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 order 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.
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 1 MHz using a laser-diode controller and wavelength scanning can be then easily achieved with a laser diode.
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.
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.
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.
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.
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.
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 novel fiber optic sensor array is proposed and demonstrated by multiplexing intensity-based long-period fiber grating
(LPG) sensors, in which a simultaneous multipoint detection of mechanical vibrations as well as a highly sensitive
operation is realized. In the intensity-based detection scheme, multiple narrowband light sources are provided by use of
a broadband amplified spontaneous emission (ASE) source and fiber Bragg gratings (FBGs) with their wavelengths
adjusted to the spectral dips of the sensing LPGs. The LPG vibration sensor array is achieved by using a combination of
wavelength division multiplexing (WDM) and space division multiplexing (SDM) technique. In the experiment, the
LPG sensor array is constructed by multiplexing two LPG sensors in tandem and simultaneous multipoint vibration
detection without crosstalk is successfully demonstrated.
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.
A long-period fiber grating (LPG) sensor is interrogated with the wavelength-to-time mapping scheme. Highspeed
scanning of the laser light source wavelength generates a train of optical dark pulses in the transmitted
light through the LPG whose peak position is modulated according to the wavelength change caused by the
environmental influence. The generated dark pulses are Fourier-analyzed and the information on the wavelength
shift is retrieved. The result in the temperature sensing shows the linearity of the sensor system and the validity
of the interrogation scheme is verified. The scheme can be also applied to the sensing of the dynamic influence.
With 20 kHz scanning frequency, dynamic influence of up to 10 kHz is expected to be interrogated. Utilizing
the characteristic advantage that a LPG can be tailored to be of higher or less sensitivity to the influence, the
proposed method can be expected to yield high performance operation of a LPG sensor.
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.
In vibration sensing using a fiber Bragg grating (FBG) based on the intensity modulation scheme, light incident on a FBG is narrow-band light tuned to the slope in a reflectance spectrum of the FBG. Since the sensor sensitivity is proportional to the slope of the spectrum curve, enhancement of the sensitivity requires the slope to be steeper. In this paper, vibration sensing by usee of an in-fiber Fabry-Perot interferometer with FBG reflectors (FBG-FPI) is presented. Because a reflectance spectrum of a FBG-EPI has much steeper slope thatn a single FBG, a FBG-FPI yields improvement in sensitivity of the sensing. High-sensitivity measurement of vibration of a piezoelectric transducer is experimentally demonstrated. Comparing to a vibration sensor using a single FBG, the use of the FPI structure has increased the sensitivity of vibration by 30dB.
Long-period fiber gratings (LPGs) are fabricated and investigated for use in an LPG vibration sensor based on intensity
modulation scheme in which a tunable laser with its wavelength tuned to a slope of an attenuation dip of an LPG is used
for an optical source and the light partially transmitted through the LPG is modulated in intensity according to the
applied vibrations. In the experiment, it is demonstrated that the LPG vibration sensor yields a clear and stable output
waveform. In addition, it is confirmed that the sensitivity of the sensor depends on the wavelength-shift of attenuation
dip per unit strain and the gradient of a slope of the attenuation dip. Since an LPG shows higher sensitivity to static or
dynamic strain when an appropriate higher-order cladding mode is adopted, proper choice of the order of cladding
mode enables a highly sensitive operation of the LPG vibration sensor.
A novel fiber Bragg grating (FBG) hydrophone array is proposed in order to realize simultaneous multipoint underwater
acoustic detection with thermal stabilization. The interrogation technique of the FBG hydrophone to be arrayed is based
on an intensity-modulation scheme, in which a tunable laser is used for an optical source with its oscillation wavelength
tuned to a slope of the reflection spectrum curve of the FBG. The light reflected at the sensing FBG is modulated by an
underwater acoustic wave and offers a waveform signal as a sensor output. For fabricating the FBG hydrophone array, a
multi-wavelength tunable fiber laser is constructed for providing multiple narrowband optical sources and the sensing
FBGs are arranged in tandem using a wavelength-division multiplexing (WDM) technique. In the experiment,
simultaneous three-point underwater acoustic detection and measurement of sound velocity in water have been
successfully demonstrated.
Long-period fiber gratings (LPG) are fabricated by illuminating 248 nm KrF excimer laser light to a single-mode optical
fiber that is photosensitized with boron doping. By applying the intensity-modulation scheme, which is known to be
effective as an interrogation method for fiber Bragg grating (FBG) vibration and underwater acoustic sensors, we
construct an optical vibration sensor of a solid that uses a LPG as a sensing element. It is confirmed that the LPG
vibration sensor yields a clear and stable signal waveform and shows good linearity when the amplitude of the vibration
strain is varied. The dynamic range of the sensor would be more than 100 dB, which is larger than that with a FBG as
a sensing element. Since a LPG is known to show higher sensitivity to static strain when one uses a higher-order
cladding mode, it is expected that with proper choice of the cladding mode a LPG vibration sensor will show much
higher sensitivity than a FBG vibration sensor.
A multi-wavelength tunable fiber laser is constructed for a wavelength-division multiplexed (WDM) fiber Bragg grating
(FBG) sensor array in which simultaneous detection of multipoint vibrations and temperature stabilization of the sensor
outputs are achieved. The laser consists of a semiconductor optical amplifier (SOA) as a gain medium and of FBGs as
wavelength selection components. Since the SOA is an inhomogeneous broadening gain medium, stable multi-wavelength
oscillation at the Bragg wavelengths of the FBGs can be realized. In addition, the oscillation wavelengths of
the laser can be tuned by applying strain to the FBGs used. In the sensor scheme, triple wavelength fiber laser is
fabricated, in which the three wavelength components provide narrowband light sources for arrayed WDM FBG sensors
and the wavelength tuning enables temperature compensation for the vibration detection. In the experiment,
simultaneous three-point vibration detection with temperature stabilization has been successfully demonstrated.
A reflective-type fiber Bragg grating (FBG) sensor based on intensity modulation technique has been developed to construct an FBG underwater acoustic sensor array, which consists of a amplified spontaneous emission (ASE) source, a four-port optical circulator and a pair of FBGs: one is used for sensing component, and the other is used to provide a narrowband light source from the incoherent ASE source. The paired FBG configuration enables us to eliminate temperature effects on the acoustic detection. In addition, the reflective-type sensor uses a single lead-in/out fiber so that it is convenient to arrange a multiplexed sensor array. In the experiment, the FBG underwater acoustic sensor with two sensing elements is fabricated and thermally stabilized time-division multiplexed detection is demonstrated.
A fiber Bragg grating (FBG) vibration sensor utilizing an intensity-modulated laser source has been devised and investigated. The research is aimed at developing a practical FBG sensor that will be applicable to high-sensitive dynamic measurements such as mechanical vibration of solid and acoustic wave in water. The FBG sensor devised is composed of a tunable laser source, an optical intensity modulator, and an FBG. In this configuration the wavelength of the optical wave from the laser source is tuned to a slope of the reflection spectrum curve of the FBG so that its Bragg wavelength shift due to vibration can be detected as a change in intensity of the optical wave reflected at the FBG. In order to increase the signal to noise ratio (SNR) in a measurement of the change in the intensity due to vibration, an intensity modulator is built into the system to make an intensity-modulated laser source and lock-in amplifier technique is employed for the vibration detection. In the experiment, high-sensitive detection for a vibration of a piezoelectric transducer (PZT) is demonstrated and discussed by comparing the sensor output with this technique to that of a conventional method without the intensity-modulation.
KEYWORDS: Fiber Bragg gratings, Sensors, Temperature metrology, Transmittance, Signal detection, Modulation, Photodiodes, Environmental sensing, Calibration, Signal to noise ratio
Simultaneous measurement of mechanical vibration and temperature are performed using a fiber Bragg grating (FBG) as a sensing element. The principle of the FBG vibration sensing is based on the fact that when an FBG is under influence of vibration, it modulates the intensity of the reflected and transmitted light if the bandwidth of the incident light is narrow and its wavelength is tuned to the slope of the FBG transmittance curve. DC component of the photocurrent produced in a photodiode by the sensor output light corresponds to the operation point of the sensor. By feeding back a part of the sensor output to the laser light source to shift the oscillation wavelength of the laser, the DC component is kept constant and the sensitivity of the vibration sensor is stabilized when the temperature varies in the environment. Its fluctuation is measured to be as small as 1 dB, though it is more than 58 dB without the stabilization. Since the oscillation wavelength has one-to-one correspondence to the environmental temperature, monitoring the laser temperature control voltage enables us to determine the temperature around the FBG sensing element.
Distribution of equal laser power into numerous optical fibers is often required for optical sensors, correlation optics, and related metrology. To do so some optical elements have been usually adopted in extracavity scheme. In this paper we shall propose a novel method to couple an equal lasing power into multioptical fibers into intracavity configuration. Mounting numerous optical fibers on the donut-shaped window of a coupling mirror of laser resonator by setting its transmittance at the optimum value, we can extrat the lasing power equally into optical fibers with in phase. As an example of experimental demonstration the numerical calculation will be given for a combination of a CO2 laser and AsS IR fiber.
Organic materials are becoming widely used in optical fields due to their many advantageous properties such as flexibility, ruggedness, and low cost. In particular, application of organic polymer to an optical amplifier and/or a laser oscillator has attracted much attention recently. In these circumstances, we have demonstrated a novel solid-state dye-laser by using organic thin film made of Rhodamine B (Rh-B)-doped UV-cured polymer as gain medium, in which distributed feedback operation has been achieved by inducing a periodic gain structure onto the gain medium. However, the characteristics of the lasing output of such a laser oscillator have still not been examined. In this paper we shall thus present a numerical analysis of lasing output of the solid-state dye laser with a photo-induced periodic-gain. Since in the photo-induced DFB operation an interference fringe of lightwave is used for pumping a periodic-gain stucture onto the gain medium, the output power strongly depends on the shape of the interference pattern. Using several parameters obtained in the experiments, dependences of output power on the visibility and average power of the pumping interference pattern are numerically calculated. In addition, the effects of gain saturation against the pump energy are also discussed from the viewpoint of practical applications.
Detection of mechanical vibration with a fiber Bragg grating (FBG) based on the intensity-modulation method provides us with simplicity and compactness of the sensor as well as high sensitivity, wide dynamic range and wide frequency response in sensing. The slope of the FBG transmittance or reflectance spectrum curve at the operation wavelength determines the sensitivity of the sensor. Since the Bragg reflection wavelength of an FBG changes as the temperature of an FBG changes, it is known that the sensitivity of the sensor is dependent on temperature of the FBG and may vary considerably when the environmental temperature varies. Except for the operation under the situation in which the temperature change is small, therefore, it is generally required to stabilize the sensor against the temperature change in the environment. In this paper we propose a new method to stabilize the sensitivity of an FBG vibration sensor in the temperaturevarying environment by automatically shifting the wavelength of the source laser. The sensitivity variation of the thermally stabilized sensor is reduced down to 3 dB, which is more than 55 dB without the stabilization.
The intensity of laser light is modulated when reflecting back from a fiber Bragg grating (FBG) which is glued onto a PZT vibrator and expands/contracts as the vibrator vibrates. The wavelength of the laser light is tuned to the slope of the FBG reflectance curve as a function of optical wavelength. Measuring the modulation component of the detected signal, we can directly observe mechanical vibration of the vibrator. The output of the sensor is stable and the involved harmonic component is below the system noise level. It is then believed that the sensor operation is linear. The sensitivity depends on the slope of the FBG reflection spectrum curve at the operating wavelength and is higher for the larger slope. The minimum amplitude of the vibrator measured in the experiment is 4.5 nm, which corresponds to the strain of 2.14 μstrain. Since not only an FBG has little influence on the object under measurement because of its small size and light weight but also its frequency characteristics are thought to be better than a PZT vibration sensor, i.e., the sensor can be used in a wide range of vibration frequency, an FBG is expected to provide us with an important tool of practicality for measuring mechanical vibration.
A fiber Bragg grating (FBG) vibration sensor is constructed with two FBGs and an incoherent optical source. One FBG is used as a sensing element which intensity-modulates incident narrow-spectrum light, of which peak wavelength coincides with the slope in the FBG reflection spectrum curve. The narrow-spectrum light is produced by the other FBG in combination with amplified spontaneous emission output of an erbium doped fiber amplifier. Direct detection of the intensity-modulated light yields real time observation of the vibration. The developed sensor shows stable output in proportion to the vibration amplitude without using an optical isolator or fusion splicing, which are commonly used for the stabilization of FBG sensors with laser light as an optical source. Thermally stabilized operation of the sensor would be realized by keeping the temperature of both FBGs the same.
Based on the intensity-modulation method, we constructed an ultrasonic vibration sensor using a fiber Bragg gratitg. The sensor can measure linearly amplitude of mechanical vibration in nm resolution without having influence on the vibrator.
An underwater acoustic sensor with fiber Bragg gratings can detect a real-time waveform and thereby amplitude and phase of an acoustic field, which in turn allows directive detection of the field with an arrayed structure.
Using two fiber Bragg gratings (FBG) in serial connection and WDM technique, multipoint detection of an acoustic field in water is demonstrated after having measured the spatial resolving power of an FBG underwater acoustic sensor. An acoustic sensor with a 24 mm ling FBG is found to have the spatial resolution on the order of 1 mm. Two FBGs in the WDM sensor work independently without interfering each other and the sensor provides us with real-time detection of the amplitudes and phases of the acoustic field at different points in water.
An optical fiber sensor with two fiber Bragg gratings in serial connection which is multiplexed in an optical wavelength domain is constructed to sense an acoustic field in water. Both the gratings are inserted between optical isolators in order to stabilize the sensor by suppressing Fabry-Perot interference in fiber that is caused by small reflections from fiber end facet, connectors and an optical coupler. The two gratings in the sensor are confirmed to work independently and have little influence on each other. Comparison of the experimental data to the theoretical prediction shows that the developed sensor can detect the relative phase between two points in the acoustic field and thereby determine the direction of the acoustic source. Since a fiber Bragg grating can make an optical sensor of high sensitivity and wide dynamic range and can be easily multiplexed with an arrayed structure in either one or two dimensions, it will then offer a convenient tool to measure distribution of amplitude and phase of an acoustic field in water.
Dynamics of a constricted filamental discharge in a discharge- excited ArF excimer laser have been examined using a two dimensional fluid model. An entire process of the filamental discharge, from its initiation and development to extinction, is shown. The filamental discharge is triggered at protrusions which would always exist on cathode surfaces, and develops in the direction of the anode assisted by the high field induced by space charge. The gas temperature in the filamental discharge in the vicinity of the cathode is found to rise. This temperature is shown to play an important role in development of the filamental discharge. The effect of preionization electron density ne0 is also examined, and significant development of the filamental discharge toward the anode is seen when the ne0 becomes lower. The results are examined properly in connection with experimental observations.
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