On Optical Frequency Domain Reflectometry Research Articles

Application of phosphate doped fibers for OFDR dosimetry

We employ phosphate doped optical fibers for distributed dosimetry based on Optical Frequency Domain Reflectometry (OFDR) and demonstrate the effect of limited Integrated Insertion Loss Dynamic Range (ILDR) on the system performance.

Long-Range High Spatial Resolution Distributed Temperature and Strain Sensing Based on Optical Frequency-Domain Reflectometry

A novel approach to realize long-range distributed temperature and strain measurement with high spatial resolution, as well as high temperature and strain resolution, is proposed based on optical frequency-domain reflectometry (OFDR). To maintain the high measurement resolution and accuracy while increasing the sensing length, an optimized nonlinearity compensation algorithm is implemented to ensure a large wavelength tuning range. The compensated OFDR trace exhibits improved sensing resolution at a short distance, and the spatial resolution gradually deteriorates at the far end due to accumulated phase noise induced by fast tuning of the laser wavelength. We demonstrated the spatial resolution of 0.3 mm over a single-mode fiber sensing length of over 300 m, and temperature and strain resolution of 0.7 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}\hbox{C}$</tex></formula> and 2.3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\varepsilon$</tex></formula> with spatial resolution of up to 7 cm, respectively.

Signal processing method based on group delay calculation for distributed Bragg wavelength shift in optical frequency domain reflectometry

A signal processing method based on group delay calculations is introduced for distributed measurements of long-length fiber Bragg gratings (FBGs) based on optical frequency domain reflectometry (OFDR). Bragg wavelength shifts in interfered signals of OFDR are regarded as group delay. By calculating group delay, the distribution of Bragg wavelength shifts is obtained with high computational efficiency. We introduce weighted averaging process for noise reduction. This method required only 3.5% of signal processing time which was necessary for conventional equivalent signal processing based on short-time Fourier transform. The method also showed high sensitivity to experimental signals where non-uniform strain distributions existed in a long-length FBG.

Lateral Load Measurements Based on a Distributed Sensing System of Optical Frequency-Domain Reflectometry Using Long-Length Fiber Bragg Gratings

In this paper, we demonstrate the measurements of lateral loads in a distributed manner using long-length fiber Bragg gratings (FBGs) based on optical frequency-domain reflectometry (OFDR). Birefringence is induced at specific areas of the long-length FBGs by lateral compression. The distributions of the Bragg peaks were monitored by the OFDR sensing system, from which we calculated the applied lateral loads. Theoretical calculation results confirmed the validity of the experimental results. In addition, position-wise beat signals of the power of the obtained spectra were observed with smaller lateral loads that were not sufficient to provide two distinguished Bragg peaks. This unique phenomenon for the frequency-domain reflectometry scheme is explained theoretically. We proposed a method for utilizing the beat cycle to calculate smaller lateral loads.

Simultaneous distributed measurement of strain and temperature by polarizationmaintaining fiber Bragg grating based on optical frequency domain reflectometry

In this paper, a simultaneous strain and temperature measurement method using apolarization maintaining and absorption reducing (PANDA) fiber is applied to adistributed sensing system based on optical frequency domain reflectometry (OFDR). Afiber Bragg grating (FBG) whose length is 100 mm is inscribed into a PANDA fiber to forma long-length PANDA-FBG. We demonstrated simultaneous measurements of strain andtemperature distributions experimentally. In experiments the long-length PANDA-FBGwas bonded on a steel plate and various strains and temperatures were applied at thesame time to the plate. Consequently strain and temperature were successfullydiscriminated. However, larger errors were observed for distributed measurement results.

Qualification of a truly distributed fiber optic technique for strain and temperature measurements in concrete structures

Structural health monitoring is a key factor in life cycle management of infrastructures. Truly distributed fiber optic sensors are able to provide relevant information on large structures, such as nuclear power plants or nuclear waste disposal facilities. The sensing chain includes an optoelectronic unit and a sensing cable made of one or more optical fibers. A new instrument based on Optical Frequency Domain Reflectometry (OFDR), enables to perform temperature and strain measurements with a centimeter scale spatial resolution over hundred of meters and with a level of precision equal to 1 μ strain and 0.1 °C. Several sensing cables are designed with different materials targeting to last for decades, either embedded in the concrete or attached to the surface of the structure. They must ensure an optimal transfer of temperature and strain from the concrete matrix to the optical fiber. Based on the European guide FD CEN/TR 14748 “Non-destructive testing – Methodology for qualification of non-destructive tests”, a qualification method was developed. Tests were carried out using various sensing cables embedded in the volume or fixed to the surface of plain concrete specimens and representative-scale reinforced concrete structural elements. Measurements were performed with an OFDR instrument, while mechanical solicitations were imposed to the concrete element. Preliminary experiments seem very promising since measurements performed with distributed sensing systems are found comparable to values obtained with conventional sensors used in civil engineering and with the Strength of Materials Modelling. Moreover, the distributed sensing system makes it possible to detect and localize cracks appearing in concrete during the mechanical loading.

J044021 分布型光ファイバセンサを用いた接着継手内部のひずみ計測([J04402]知的材料・構造システム(2))

We have developed the distributed strain measurement system based on optical frequency domain reflectometry (OFDR) system. It can measure a strain distribution with the high spatial resolution at an arbitrary position along long-length fiber Bragg grating (FBG) sensors. In this study, we embedded the long-length FBG sensors into the adhesive of a single-lap joint. We could successfully measure the strain distribution at the interface between the adhesive and the adherend in the tensile test.

Performance of a solid-state frequency-shifted feedback laser in optical ranging

The performance of a frequency-shifted feedback laser (FSFL) using a Nd:YVO4 crystal as gain medium was investigated as light source in high accuracy optical ranging based on optical-frequency domain reflectometry (OFDR). The FSFL generates a comb of chirped frequency components over a bandwidth of 45GHz with chirp rates of 3.8×1017Hz/s. In OFDR, distance accuracies better than 25um at a data measurement time of 2ms were demonstrated at a standoff distance of 5m. The results show that the FSFL is a promising light source for high accuracy, high speed 3D measurement applications.

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.

Structural health monitoring using fiber optic distributed sensors for vacuum-assisted resintransfer molding

In this study we implemented manufacturing process and strain monitoring of a compositestructure by optical fiber sensors for vacuum-assisted resin transfer molding (VaRTM).Optical fibers with fiber Bragg gratings were embedded into a glass fiber reinforced plasticspecimen made by VaRTM and the applicability of structural health monitoring with fiberBragg grating (FBG) sensors based on optical frequency domain reflectometry(OFDR) was investigated. In this study, long-gage FBGs which are 10 timeslonger than ordinary FBGs (which are about 10 mm long) were employed fordistributed sensing. We can easily map the strain or temperature profile alonggratings by OFDR and the spatial resolution of this sensing technique is about1 mm. The resin flow process in VaRTM could be monitored by measuring thedifference in temperature between the resin and preform. Then, the shrinkageof resin could be also monitored during the curing process. The specimen wasthen subjected to a bending load in a three-point bending test and the straindistributions along the FBGs were measured. From these results we could show theapplicability of distributed sensors to quality assurance of a composite structure madeby VaRTM and assessment of the structural integrity of in-service compositestructures.

2504 分布型光ファイバセンサによる溶接部のひずみモニタリング(J10-1 計測・モニタリング・解析(1),J10 知的材料・構造システム)

Distributed strain sensor with particularly high spatial resolution is required for assessing integrity of welded joint by monitoring its strain distribution. A distributed strain measurement system based on optical frequency domain reflectometry (OFDR) can measure a strain at arbitrary position along a long gauge FBGs (fiber Bragg grating) with high spatial resolution less than 1mm. In this study, we evaluated the applicability of the system to strain monitoring of welded joints in static tensile tests. We could successfully measure the strain distribution along the weld line for 100mm, and could show the process of degradation of the specimen. In addition, we could observe that the strain distribution sharply fluctuates with acoustic emissions in the area where anomalous distortions in the strain distribution were observed under the relatively lower load. We could also measure strain distribution ranged over 8000με within the FBG with gauge length of 100mm. The measured strain was mare than -10000με at maximum. Thus, the system was proved to have excellent spatial resolution and the measurement range sufficient to monitor the strain distribution of the welded joints.

2502 OFDRによるひずみ分布計測の測定範囲の拡張に関する考察(J10-1 計測・モニタリング・解析(1),J10 知的材料・構造システム)

We have developed a new distributed strain measurement system with a long gauge Fiber Bragg Grating (FBG) sensor based on Optical Frequency Domain Reflectometry (OFDR). This sensing system enables us to measure strain at an arbitrary position along the FBG sensor with a high spatial resolution below 1mm. However, the sensing range of this system is confined to 〜10m at present. To expand the measurement range, we have problems in several components of the measurement system, such as the signal processing capability, limitation of measurable range due to reflection properties of the FBG sensor, and the effect of length fluctuation in the arm length of the interferometer. This paper discusses these factors using a simulation model of based on coupled mode theory. The signal analysis time was shortened by confining the analyzing section. The simulated result showed that, the measurable range is approximately 1.5m, and the length fluctuation in the arm length of measurement interferometer has a significant effect on the measured result.

Distributed Measurements with a Long Gauge FBG Sensor Using Optical Frequency Domain Reflectometry (1st Report, System Investigation Using Optical Simulation Model)

Optical fiber sensors are used in order to monitor strain distribution of a structure, because they have many advantages for structural health monitoring. By using Fiber Bragg Grating (FBG) sensors with a general interrogation system e.g. Wavelength Division Multiplexing (WDM) makes it possible to measure only average strain within the gauge length. In addition, it is necessary to allocate enough numbers of FBG sensors for monitoring stress concentration overall. Therefore, it is expected to apply a distributed strain sensor with the higher spatial resolution and the longer sensing length to accurate and effective monitoring of stress concentration. We present a new strain measurement system with a long gauge FBG sensor based on Optical Frequency Domain Reflectometry (OFDR), which enables us to measure fully distributed strain at special high spatial resolution. In this paper, we describe the principle and optical simulation model of the proposed measurement system and show results of numerical calculations.

3208 長ゲージFBGによる応力分布計測(J06-2 計測・モニタリングI,J06 知的材料・構造システム)

We reported the applicability of the distributed sensing system based on optical frequency domain reflectometry (OFDR) to structural health monitoring. We could principally obtain strain or temperature distribution along an optical fiber with the spatial resolution of less than several mm by using the system. In this study, we implemented experiments to measure the strain distributions on the open hole specimen to monitor stress concentration, and we also analyzed the response distance using finite element method (FEM) to investigate the limitation on the spatial resolution under the influences of physical characteristics of coating materials and adhesives. Then it is demonstrated that the current system has the high spatial resolution.

High-resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer

A novel differential mode delay (DMD) measurement technique for a multimode optical fiber based on optical frequency domain reflectometry (OFDR) has been proposed. We have obtained a high-resolution DMD value of 0.054 ps/m for a commercial multimode optical fiber with length of 50 m by using a modified OFDR in a Mach-Zehnder interferometer structure with a tunable external cavity laser and a Mach-Zehnder interferometer instead of Michelson interferometer. We have also compared the OFDR measurement results with those obtained using a traditional time-domain measurement method. DMD resolution with our proposed OFDR technique is more than an order of magnitude better than a result obtainable with a conventional time-domain method.

長ゲージ長FBGによるひずみの分布計測(OS1-1 光ファイバセンサ,OS1 構造ヘルスモニタリング)

We developed a new sensing technique with long gauge FBG sensors and we can obtain strain distributions of the sensing region with high spacial resolution by using this technique. The sensing region is equivalent to the gauge length of FBCG An FBG sensor with 10 cm gauge length was made and bonded on a tensile test specimen. The FBG sensor was connected to the measurement system. This system is based on Optical Frequency Domain Reflectometry (OFDR). In tensile tests, we could obtain very high resolution strain distribution on the specimen from the system. This new sensing technique can be apply to health monitoring of a structure which may have stress concentration.

A new method for measurement of group velocity dispersion of optical fibers by using a frequency-shifted feedback fiber laser

A new method for the measurement of the group velocity dispersion (GVD) in optical fibers, based on optical frequency domain reflectometry (OFDR), is proposed and demonstrated. Frequency chirped light is used as a light source, which allows the dispersion to be determined from changes in the frequency chirp rate. Accurate measurement of the average dispersion, at a single terminal, in fibers up to several tens of kilometers, is accomplished using a frequency-shifted feedback fiber laser as a light source.