Xiaohua Lei

Effects of bonding layer on the available strain measuring range of fiber Bragg gratings

Stress-induced birefringence can lead to distortion in the reflection spectra of fiber Bragg grating (FBG) sensors, thereby resulting in the loss of accuracy and stability of strain measurements. The bonding layer is a direct factor in producing stress birefringence within FBGs. To assess the impacts quantitatively, a theoretical model that links the bonding layer and the reflection spectrum was established. At the same time, the finite element method, based on the theoretical model, was used to study the relationships between characteristics of the bonding layer and reflection spectrum in detail. The analytical results indicate that high elastic modulus and mismatched Poissons ratio of bonding layer decrease the available strain measuring range of FBGs remarkably, and that unreasonable geometric parameters of the bonding layer should be avoided. In addition, a validation experiment was conducted and experimental results proved the prediction of the theoretical analysis. It can be concluded from the results that the bonding layer is the major limiting factor for the application of surface-bonded FBG sensors in large strain measurements. The bonding materials and bonding processes used in producing FBG sensors deserve serious consideration.

Far-field distance for surface light source with different luminous area

Most of the lighting designs are based on the far-field characteristics of light sources. It is important to know whether the measurement distance is far enough to simulate the light source as a point source. In this paper, we deduce the far-field conditions for surface light sources with continuous luminous area. The relative far-field distance for several typical flat and curved surface light sources, such as the round, rectangular, annular, hemispherical, and semi-cylindrical sources are calculated. Results show that the classical five times rule is not applicable to surface light sources with hemispherical, arc-shaped semi-cylindrical, and narrow annular luminous areas.

Thermal Characteristics Analysis of Die Attach Layer Based on Time-Constant Spectrum for High-Power LED

The performance and reliability of the high-power light-emitting diodes (HP LEDs) are closely related to the quality of the die attach adhesive (DAA) layer, because voids or delaminations in the layer may cause higher junction temperature, and even result in the break of the chip. In this paper, a dynamic compact thermal model is constructed for an independent HP LED without being attached on a cold plate. The time-constant spectrum method based on the thermal model is applied to characterize the transient thermal behavior of the DAA layer and other layers along the heat flow path. Thermal transient experiments are carried on HP LED samples with different DAA dosages. The theoretical and experimental results demonstrate that the time-constant spectrum for LEDs tested without cold plate can reflect the thermal characteristics of the DAA layer as well as that with cold plate. Differences in the HP LED samples with different DAA layers can be obviously distinguished in the time-constant spectrums as well. Compared with the traditional testing method with cold plate, direct test in air for HP LEDs has the advantages of easy operation, time saving, and no pollution to samples.

Degradation of sensing properties of fiber Bragg grating strain sensors in fatigue process of bonding layers

Abstract. Bonding layers, serving as the strain transmission mediums, may bring undesirable effects to the sensing properties of fiber Bragg grating (FBG) strain sensors during their fatigue process. To analyze the strain sensitivity and the reflected spectrum of FBG strain sensors in different fatigue stages of bonding layers, their strain sensitivities were derived according to strain transfer models. Resorting to T-matrix formalism, the affected reflected spectra were stimulated. Theoretical analysis results show that there is a gradual decline in the strain sensitivity during the stage of fatigue crack initiation, and that significant distortions emerge in the reflected spectrum during the stage of fatigue crack propagation. In addition, a cyclic loading fatigue test was conducted and the phenomenon observed in the test showed a good agreement with the theoretical prediction.

Transient multiexponential signals analysis using Bayesian deconvolution

A new method based on Bayesian deconvolution is proposed for multiexponential transient signal analysis. The multiexponential signal is initially converted to a convolution model using logarithmic and differential transformation after which the Bayesian iteration is used to deconvolve the data. The numerical simulation is applied on four different multiexponential signals with different levels of noise. Thermal transient experiment data of the high power light emitting diodes are also analyzed using the proposed method. Simulation and experimental results indicate that the present method performs efficiently in accurately estimating the decay rates except at low SNR case.

Analysis of misalignment-induced measurement error for goniophotometry of light-emitting diode arrays

The luminous distribution characteristics of light sources can be measured by goniophotometry. In this work, the misalignment of luminary-induced measurement errors as the main factor affecting the measurement accuracy is analyzed. A calculation method for measurement error is proposed. Then, the translational and angular misalignment-induced measurement errors are calculated and analyzed. Results show that the measurement errors induced by misalignments may be great in some cases even if the far-field condition is satisfied. For luminaries with different radiation patterns, the acceptable misalignment tolerances corresponding to measurement error of less than 1% are given.

Improvement of light extraction efficiency of GaN-based flip-chip light-emitting diodes by patterning the double sides of sapphire

The double-side patterned sapphire structure was proposed to improve the light extraction efficiency (LEE) of flip-chip GaN-based light-emitting diodes (LEDs). The influences of sapphire substrate thickness, pattern shapes and sizes on LEE were analyzed by Monte Carlo simulation method. Using silicon oxide as mask membrane, double-side patterned sapphires were processed by the standard lithography and the reaction-ion-etching (RIE) technology. The LEDs with patterned sapphire were packaged. The measured light outputs of LEDs verified our predicted effects.

Quantum efficiency measurement of luminescence glasses for white LED

Quantum efficiency measurement of luminescence glasses for high-power white LED was investigated. Luminescence glasses have transparent and anisotropic characteristics, for this reason, we adopted an integrating sphere with 20cm diameter which was connected to a CCD spectrometer to obtain fluorescence spectra of the sample. The relative spectral intensity distribution of the sample under the light source excited was derived from the measured spectra firstly, then using the standard halogen lamp to calibrate the system, we got the absolute spectral intensity distribution, finally the quantum efficiency of the sample can be calculated based on the distribution. It provides an accurate method to measure the luminescence materials’ emission characters.

Optimal design of light distribution of LED luminaries for road lighting

Conventional road lighting luminaries are gradually upgraded by LED luminaries nowadays. It is an urgent problem to design the light distribution of LED luminaries fixed at the former luminaries arrangement position, that are both energysaving and capable of meeting the lighting requirements made by the International Commission on Illumination (CIE). In this paper, a nonlinear optimization approach is proposed for light distribution design of LED road lighting luminaries, in which the average road surface luminance, overall road surface luminance uniformity, longitudinal road surface luminance uniformity, glare and surround ratio specified by CIE are set as constraint conditions to minimize the total luminous flux. The nonlinear problem can be transformed to a linear problem by doing rational equivalent transformation on constraint conditions. A polynomial of cosine function for the illumination distribution on the road is used to make the problem solvable and construct smooth light distribution curves. Taking the C2 class road with five different lighting classes M1 to M5 defined by CIE for example, the most energy-saving light distributions are obtained with the above method. Compared with a sample luminary produced by linear optimization method, the LED luminary with theoretically optimal lighting distribution in the paper can save 40% of the energy at the least.

A study on the cross-correlating demodulation of frequency division multiplexed optical fiber Fabry-Perot sensors

In order to demodulate frequency division multiplexed optical fiber Fabry-Perot sensors with high accuracy and absolute cavity length, an improved cross-correlation method was presented. As the applying of a special correlating factor in it, the cross correlation function at every sensors cavity length is a single peak and multiplexed sensors that cavity length is close to each other can be easily distinguished, and segment golden search method was employed to reduce computation and demodulating speed is fast. The multiplexing experiment of two sensors was accomplished with the help of general demodulating system constituted by broadband light source, spectrometer etc. The results showed that the system can demodulate the frequency division multiplexed signal of optical fiber Fabry-Perot sensors, and the least cavity length difference that can be distinguished among multiplexed sensors is less than 6μm and fluctuation of the cavity length is less than 0.073%.