Dae-Cheol Seo

Novel auto-correction method in a fiber-optic distributed-temperature sensor using reflected anti-Stokes Raman scattering.

A novel method for auto-correction of fiber optic distributed temperature sensor using anti-Stokes Raman back-scattering and its reflected signal is presented. This method processes two parts of measured signal. One part is the normal back scattered anti-Stokes signal and the other part is the reflected signal which eliminate not only the effect of local losses due to the micro-bending or damages on fiber but also the differential attenuation. Because the beams of the same wavelength are used to cancel out the local variance in transmission medium there is no differential attenuation inherently. The auto correction concept was verified by the bending experiment on different bending points.

Monitoring of fatigue damage of composite structures by using embedded intensity-based optical fiber sensors

In this paper, the feasibility of real-time monitoring of the fatigue damage of composite laminates under fatigue loading using the embedded intensity-based optical fiber (IBOF) sensors was evaluated. The IBOF sensor system used in this study has a relatively simple sensing principle. Thus the system can be composed of inexpensive components compared with other optical fiber sensors such as Fabry-Perot and Bragg grating sensors. The performance of the fabricated IBOF sensors was evaluated by comparison with a surface-mounted extensometer. The process of conversion of the detected IBOF sensor signal into the stiffness was proposed. Experimental results showed that the sensing response of embedded IBOF sensors showed good agreement with that of the surface-mounted extensometer. The IBOF sensor system showed good potential as a health monitoring system which can give composite structures the capability of fatigue damage monitoring by detecting the stiffness change of the composite structures under fatigue loading. The IBOF sensors showed relatively good durability under fatigue loading compared with commercial electrical strain gages.

Terahertz spectrum analyzer based on frequency and power measurement

We demonstrate a terahertz (THz) spectrum analyzer based on frequency and power measurement. A power spectrum of a continuous THz wave is measured through optical heterodyne detection using an electromagnetic THz frequency comb and a bolometer and power measurement using a bolometer with a calibrated responsivity. The THz spectrum analyzer has a frequency precision of 1x10(-11), a frequency resolution of 1Hz, a frequency band up to 1.7THz, and an optical noise equivalent power of approximately 1 pW/Hz(1/2).

Sensitivity enhancement of fiber optic FBG sensor for acoustic emission

A fiber optic Bragg grating based acoustic emission sensor system is developed to provide on-line monitoring of cracks or leaks in reactor vessel head penetration of nuclear power plants. Various type of fiber Bragg grating sensor including the variable length of sensing part was fabricated and prototype sensor system was tested by using PZT pulser and pencil lead break sources. In this study, we developed a cantilever type fiber sensor to enhance the sensitivity and to resonant frequency control. Two types of sensor attachment were used. First, the fiber Bragg grating sensor was fully bonded to the surface using bonding agent. Second one is that one part of fiber was partially bonded to surface and the other part of fiber will be remained freely. The resonant frequency of the fiber Bragg grating sensor will depend on the length of sensing part. Various kinds of resonant type fiber Bragg grating acoustic emission sensors were developed. Also several efforts were done to enhance the sensitivity of FBG AE sensor, which include FBG spectrum optimization and electrical and optical noise reduction. Finally, based on the self-developed acquisition system, a series of tests demonstrate the ability of the developed fiber sensor system to detect a pencil lead break event and continuous leak signal.

Novel fiber optic sensor probe with a pair of highly reflected connectors and a vessel of water absorption material for water leak detection.

The use of a fiber optic quasi-distributed sensing technique for detecting the location and severity of water leakage is suggested. A novel fiber optic sensor probe is devised with a vessel of water absorption material called as water combination soil (WCS) located between two highly reflected connectors: one is a reference connector and the other is a sensing connector. In this study, the sensing output is calculated from the reflected light signals of the two connectors. The first reflected light signal is a reference and the second is a sensing signal which is attenuated by the optical fiber bending loss due to the WCS expansion absorbing water. Also, the bending loss of each sensor probe is determined by referring to the total number of sensor probes and the total power budget of an entire system. We have investigated several probe characteristics to show the design feasibility of the novel fiber sensor probe. The effects of vessel sizes of the probes on the water detection sensitivity are studied. The largest vessel probe provides the highest sensitivity of 0.267 dB/mL, while the smallest shows relatively low sensitivity of 0.067 dB/mL, and unstable response. The sensor probe with a high output value provides a high sensitivity with various detection levels while the number of total installable sensor probes decreases.

Monitoring of fatigue crack growth of cracked thick aluminum plate repaired with a bonded composite patch using transmission-type extrinsic Fabry–Perot interferometric optical fiber sensors

Recently, optical fiber sensors have been increasingly applied to monitor various engineering and civil structural components. These fiber optic smart structures allow engineers to add nervous systems to their designs, giving structures capabilities that would be very difficult to achieve by other means, including continuous assessment of damage processes. Several studies associated with crack monitoring using optical fiber sensors have been reported. In this study, we used recently developed transmission-type extrinsic Fabry–Perot interferometric (TEFPI) optical fiber sensors for the monitoring of fatigue crack growth behavior of cracked thick aluminum plate repaired with a bonded composite patch. The TEFPI optical fiber sensor has both the advantages of reflection-type EFPI optical fiber sensors and a simpler and more effective function to distinguish strain direction than do reflection-type EFPI optical fiber sensors. The objective of this study is to evaluate the potentiality of the application of TEFPI optical fiber sensors to the monitoring of the fatigue crack growth behavior of composite patch repaired structures. The sensing principle and the sensor construction of the TEFPI optical fiber sensor are presented. The experimental results from fatigue tests of center cracked tension aluminum specimens repaired with a bonded composite patch are presented and discussed. TEFPI optical fiber sensors are embedded and surface bonded to the composite patch at several locations. The experimental results show that it is possible to monitor the fatigue crack growth behavior of composite patch repaired structures using TEFPI optical fiber sensors.

Restoration of Reflection Spectra in a Serial FBG Sensor Array of a WDM/TDM Measurement System

A restoration method for reflection spectra in a serial FBG sensor array with spectral shadowing is proposed and experimentally demonstrated in a WDM/TDM combined multiplexing system. The SNR of each FBG sensor is formulated and analyzed as a function of the number and reflectivities of serial FBG sensors. The maximum number of FBG sensors in a single fiber line can be determined by the approximate formula. In the test using two FBG sensors, the restored reflection spectrum of second FBG sensor is shown to be very well matched with the original reflection spectrum. Using the proposed restoration method, the maximum peak detection error in a strain experiment is suppressed drastically by almost seven-fold, from 0.074 nm to 0.011 nm.

Study on the Nonlinear Electromagnetic Acoustic Resonance Method for the Evaluation of Hidden Damage in a Metallic Material

Recently, much attention has been paid to nonlinear ultrasonic technology as a potential tool to assess hidden damages that cannot be detected by conventional ultrasonic testing. One nonlinear ultrasonic technique is measurement of the resonance frequency shift, which is based on the hysteresis of the material elasticity. Sophisticated measurement of resonance frequency is required, because the change in resonance frequency is usually quite small. In this investigation, the nonlinear electromagnetic acoustic resonance (NEMAR) method was employed. The NEMAR method uses noncontact electromagnetic acoustic transducers (EMATs) in order to minimize the effect of the transducer on the frequency response of the object. Aluminum plate specimens that underwent three point bending fatigue were tested with a shear wave EMAT. The hysteretic nonlinear parameter , a key indicator of damage, was calculated from the resonance frequency shift at several levels of input voltage. The hysteretic nonlinear parameter of a damaged sample was compared to that of an intact one, showing a difference in the values.

Feasibility study for monitoring of off-shore pipelines using BOTDA system

The oil leakage of off-shore pipelines will cause ocean contamination and economic losses. These accidents may happen by the failures of offshore pipelines due to corrosion, impulse and free-spanning. So, it is very urgent on pipeline health monitoring. Fiber optic distributed sensors should be used to know when and where failures may occur. In this study, a feasibility of BOTDA (Brillouin Optical Time Domain Analysis) system is studied on off-shore pipeline distributed strain monitoring influenced by free spanning. Strain distribution of an off-shore pipeline is calculated by numerical analysis and strain measurement experiments are carried on a beam bending test using BOTDA system. BOTDA could be an excellent tool to monitor the long-distance pipeline.

Image Enhancement for Sub-Harmonic Phased Array by Removing Surface Wave Interference with Spatial Frequency Filter

Closed cracks are difficult to detect using conventional ultrasonic testing because most incident ultrasound passes completely through these cracks. Nonlinear ultrasound inspection using sub-harmonic frequencies a promising method for detecting closed cracks. To implement this method, a sub-harmonic phased array (PA) is proposed to visualize the length of closed cracks in solids. A sub-harmonic PA generally consists of a single transmitter and an array receiver, which detects sub-harmonic waves generated from closed cracks. The PA images are obtained using the total focusing method (TFM), which (with a transmitter and receiving array) employs a full matrix in the observation region to achieve fine image resolution. In particular, the receiving signals are measured using a laser Doppler vibrometer (LDV) to collect PA images for both fundamental and sub-harmonic frequencies. Oblique incidence, which is used to boost sub-harmonic generation, inevitably produces various surface waves that contaminate the signals measured in the receiving transducer. Surface wave interference often degrades PA images severely, and it becomes difficult to read the closed crack’s position from the images. Various methods to prevent or eliminate this interference are possible. In particular, enhancing images with signal processing could be a highly cost-effective method. Because periodic patterns distributed in a PA image are the most frequent interference induced by surface waves, spatial frequency filtering is applicable for removing these waves. Experiments clearly demonstrate that the spatial frequency filter improves PA images.