A metallic packaging technique of fiber Bragg grating (FBG) sensors is developed for measurement of strain and temperature, and it can be simply achieved via one-step ultrasonic welding. The average strain transfer rate of the metal-packaged sensor is theoretically evaluated by a proposed model aiming at surface-bonded metallic packaging FBG. According to analytical results, the metallic packaging shows higher average strain transfer rate compared with traditional adhesive packaging under the same packaging conditions. Strain tests are performed on an elaborate uniform strength beam for both tensile and compressive strains; strain sensitivities of approximately 1.16 and 1.30 pm/μϵ are obtained for the tensile and compressive situations, respectively. Temperature rising and cooling tests are also executed from 50°C to 200°C, and the sensitivity of temperature is 36.59 pm/°C. All the measurements of strain and temperature exhibit good linearity and stability. These results demonstrate that the metal-packaged sensors can be successfully fabricated by one-step welding technique and provide great promise for long-term and high-precision structural health monitoring.
The rare earth-doped active fibers owning ten thousands of square-micron core-area but also delivering laser with high beam quality have little been reported. In this paper, we have designed a large-mode-area Yb3+-doped photonic crystal fiber in the cladding region with square-array air holes. Simulations demonstrate that only fundamental mode (FM) with mode-field-area (MFA) of ~15500 μm2 can be amplified and propagated at the gain saturation, and the beam quality M2 is less than 1.5. It is predicted that almost 58 mJ per-pulse can be available from such a 1.0 meter-length fiber, and the beam shape of amplified laser is near squared. It will be potential for so huge pulse-energy output from the VLMA LPF to be applied in the remote detecting, high-intensive welding and so on.
A Q-switched fiber laser with all-fiber configuration is proposed in which an all-fiber Q-switching modulator is
developed based on the long period fiber grating. The fiber laser cavity comprises a pair of fiber Bragg gratings which
are used to confine oscillation wavelength of the laser, gain fiber and a long period fiber grating based modulator. The
long period fiber grating modulator is inserted in the laser cavity as Q-switching device, where the long period fiber
grating modulator contains a long period fiber grating and an actuator, the initial resonance loss of the long period fiber
grating is matched to the fiber Bragg grating wavelength. While the actuator applies stress to a section of the long period
fiber grating, transmission spectrum of the long period fiber grating can be changed accordingly; Q-factor of the fiber
laser cavity can be alternately modulated thereby.
A simple Q-switch, consisting of a rotating planar-mirror where the reflectivity of portions of the surface are high and
the reflectivity of the remaining portions are low, is reported. This novel Q-switch is demonstrated using a fiber laser.
Using two meters of single-mode, 5 &mgr;m core, Yb-doped fiber, a 40% fiber Bragg-grating (FBG) at 1064 nm, and a
single-mode diode-pump at 976 nm, a fiber laser with a threshold power and slope efficiency of approximately 25 mW
and 33% was achieved, respectively, with the rotary mirror held stationary at the high-reflectivity region to form part of
the laser cavity. With the mirror rotating at a fixed speed of 7200 rpm, pulses with a 140 ns full-width-half-maximum
(FWHM) at a repetition rate of 480 Hz were observed. At an optical pump-power of 80 mW, the average power from
the Q-switched fiber laser was 3.5 mW resulting in a calculated, peak pulse-power of 48.6 W.
In this work, we demonstrate how a polarization switching technique can be used to create multiple fiber ends, and allow the radiation to pass twice through each amplifying section for more efficient energy extraction. The technique uses a polarizing beam splitter combined with polarization switching in each arm of the cavity to define a ring-like cavity with multiple gain sections that can be end pumped. Polarization-maintaining double-clad rare-earth-doped fiber with slightly multi-mode core was used as the gain sections. A laser system based on the in-cavity polarization switching design has been demonstrated with maximum measured 62% slope efficiency and close to 30W output. The relatively low output power is only limited by the available pump sources.
High energy laser systems, both pulsd and CW, have become of significant interst in the recent past. To achieve higher powers in a laser system, it is often necessary to consider means by which individual lasers can be made coherent with one another. This can be achieved through the use of a master oscillator concept, which can have problems with overall stability, or by monitoring the phases of each individual laser and using feedback technique that can be used to combine individual pumped fiber gain sources into a cavity with a single output and a single set of longitudinal modes. We discuss the advantages of end pumping of double clad fiber lasers and speculate on means by which an all-glass double clad fiber laser may be developed.
The distributed strain sensor has significant application in real-time monitoring of the status of large and important engineering structures such as bridges and dams. In this paper, a quasi-distributed optical fiber sensor system is set up using OTDR technique. The strain sensors that base on a novel microbend configuration are designed and used to measure the local strains along the optical fiber. In experimental studies, we are concerned about two questions: the monitoring of the safety status and the strain measurement of the member to be measured. The experimental results are given.
We have developed and demonstrated a reflection-mode optical fiber-based instrument for in situ monitoring and feedback control of thin film dielectric deposition processes. The instrument operates in single-wavelength or multi-wavelength mode. One end of the fiber is placed in the deposition zone, close to the samples being coated. Single or multi-wavelength light is sent down the fiber and the reflected light from the end being coated is analyzed for intensity vs. wavelength. The fiber end being coated features an easily replaced tip to prevent loss of resolution when the coating becomes too thick. For processes in which the index of refraction or composition of the thin films is fixed, the less expensive single wavelength instrument is sufficient and measures thickness of the films by counting interference fringes. For processes in which film composition or index of refraction are variable, we use a white light source and compact spectrograph to measure reflectance vs. wavelength. For critical applications like diode laser facet coating where yield loss is significant cost driver, this monitor measures the thickness and index of refraction of single and multi-layer thin films as they are deposited. More importantly, it measures the critical parameter of interest: reflectance at the actual laser emission wavelength. This instrument replaces quartz crystal oscillators and other, more complex instruments.
Two fiber-optic cure sensors based on the refractive index measurement are presented. An optical fiber distal end reflectometer and a stripped claddind fiber refractive index sensor, both made by an optical fiber with the core refractive index of 1.558, are tried and tested to be calibrated and provide quantitative information during cure. The fiber reflectometer is confirmed to be suitable to be calibrated during cure and may be possible to provide quantitative cure extent when hybrided with an Optic Time Domain Reflector (OTDR). The stripped cladding refractive index sensor is found to be unsuitable for the purpose of being calibrated during cure.
This paper presents a fiber-optic temperature measuring system which can be used for measuring the temperature in many occasions. The system is of reflective type and composed of thermostatic bimetal-plate, lever-piston framework, optical grating and optical fiber probes. When the temperature changes, the thermostatic bimetal-plate deforms. Through lever-piston framework, the optical grating produces displacement in the upright direction. Thus the change of the temperature is transformed into the upright displacement of the optical grating. Optical fiber probes are used for detecting the number of streak lines of the optical grating's displacement depending on the change of temperature. The detected signal can be transmitted to the control center through optical fiber cable up to distance of 1km. The measurable range of this system reaches 100 degrees C with accuracy of +/- 0.2 degrees C.
The composite structures with embedded optical fiber sensors construct a smart composite structure system, which may have the characteristics of the in-service self-measurement, self- recognition and self-judgement action. In the present work, we studied the microstructures of carbon/epoxy composite laminates with embedded sensing optical fibers, and the integration of optical fiber with composites was also discussed. The preliminary experiment results show that because of the difference between the sensing optical fibers and the reinforcing fibers in their size, the microstructure of the composites with embedded optical fibers will produce partial local changes in the area of embedded optical fiber, these changes may affect the mechanical properties of composite structures. When the optical fibers are embedded parallel to the reinforcing fibers, due to the composite prepregs are formed under a press action during its curing process, the reinforcing fibers can be arranged equably around the optical fibers. But when the optical fibers are embedded perpendicularly to the reinforcement fibers, the resin rich pocket will appear in the composite laminates surrounding the embedded optical fiber. The gas holes will be easily produced in these zones which may produce a premature failure of the composite structure. The photoelastic experiments are also given in the paper.
The distributed optical fiber sensing technique has significant application in the real-time monitoring and damage detection of the large and critical engineering structures. In this paper, a distributed optical fiber sensor based on the optical time domain reflectometry technique has been used for monitoring structural deformations, where a novel structure of the microbend sensor was developed for measuring of both tensile and compressive strain in structures.
An optical fiber sensing scheme used for cure monitoring during composite curing cycle is presented. The sensing mechanism is based on the change in refractive index during cure process of the composite. An optical fiber sensor embedded in the composite was used for detecting the variation of the refractive index of cure resin in composite materials during its curing cycle. A carbon fiber/epoxy composite laminate was used for experiments in this paper.
A new kind of practical fiber optic flow sensor system is studied in this paper. Combining the matured technology of conventional Rhodes flowmeter with advanced technology of fiber optic sensor, we proposed a practical scheme of hybrid fiber optic flow sensor system which remains the advantage of high accuracy of conventional Rhodes flowmeter, improves its characteristic in low speed and has large measuring range well over that the magneto-electric Rhodes flowmeter has. Thanks to the advantages of isolation, nonconduction and immune from electromagnetical interference, fiber optic flow sensor has steady output signal and can work without electricity on worksite. So it is used specifically in hazardous environments such as oil depot for oil and chemicals measuring. In this paper, the working principle of the flow sensor is introduced, the structure of the fiber optic Rhodes wheels rotary displacement probe is designed, and the signal-processing system is described. Finally the in-situ test of the system in the oil terminal of Zhanjiang Harbor in China is introduced.
It is significant to study and employ interferometric optical fiber sensors which are most sensitive to measurand and can be used to measure some critical parameters of measurands, it is, however, not practical because of the difficulties that lie in the aspect of interferometric signal demodulation, which may be overcome by several different ways extensively studied such as active homodyne phase detection, passive homodyne phase detection and heterodyne phase detection. Passive homodyne phase detection for interferometric sensors uses a passive optical element to provide optical pi/2 phase shift output, the measurand induced phase change is recovered by the detection electronics. A 3 multiplied by 3 directional coupler can be used to obtain the optical pi/2 phase shift output and hence to form interferometric optical sensor. In this article, fundamental principles of the sensor were analyzed using it, which possess characteristics of all-optical fiber, high- sensitivity, high-distinctiveness and easy integration, strain measurements were completed. The experimental results were given and also characteristics of the sensor were analyzed.
In order to advance the safety of some important structures that will be required to perform under increasingly stringent conditions, it will be necessary to use materials that are lighter weight, have superior strength, and have the ability to change such parameters as shape, degree of stiffness, and electrical and mechanical properties as needed. These future materials had been named 'smart' material. It is important to measure strain within the materials. A fiber optic sensor may be used to support the necessary sensing functions of these 'smart' materials and has several compelling advantages with respect to electrical alternatives, which are tolerant to corrosion, resisted to disturvance of electric and magnetic, easily to be embedded into composites. Unless the (pi) /2 nonreciprocal phase bias and the phase shifting become a problem, interferometers using single mode fibers as optical propagation paths will be superior. It is well known that the oscillating frequency of coupled cavity semiconductor laser can be modulated by induced current. In this paper we studied the coherent frequency modulated heterodyne interferometer, (used to overcome the problem of (pi) /2 nonreciprocal phase bias, and the phase shifting problem without help of active control by ceramics), and the measuring of strain of smart structures when embedded into materials. The maximum defection range is limited by the coherent length and the dependence of the semiconductor laser.
A novel fiber-optic sensor for measuring external pressure distribution along a high-birefringent fiber is proposed, based on the polarization mode conversion due to pressure. The sensor exhibits spatial resolution better than 0.15 m, high signal-to-noise ratio, and wide dynamic range.
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