Kefa Cen

Design and characteristics of refractive index sensor based on thinned and microstructure fiber Bragg grating

A refractive index sensor based on the thinned and microstructure fiber Bragg grating (ThMs-FBG) was proposed and realized as a chemical sensing. The numerical simulation for the reflectance spectrum of the ThMs-FBG was calculated and the phase shift down-peak could be observed from the reflectance spectrum. Many factors influencing the reflectance spectrum were considered in detail for simulation, including the etched depth, length, and position. The sandwich-solution etching method was utilized to realize the microstructure of the ThMs-FBG, and the photographs of the microstructure were obtained. Experimental results demonstrated that the reflectance spectrum, phase shift down-peak wavelength, and reflected optical intensity of the ThMs-FBG all depended on the surrounding refractive index. However, only the down-peak wavelength of the ThMs-FBG changed with the surrounding temperature. Under the condition that the length and cladding diameter of the ThMs-FBG microstructure were 800 and 14 mum, respectively, and the position of the microstructure of the ThMs-FBG is in the middle of grating region, the refractive index sensitivity of the ThMs-FBG was 0.79 nm/refractive index unit with the wide range of 1.33-1.457 and a high resolution of 1.2 x 10(-3). The temperature sensitivity was 0.0103 nm/ degrees C, which was approximately equal to that of common FBG.

Sizing of particles smaller than 5 μm in digital holographic microscopy

The holographic microscopy technique is a strong contender for dynamic three-dimensional (3D) measurement of small particles (typically smaller than 5 μm) in microchannels. However, there is a big challenge to accurately measure the size of such small particles. The traditional hologram reconstruction method was numerically investigated. It is found that the error level, especially for the size measurement, is higher than expected, even in an ideal situation without consideration of noise. An alternative way based on Lorenz-Mie (LM) calculations was then presented. The intensity distribution of the fringe pattern on the particle hologram is directly used and compared with the ones calculated using an LM-based program. Various cases for particle sizes from 0.5 to 5 μm and recording distances from 5 to 500 μm are tested. The results show that the accuracy in particle sizing can be significantly improved.

Expansion of digital holography measurement area in particle field

Digital holography shows great potential in flow field diagnostics due to its non-intrusive and instant three-dimensional and multi-parameter measurement. However, the low spatial resolution of the existing CCD, leading to small measurable area, is one of the limitations. Based on angular distribution of light scattering intensity, expansion of the measurement cross section of digital in-line holography was investigated at the condition of the existing CCD-resolution. Airy disk of forward light scattering of a particle limits the recording of interference pattern with high frequency, reducing the spatial resolution requirement of CCD. The measurement section of a flow field can be expanded by pre-amplification of CCD using imaging. The bigger the particle is, the larger the measurement section can be extended to. Effect of the CCD pixel size on particle measurement with different size was numerically studied, and coal particle flow was measured by digital in-line holography experimentally. Results show that the analysis is reasonable, and that the proposed method expanding the measurement cross-section is feasible.

Measuring Frequencies of the Turbine Blade Based on Fiber Bragg Grating

Fiber Bragg grating (FBG) is a novel optics fiber sensing unit with many advantages such as light weight, slim body, low loss transmission, chemical stability, immunity to electromagnetic interference etc., and possesses special virtues, immunity to laser resource instability, buried easily into the materials and made a distributed sensing network. This paper presents a measuring technique about the frequencies of the turbine blade based on the FBG. The blade is a twisted one made up of hardening and tempering 2Cr13-5 with T-type blade root in low pressure cylinder, and 25 2mm length without shroud band and lashing wire. In this work, two FBGs were used, one was annealed with package, while the other one wasn’t. The frequencies of the blade have been measured without constraint condition by that two gratings, the former are 1005.00Hz, 2048.00Hz, 2297.25Hz, 2786.00Hz, the latter are 1005.00Hz, 2048.00Hz, 2297.25Hz, 2786.00Hz. The results were obtained with constraint condition by that two FBGs, the former are 921.50Hz, 1482.00Hz, 2454.75Hz, the latter are 927.00Hz, 1479.75Hz, 2472.75Hz. An acceleration transducer was also employed in this work to make a comparison, the results without constraint condition are 1005.00Hz, 2048.00Hz, 2297.00Hz, 2786.00Hz, while the ones with constraint condition are 917.00Hz, 1480.75Hz, 2454.25Hz. Obviously, it is a feasible method to measure the frequencies based on the FBG because their results are approximately equal. Furthermore, interrogator and wavelength division multiplexing (WDM) of the FBG, which is beneficial to make sensing network to measure many parameters simultaneously, have also been investigated.

Sizing of irregular particles using a near backscattered laser Doppler system

A near backscattered laser Doppler system was presented to carry out velocity and size distribution measurements for irregular particles in two-phase flows. The technique uses amplitudes of particles Doppler signals to estimate the particle size distribution in a statistical manner. Holves numerical inversion scheme is employed to unfold the dependence of the scattered signals on both particle trajectory and orientation through the measurement volume. The performance and error level of the technique were simulated, and several parameters including the number of particle samples, the fluctuation of irregular particle response function, inversion algorithms, and types of particle size distribution were extensively investigated. The results show that the size distributions for those irregular particles even with strong fluctuations in response function can be successfully reconstructed with an acceptable error level using a Phillips-Twomey-non-negative least-squares algorithm instead of a non-negative least-squares one. The measurement system was then further experimentally verified with irregular quartz sands. Using inversion matrix obtained from the calibration experiment, the average measurement error for the mixing quartz sands with a size range of 200-560 microm are found to be about 23.3%, which shows the reliability of the technique and the potential for it to be applied to industrial measurement.