In this paper, we propose techniques to design and fabricate polymer micro-cantilevers for attachment onto the end of standard single mode fibers using laser machining. The polymer cantilever is fabricated by laser micro-machining a sheet of polymer into the required shape and then bonded onto the top of a ceramic ferrule by photo resist as a flat supporting and bonding layer. The dimension of resulting cantilever is ~1.2 mm long, ~300 μm wide, and 25 μm thick. In this work we describe the fabrication of single sensors, however the process could be scaled to offer a route towards mass production. Cantilever vibration caused by vibration signal are monitored by a DFB laser based phase interrogation system. Proof-of-concept experiments show that the sensor is capable of detecting vibration signal with a frequency range of 0-800Hz. By using thinner polymer sheet and machining longer cantilever, the frequency response range can be extended up to a few kHz.
Photo-acoustic spectroscopy gas detection technology has the advantages of high sensitivity, good selectivity, small size and real time monitoring and has been widely used in environmental monitoring, industrial production, medical diagnosis, biological technology and monitoring of power facilities. In this paper, a method to improve the sensitivity of photo-acoustic spectroscopy system is presented, which is combined with the technique of Herriott type multiple pass cell. In this experimental apparatus, the design of the experimental device can make the beam pass the cell 18 times. By comparing the signal of one time pass through the photoacoustic cell and the signal of 18 times passes pass through the photoacoustic cell, we can confirm that the signal is increased and this method is feasible.
Optical fiber Bragg grating (FBG) displacement sensors play an important role in various areas due to the high
sensitivity to displacement. However, it becomes a serious problem of FBG cross-sensitivity of temperature and
displacement in applications with FBG displacement sensing. This paper presents a method of temperature insensitive
measurement of displacement via using an appropriate layout of the sensor. A displacement sensor is constructed with
two FBGs mounted on the opposite surface of a cantilever beam. The wavelengths of the FBGs shift with a horizontal
direction displacement acting on the cantilever beam. Displacement measurement can be achieved by demodulating the
wavelengths difference of the two FBGs. In this case, the difference of the two FBGs’ wavelengths can be taken in order
to compensate for the temperature effects. Four cantilever beams with different shapes are designed and the FBG strain
distribution is quite different from each other. The deformation and strain distribution of cantilever beams are simulated
by using finite element analysis, which is used to optimize the layout of the FBG displacement sensor. Experimental
results show that an obvious increase in the sensitivity of this change on the displacement is obtained while temperature
dependence greatly reduced. A change in the wavelength can be found with the increase of displacement from 0 to
10mm for a cantilever beam. The physical size of the FBG displacement sensor head can be adjusted to meet the need of
different applications, such as structure health monitoring, smart material sensing, aerospace, etc.
Spatial temperature and gas concentration distributions are crucial for combustion studies to characterize the combustion position and to evaluate the combustion regime and the released heat quantity. Optical computer tomography (CT) enables the reconstruction of temperature and gas concentration fields in a flame on the basis of line-of-sight tunable diode laser absorption spectroscopy (LOS-TDLAS). A pair of H2O absorption lines at wavelengths 1395.51 and 1395.69 nm is selected. Temperature and H2O concentration distributions for a flat flame furnace are calculated by superimposing two absorption peaks with a discrete algebraic iterative algorithm and a mathematical fitting algorithm. By comparison, direct absorption spectroscopy measurements agree well with the thermocouple measurements and yield a good correlation. The CT reconstruction data of different air-to-fuel ratio combustion conditions (incomplete combustion and full combustion) and three different types of burners (one, two, and three flat flame furnaces) demonstrate that TDLAS has the potential of short response time and enables real-time temperature and gas concentration distribution measurements for combustion diagnosis.
Spatial resolution determines the minimum space unit that a distributed temperature sensor system can distinguish along the fiber thus it is an important parameter to evaluate the performance of the distributed temperature sensor system. A typical distributed temperature sensor system with a spatial resolution of 5m is built and an algorithm of linear fitting correction is proposed to realize temperature measurement of fiber length shorter than 5m accurately. With the method of linear fitting correction, the spatial resolution of the distributed temperature sensor system has been improved from 5m to 1m. The measured temperature of the DTS system is well calibrated by using linear fitting correction algorithm with a fiber length of 4m, 3m, 2m and 1m respectively. The maximum error of the corrective temperature is 2℃ for long term measurement.
Focused Ion Beam (FIB) machining has been demonstrated to be capable of fabricating nano and micro scale elements
onto optical fibres. In this paper we exploit FIB to fabricate core aligned 45° mirrors at the end of multi-core fibres
(MCF). The resulting fibre is used as a component in a two dimensional optical fibre accelerometer. The mirror is
produced using a two step process: first a scanning process is used to make a rough cut to define the overall mirror
structure. This is followed by a polishing process to create an optical surface finish. The machined 45° mirror can be
accurately aligned with optical fibre core, which avoids issues associated with the alignment of external turning mirror
components. Proof-of-concept tests demonstrate the use of such a fibre as a two axis acceleration sensor that is
interrogated interferometrically. The sensor operated between 0.5g and 4.5g with a cross talk of -24.3dB between axes.
KEYWORDS: Temperature metrology, Calibration, Signal attenuation, Optical fibers, Raman spectroscopy, Temperature sensors, Optical calibration, Error analysis, Signal detection, Demodulation
The effect of optical fiber attenuation differences (AD) induced temperature error of Raman distributed temperature sensor (RDTS) is analyzed using the temperature demodulation algorithm. First of all, a novel method to address the effects caused by the AD between Stokes and anti-Stokes light is proposed. Furthermore, the temperature measurement error caused by additional AD of fiber temperature is also reduced by using a formula obtained by experimental data. The experimental results demonstrate that the RDTS system can measure different temperature zones more accurately.
W. Chen, T. Wu, W. Zhao, G. Wysocki, X. Cui, C. Lengignon, R. Maamary, E. Fertein, C. Coeur, A. Cassez, Y. Wang, W. Zhang, X. Gao, W. Liu, F. Dong, G. Zha, Xu Zheng, T. Wang
Chemically reactive short-lived species play a crucial role in tropospheric processes affecting regional air quality and
global climate change. Contrary to long-lived species (such as greenhouse gases), fast, accurate and precise monitoring
changes in concentration of atmospheric short-lived species represents a real challenge due to their short life time (~1 s
for OH radical) and very low concentration in the atmosphere (down to 106 molecules/cm3, corresponding to 0.1 pptv at
standard temperature and pressure).
We report on our recent progress in instrumentation developments for spectroscopic sensing of trace reactive species.
Modern photonic sources such as quantum cascade laser (QCL), distributed feedback (DFB) diode laser, light emitting
diode (LED), difference-frequency generation (DFG) parametric source are implemented in conjunction with highsensitivity
spectroscopic measurement techniques for : (1) nitrous acid (HONO) monitoring by QCL-based long optical
pathlength absorption spectroscopy and LED-based IBBCEAS (incoherent broadband cavity-enhanced absorption
spectroscopy); (2) DFB laser-based hydroxyl free radical (OH) detection using WM-OA-ICOS (wavelength modulation
off-axis integrated cavity output spectroscopy) and FRS (Faraday rotation spectroscopy), respectively; (3) nitrate radical
(NO3) and nitrogen dioxide (NO2) simultaneous measurements with IBBCEAS approach.
Applications in field observation and in smog chamber study will be presented.
There have been a number of papers focusing on fiber distributed sensing with coherent Rayleigh backscattering
published. However, up to now, very limited research articles on investigation of coherent Rayleigh backscattering signal
waveform and its physical mechanism have been reported. This paper first proposes a theoretical derivation to illustrate
coherent Rayleigh backscattering waveform. The theoretical model is then proved with numerical simulation and
experimental measurement. In addition, signal processing method is an important factor on the performance of a phasesensitive
OTDR system. An improvement of signal processing method, which is consisted of digital average, moving
average and interval subtraction, with good effect on locating external perturbation is also introduced.
For any tunable diode laser absorption spectroscopy (TDLAS) system, its performance is often degraded by long-term
distortion and fluctuations of light intensity. The source of long-term distortion and the corresponding proposal are
discussed at first in this paper. It is proved that the long-term distortion of the system is mainly induced by the change of
wavelength dependent transfer function. A balanced optical path can be used to reduce it effectively. In order to decrease
the disturbance of intensity fluctuation, a novel method for online correction is presented. It is developed according to
the linear relation between the peaks of harmonic power spectra and the incident light intensity. It is demonstrated by the
experiments and explained as residual sum frequency and difference frequency power of signal and reference after the
lock-in amplifier. This method could achieve real-time light intensity correction with only little calculation. By using a
17.5m multi-pass cell, the experiments show that the system can achieve about 20ppmv stability for long-term continual
monitoring. Allan variance indicates that the detection limit for short-term measurement is between 0.3ppmv and 1.5ppmv depending on the response time allowed by the instrument.
KEYWORDS: Signal processing, Signal detection, Absorption, Wavelet transforms, Interference (communication), Wavelets, Absorption spectroscopy, Tunable diode lasers, Linear filtering, Signal analyzers
Gas analysis based on tunable diode laser absorption spectroscopy (TDLAS) provides features of high sensitivity, fast
response and high selectivity. When target gas concentration is below a few parts-per-million spectrometers become more
and more sensitive towards noise, interference, drift effects and background changes associated with low level signals. It
is purpose of this paper to address some of the problems which are encountered at this low signal levels and to describe a
signal processing strategy for gas monitoring with wavelet transform. Different parameters of wavelet by taking gas
detection of H2S as an illustration are studied and an improved wavelet-based signal enhancement process is proposed
based on the feature of TDLAS second-harmonic signal. The algorithm uses bior3.9 wavelet basic function and
multi-resolution decomposition. The results show a plausible improvement in performance of TDLAS system and
enhancement of detection limit from 10ppm to hundreds of ppb level under various noise conditions.
KEYWORDS: Digital signal processing, Signal processing, Data acquisition, Signal generators, Absorption, Tunable diode lasers, Gases, Pollution, Calibration, Analog electronics
Increased demands on air quality have created incentives for new methods to monitor gas pollution. In this paper we will
present an online gas analyzer for industrial flue pollution monitoring based on tunable diode laser absorption
spectroscopy (TDLAS) technology. Signal measurements with a sensitive device inevitably suffer from the predictable or
unpredictable sources such as intensity fluctuations and the laser output wavelength dithers. In order to eliminate or at
least reduce the measurement uncertainty and gain high reliability, a close-circle digital-control module with functions of
digital signal generator, digital lock-in-amplifier (D-LIA), data acquisition and data processing has been developed to
substitute the previous independent signal generator board, analog lock-in-amplifier and data acquisition card in our
TDLAS system. With the help of the digital-control module and the advanced digital signal processing techniques the
performance of the TDLAS system has demonstrated great improvement in long term field tests.
Carbon monoxide (CO) is a very important gas generated in the industrial process; therefore to implement CO
concentration on-line monitoring is a key factor for industrial process control. Tunable diode laser absorption
spectroscopy (TDLAS) is a high sensitive, high selective and fast trace gas detection technique. With the features of
tunability and narrow linewidth of distributed feedback (DFB) laser and by precisely tuning its wavelength to a single
isolated absorption line of the gas, TDLAS technique can be utilized to accurately perform online gas concentration
monitoring with very high sensitivity. In this paper, a system for online monitoring of CO concentration is developed by
our group employing TDLAS technique. The experimental results are present and discussed in this report. The
characteristics of the system are: the sensitivity, 10ppm; detection accuracy, 0.02%; long term stability, 1%.
Knowing the quantity of pollutants that the vehicle fleet is emitting to the air has become a vital question in almost every major city in China. Finding and fixing gross polluters is therefore very important to control the urban air quality and protect the human health and the environment. Remote sensing is an important advance in the technology of on-road vehicle emissions testing because it is fast, mobile, and unobtrusive. This on-road vehicle emissions remote system is designed to measure the carbon monoxide, carbon dioxide and opacity from the vehicles's tailpipe based on the Tuneable Diode Laser Absorption Spectroscopy (TDLAS). There are several advantages of this system such as compact design and ease of use. The measurement principle and optical layout of the instrument has been described in this paper. Field testing at Beijing and Hefei were conducted over one year, more than 6000 vehicles were tested. This vehicle emissions remote system has been shown to be able to measure CO,CO2 and opacity from individual at highway speeds. In parallel, the plate license, speed, acceleration and length of vehicle are recognised by computer so that the owners of vehicles exceeding the permissible level of emissions can be identified.
With increased interest in the use of compressed gas as a vehicle fuel, attention has been focussed on the safety issues surrounding the tanks used to store the fuel. Currently it is necessary to remove the tanks from the vehicle in order to inspect them, which entails a considerable cost in manpower and takes the vehicle being out of service. We have been developing a sensor scheme that can provide in situ monitoring of the tanks condition. This entails bonding optical fibre sensors to the tank and using them to measure the strains experienced by the tank during pressurisation. If the tank is significantly damaged, then the tank will expand in a distorted manner. We therefore measure the strain characteristics of a healthy tank and use them as a reference for future measurements. The method of strain measurement is the well established rf subcarrier phase detection technique, however in this application the changes in optical power caused by microbending of the fibres during pressurisation produces inaccuracies. In order to overcome this problem we use both in-phase and quadrature mixing and then take the ratio of the outputs to obtain a value of arctangent that is independent of amplitude.
On-road motor vehicle emissions have been estimated being the single largest contributor of major atmospheric pollutants worldwide. The amount of motor vehicles in Beijing has rapidly grown in the past few years and will increase even more noticeably in the near future. It is therefore clearly required that a targeted approach to identifying the grossly polluting vehicles will have maximum impact on reducing the detrimental effects of the emissions. In this paper we will introduce an on-road motor vehicle exhaust emission monitor built by our group. This monitor was fabricated with tunable diode laser absorption spectroscopy (TDLAS) technique. On-road remote sensing of CO and CO2 of the motor vehicle exhaust emissions on Xueyuan road and at Dayangfang motorway toll station in Beijing were performed with this monitor. Analysis of the results shows that only 73.4% on-road motor vehicles are clean. Among the rest more than 15% are heavy gross polluters which should be warn for repair or removed from service.
In the past decade interest has focused on the remote analysis of emissions from motor vehicles using spectrometric techniques, driven by recognition of the fact that a very large proportion of the environmental damage done in this way originates with a small percentage of vehicles. Several instrumentation manufacturers now market such devices, and others are developing new technologies that will offer opportunities for enhanced performance and lower cost.
In this paper we demonstrate the prototype design of vehicle emission remote sensing system by differential optical absorption spectroscopy (DOAS) and tunable diode laser absorption spectroscopy (TDLAS). In this design, vehicle emissions will be measured when they past a on road remote sensing station while driving along a road. The concentrations of hydrocarbons (HC) and Oxides of Nitrogen (NOx) are measured by DOAS, and the concentrations of Carbon Monoxide (CO) and Carbon Dioxide (CO2) are measured by TDLAS. The measurement precisions for CO and CO2 are ±0.25%, and the detection limits for HC and NO are ±250ppm.
The structure and performance of an all polarization maintaining optical fibre hydrophone element is described and a sensitivity of -158±1.5dB is achieved. A 32-element, spatially multiplexed system is constructed with a noise-equivalent sound pressure of ~3.58x10-4 Pa per square root Hz at 1kHz and the major results of sea trials indicate that the system is useful for research and industrial applications.
This paper describes an all optical non-contact technique to determine material properties in a mechanical structure. Using optical generation of ultrasound and interferometric optical detection to determine the dispersion curves for acoustic (Lamb) waves, readily available numerical algorithms have produced values of Young's Modulus and Poisson ratio with confidence limits of a few percent. The approach may be applied to inspection regions with dimension in the range microns to centimetres.
We describe a novel but simple method of detecting Lamb waves through the measurement of the changes in the polarimetric state of light propagating through an optical fibre which has been either embedded into or bonded onto the plate to be tested. The directional properties of the sensor are described, as is its ability to detect a hole produced in the plate. We also show how the relative sizes of the detected signal amplitudes from the source PZT and from the hole vary according to the alignment of the source with the sensor axis.
Ultrasound has been demonstrated to be a perfect tool for NDT. There are several detectors that can be applied in NDT, for example fibre Bragg grating, interferometry, etc. Here we concentrate in polarimetric optical fibre detection.
In this paper we develop a simple but realistic analysis of the ultrasonic wavefront integration technique along an optical fibre for acoustic detection. Our model considers the perturbation caused by the acoustic wave as an isotropic change in the effective refractive index of the sensing fibre used as the detection system and neglects the polarization modulation. Also we assume the stress homoegeneous through the section of the fibre.
The theoretical analysis has been simulated in MATLAB. In this program we have analyzed the relation between the length of the sensing fibre, its distance to the ultrasound source and its sensitivity to ultrasound detection, for different orientations of the source with respect to the sensing fibre. The results indicate that optimum ultrasonic detection may be achieved through careful positioning and orientation of the optical fibre. These results may be applied, for example in NDT, where scattered ultrasound from defects introduces new effective sources that may be characterized by arrays of these integrating sensors.
In this report we will present some preliminary results on a RF sub-carrier based fibre strain sensing system for on-line monitoring of composite high pressure storage tanks for natural gas or hydrogen. By employing a temperature compensation scheme to the system developed has shown stable performance with better than 20μm length resolution and comparable long-term drift. The system uses readily available components and could be used for low-cost high volume applications such as on-line continuous monitoring.
The problem of damage detection and identification has a natural hierarchical structure. At the higher levels, one might require the diagnostic to return say, information about the expected time to failure of a structure, while at the lowest level, the question is simply of whether a fault is present or not. In many ways, the latter is the most fundamental. In response to the need for robust low-level damage detection strategies, the discipline of novelty detection has recently evolved. The problem is simply to identify from measured data if a machine or structure has deviated from normal condition, i.e., if the data are novel. The method requires a bank of normal condition data against which the possible damage condition data are compared.
This paper presents the basic principles of the laser generation of ultrasound (LGU) through thermal conversion and illustrates the approaches to its use in material evaluation using the broadband features of the source. Traditional LGU involves high energy optical pulse irradiation which often induces surface damage, especially in carbon or glass fiber composites. We therefore expand the concept into low power excitations using laser diode sources. This enables excitation without damage but requires coded temporal signals. Arrays of semiconductor laser sources can also produce very broadband acoustic signals, both temporally and spatially. Piezoelectric sources are usually the opposite constrained in both space and time. This basic observation opens new avenues of material investigation, some of which feature in this paper.
The DAMASCOS (DAMage Assessment in Smart COmposite Structures) project is a European Union funded program of work bringing together a number of academic and industrial partners throughout Europe. The aim of Damascos is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realize damage assessment and ageing characterization in composite structures. This paper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.
Ultrasonic Lamb waves have been proved to be a potentially desirable method for quick interrogation of large plate structures. Integrating fiber optic sensors offer great potential to monitor large areas due to their geometric flexibility. To date this potential has not been widely exploited and there have been only a few reports of non-destructive testing with integrating fiber optic sensors. In this report we present our preliminary measurements for the interrogation of hole defects in plate-like materials with surface-bonded integrating fiber optic sensors. Due to the signal integrating characteristics of fiber sensors very complicated signals were obtained. Therefore signal processing is needed to interpret waveforms for flaw detection.
The simultaneous detection of in-plane and out-of-plane ultrasound displacements is crucial for the interpretation of Lamb wave mode pattern and their interaction with defects within the plate. In this report we present our preliminary experiments for the simultaneous measurement of these two components. A two-channel fibre optic interferometer system has been built which enable the measurements of in-plane and out-of-plane displacements separately and simultaneously. One channel is a Michelson and the other a modified Michelson interferometer or a Mach-Zehnder interferometer. The Michelson interferometer allowed direct measurement of the absolute out-of-plane displacement while the modified Michelson interferometer measured the in-plane displacement. The Mach-Zehnder interferometer measured both components. This two-channel fibre optic interferometer allows a directly calibrated measurement of the two components of displacement simultaneously and offers a great insight into ultrasonic flaw interrogation in plate-like materials.
Ultrasonic Lamb waves have been extensively investigated for non-destructive testing of materials. Embedded or surface bonded optical fiber, acting as the signal arm of a Mach- Zehnder interferometer, is one method previously utilized to detect the Lamb waves. Optical fibers therefore have potential as permanent sensors for structural monitoring of damage and defects in materials. A greater understanding of the ultrasound interaction with the optical fiber sensor will bring application closer. In order to probe this interaction we built a two channel interferometer allowing ultrasound traveling through a material to be monitored simultaneously by a Mach-Zehnder interferometer and also a Michelson interferometer. The Michelson interferometer allows a non- constat measurement to be made of the absolute surface displacement associated with an ultrasonic Lamb wave. Comparison of the ultrasound signals detected by the two different interferometers provides a greater insight into the detection mechanism and sensitivity of the Mach-Zehnder interferometer. The work is then extended to look at embedded fibers in composite materials and damage detection.
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