Optical Interrogation System Research Articles

Low-cost optical interrogator system for structural health monitoring

Optical fiber sensors have great potential for application in civil construction, especially in the field of structural health monitoring. However, one of the barriers to the diffusion of these sensors is allied to the fact that their implementation is quite expensive, mainly due to the high price of commercial optical interrogators. Therefore, the present work seeks to develop a low-cost optical interrogation system. For this, the present study proposes the development of this system based on the use of tunable filters and edge filters in the monitoring of low and high frequencies, and consequent digital processing of the collected signals through the find_picks algorithm and the Fourier transform. As a result, it was possible to observe the adequate functioning of the tunable filters, with the validation of the system using a FBG, and a previous simulation for the edge filter system with frequencies superior to 100 kHz.

Thermally Stimulated Desorption Optical Fiber-Based Interrogation System: An Analysis of Graphene Oxide Layers’ Stability

Thin graphene oxide (GO) film layers are being widely used as sensing layers in different types of electrical and optical sensor devices. GO layers are particularly popular because of their tuned interface reflectivity. The stability of GO layers is fundamental for sensor device reliability, particularly in complex aqueous environments such as wastewater. In this work, the stability of GO layers in layer-by-layer (LbL) films of polyethyleneimine (PEI) and GO was investigated. The results led to the following conclusions: PEI/GO films grow linearly with the number of bilayers as long as the adsorption time is kept constant; the adsorption kinetics of a GO layer follow the behavior of the adsorption of polyelectrolytes; and the interaction associated with the growth of these films is of the ionic type since the desorption activation energy has a value of 119 ± 17 kJ/mol. Therefore, it is possible to conclude that PEI/GO films are suitable for application in optical fiber sensor devices; most importantly, an optical fiber-based interrogation setup can easily be adapted to investigate in situ desorption via a thermally stimulated process. In addition, it is possible to draw inferences about film stability in solution in a fast, reliable way when compared with the traditional ones.

Optical sensor network interrogation system based on nonuniform microwave photonic filters.

Based on the nonuniformly spaced microwave photonic delay-line filter technology, a new design of a generic optical fiber sensor network interrogation platform is proposed and demonstrated. Sensing information from different types of optical sensors embedded in filter taps is converted into the variations of delay time and amplitude of each filter tap individually. Information to be measured can be decoded from the complex temporal impulse response of the microwave photonic filter. As proof-of-concept, our proposed approach is verified by simulations and experimental demonstrations successfully. Four optical sensors of different types are simultaneously interrogated via inverse Fourier transform of the filter frequency response. The experiment results show good linearity between the variation of temporal impulse response and the variations of the twist, the lateral pressure, the transversal loading and the temperature. The sensitivity of the sensors in the proposed platform is -2.130×10-5 a.u/degree, 6.1039 ps/kPa, -1.9146×10-5 a.u/gram, and 5.1497 ps/°C, respectively. Compared to the conventional optical sensors interrogation system, the presented approach provides a centralized solution that works for different types of optical sensors and can be easily expanded to cover larger optical sensor networks.

High-speed interrogation of multiplexed fiber Bragg grating sensors for monitoring dynamic strain responses during a flexible plate impact on a water surface

The investigation of fluid-structure interaction during the impact of a flexible plate on a water surface has received much attention. Measurement of highly transient, distributed strain of the plate during the slamming event is of great interest. Multiplexed fiber Bragg grating (FBG) strain sensors provide a promising solution for such measurement since these sensors are inherently waterproof and are immune to electromagnetic interference. However, in order to monitor the highly transient, distributed strain responses (up to 20 kHz), high-speed simultaneous interrogation of multiplexed FBG sensors is required, which is challenging by using commercial optical interrogators. We present a tunable-wavelength-filter-based optical interrogation system for high-speed simultaneous interrogation of multiplexed FBG strain sensors and demonstrate its application for structural monitoring of a flexible plate during the slamming event. The interrogation system employs a piezoelectric-transducer-controlled Fabry–Perot tunable filter. By operating the tunable filter at its resonant frequency and demodulating the sensor signal based on a peak tracing method, we demonstrated an interrogation speed of 100 kHz, an interrogation range of 98 nm, and an interrogation resolution of 5 pm. For proof-of-performance, the interrogation system was used to monitor the vibrational responses of a cantilever plate under impact loading and the measurement of vibration modes up to 6.785 kHz was demonstrated. Finally, the slamming experiments were carried out with six multiplexed FBG strain sensors mounted on a flexible plate. The dynamic strain measurement of the plate during the slamming event was successfully demonstrated by using the high-speed FBG interrogation system.

Smartphone-Based Interrogation of a Chirped FBG Strain Sensor Inscribed in a Multimode Fiber

Smartphone-based interrogation of a fiber Bragg grating sensor is, to the best of our knowledge, reported for the first time. The smartphone flashlight LED was used as a light source and a transmissive diffraction grating projected the CFBG spectra on the smartphone camera. In order to efficiently couple light from the smartphone LED to the fiber with CFBG, multimode fiber was used for inscription. Interrogation setup consists of a smartphone and low-cost off-the-shelf available components. Measurement principle was illustrated through the fiber strain caused by applied longitudinal force. Attained measurement sensitivity and resolution were validated via comparison with commercial spectrometer and theoretical results based on Cramer-Rao approach. Also, to the best of our knowledge, the influence of modal noise on the smartphone-based fiber optic sensor interrogation system performance is considered for the first time.

High Enhancement Strain Sensor Based on Vernier Effect Using 2-Fiber Loop Mirrors

In this letter, a strain sensor with high sensitivity enhancement using a special case of Vernier effect is presented. The sensor configuration is composed of two-fiber loop mirrors in a cascaded configuration with opposite strain responses when individually characterized. Thus, the enhanced Vernier effect is explored, which is the most sensitive of three possible cases Vernier effect. Here, the Vernier response depends on the difference between the sensitivities of each Hi-Bi optical fiber. In addition to this, the fundamental and the first harmonic were also explored. The results obtained are a strain sensitivity of (13.3 ± 0.3) pm/ $\mu \varepsilon $ for the carrier, (80.0 ± 0.3) pm/ $\mu \varepsilon $ or the Vernier envelope of the fundamental case and (120 ± 1) pm/ $\mu \varepsilon $ for the Vernier envelope of the first harmonic. The first harmonic could achieve a magnification factor of 8. Considering that the optical interrogation system allows a minimum resolution of 0.02 nm, the minimum measurement step achievable is $0.2~\mu \varepsilon $ . This work proves the possibility of applying the concept of enhanced Vernier effect to fiber loop mirrors, obtaining higher sensitivity than a standard fiber loop mirror alone. Besides, the sensitivity can be increased through the usage of harmonics of the Vernier effect. Moreover, the use of large interferometers allows a better discretization of the envelope, which implies a greater ease of analysis.

In-situ strain measurement of ballistic fabrics during impact using fiber Bragg gratings

Abstract In previous experiments, the authors demonstrated that strain values collected from fiber Bragg gratings (FBG) integrated into a single layer of Kevlar fabric, placed between a soft armor test specimen and backing material, could be related to the time dependent back-face deformation (BFD) of the armor sample. In this paper, we investigate the specific fabric deformation and failure mechanisms that cause observed events in the FBG measured strain behavior and the FBG spectral profile throughout the impact event. For these experiments, the standard clay backing material was replaced with a 20% clear ballistic gel to provide visual access to the back-face. The test specimen was impacted by an 8.24 g steel ball bearing travelling at 248.8 m/s, during which strain was calculated from the measured full spectrum response of the FBG using a high-speed optical interrogation system. The strain response was compared to the BFD of the Kevlar sample. The BFD was measured through the clear ballistic gel using two high speed cameras recording at 100,000 fps. The results from these tests can be used for future testing using a non-transparent backing material to obtain a detailed strain–time history, back-face deformation history and an understanding of the time sequence of physical energy dissipation mechanisms in the fabric.

Optical Fibre Micro/Nano Tips as Fluorescence-Based Sensors and Interrogation Probes

Optical fibre micro/nano tips (OFTs), defined here as tapered fibres with a waist diameter ranging from a few microns to tens of nanometres and different tip angles (i.e., from tens of degrees to fractions of degrees), represent extremely versatile tools that have attracted growing interest during these last decades in many areas of photonics. The field of applications can range from physical and chemical/biochemical sensing—also at the intracellular levels—to the development of near-field probes for microscope imaging (i.e., scanning near-field optical microscopy (SNOM)) and optical interrogation systems, up to optical devices for trapping and manipulating microparticles (i.e., optical tweezers). All these applications rely on the ability to fabricate OFTs, tailoring some of their features according to the requirements determined by the specific application. In this review, starting from a short overview of the main fabrication methods used for the realisation of these optical micro/nano structures, the focus will be concentrated on some of their intriguing applications such as the development of label-based chemical/biochemical sensors and the implementation of SNOM probes for interrogating optical devices, including whispering gallery mode microcavities.

Label-Free Multi Parameter Optical Interrogation of Endothelial Activation in Single Cells using a Lab on a Disc Platform

Cellular activation and inflammation leading to endothelial dysfunction is associated with cardiovascular disease (CVD). We investigated whether a single cell label-free multi parameter optical interrogation system can detect endothelial cell and endothelial progenitor cell (EPC) activation in vitro and ex vivo, respectively. Cultured human endothelial cells were exposed to increasing concentrations of tumour necrosis factor alpha (TNF-α) or lipopolysaccharide (LPS) before endothelial activation was validated using fluorescence-activated cell sorting (FACS) analysis of inflammatory marker expression (PECAM-1, E-selectin and ICAM-1). A centrifugal microfluidic system and V-cup array was used to capture individual cells before optical measurement of light scattering, immunocytofluorescence, auto-fluorescence (AF) and cell morphology was determined. In vitro, TNF-α promoted specific changes to the refractive index and cell morphology of individual cells concomitant with enhanced photon activity of fluorescently labelled inflammatory markers and increased auto-fluorescence (AF) intensity at three different wavelengths, an effect blocked by inhibition of downstream signalling with Iκβ. Ex vivo, there was a significant increase in EPC number and AF intensity of individual EPCs from CVD patients concomitant with enhanced PECAM-1 expression when compared to normal controls. This novel label-free ‘lab on a disc’ (LoaD) platform can successfully detect endothelial activation in response to inflammatory stimuli in vitro and ex vivo.

High-speed optical sensor interrogator with a silicon-ring-resonator-based thermally tunable filter.

On-chip optical interrogators employing a silicon-ring-resonator-based thermally tunable filter (SRRTF) offer a promising solution for realizing portable, compact optical sensing systems. However, the slow interrogation speed of conventional SRRTF-based interrogators (less than a few Hz) hinders real-time sensing of dynamic parameters. In this Letter, we report a 100 kHz of high-speed optical interrogation system based on the SRRTF. The speed enhancement is achieved by using the nonlinear transient response of the SRRTF to a square-wave input voltage. The entire spectral range of the SRRTF is scanned twice during its thermal response cycle. The time-domain sensor output signal, which is obtained by scanning the SRRTF over the sensor spectrum, is converted into spectrum domain based on the experimentally characterized, time-dependent resonance wavelength shifts of the SRRTF. With this system, we demonstrate high-speed interrogation of a fiber Bragg grating sensor under dynamic temperature change (200 Hz).

Analysis and evaluation of the Full Disk Telescope refocusing mechanism for the Solar Orbiter mission

The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. It will provide full-disk measurements of the photospheric magnetic field vector and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image contrast analysis and it allows for a lens displacement in order to locate the best focus position. The figure of merit chosen for the image quality evaluation is presented. The influences of attitude instability and mechanical uncertainties are considered in a refocusing process simulation. In addition, an engineering model of the mechanism is tested at flight operating conditions. To check its performance, an optical interrogation system is set up. Determination of accuracy and repeatability of the mechanism positioning is experimentally evaluated and discussed according to the ISO standard. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities.

Fiber-optic ferrule-top nanomechanical resonator with multilayer graphene film.

Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN/Hz1/2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements.

POLICRYPS composite structures: realization, characterization and exploitation for electro-optical and all-optical applications

The acronym POLICRYPS indicates a nano/micro composite structure made of films of well aligned nematic liquid crystal (NLC) alternated to slices of almost pure polymer. Structures are fabricated by curing a homogeneous mixture of mesogenic material, monomer and a curing agent with UV radiation, under suitable physical and geometrical conditions. In particular, geometrical conditions determine (in the range 0.2 ÷ 15 μ m) the spatial periodicity of the realized sample, which can be utilized for transmitting, diffracting or reflecting an impinging light beam, with negligible scattering losses. The spatial modulation of the refractive index (from polymer to NLC) can be switched ON and OFF both by applying an electric field of few V/μ m or, in some cases, by irradiating the sample with a light beam of suitable wavelength. This electrical/optical tuneability is responsible for a series of distinctive characteristics of the optical effects produced by the structure, and determines the range of possible applications. In fact, in different geometries, the POLICRYPS can be exploited as a switchable holographic grating, a switchable optical phase modulator, a switchable beam splitter, a tuneable Bragg filter or it can be exploited as an electro-optical edge filter in an optical interrogation system.

Optical interrogation system based on holographic soft matter filter

We present an optical interrogation system based on a holographic soft-matter filter. It provides a wavelength-dependent transmittance, with a linear relation between Bragg wavelength shift and variation in the filter output intensity. This is the demonstration of a simple and inexpensive technology to implement a, low cost, monitoring system based on a fiber Bragg grating sensor.

High-resolution spectrally-resolved fiber optic sensor interrogation system based on a standard DWDM laser module

This paper presents a spectrally-resolved integration system suitable for the reading of Bragg grating, all-fiber Fabry-Perot, and similar spectrally-resolved fiber-optic sensors. This system is based on a standard telecommunication dense wavelength division multiplexing transmission module that contains a distributed feedback laser diode and a wavelength locker. Besides the transmission module, only a few additional opto-electronic components were needed to build an experimental interrogation system that demonstrated over a 2 nm wide wavelength interrogation range, and a 1 pm wavelength resolution. When the system was combined with a typical Bragg grating sensor, a strain resolution of 1 με and temperature resolution of 0.1 °C were demonstrated experimentally. The proposed interrogation system relies entirely on Telecordia standard compliant photonic components and can thus be straightforwardly qualified for use within the range of demanding applications.

An optical system with potential for remote health monitoring of subsea machinery

A prototype fibre-optic sensing system is described with potential to remotely monitor the condition of three-phase variable frequency subsea motors and electric submersible pumps. An indication that the integrity of a powerful electric motor may be compromised can be gained by spectral analysis of the stator drive current, the phases of the currents, the measurement of vibration at specific locations on the motor and the temperature of the bearings. The optical interrogation system is based on an imbalanced Mach–Zehnder fibre interferometer, illuminated with a broadband source with fibre Bragg gratings (FBGs) used as the basic transducers for the current and vibration measurements. Signals were recovered at a distance of 7 km, for the ‘in house’ current sensor at an effective ac current of 500 A at frequencies from 10 to 1000 Hz with a S/N ratio of 45 dB, together with the relative phases between the currents. These signals were generated with a simulated high power three-phase electric motor. A commercial accelerometer was incorporated into the system which demonstrated a sensitivity of 1 mg Hz−1/2 also from 10 to 1000 Hz. As the interrogation system can only be used to detect dynamic signals, a second interrogation for quasi-static temperature measurements is required; although this was not deployed, possible applications other than bearing temperature measurement are considered.

Detection technique uses total reflectivity of a fiber Bragg grating

Any sensing apparatus includes two basic parts: a sensor enclosed in a suitable package and an interrogation system to detect and convert a physical parameter into a convenient signal. Optical fiber sensors are attractive both because they can be very compact and because numerous detection techniques with various levels of resolution and performance are available. Unfortunately, the packaging of the sensor and optical interrogation system is often complex and expensive. As a result, the overall cost of the optical sensing apparatus can hardly compete with other sensing technologies (e.g., electrical sensors). Fiber Bragg gratings (FBGs) show great potential for detecting strain, pressure, and temperature variations in sensing applications. 2 An FBG is a periodic modulation of the fiber core refractive index and is typically used as a light-reflective device. More particularly, if the period is not constant along its length, the grating is said to be chirped. The FBG reflection spectrum can then be shifted or broadened depending on how the external perturbation is applied to the grating. The current work proposes a very simple and low-cost interrogation technique for use in FBG-based high-g accelerometers3 and dispersion compensators.4 The detection technique is based on the measurement of the total reflectivity of a uniform Bragg grating that is linearly chirped under strain. In the high-g accelerometer application, the fiber sensor is free to stretch (or compress) under acceleration. As illustrated in Figure 1, one end of the fiber is tightly attached to a holder, while the unattached part can deform under its own weight. In this case, the applied strain is linearly distributed along the free section of the fiber sensor.5 Figure 1. When the free fiber section stretches under strain, the fiber elongation creates an uneven grating period .

Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.

The fully digital evaluation of photographic PIV recordings

The performance of a purely digital evaluation system for photographic PIV recordings is described. High resolution digital images are obtained from the 35 mm negatives using a commonly available slide scanner. Together with the continually improving capabilities of standard computers, this evaluation system is a cost effective alternative to the traditional analog optical/digital (Young's fringe method) and purely optical PIV interrogation systems. Compared to the optical systems the fully digital evaluation can provide a higher spatial resolution while maintaining a similar measurement uncertainty. Using actual PIV recordings absolute measurement uncertainties are obtained and further predictions toward optimal displacement data recovery are made with the aid of Monte-Carlo simulations. Measurement uncertainties are minimized for particle image diameters on the order of 2 pixels while the reduction of the image depth (i.e. bits/pixel) has little effect. The overall performance of the described digital evaluation is compared to two types of optical evaluation systems.