Different from the previously reported cryogenic temperature measurement based on the fiber Bragg grating (FBG) sensor, we propose a novel cryogenic temperature measurement scheme with a fiber interferometric sensor. The proposed sensor is constructed by the extrinsic Fabry–Perot interferometer (EFPI) which is consisted of a ceramic ferrule, a copper sleeve and two fiber ends. Under the cryogenic surrounding, the cavity length of the EFPI will change with the thermal deformation of the copper sleeve. With liquid nitrogen, the surrounding temperature can be changed from 77K to room temperature, and as high as 2.246nm/K of the temperature sensitivity can be achieved at a temperature range from 113K-153K in the experiment, with a 12.38μm EFPI cavity. The proposed interferometric sensor will still have relatively high sensitivity at a temperature lower than 77K according to our numerical simulation, so, has good application prospect in cryogenic temperature environment.
A novel sensitivity-enhanced intrinsic fiber Fabry-Perot interferometer (IFFPI) high temperature sensor based on a hollow- core photonic crystal fiber (HC-PCF) and modified Vernier effect is proposed and experimentally demonstrated. The all fiber IFFPIs are easily constructed by splicing one end of the HC-PCF to a leading single mode fiber (SMF) and applying an arc at the other end of the HC-PCF to form a pure silica tip. The modified Vernier effect is formed by three beams of lights reflected from the SMF-PCF splicing joint, and the two air/glass interfaces on the ends of the collapsed HC-PCF tip, respectively. Vernier effect was first applied to high temperature sensing up to 1200°C, in this work, and the experimental results exhibit good stability and repeatability. The temperature sensitivity, measured from the spectrum envelope, is 14 to 57 times higher than that of other configurations using similar HC-PCFs without the Vernier effect. The proposed sensor has the advantages of high sensitivity, good stability, compactness, ease of fabrication, and has potential application in practical high-temperature measurements.
The most important part of composite insulator is the joint of the composite rod and the metal end-fitting because most of mechanical faults take place here. Thus it is necessary to on-line monitoring of the joints. This paper theoretically analyzes the response behavior of the embedded fiber Bragg grating (FBG) in the joint based on our preliminary analysis of the stress distribution of the composite rod. Comprehensive considering the thermal stress and the material thermal coefficient, the reflection spectrum of FBG embedded in three typical locations is simulated. The simulation result shows that the wavelength shift of the FBG embedded in the joint is about 180pm larger than those embedded out of joint when the temperature rises 10°C. This result has the directive significance for the embedding position and the embedded way. It can be used to monitor the mechanical strength changing with temperature, to improve the manufacture technology of the joint, and to detect the change of electrical property.
Influence of splicing the highly birefringent photonic crystal fiber (HB-PCF) with single mode fiber (SMF) under two
different experimental conditions is studied in details. The result shows the birefringence of the HB-PCF can be either
increased or decreased significantly, depending on the connection conditions of the HB-PCF end, which are classified as
case I (the end is closely butted by another fiber) and case II (the end is in open air). From the experiment and
theoretical analysis, it has shown that in case I the retardation change of the spliced section of HB-PCF with 0.2mm in
length can be 3.2 times larger than the original value. However, in case II the retardation may be reduced to 72.12% of
the original one. The obtained result is important for the design and fabrication of optical fiber devices and sensors based
on HB-PCFs.
We discussed how to design the typical trip-clad high-order mode fiber (HOMF) profiles to achieve the required
dispersion properties based on LP02 mode, to compensate all modern transmission fibers, without sacrificing other
important properties, such as effective area. Finally, HOMF compensating 100km eLEAF fiber has been designed. Its
dispersion at 1550nm is -1217ps/nm/km, and the relative dispersion slope (RDS) is 0.02nm-1. Only ~345m of HOMF is
needed to achieve full dispersion and dispersion slope compensation of the span, while maintaining effective area above
52μm2 over the entire C-band.
The measurement of alternating current (AC) is realized by using fiber Bragg grating (FBG), giant magnetostriction
material (GMM), and the technology of fiber coupler demodulation (fiber wavelength division multiplexing filter,
FWDMF). The magnetostrictive effect of GMM, the theoretical analysis of the FBG current measurement and the
interrogation technology of FWDMF are introduced as well. The sensing unit of AC current measurement is constructed
with magnetostrictive material and FBG. The feasibility of AC current measurement by using this method is proved, when
the AC exciting current is in the scope of 0~2A.
A novel narrow linewidth, discretely tunable and stable fiber ring laser based on the matching of the transmission spectra
of two fiber Bragg grating Fabry-Perot (FBG-FP) filters is proposed in this paper. We studied the characteristics of this
sort of tunable fiber laser both theoretically and experimentally, and obtained eight discretely tunable lasing wavelengths
from 1552.24 nm to 1552.912 nm with an average 96 pm spacing, by pulling one of the FBG-FPs. The output of the
laser at the discrete wavelengths specified by the transmission maximum of the fixed FBG-FP is quite stable, and the
fluctuation of the output power is less than 0.1dB.
The Sagnac loop filter composed of the highly birefringent (Hi-Bi) fiber and a 3-dB coupler has been employed in the novel structure of Q-switched fiber laser in this paper. We use square-wave-driven PZT to modulate the filter, whose period is changed according to that of the wave. Therefore, the Hi-Bi fiber Sagnac loop filter will not only fulfill the function of choosing the wavelength of the laser but also serve as a Q-switching that changes the transmission ratio of the whole resonator. In the experiment of Q-switched fiber laser based on the ring resonator, the output laser pulse remains stable at an external frequency of 3 kHz-8 kHz. When the frequency is 3.2 KHz and the pump power is 19mw, the FWHM (Full Width at Half Maximum) of the pulse is approximately 10.5μs, and the average output power is 685.3μW, and the peak power is 41.1mW. Meanwhile, another novel structure of Q-switched fiber with linear resonator has also been studied in the paper.
A novel method to detect the small wavelength shift of the fiber Bragg grating used in an electric current sensor with a comb-like fiber filter is proposed in this paper. The wavelength filter was constructed by joining a piece of hi-bi fiber in the loop of a normal fiber Sagnac interferometer, which may result to a periodically varied spectrum in the reflection or transmission light. Therefore the small wavelength shift of the fiber grating caused by the change of the applied magnetic field can be detected by monitoring the intensity of the light traveling through the comb-like filter. With a 0.8-nm spacing filter used in the measurement, a sensitivity better than 0.01 nm for the Bragg wavelength shift of the fiber grating (limited by the noise of the electronic circuit and detection instruments) was obtained in the experiment. The proposed demodulation scheme provides a new approach to the detection of small wavelength shift of fiber grating.
We propose a simple, passive signal-processing scheme for stabilizing the Faraday effect current sensor that relies on the use of a twisted fiber or a spun highly birefringent fiber as the sensing element. Experimental results are presented to demonstrate the effectiveness of the scheme.
A computer-controlled optical fiber sensor system has been developed for the characterization of ultrasonic transducers in the megahertz range. The sensing element used is a short length of highly birefringent optical fiber. When an ultrasonic wave is incident normally upon the fiber, the birefringence, or the phase difference between the two polarized light waves in the fiber, is modulated. The phase modulation is converted into an intensity-modulated signal by means of polarimetry. To optimize and stabilize the signal against environmental effects such as temperature variations, a liquid-crystal phase retarder, controlled by feedback electronics, is placed in the optical path. Intensity noises arising from the laser source and the fiber leads are also eliminated by using signal processing techniques. The spatial intensity profile of an ultrasonic beam can be determined by scanning the sensing fiber across the ultrasonic beam. Fiber scanning and data acquisition are controlled by a personal computer. In this paper, the principle and the implementation of the sensor system are described. Experimental results obtained from characterizing commercial ultrasonic transducers are presented.
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