimized sources for the interrogation of other sensors based on wavelength change or wavelength filtering, such as fiber Bragg grating (FBG) sensors. In this paper, a dual-OFC FBG interrogation system based on a single laser and two optical-phase modulators is presented. This architecture allows for the configuration of multimode optical source parameters such as the number of modes and their position within the reflected spectrum of the FBG. A direct read-out is obtained by mapping the optical spectrum onto the radio-frequency spectrum output of the dual-comb. This interrogation scheme is proposed for measuring fast phenomena such as vibrations and ultrasounds. Results are presented for dual-comb operation under optimized control. The optical modes are mapped onto detectable tones that are multiples of 0.5 MHz around a center radiofrequency tone (40 MHz). Measurements of ultrasounds (40 kHz and 120 kHz) are demonstrated with this sensing system. Ultrasounds induce dynamic strain onto the fiber, which generates changes in the reflected Bragg wavelength and, hence, modulates the amplitude of the OFC modes within the reflected spectrum. The amplitude modulation of two counterphase tones is detected to obtain a differential measurement proportional to the ultrasound signal.