Search

In smart structure monitoring, twist angle is one of the most critical mechanical parameters for infrastructure deterioration. A compact temperature-insensitive optical fiber twist sensor based on multi-phase-shifted helical long period fiber grating has been proposed and experimentally demonstrated in this paper. A multi-phase-shifted helical long period fiber grating is fabricated with a multi-period rotation technology. A π / 2 and a 3π / 2 phase shift is introduced in the helical long period fiber grating by changing the period. The helical pitch can be effectively changed with a different twist rate, which is measured by calculating the wavelength difference between two phase shift peaks. Although the wavelength of the phase shift peak also shifts with a change of the temperature, the wavelength difference between two phase shift peaks is constant due to two fixed phase shifts in the helical long period fiber grating, which is extremely insensitive to temperature change for the multi-phase-shifted helical long period fiber grating. The experimental results show that a sensitivity of up to 1.959 nm/(rad/m) is achieved.

The emerging concept of structural health management relies on extensive onboard diagnostic sensors that can provide near real-time information about the state of a structure so that informed prognostic assessment can be made of the continuing reliability of the structure. In this paper, we will discuss two types of sensing platforms that can provide valuable information about the state of a structure: 1D fiber-optic sensors and 2D thin-film sensors. Both fiber-optic and thin film sensors are easily integrated with structures, and can offer local and/or distributed sensing capabilities. Parameters that can be sensed include: static and dynamic strain, acoustic emission, vibration, corrosion products, moisture ingression etc. We will first describe some recent developments in dynamic strain sensing using optical fiber Bragg grating (FBG) sensors. Applications to detection of acoustic emission and impact will be described. In the area of chemical sensing, we will describe a nanofilm-coated photonic crystal fiber (PCF) long-period grating (LPG) sensing platform. PCF-LPG sensors can be designed to provide greater interaction between the analyte of interest and the light propagating in the fiber, thereby increasing the sensitivity of detection. Applications to humidity sensing will be described. Finally, 2D thin-film sensors on polymer substrates will be discussed. One type of sensor we have been fabricating is based on reduced graphene oxide for large-area chemical sensing applications. It is expected that these 1D and 2D sensing platforms will form part of a suite of sensors that can provide diagnostic structural health information.

Fiber-optic gas sensing techniques are commonly based on the recognition of a wide range of chemical species from characteristic absorption, fluorescence or Raman-scattering spectra signatures. By tuning over the vibrational lines of species in the path of laser beam, tunable diode laser gas sensors measure signal spectroscopic intensity, gas concentration, and other properties. However, they have limitations of bulk architecture, small change of signal on top of large background, and low sensitivity of direct absorption. Here we report the fabrication and optical measurements of tunable Er-doped fiber ring laser absorption spectroscopic sensor featuring a gas cell that is a segment of photonic crystal fiber (PCF) with a long-period grating (LPG) inscribed. The tunable laser beam is coupled into the cladding of the PCF by the LPG where the gas in air holes absorbs light. The light travels along the PCF cladding and reflects at the end of the fiber where a silver film is coated as a mirror at one end facet. The light propagates back within cladding and passes through the gas one more time thus increasing the interaction length. This light is finally recoupled into the fiber core for intensity measurement. The proposed fiber gas sensors have been experimentally used for ammonia (NH 3 ) concentration detection. They show excellent sensitivity and selectivity, and are minimally affected by temperature and/or humidity changes. The sensors using PCF-LPG gas cell are simple to fabricate, cost-effective, and are deployed for a variety of applications not possible using conventional optical fibers.

Fiber-optic accelerometers have attracted great attention in recent years due to the fact that they have many advantages over electrical counterparts because all-fiber accelerometers have the capabilities for multiplexing to reduce cabling and to transmit signals over a long distance. They are also immune to electromagnetic interference. We propose and develop a compact and robust photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) that can be implemented as an accelerometer for measurements of vibration and displacement. To excite core mode to couple out with cladding modes, two long-period gratings (LPGs) with identical transmission spectra are needed to be written in an endless single-mode PCF using a CO2 laser. The first LPG can couple a part of core mode to several cladding modes. After the light beams travel at different speeds over a certain length of the core and cladding, the cladding modes will be recoupled back to the core when they meet the second LPG, resulting in interference between the core mode and cladding modes. Dynamic strain is introduced to the PCF-MZI fiber segment that is bonded onto a spring-mass system. The shift of interference fringe can be measured by a photodetector, and the transformed analog voltage signal is proportional to the acceleration of the sensor head. Based on simulations of the PCF-MZI accelerometer, we can get a sensitivity of ~ 0.08 nm/g which is comparable with fiber Bragg grating (FBG) accelerometers. The proposed accelerometer has a capability of temperature insensitivity; therefore, no thermal-compensation scheme is required. Experimental results indicate that the PCF-MZI accelerometer may be a good candidate sensor for applications in civil engineering infrastructure and aeronautical platforms.

The interrogation systems based on fiber-optic sensors are very attractive for the practical applications in structural health monitoring owing to a number of advantages of optical fiber elements over their electronic counterparts. Among the fiber-optic sensors, the fiber Bragg gratings (FBGs) have their own unique features to be widely used for detection of acoustic emission. We have developed a dynamic strain sensing system by using a tunable single longitudinal mode Erbium-doped fiber ring laser to be locked to the middle-reflection wavelength of the FBG as the demodulation technique. A proportional-integral-derivative device continuously controls the laser wavelength that is kept at the FBG middle-reflection wavelength, thus stabilizing the operating point against quasi-static perturbation, while the high frequency dynamic strain shifts the FBG reflection profile. The reflected power varies in proportion to the applied strain which can be derived directly from AC photocurrent of the reflected signal. We have designed and assembled a fourchannel demodulator system for simultaneous high frequency dynamic strain sensing.

Using long-period gratings (LPGs) inscribed in an endless single-mode photonic crystal fiber (PCF) and coating nanostructure film into air channels in the PCF cladding with modal transition of the LPG, we have developed a fiber-optic sensing platform for detection of chemicals. PCF-LPG possesses extremely high sensitivity to the change in refractive index and chemical selectivity by localizing binding and/or absorption events in analyte solution. In this work, we numerically and experimentally investigate the behaviors of modal transition in the PCF-LPG where the air channels of PCF cladding are azimuthally coated with two types of nanostructure polymers as primary and secondary coatings by electrostatic self-assembly (ESA) deposition technique. The primary coating does not affect PCF-LPG parameters such as grating resonance wavelengths and its intensity that can be used for sensing, but it increases the sensitivity to refractive index of chemical analytes in the air channels. The secondary coating is for selective absorption of analyte molecules of interest. Those two coatings significantly modify the cladding mode distribution of PCF-LPG and enhance the evanescent wave interaction with the external environment, leading to a highly sensitive and selective chemical sensor. The integrated sensor has potential in a variety of applications, especially for nano-liter scale measurement in situ. The functional nanostructure films which respond to different parameters can be introduced into the air channels of the PCF-LPGs as transducers with chemical selectivity. In this paper, we demonstrate a fiber-optic humidity sensor with the proposed nanofilm-coated PCF-LPG for detection of corrosion in civil infrastructural health monitoring.

Long-period gratings (LPGs) have shown their significant promising applications in sensors owing to the attractive features that they posses such as small size, immunity for electromagnetic interference, geometric versatility, multiplexing capability, and resistance to corrosive and hazardous environments. Recent researches have revealed that LPGs written on the standard optical fibers could be used as a powerful sensing platform for structural health monitoring. In this work, we inscribe LPGs into SMF-28 optical fiber by focused-beam CO 2 laser, demonstrating as a refractive index sensor for nondestructive chemical detections in the civil infrastructures. Although evanescent-field based LPG sensors have been applied in quantitatively monitoring chemical analytes including moisture, chloride, and corrosion by-product, etc., the sensitivity, selectivity, and response time as well as thermo-stability of such sensors are still the issues for some special purposes. In order to improve those characteristics of the sensors, we propose two types of nano-film to be coated in grating region by electrostatic self-assembly (ESA) deposition processing. The primary coating does not affect on LPG transmission parameters such as resonance wavelength and its intensity that can be used for sensing, but it increases the sensitivity to refractive index change of surrounding material. The secondary coating is for selectively absorption of analyte molecule of interest. Response time of the nanofilm-coated LPG sensor is dependent on the analyte absorption and de-absorption rates as well as the thicknesses of the coating materials, which is also investigated. Multi-channel sensor system is being designed to monitor different analytes simultaneously, which is continuing to further explore the monitoring of structural health conditions through in situ measurements of corrosion in the concrete structures.

Fiber optic sensors have become widely used for structural health monitoring in recent decades. The aim of this research is to characterize the dynamic failure signals emitted in fiber reinforced polymer (FRP) stay cable and specimens using Fiber Bragg Gratings (FBGs) and two types of interferometric demodulation systems, namely Michelson interferometer (MI) and two-wave mixing interferometer (TWMI) for detection. Due to its one-dimensional form, only one FBG and the Michelson interferometer are used for damage monitoring in a carbon FRP stay cable under various types of loading. Michelson interferometer is capable of detecting frequency contents extending up to 500 kHz, where frequency contents below 250 kHz are categorized as matrix failure and those above 300 kHz corresponded to fiber failure. Two channels of FBGs are used with the TWM interferometer to track local damage in coupon-size FRP samples. Using TWM scheme, continuous and burst acoustic emission events are detected with frequency responses extending up to 125 kHz in coupon-size GFRP specimens, limited only by the sampling rate of the data acquisition system. The experimental results suggest that both types of FBG demodulation systems may be suitable for monitoring high frequency mechanical strains in civil structures, providing a tool for local structural damage detection.

An evanescent field sensing platform is being pursued through excitation of cladding modes using long-period (LPGs) inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO 2 laser irradiation. Core-cladding mode coupling and recoupling has resulted in significant improvement in the evanescent field overlap throughout the cladding air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization of the PCF-LPG configuration can lead to a field power overlap as high as 23% with a confinement loss of less than 1 dB/m in the cladding mode.

The array of axially aligned air channels and the robust waveguide characteristics of index-guiding photonic crystal fibers (IG-PCFs) integrated with long-period gratings (LPGs) make them a powerful platform for chemical sensing and detection. Compared to their conventional all-solid fiber counterpart, the IG-PCFs are a particularly attractive sensing device as they are both a waveguide and a vapor/aqueous transmission cell, permitting light intensity-analyte interaction over long path length without the removal of fiber cladding. While the fundamental core-mode in the IG-PCF has been utilized for evanescent field based sensing, there exist two inherent limitations: (1) only short distance extended by evanescent waves from the guiding core to the surrounding PCF cladding air channels to restrict the probing of an analyte only in the inner most ring of the air channels in cladding, and (2) less than 1% power of the core-mode overlap with the surrounding air channels leading to weak light intensity-analyte interactions due to the localization of the coremode in the fiber core area. Should a cladding-mode with maximum overlap in air channels be excited by an LPG, it would fundamentally increase the evanescent field sensitivity. In this work, we present the simulation for the mode properties of selected IG-PCF for optimization of mode field distribution and light power overlap with air channels in fiber cladding. The numerical calculation reveals that if the optimized cladding-mode is selectively coupled, the evanescent wave overlap (at wavelength of 1550 nm) with cladding air channels of the round and hexagonal structures can be increased from 0.11% and 0.13% up to 4.01% and 6.54%, respectively.

Numerical optimization of photonic crystal fiber (PCF) structures for refractive index sensors based on long period gratings inscribed in PCFs has been performed. The optimization procedure employs the Nelder-Mead downhill simplex algorithm. This direct-search method attempts to minimize a scalar-valued nonlinear function of N real variables (called the objective function) using only function values, without any derivative information. An inverse design approach utilizes the objective function constructed using desired sensing characteristics. For the modal analysis of the PCF structure a fully-vectorial solver based on the finite element method is called by the objective function. The dispersion optimization of PCFs is aimed at achieving a high sensitivity of measurement of refractive index of analytes infiltrated into the air holes for the refractive index and probe wavelength ranges of interest. We have restricted our work to the index-guiding solid-core PCF structures with hexagonally arrayed air holes.

We have explored the use of index-guiding liquid-core photonic crystal fiber (LC-PCF) as a platform for sensing and measurements of analyte solutions of minute volume by normal and surface-enhanced Raman scattering (SERS). The index-guiding LC-PCF was fabricated by selectively sealing via fusion splicing the cladding air channels of a hollow-core PCF (HC-PCF) while leaving the center core open at both ends of the fiber. The center core of the resultant fiber was subsequently filled with water-ethanol solution mixtures at various ethanol concentrations for normal Raman scattering measurements and with water-thiocynate solutions containing Ag nanoparticle aggregates for SERS detection of thiocynate at trace concentrations. The light-guiding nature in the solution phase inside the LC-PCF allows direct and strong light-field overlap with the solution phase over the entire length of the PCF (~30 cm). This detection scheme also dramatically reduces the contribution of silica to Raman spectral background, compared with the solid-core counterpart, thus its potential interference in spectral analysis. These features attribute to ready normal Raman measurements of water, ethanol, and water (99 vol.%)-ethanol (1 vol.%) solutions as well as sensitive and reproducible SERS detection of ~10 ppb thiocynate in water, all at a volume of ~0.1 μL.

In this paper, we present the numerical analyses of moda l coupling properties and resonance spectral response of long-period gratings (LPGs) in solid-core photonic crystal fibers (PCFs) with respect to their sensitivity to refractive indices of measurands in the air channels using a full-vectoral mode solver combined with frequency-domain method. The calculated results show that the wavelength resonance of a PCF-LPG can be extremely sensitive to the refractive index change. The PCF-LPG refractive index sensors, inscribed by residual stress relaxation using a scanning CO 2 laser, are also experimentally investigated in terms of resonance shift with the PCF-LPG structure filled with sodium chloride (NaCl) solutions at concentrations ranging from 0-26% (w/w). The spectral features were greatly improved by inscribing the LPG structure in PCF prefilled with water. The PCF-LPG se nsors are shown to be able to detect the change of ~10 -7 refractive index unit (RIU) in the index range of 1.33 to 1. 35, in a good agreement with the numerical simulation. Keywords: Photonic crystal fibers, microstructured optical fibers, holey fibers, fiber Bragg gratings, long-period gratings, refractive index sensors, refractometers

Long-period gratings (LPGs) have been fabricated in endlessly single mode photonic crystal fibers (ESM-PCFs) utilizing focused CO 2 laser irradiation and residual stress relaxation technique. The responses of the ESM-PCF-LPGs to external refractive index and applied bending curvature have been investigated. As compared with the conventional fiber-based LPGs inscribed under the same condition, the ESM-PCF-LPGs exhibit higher sensitivity to external refractive index change and macro-bending, making them attractive candidates for chemical and biological sensing applications.

Although long-period gratings (LPGs) and photonic crystal fibers (PCFs) have emerged at the same time and been around for almost ten years, the fabrications of fiber components in PCFs have attracted great attention in recent years. Post processing of PCFs with a CO 2 laser is very powerful and versatile method for making miniature compact fiber-based devices including LPGs and phase-shifted LPGs. This article will review our research work on fabrication of those gratings in PCFs by use of focused beam from a CO 2 laser and point-by-point writing fashion. Either the mechanical stress relaxation technique or surface deformation method is employed in the design and fabrication of the gratings. The characterizations of the inscribed LPGs in PCFs at high temperature and high strain are also described. The potential applications of PCF-LPG devices for gas sensing have been discussed. Unlike the PCF-based gas sensor that detects the analytes by the interaction of light with gases through the absorption of the evanescent wave in the holes of fiber cladding, the PCF-LPG gas sensing works by the interrogating of the shifts of different resonance wavelength and strength of core-cladding mode coupling in the transmission spectrum. The advantages of the PCF-LPG sensing devices are: (1) high temperature insensitive and stability; (2) compactness when packaged; (3) practical use under hazardous conditions and in high temperature environment.

We demonstrate the first fabrication of highly-compact and temperature-insensitive phase-shifted and multiple phase-shifted long-period gratings using the focused pulses of CO2 laser with point-by-point technique. By introducing the 3π/2, π, or π/2 phase shifts in the different location of the gratings with 480 μm of a period-length and total number about 16 - 21 periods, the symmetrical and asymmetrical pandpass filters based on those gratings can be obtained. The temperature sensitivity of formed gratings is about 8.8 pm/°C at a temperature range of 23°C - 190°C. Such fiber grating component could be used as an all-fiber oscillator in the laser cavity. Long-period gratings with different phase shifts are also investigated with various notches of resonance attenuations. All gratings fabricated are compactness to be packaged into the fiber-optic based devices.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical communications and sensing systems have required optical fibers to meet the demanding applications day by day, such as high bit rates, dense wavelength division multiplexing (DWDM) and long distance, so that there could be always renewed interest in fiber designs to lift fundamental limitations of silica optical fibers. One kind of fiber, which is different from the conventional fiber, is silica photonic crystal fiber (PCF) that has an array of microscopic air-holes running along its length. For PCF, there are two distinct classes as the guidance; one is using a type of total internal refection which is referred to the terms microstructured fiber, holey fiber, effective-index fiber, and hole-assisted fiber, and the other is employing photonic band-gap which is referred to photonic band-gap, Bragg fiber, and omni-directional waveguide. In this paper, we will address recent advances in PCFs. While the potential applications are also addressed, the emphasis will be on PCFs for applications in optical communications and sensing systems.

A novel wavelength-tunable add/drop multiplexer is designed and proposed to be used as a signal selector in dense wavelength-division multiplexing systems, which is based on four identical long-period gratings assembled on four same structured piezoelectric ceramic fiber stretchers, and their operation of wavelength-selective coupling modes. By contrast to fiber Bragg gratings used in add/drop multiplexer, long-period gratings could be specifically designed to operate in transmission mode.

A novel technique has been demonstrated for making chirped fiber Bragg gratings (CFBG), which was used as the means for in situ absolute center wavelength shifting and reflection bandwidth broadening of a CFBG.

The transmission characteristics of phase-shifted long-period gratings (LPGs) are simulated theoretically by a combination of coupled-mode theory and fundamental-matrix method. It is suggested that a phase-shifted LPG device cascaded with another normal LPG can be used to flatten the gain spectrum of an erbium-doped fiber amplifier (EDFA) with all three gain peaks. The results of simulations have shown that a broadband amplifier with peak-to-peak 0.7 dB gain variation over 36 nm can be practically realized. It proves that such a gain- flattening fiber filter is indeed plausible, and has a potential application in gain-flattening in dense wavelength- division multiplexed (DWDM) telecommunication systems.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A uniform period fiber Bragg grating can be chirped by applying an axially strain gradient. We have demonstrated a novel technique for making the chirped fiber Bragg gratings (CFBG), which involves bonding an unchirped fiber grating to the surface of a tapered stainless steel plate which is strained by loading. Three kinds of tapered profiles (convex, gradient and concave) of plates and two kinds of loading procedures (bending and dilating) were used as the means of an in situ absolute center wavelength shifting and reflection bandwidth broadening of a CFBG. By this method, the center wavelength of a CFBG could be tuned from 1548.92 nm to 1550.99 nm, and a maximum adjustable chirp bandwidth of 2.00 nm was obtained for a fiber Bragg grating in 50 mm length. An application in the accelerometer was introduced.

A cantilever beam and fiber Bragg grating is used to measure acceleration. The cantilever induces strain on the grating resulting in a Bragg wavelength modification which is subsequently detected. The output signal is insensitive to temperature variations and for a temperature change from —20 °C to 40 °C, the output signal fluctuated less than 5%without any temperature compensation schemes. Because the sensor does not utilize expensive and complex demodulationtechniques it is potentially inexpensive. For the experimental system a linear output range of 8 g could be detected.Keywords: Optical fiber sensors, fiber Bragg gratings, accelerometer, chirped gratings 1. INTRODUCTION The first fiber gratings were reported approximately two decades ago and since then applications of fiber Bragg gratingsincreased' . Because the optical linewidth ofthe reflected Bragg wavelength can be in the order of 0. 1 urn, the main area ofinterest has been in optical fiber telecommunkation systems such as wavelength division multiplexing, fibre lasers, opticalamplifiers as well as dispersion compensation elements 2•

A permanent index change of 0.0051% was induced in Sm+2/Sm+3 doped aluminosilicate fiber by irradiating 2W of multi-line output from an Ar ion laser. The photosensitivity was detected by measuring a shift in LP11 mode cutoff wavelength. Bleaching of a broad absorption band in the visible was also observed. The origin of the photosensitivity in the Sm doped optical fiber is the bleaching of Sm+2 ion absorption bands enhanced by multiphoton processes.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.