Bragg Resonance Wavelength Research Articles

Strain gauge based on n-pairs of chirped fiber Bragg gratings

Strain gauges based on fiber Bragg gratings are not only a well-developed field of research, but also gain a wide market due to their high sensitivity, response speed, immunity to electromagnetic radiation and flexible characteristics. In this work, we studied the strain gauge sensor based on n-pairs of chirped fiber Bragg gratings. These structures are inscribed at the same Bragg resonance wavelength within each individual pair. One grating is sensitive to applied load, the other is in the free position. Under mechanical impact, load-sensitive grating shifts to the region of long wavelengths. In this regard, the total reflection spectrum becomes wider and the reflected optical power increases. Bragg gratings are inscribed with reflection coefficients close to 100%. The experimental results to identify the dependencies between the applied mechanical stress and the reflected optical power are obtained. The proposed implementation of the sensor does not require spectral scanning of the gratings, which greatly simplifies its operation.

Filament-arrayed Bragg gratings for azimuthally resolved displacement sensing in single-mode fibers.

Filament arrays were inscribed off-axis in the core of standard single-mode telecommunication fiber, using femtosecond laser pulses. The flexible line-by-line writing formed uniform, parallel filaments, permitting Bragg grating sensing of the photoelastic response from inside of the narrow grating plane. Active monitoring of the Bragg resonance wavelength while driving a lateral fiber tip displacement directly informed on the fiber mechanics when coupled with opto-mechanical modelling. Overlaying of parallel and orthogonal gratings further provided a strongly contrasting azimuthal sensitivity, which paves the way for multi-dimensional displacement sensing with improved precision.

High-performance fiber Bragg gratings arrays inscription method

This paper proposes the interferometric method for arrays inscription of type I Bragg gratings on the unified segment of the standard telecommunication single-mode optical fiber, using preliminary inscription of the gratings with low diffraction efficiency. The accuracy of setting the Bragg resonance wavelength of each individual diffraction structure is at least 50 pm with the reflection coefficient close to 100%. Also, the results of chirped fiber Bragg gratings arrays inscription in a fiber with a high content of germanium dioxide (GeO2) in the core are obtained. An analysis of the spectral characteristics of the inscribed periodic structures of modulation of the refractive index is made.

Methods of hot wire creation for fiber-optical thermal anemometer

Subject of Research. The paper presents the study of various methods of optical radiation coupling from the fiber core into its cladding with the aim of creating a fiber-optic anemometer hot wire. The effectiveness of the described methods is evaluated by analyzing the obtained dependencies. Method. The structures of two optical fibers were studied, one of which was used to transmit an information signal, and the other — to create a hot wire. A Bragg grating was formed in the core of the first fiber, and a radiation coupling region was created in the second fiber. These fiber sections were aligned with each other and fixed by a tin-lead alloy. The temperature was measured by monitoring the spectral characteristics of fiber Bragg gratings inscribed by the phase mask method. The areas of radiation coupling were formed either by changing the fiber geometry using custom modes of the arc fusion splicer, or by creating a SMF-MMF-SMF transition. Main Results. Structures are developed which can be used as a hot wire of a fiber-optic anemometer sensitive element. The dependencies of the Bragg resonance wavelength shift on the pump laser power are obtained; the Bragg wavelength shift varies in the range from 0.15 nm to 3 nm as a result of a hot wire creation. The resulting curves are constructed after three experiments for each type of structure, taking into account the standard deviation. They make it possible to judge the effectiveness of various methods of radiation coupling. Comparative analysis of the studied methods is presented. Practical Relevance. The study describes the capabilities of fiber-optic anemometry when creating point and distributed sensors. The results of this work can be used in the areas where the creation of quasi-distributed and point diffusers is necessary, as well as partial or complete coupling of radiation from a fiber core.

Sense high refractive index sensitivity with bragg grating and MIM nanocavity

The study of optical microcavity sensors in current micro optical electro mechanical systems (MOEMS) devices is an important research direction in this field. This study investigates a new type of nanocavity temperature sensor based on the S-shaped metal-insulator-metal (MIM) and Bragg grating. The analysis of the original sensor model shows that the resonance wavelength of the proposed grating's reflection spectrum shifts to the long-wave direction with the change of refractive index. The value of the high refractive index sensitivity is 1960 nm/RIU, which is obtained near the Bragg resonance wavelength of 1950 nm, and the figure of merit (FOM) is 14.6. The structure is further optimized by adding nanocavity in the middle of the S-shaped MIM Bragg grating. Results show that the refractive index sensitivity is basically unchanged, but the corresponding FOM is 27.7, showing an improvement of 89.7%. The temperature sensor sensitivity is as high as 4.95 nm/°C, and the reflection spectrum resonance wavelength of the proposed grating shifts to the short-wave direction as the temperature changes. High sensitivity can be obtained when the Bragg resonance wavelength is long. Moreover, the Bragg wavelength has a good linear relationship with external physical parameters, such as refractive index and temperature. The findings of this study have certain theoretical and practical values in realizing high-precision, anti-harsh environment, and multi-sensor integrated MOEMS devices.

Bragg Gratings in Suspended-Core Photonic Microcells for High-Temperature Applications

We report a novel type-II photonic crystal fiber Bragg grating for high-temperature applications. The Bragg grating is inscribed in a low-loss in-fiber structure named suspended-core photonic microcell, which is postprocessed from a commercial pure-silica photonic crystal fiber. Grating samples with core diameters of about 4 μm were made by using a focused near-infrared femtosecond laser and a phase mask, and then tested in a tube furnace from room temperature to about 1200 °C. The thermal response of the Bragg resonant wavelength was measure to be about 12 and 16 pm/°C, respectively, at the temperature around 100 °C and 1000 °C. The grating spectrum remained stable in a 10-h isothermal annealing at 1000 °C and started decaying at about 1120 °C with the rate of about 0.02 dB/min. This type of grating possesses flexibilities in both waveguide and grating structure design, exhibits good high-temperature performance, hence would be promising platform for building wavelength-division-multiplexed fiber sensors and tunable devices with a wide working temperature range.

Humidity responsive photonic sensor based on a carboxymethyl cellulose mechanical actuator

We describe an intuitive and simple method for exploiting humidity-driven volume changes in carboxymethyl cellulose (CMC) to fabricate a humidity responsive actuator on a glass fiber substrate. We optimize this platform to generate a photonic-based humidity sensor where CMC coated on a fiber optic containing a fiber Bragg grating (FBG) actuates a mechanical strain in response to humidity changes. The humidity-driven mechanical deformation of the FBG results in a large linear change in Bragg resonance wavelength over the relative humidity range of 5%–40%. The measurement uncertainty over this relative humidity range is ±2% (k = 1).

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 a compact and high sensitive refractive index sensor base on metal-insulator-metal plasmonic Bragg grating.

A nanometric and high sensitive refractive index sensor based on the metal-insulator-metal plasmonic Bragg grating is proposed. The wavelength encoded sensing characteristics of the refractive index sensor were investigated by analyzing its transmission spectrum. The numerical results show that a good linear relationship between the Bragg wavelength and the refractive index of the sensing material can be obtained, which is in accordance with the analytical results very well. A high refractive index sensitivity of 1,488 nm/RIU around Bragg resonance wavelength of 1,550 nm was obtained. Besides, the simulation results show that the sensitivity is depended on the Bragg resonance wavelength and the longer the Bragg resonance wavelength, the higher sensitivity can be obtained. Furthermore, the figure of merit of the refractive index sensor can be greatly increased by introducing a nano-cavity in the proposed plasmonic Bragg grating structure. This work pave the way for high sensitive nanometric refractive index sensor design and application.

Fiber Bragg grating-based high temperature sensor and its low cost interrogation system with enhanced resolution

A fiber Brag grating-based high temperature sensor together with a low cost, high speed and compact size interrogation system using a long period grating was designed, developed and tested. The designed sensor measures the temperature from room temperature to 550 °C. The sensor head was configured by encapsulating an fiber Brag grating (type-I) of Bragg resonance wavelength at 1552.88 nm in a capillary tube made of copper. Long period grating with peak transmission loss at 1550 nm was employed to convert the wavelength information from fiber Brag grating into an intensity modulated signal. Temperature related optical intensity information was again converted into its equivalent electrical signal by using a photodiode. The achieved resolution of the sensor was found to be 0.5 °C.

Robust high temperature sensor probe based on a Ni-coated fiber Bragg grating

A robust high temperature sensor probe based on a Ni-coated fiber Bragg grating(FBG) was fabricated by Ni electroplating of femtosecond laser written FBG. The probe can resist high temperature up to 800 oC with a high sensitivity of 32.2 pm/oC. It also has a good mechanical strength even after high temperature annealing. The thermal strain of the probe was simulated by the finite element method(FEM). The Bragg resonant wavelength shift with temperature and its sensitivity change with the thickness of the Ni-coated layer were also calculated. Keywords Fiber Bragg grating(FBG); Femtosecond laser; Electroplating

Rapid fabrication of microhole array structured optical fibers

A microhole array in a common single-mode fiber is fabricated by selective chemical etching of femtosecond-laser-induced fiber Bragg grating (FBG), which has a laser-modified region extending from the fiber core to the cladding-air boundary due to laser self-focusing. The shape and size of the orderly microhole on the fiber surface are controlled via changing conditions of FBG fabrication and chemical etching. A simultaneous sensing for surrounding refractive index and temperature is demonstrated by this microhole array FBG through measurement of the transmission power change and Bragg resonant wavelength shift.

Femtosecond laser written optofluidic sensor: Bragg grating waveguide evanescent probing of microfluidic channel

Microfluidic channels and Bragg Grating Waveguides (BGWs) were simultaneously fabricated inside fused silica glass by means of femtosecond laser exposure followed by chemical etching. Evanescent field penetration of the waveguide mode into the parallel microfluidic channel induced Bragg resonant wavelength shifts to enable refractive index characterization of the fluidic medium in the 1 to 1.452 range. Laser exposure was optimized to fabricate devices with optically smooth channel walls and narrow Bragg resonances for high sensing response at 1560 nm wavelength. Reference gratings were also employed in the optical circuit for temperature and strain compensation. These devices open new directions for optical sensing in three-dimensional optofluidic and reactor microsystems.

Observation of spectral enhancement in a soliton fiber laser with fiber Bragg grating

We demonstrate that soliton generation in a fiber laser containing a fiber Bragg grating exhibits spectral enhancement near the Bragg resonance wavelength. The Bragg grating leads to a spectral hole on the soliton spectrum while the observed enhancement is always located at the long wavelength side of the Bragg resonance.

Effect of interstitial air holes on Bragg gratings in photonic crystal fibre with a Ge-doped core

The effect of interstitial air holes on Bragg gratings in photonic crystal fibre (PCF) with a Ge-doped core is numerically investigated by using the beam propagation method (BPM). It is shown that the interstitial air holes (IAHs) can make Bragg resonance wavelength λB shift a little towards short wavelengths and increase λB – λ1 (the wavelength spacing between the main peak with Bragg resonance wavelength λB and the first side peak with wavelength λ1) and the coupling coefficient κ of Bragg resonance. Moreover, when the ratio of air hole diameter (d) to pitch (Λ), d/Λ, is small, IAHs can suppress the cladding mode resonance. When d/Λ is large, IAHs increase the number of mode that could strongly interact with the fundamental mode. By comparing the transmission spectral characteristics of PCF-based fibre Bragg grating (FBG) with IAHs with those without IAHs at the same air-filling fraction, it is clarified that the change of transmission spectral characteristics of PCF-based FBG with IAHs is not due to a simple change in air-filling fraction. It is also closely related to the distribution of interstitial air holes.

Fiber Bragg grating sensor for simultaneous measurement of flow rate and direction

A new fiber-optic sensor system consisting of a fiber Bragg grating cantilever as a transducer is proposed and demonstrated to realize simultaneous measurement of fluid flow rate and direction. For the fiber Bragg grating mounted on either a stainless steel or a spring steel substrate, a change in the water flow rate gives rise to a monotonic shift in the Bragg resonance wavelength of the grating while the flow direction results in either a redshift or a blueshift in the Bragg wavelength due to a stretched or shrunk state of the grating. Shifts in the Bragg resonance wavelength of 0.077 and 0.826 nm at a water flow rate of 90 cm3 s−1 were achieved with the fiber Bragg grating stainless steel and spring steel cantilever sensors, respectively. The experimental results are in good agreement with the theoretical analysis.

파이버 브래그 격자를 이용한 곡률 측정

본 논문에서는 곡률을 측정하기 위해 파이버 브래그 격자를 이용한 곡률 센서에 대해 연구하고자 한다. 곡률의 변화를 측정하기 위해 브래그 공진파장의 이동 변화를 측정하였다. 실험견과로부터 곡률이 <TEX>$0\;m^{-1}$</TEX>에서 <TEX>$10\;m^{-1}$</TEX>까지 증가함에 따라 반사 스펙트럼은 감소하나. 브래그 공진 파장의 변화량은 선형적으로 증가함을 알 수 있었으며, 기울기는 대략 <TEX>$8.8\;pm/m^{-1}$</TEX> 이었다. 한편, 반사 스펙트럼은 곡률이 증가함에 따라 감소하였다. To measure the curvature, in this paper, we investigate an optical curvature sensor based on the fiber Bragg gratings. We observed the variation of the Bragg resonant wavelength shift to measure the curvature change. From the experimental results, we knew that the Brags resonant wavelength shift was lineally increased with the increase of the curvature from <TEX>$0\;m^{-1}$</TEX> to <TEX>$10\;m^{-1}$</TEX>. It's slope is about <TEX>$8.8\;pm/m^{-1}$</TEX>. On the other hand, the spectral reflection decreased with the increase of the curvature.

Photoenhanced polarization mode separated fiber Bragg gratings inscribed by femtosecond laser

In this paper, we report the fabrication of photoenhanced polarization mode separated fiber Bragg gratings (FBGs) in polarization maintaining (PM) fibers using IR femtosecond laser illumination. The separation of the Bragg resonant wavelengths between the two polarization modes is as large as 1.78nm due to the photoenhanced birefringence effect generated by the strong ultrashort laser pulses. This large polarization mode separation solves one of the major problems of the conventional PM Bragg gratings (i.e., the narrow spacing or even the partial spectral overlap between spectra of the two polarization eigenmodes) and substantially enhances the multiparameter sensing capability of FBGs by offering a wider sensing range and higher discrimination. Furthermore, the high thermal stability of FBGs (up to 1000°C for silica fibers) inscribed by IR femtosecond lasers provides for multiparameter sensing in harsh, high temperature environments.

Supercontinuum generation in a fiber grating

We show that supercontinuum generation in a fiber containing a Bragg grating exhibits &amp;gt;10× enhancement near the Bragg resonance wavelength. We also show that the grating dispersion exceeds the waveguide dispersion over a bandwidth far in excess of its photonic band gap. The observed enhancement is consistent with nonlinear Schrödinger equation simulations of the supercontinuum formation that combine grating and waveguide dispersion.

Wide tuning range acousto-optical fiber Bragg grating filter (FBGF)

An acousto-optical filter having a wide tuning range and a method of making the same. An acoustic transducer is provided for generating an acoustic pressure wave of a selected frequency that is propagated longitudinally along an optical fiber member. The pressure wave generates a plurality of alternating localized compressions and rarefactions in the optical fiber such that a grating (i.e., periodic changes in the fiber's refractive index) is created therein. The grating reflects optical signals of a particular wavelength depending upon its period or pitch (i.e., Bragg resonance wavelength). The acoustic pressure wave's frequency is modulated by controlling the acoustic transducer such that a variable grating pitch is obtained, thereby causing a corresponding change in the Bragg resonance wavelength of the grating. In response, a reflected optical signal selected from incoming multiplexed optical signals tunes to a different wavelength. A closed-loop controller is provided for controlling input signals to the acoustic transducer/actuator so as to modulate the tuning of the reflected optical signals.