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A novel tunable erbium-doped fiber ring laser (EDFRL) using a cascaded-chirped long period fiber grating (C-CLPG) as a wavelength selection element is proposed. The oscillation wavelength is determined by the one of the spectral peaks of the C-CLPG used, and the oscillation output provides a high signal to noise (S/N) ratio detection and a highly sensitive measurement of the temperature due to its high power and narrow spectral output. In the experiment, it is confirmed that the wavelength of the output shifts in accordance with the temperature-induced spectral shift of the C-CLPG transmittance spectrum and the temperature sensitivity is obtained to be ∼-0.2 nm/°C within the wavelength range of 1567.30 ∼ 1575.78 nm. The oscillation wavelength range is to be limited depending on the fringe spacing of the channeled spectrum of C-CLPG, which limits the temperature measurement range, but a data processing approach to solve this problem is additionally proposed and its availability is also presented.

A graphene oxide-based α-K6P2W18O62 (Dawson-type polyoxometalate) nanocomposite was formed by using two types of graphene oxide (GO) samples with different C/O compositions. Herein, based on the interaction of GO, polyoxometalates (POMs), and their nanocomposites with the Cs cation, quantitative data have been provided to explicate the morphology and Cs adsorption character. The morphology of the GO-POM nanocomposites was characterized by using TEM and SEM imaging. These results show that the POM particle successfully interacted above the surface of GO. The imaging also captured many small black spots on the surface of the nanocomposite after Cs adsorption. Furthermore, ICP-AES, the PXRD pattern, IR spectra, and Raman spectra all emphasized that the Cs adsorption occurred. The adsorption occurred by an aggregation process. Furthermore, the difference in the C/O ratio in each GO sample indicated that the ratio has significantly influenced the character of the GO-POM nanocomposite for the Cs adsorption. It was shown that the oxidized zone (sp2/sp3 hybrid carbon) of each nanocomposite sample was enlarged by forming the nanocomposite compared to the corresponding original GO sample. The Cs adsorption performance was also influenced after forming a composite. The present study also exhibited the fact that the sharp and intense diffractions in the PXRD were significantly reduced after the Cs adsorption. The result highlights that the interlayer distance was changed after Cs adsorption in all nanocomposite samples. This has a good correlation with the Raman spectra in which the second-order peaks changed after Cs adsorption.

A novel tunable Erbium-doped fiber ring laser (EDFRL) with a cascaded-chirped long-period fiber grating (C-CLPG) as a wavelength selection filter is proposed from the viewpoint of the sensor use, in which a variable optical attenuator (VOA) is employed as an intracavity loss modulator to change the oscillation wavelength region so that the resultant tuning wavelength range is widened. In the demonstrative experiment for temperature measurements, oscillation over the wavelength range of 12.85 nm (1557.62~1570.47 nm), which is more than three times range of the previously presented laser and is equivalent to 64 °C in terms of temperature change, was achieved, while a single-wavelength oscillation was maintained. In addition, a practical technique for realizing a temperature measurement by combining with the VOA control is also discussed.

In this paper we have investigated optical frequency-shift sensitivities of the transmittance channeled spectra of cascaded-chirped long period fiber gratings (C-CLPGs) to the applied strain and temperature change from the viewpoint of the sensor use. In the experiment, several kinds of C-CLPGs have been fabricated by changing the cascaded distance between CLPGs and the strain- or temperature-induced spectral shifts for three different order of the cladding modes are measured simultaneously by use of a cross-correlation technique which provides highly precise determination of the spectral shift. As a result, it is confirmed that the frequency-shift sensitivities of the C-CLPGs to the strain and temperature are larger with the higher order of cladding mode, while the dependences for the temperature sensitivity is much less than that for the strain. In addition, the dependences for the cascaded distance are also investigated.

We present a multipoint vibration sensing system using fiber Fabry-Perot interferometers consisting of low-reflective fiber Bragg gratings (FBG-FPI) and a current-modulated laser diode. The lasing wavelength of a laser diode can be modulated by changing the injection current. Therefore, high-speed current modulation makes possible to achieve high-speed wavelength sweep exceeding 100 kHz. Using a current modulated laser diode as the source for interrogation of FBG-FPI sensor array, high-speed multipoint vibration measurement can be realized.

We have investigated optical frequency-shift sensitivity of the transmittance channeled spectrum of cascaded-chirped long period fiber gratings (C-CLPGs) to an applied strain from the viewpoint of the sensor use. In the experiment, eight different kinds of C-CLPGs have been fabricated by changing the cascaded distance for two types of CLPGs and the strain-induced spectral shifts are measured by use of a cross-correlation technique which provides highly precise determination of the spectral shift. As a result, it is confirmed that the higher frequency-shift sensitivities of the C-CLPGs to the strain obtain with the higher order of cladding mode.

A novel tunable and switchable Erbium-doped fiber ring laser (EDFRL) is constructed from the viewpoint of sensor applications. The laser consists of a cascaded-chirped long period fiber grating (C-CLPG) and a variable optical attenuator (VOA). The oscillation wavelength is used for sensing a measurand. As a result, wavelength range selecting over about 1557.57 ~ 1570.68 nm with the tuning range within ~5.6 nm is demonstrated, and the sensitivity of ~ −0.22 nm/°C is recorded.

A fast interrogation method using a sinusoidal modulated laser diode for a fiber Fabry–Perot interferometric sensor consisting of Bragg gratings (FBG–FPI) is proposed.. The FBG–FPI has sharp transmittance peaks in the reflection band of the FBGs. Wavelength sweep produced by current modulation of a laser diode can be used to detect the peak position. This enables high-resolution strain or temperature measurement. To precisely control the current, the current modulation is realized using a laser diode controller (LDC) with external modulation function. In the modulation by a sawtooth wave, the possible speed of wavelength sweeping is limited to 100 kHz or less due to the bandwidth limitation of an LDC and thermal effect in a laser diodeUsing a sinusoidal wave as a modulation waveform enables wavelength sweeping at speeds exceeding 100 kHz. The modulation characteristics of the laser wavelength is evaluated experimentally and the operating wavelength is monitored using an asymmetric Mach–Zehnder interferometer. The resolution of 0.2 fm/ $$\sqrt{\mathrm{Hz}}$$ and measurement time of 1 $$\upmu$$ s were experimentally demonstrated in the present sensor.

Fast and high-resolution simultaneous measurement of temperature and strain is experimentally demonstrated. A Fabry–Perot interferometer fabricated in a polarization-maintaining fiber has sharp transmission peaks, which are divided into two sets corresponding to the polarization of the transmitted light. The sharp transmission peaks shift in response to changes in temperature and strain, and because the dependence coefficients are different for each set, temperature and strain can be simultaneously measured with high resolution. To read the position of a sharp transmission peak, wavelength sweeping with narrowband light is necessary. By using injection current modulation in a laser diode, high-speed wavelength sweeping and short measuring time are realized. We performed simultaneous measurements with a measurement time of 20 $\mu$ s; measurement resolution of 0.1 $^\circ$ C and 0.8 $\mu \epsilon$ for temperature and strain, respectively; and noise spectral density of 30 n $\epsilon /\sqrt{\rm Hz}$ . We further experimentally demonstrated that vibration with an amplitude of 20 $\mu \epsilon$ and a frequency of 1 Hz and heating at a rate of 0.7 $^\circ$ C $/$ s can be simultaneously measured.

著者人数: 35名, 形態: カラー図版あり, Number of authors: 35, Physical characteristics: Original contains color illustrations, Accepted: 2018-01-22, 資料番号: PA1810022000

Multipoint vibration sensing is demonstrated using fiber Bragg gratings and optical-frequency-domain reflectometry (OFDR). Using OFDR, a few thousand arrayed sensors can be monitored with a measurement time limited by the wavelength-scanning rate of the light source. Here, a laser diode serves as the source. Its wavelength can be modulated by varying the injection current at high frequency using a laser-diode controller, so that high-speed scanning can be achieved. It is experimentally demonstrated that the proposed sensor achieves vibration measurement with a noise density of 7 fm/ $\sqrt{\rm Hz}$ , measurement time of 10 $\mu$ s.

A fiber Bragg grating (FBG)-based multi-point sensing system for the simultaneous measurement of strain and temperature is proposed, with a Fabry-Perot interferometer (FPI) consisting of low reflective FBGs inscribed inside a polarization-maintaining (PM) fiber is introduced as a sensor head. A multi-point sensing method with high dynamic range and short measurement time can be realized by analyzing the reflection spectrum of low reflective FBG-FPIs, and a simultaneous measurement of strain and temperature using two orthogonal polarization modes is enabled by the high birefringence of a PM fiber. An experimental demonstration of multi-point measurement utilizing the proposed system is reported.

Cascaded chirped long period fiber gratings (C-CLPG) are provided as sensing elements for use in intensity-based underwater acoustic fiber-optic sensors, in which C-CLPGs are inscribed in photosensitive fiber by means of point-by-point technique with UV radiation. For a demonstration, C-CLPGs are immersed in a water vessel and exerted by underwater acoustic waves. In the demonstration, higher sensitive underwater acoustic detection is obtained by employing a C-CLPG with the steeper channeled spectrum.

By using the optical frequency reflectometry method and modulated laser diode, it is possible to read many fiber Bragg grating (FBG) sensors at high speed. In this method, the measurement range by the sensor is limited by the wavelength sweep range of the light source. Since the range of the wavelength sweep by the modulated laser diode is as narrow as several hundreds of pm, the measurement range also becomes narrow. In order to overcome this limitation, we are considering using multiple modulated laser diodes at the same time. Light beams from multiple modulated laser diodes modulated with different frequencies are superimposed and used for sensor reading. Signals for each light source can be separated by using frequency analysis. In experiments, light of multiple laser diodes was superimposed and made incident on the sensor, and frequency analysis of the reflected light from the sensor was successfully performed to separate the signals for each light source.

A precise measurement technique of long period of fiber grating sensor (LPG) is demonstrated by employing a cascaded chirp LPG (C-CLPG) as a sensing device and by adopting a Fourier transform method for an interrogation technique. In this approach, transmittance of the C-CLPG exhibits periodic channeled spectrum and its strain- and/or temperature-dependent wavelength shift is analyzed by obtaining a cross-spectrum function between the original and the shifted channeled spectra, in which the phase term of the cross-spectrum corresponds to the wavelength shift. Since the phase component is determined by using all the channeled spectrum over a wider bandwidth of the C-CLPG, more precise measurements of the strain and temperature are expected than that of the peak search method in that the wavelength shift is determined by the shift of one of peaks of the channeled spectrum. In the experiment, the highly precise strain and temperature measurements are presented by comparing with the results of the peak search method.

High-resolution simultaneous measurement of vibration and temperature using an optical fiber sensor is presented. Strain and temperature can be measured simultaneously by using two types of reflection spectra of a Fabry-Perot interferometer consisted of fiber Bragg gratings in a polarization maintaining fiber. The fine structure of reflection spectrum of the interferometer, i.e. PM-FBG-FPI, enables high-resolution detection of wavelength shifts. In oder to measure solid vibration, we present a fast interrogation method using current modulation of a laser diode for PM-FBG-FPI sensors. The resulting fast measurement is demonstrated experimentally

We present a novel optical fiber sensor based on a Fabry-Perot interferometer with chirped fiber Bragg gratings (CFBG-FPI) using a narrow wavelength sweep range. In a fiber Bragg grating (FBG) sensor, wavelength shifts in the reflection spectrum are detected. Using a fiber Fabry-Perot interferometer with FBGs (FBG-FPI), the resolution can be improved, but its dynamic range is reduced. Here, a CFBG-FPI is used to expand the dynamic range. It has many sharp transmittance peaks in its wide reflection band. Combining precise detection of the position of a transmittance peak position in a narrow wavelength sweep with the change in the number of peaks entering the sweep range, measurements as precise as FBG-FPI sensors are achieved over a wide dynamic range, a hundred times larger than that of FBG-FPI sensors.

Soft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from micro-vibration from cryocoolers mounted on the dewar. This is mitigated for the flight model by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the flight model was verified before launch of the spacecraft in both ambient condition and thermal-vac condition, showing no detectable degradation in energy resolution. The in-orbit performance was also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the micro-vibration could degrade the cryogenic detector is shown.

Simultaneous multi-parameter sensing using a cascaded chirp LPG is proposed, in which two resonance peaks are utilized for discriminating between strain and temperature and applied strain and temperature changes are simultaneously determined within ±23 micro-strain and ±0.14 degree C, respectively.

An in-fiber Fabry-Perot interferometer with fiber Bragg grating (FBG) mirrors (FBG-FPI) yields extremely narrow transmission peaks within the FBG reflection wavelength range. Periodical scanning of the laser light source wavelength produces a train of optical pulses and the occurrence time of the pulse is modulated as the FBG-FPI is under influence of strain. When dynamic strain due to mechanical vibration is applied to the FBG-FPI, the detected signal of the pulse train is Fourier transformed and processed to reproduce the waveform of the vibration-induced strain. Furthermore, an inexpensive DFB laser can be utilized as a light source because the narrowness of the transmission peaks does not require broad wavelength scanning.

A measurement method for many sharp transmittance peaks of a fiber Fabry–Perot interferometer (FPI) with high resolution in a short time is proposed. Using the proposed method, temperature and strain dependences of an FPI constructed using polarization maintaining type fiber Bragg grating (PM-FBG-FPI) are evaluated. By detecting wavelength shift occurring in many peaks of a PM-FBG-FPI with high resolution, it is possible to quantitatively evaluate a slight shift coefficient difference due to the difference in resonance order of each peak. Simultaneous measurement resolution of temperature and strain by the PM-FBG-FPI sensor is also improved by using the average value of the wavelength shift detected at many peaks. The resulting high-resolution measurements are demonstrated experimentally.

We present multipoint vibration sensing using fiber Bragg gratings and optical frequency domain refrectometry (OFDR). In OFDR based method, the maximum number of arrayed sensor can be few thousands and the measurement time is determined by wavelength scanning rate of a light source. In our sensor system, a laser diode is used as a wavelength scanning light source. Lasing wavelength of a laser diode can be modulated by changing its injection current. The injection current can be precisely modulated at high frequency up to 1 MHz using a laser-diode controller and wavelength scanning can be then easily achieved with a laser diode.

A fiber-optic strain sensor array using long period fiber gratings (LPGs) is proposed and demonstrated, in which cascaded LPGs (C-LPGs) are employed as the sensing elements and a Fourier transform technique is applied to their periodic channeled spectra for the interrogation scheme. In this technique, strain-induced shift of the channeled spectrum of C-LPG can be determined precisely from the cross-spectrum (i.e. Fourier transform of cross-correlation) between the original and shifted channeled spectra and multiplexing operation of C-LPG sensor array can be achieved by using the different C-LPGs having the channeled spectra with various periods. In the experiment, several kinds of CLPGs are fabricated by UV-irradiation technique and simultaneous multipoint strain measurements are successfully demonstrated.

We present multipoint vibration sensing using fiber Bragg gratings and optical frequency domain refrectometry (OFDR). In OFDR based method, the maximum number of arrayed sensor can be few thousands and the measurement time is determined by wavelength scanning rate of a light source. In our sensor system, a laser diode is used as a wavelength scanning light source. Lasing wavelength of a laser diode can be modulated by changing its injection current. The injection current can be precisely modulated at high frequency up to 100 kHz using a laser-diode controller and wavelength scanning can be then easily achieved with a laser diode.

Highly precise fiber optic strain and temperature measurements using chirped long period fiber grating (CLPG) are proposed and demonstrated, in which cascaded CLPGs (C-CLPGs) are employed as the sensing element and a Fourier transform technique is applied for the interrogation scheme. In this technique, strain and/or temperature-induced wavelength shift is determined precisely from the cross-correlation or cross-spectrum between the original and shifted channeled spectrum. In the experiment, C-CLPGs are fabricated by UV-irradiation technique, and strain and temperature characteristics are investigated. The highly precise measurements are confirmed by comparing with the results of the peak tracking method.

This paper reviews a series of our research works to have dealt with a compression bending test method during these years. Starting from the basic concept, some refinements of data reduction methodology and some advancements of test fixtures are described in conjunction with the application to flat coupons as well as pipe configurations. The most recent work of an eccentric compression bending is also reviewed.

This paper reports on a test method to evaluate the shearing modulus of foamed core which is used for sandwich plates. Although this kind of relatively soft core is widely used for various applications, a suitable test method has not necessarily been established. In the present study, a symmetric test sample with four pieces of core is used to eliminate undesirable deformation due to eccentric load. By varying the core length and the core thickness, we found that the shearing modulus strongly depends on the specimen configuration. This tendency was also recognized from the results of FEM and beam theory analyses where the shearing deformation is incorporated. Finally we propose a methodology for obtaining a reliable shearing modulus with the aid of FEM analysis.

A compression bending test method for CFRP pipes is described in this paper. Although CFRP pipes are widely used in such applications as truss members of architecture, golf clubs and fishing rods, there seem to be no suitable test methods for CFRP pipes. A conventional bending test of three- or four-point bending is not necessarily suitable because a high stress concentration due to a loading device takes place. In the past, we have developed a new bending test method, namely, a compression bending test and it was successfully applied to a flat specimen. In the present paper, we propose to apply this method to a pipe. The results show that the bending modulus was close to that measured by the three-point bending test and that the bending strength was higher than that measured by the three-point bending test. Hence, the proposed test method is well-suited for testing of pipe structures.

This paper reports a new test method to evaluate the bending strength of composite pipes. In the past, we proposed a compression bending test method to get more reliable bending strength values than the conventional three- or four-point bending test. This compression bending was successfully applied not only to flat CFRP coupons but also to slender CFRP pipes. However, if the diameter of the pipe is relatively large, this compression bending is not necessarily appropriate. Therefore, as a step forward to accurate determination of compression bending, an eccentric compression bending test method is tried in the present paper.

A fast and high-resolution simultaneous measurement of strain and temperature using an optical fiber sensor is presented. Temperature and strain can be measured simultaneously by using two types of reflection spectra of a Fabry–Perot interferometer consisting of fiber Bragg gratings in a polarization-maintaining fiber (PM-FBG-FPI). The fine structure of a reflection spectrum of the PM-FBG-FPI enables the high-resolution detection of wavelength shifts. We present a fast interrogation method with current modulation of a laser diode for PM-FBG-FPI sensors. The resulting fast measurement is demonstrated experimentally.

A highly sensitive fiber-optic mechanical vibration sensor is constructed by using a cascaded long period fiber grating (LPG) based on an intensity modulation scheme. In the fabrication process, the cascaded LPG, which is composed of a pair of identical LPGs with a certain distance, is inscribed in a length of photosensitive single-mode optical fiber by means of a point-by-point technique using a KrF excimer laser. Since the sensitivity of the intensity-based LPG sensor depends on a gradient of the slope of transmittance spectrum curve as well as the strain-sensitivity of the spectral shift, the channeled spectrum of the cascaded LPG provides a highly sensitive operation for the vibration detection. In the experiment, several kinds of cascaded LPGs have been fabricated and examined in terms of the sensor sensitivity. In addition, highly sensitive mechanical vibration detection has been successfully demonstrated.

A Fiber Bragg Gratings(FBG) have been used as a sensor head for measurement of temperature and static strain. However, a standard FBG sensor, which is constructed on single-mode fiber, cannot simultaneously measure both temperature and static strain since the sensor has cross-sensitivity between them. The cross-sensitivity problem can be solved by using an FBG constructed on a polarization maintaining fiber(PM-FBG) instead of a standard FBG. In this paper, we report improvement on the sensing resolution for the simultaneous measurement of temperature and static strain. An Fabry-Perot interferometer constructed with PM-FBG(PM-FBG-FPI) is introduced as a sensor head. The fine structure of an PM-FBG-FPI reflection spectrum enables high resolution detection of wavelength shifts. The resulting high resolution measurement is demonstrated experimentally.

Operation of an optical fiber sensor based on an in-fiber Fabry-Perot interferometer using chirped fiber Bragg gratings is examined in the pulse-position modulation scheme, especially for mechanical vibration measurement. Emphasis is placed on the ability to measure vibration of larger amplitude. Although the magnitude of vibration that can be measured with a single Fabry-Perot resonance peak is rather limited, the limitation is expected to be overcome by use of multiple resonance peaks in the operation. The experiment with five resonance peaks shows the successful operation of the sensor and therefore the validity of the method proposed.

Fiber reinforced plastics are essentially heterogeneous materials where plastic matrix is reinforced by fibers. Therefore, it is important to know the properties of fiber/matrix interaction in addition to the properties of fiber and matrix themselves. To measure the fiber/matrix adhesive strength, various kinds of tests such as fiber pull-out, fragmentation, microbond, etc. have so far been tried. According to our previous study, the interfacial shearing strength by means of the microbond test was a little smaller than that of the fragmentation test, which coincided with the other researchers' result. In the present study, we tried a microbond test at four kinds of condition to establish reliable microbond test method. They are the combination of sharp or blunt microvise and two widths of microvises. We made clear that the interfacial shearing strength was different for the shape and the width of the microvise and the best method of the microbond test was to use a blunt microvise and to keep a broad width of microvise. We also made clear that the microbond test should be conducted using large microdrops to get real shearing strength. We also carried out a finite element analysis to understand the stress distribution at the fiber/matrix interface and the numerical results qualitatively coincided with the experimental result.

This paper presents a methodology to measure the strength of monofilaments which are commonly used for fiber-reinforced composite materials. A so-called loop test is adopted for the present test. B...

This paper addresses the problem of determining the probabilistic strength of unidirectional composites. We first calculated the stress-concentration factors (SCFs) around broken fibers in a three-dimensional, hexagonal-array model. Two approaches were tried to calculate the SCFs. One is a shear-lag analysis originated by Hedgepeth and Van Dyke and the other is close to a local load-sharing rule (LLS). These two sets of SCFs provide the upper and lower bounds of the strength. With these SCFs, we next carried out a Monte Carlo simulation to evaluate the strength of unidirectional composites in which a chain-of-bundles probability model and a Weibull distribution were used. Parametric studies were first conducted and a comparison with experiments and other existing theories was next done. Our `approximate' upper bound was lower than Rosen's prediction and lower bound, higher than Zweben's value. The value from the simulation was also close to the experimental results.

Fiber reinforced plastics are essentially heterogeneous materials where plastic matrix is reinforced by fibers. Therefore, it is important to know the properties of fiber/matrix interaction in addition to the properties of fiber and matrix themselves. To measure the fiber/matrix adhesive strength, various kinds of tests such as fiber pull-out, fragmentation, microbond, etc. have so far been tried. However the correlation of the strength among these test methods is not necessarily clear although the argument of Herrera-Franco and Drzal [1] has a point. In the present study, we tried two kinds of tests, microbond test and fragmentation test, to get reliable interfacial shear strength. According to the present study, the interfacial shear strength by means of the microbond test was a little smaller than that of the fragmentation test, which coincided with the other researchers' result. The interfacial shear strength of a sample where siging agent was removed was smaller than that of as-received specimen. It was also demonstrated from two-fiber fragmentation tests that the interfacial property affects the failure process of unidirectional composites.

Cascaded long period fiber gratings are fabricated and investigated for use in intensity-based fiber optic mechanical vibration sensors. The cascaded LPGs are inscribed in photosensitive fiber by means of point-by-point technique with UV KrF excimer laser and examined in terms of axial strain sensitivity. In the experiment, much higher strain-sensitivity than that of single LPG is obtained and mechanical vibration sensing is successfully demonstrated.

Guiding and triggering of the electrical discharges in lm air gaps by excimer lasers (KrF (λ=248nm), ArF (λ=193nm)) have been performed. A laser pulse (KrF: 750mJ, ArF: 550mJ) produced a plasma channel over a distance of more than 0.5m by focused with a lens. Characteristics of guided discharges along the plasma channel were investigated in detail. From these results, it is found that an excimer laser pulse can guide the positive discharges and propagate in the plasma channel produced by itself without loss. However the excimer lasers have little ability to trigger the discharges. Further the guiding ability is not maintained for a long time compared with that of infrared lasers. On the basis of these experimental results, we discussed the basic concept of the laser triggered lightning with an excimer laser.

An in-fiber Fabry-Perot interferometer with fiber Bragg grating mirrors (FBG-FPI) yields extremely narrow transmission peaks within the FBG reflection wavelength range. Periodical scanning of the laser light source is under influence of strain. A high signal-to-noise ratio interrogation is possible with high resolution and a wide range of vibration frequency can be analyzed since the laser wavelength can be easily scanned at high frequency. Furthermore, an inexpensive DFB laser can be utilized as a light source because the narrowness of the transmission peaks does not require broad wavelength scanning. In this paper, experimental investigation of dynamic characteristics of wavelength of DFB-LD is reported.

A Fabry-Perot interferometer with chirped FBGs as reflectors has hundreds of resonance peeks in the transmittance spectrum and can be utilized as a sensor based on the dependency of the peak wavelengths on the influence applied to the fiber. Because the width of the peak is narrow and the number of the peak is large, one may well expect high resolution and/or large dynamic range in its operation. In this paper, we propose to use it for measuring vibration of solid. We characterize the sensor for two kinds of interrogation methods: the intensity-modulation and wavelength-sweep schemes. The sensor works well and the former scheme yields a lower minimum detectable strain while the latter gives flexibility in the magnitude of the dynamic range. In addition, difference in finesse between shorter- and longerwavelength regions allows us to design a sensor of different sensitivity by changing the operation wavelength.

Bending-characteristics of long period fiber gratings (LPGs) are investigated for use in LPG vibration sensors based on an intensity modulation scheme, in which a tunable laser is used for an optical source with its wavelength tuned to a transmission spectrum curve of an attenuation dip of an LPG and the partially transmitted light through the LPG is modulated in intensity by applied vibrations. In the experiment, LPGs are fabricated by use of an UV irradiation technique and their bending-induced spectral changes are examined in terms of the wavelength and transmittance of the attenuation dip. In contrast to the axial strain, it is confirmed that the much larger spectral changes are obtained when bending deformations are applied to the LPGs. The intensity-based vibration sensor is then demonstrated by adopting the bending effects on a LPG spectrum and its highly sensitive operation is successfully performed.

We construct an intensity-based FBG sensor for a vibration measurement of a rotating structure by using an optical fiber rotary joint (OFRJ) which enables free rotation of fiber while maintaining low-loss coupling efficiency. Although a number of FBG sensors have been proposed for use in various applications, they have not yet been applied to a rotating structure especially for dynamical measurement. In the proposed sensor system, an SOA-based dualwavelength fiber laser is employed as an optical source and an additional reference FBG sensor is multiplexed with the intensity-based FBG sensor by use of a wavelength division multiplexing (WDM) technique. The reference sensor enables to compensate the insertion-loss variation due to the rotating OFRJ so that a real-time vibration measurement of a rotating structure is achieved in the intensity-modulation scheme. In the experiment, the operation principle is successfully confirmed, and the vibration measurements of a rotating cantilever beam and a propeller-like plastic blade are demonstrated.

An in-fiber Fabry-Perot interferometer with fiber Bragg grating mirrors (FBG-FPI) yields extremely narrow transmission peaks within the FBG reflection wavelength range. Periodical scanning of the laser light source wavelength produces a train of optical pulses and the occurrence time of the pulse is modulated as the FBG-FPI is under influence of strain. When dynamic strain due to mechanical vibration is applied to the FBG-FPI, the detected signal of the pulse train is Fourier transformed and processed to reproduce the waveform of the vibration-induced strain. A high signal-to-noise ratio interrogation is possible with high resolution and a wide range of vibration frequency can be analyzed since the laser wavelength can be easily scanned at high frequency. Furthermore, an inexpensive DFB laser can be utilized as a light source because the narrowness of the transmission peaks does not require broad wavelength scanning.