Theory of noise in a kilo-Hz cascaded high-energy Yb-doped nanosecond pulsed fiber amplifier

A theoretical analysis of noise in a high-power cascaded fiber amplifier is presented. Unlike the noise theory in low power communication, the noise of a high power system is redefined as the leaked output energy between pulses with coherent beat noise uncounted. This definition is more appropriate for high power usage in which the pulse energy receives more attention than the pulse shape integrity. Then the low power pre-amplifying stages are considered as linear amplification and analyzed by linear theory. In the high-power amplification stages, the inversion is assumed to recover linearly in the time interval between pulses. The time shape of the output pulse is different from that of the input signal because of different gains at the front and back ends of the pulse. Then, a criterion is provided to distinguish the nonlinear and linear amplifications based on the signal-to-noise ratio （SNR） analysis. Then, an experiment that shows that the output SNR actually drops off in nonlinear amplification is performed. The change in the noise factor can be well evaluated by pulse shape distortion.     

Synthesis of Au nanorods in a low pH solution via seed-media method

The gold(Au) nanorods with various aspect ratios are obtained by a seed-media method in low pH growth solution.Transmission electron microscopy(TEM) and UV-visible spectrophotometry are utilized to characterize the Au nanorods,and the longitudinal absorption peak positions of Au nanorods show different shifting trends of the growth evolutions in various low pH(1～3) solutions. Other influential factors on the shape of Au nanorod are also systematically studied under low pH reaction condition. The positions of longitudinal peak shift between 600 nm and 900 nm, with the aspect ratios of Au nanorods varying from 2 to 5 both in the simulation and experimental results. The simulation results are in agreement with experimental ones.     

Stochastic resonance in an over-damped linear oscillator

For an over-damped linear system subjected to both parametric excitation of colored noise and external excitation of periodically modulated noise, and in the case that the cross-correlation intensity between noises is a time-periodic function,we study the stochastic resonance(SR) in this paper. Using the Shapiro–Loginov formula, we acquire the exact expressions of the first-order and the second-order moments. By the stochastic averaging method, we obtain the analytical expression of the output signal-to-noise ratio(SNR). Meanwhile, we discuss the evolutions of the SNR with the signal frequency, noise intensity, correlation rate of noise, time period, and modulation frequency. We find a new bona fide SR. The evolution of the SNR with the signal frequency presents periodic oscillation, which is not observed in a conventional linear system. We obtain the conventional SR of the SNR with the noise intensity and the correlation rate of noise. We also obtain the SR in a wide sense, in which the evolution of the SNR with time period modulation frequency presents periodic oscillation. We find that the time-periodic modulation of the cross-correlation intensity between noises diversifies the stochastic resonance phenomena and makes this system possess richer dynamic behaviors.     

Direct numerical simulation methods of hypersonic flat-plate boundary layer in thermally perfect gas

High-temperature effects alter the physical and transport properties of air such as vibrational excitation in a thermally perfect gas,and this factor should be considered in order to compute the flow field correctly.Herein,for the thermally perfect gas,a simple method of direct numerical simulation on flat-plat boundary layer is put forward,using the equivalent specific heat ratio instead of constant specific heat ratio in the N-S equations and flux splitting form of a calorically perfect gas.The results calculated by the new method are consistent with that by solving the N-S equations of a thermally perfect gas directly.The mean flow has the similarity,and consistent to the corresponding Blasius solution,which confirms that satisfactory results can be obtained basing on the Blasius solution as the mean flow directly in stability analysis.The amplitude growth curve of small disturbance is introduced at the inlet by using direct numerical simulation,which is consistent with that obtained by linear stability theory.It verified that the equation established and the simulation method is correct.     

Synthesis and Properties of Tannic Acid-Based Hydrogels

Hydrogels were synthesized by using tannic acid (TA) as a chemical cross-linker for the copolymer of allylglycidyl ether and acrylamide. The swelling ratio of the hydrogels increased with increasing amount of TA and decreased with increasing amounts of allylglycidyl ether. The hydrogels exhibited pH sensitivity; the swelling ratio increased with pH. Adsorption experiments of the hydrogel for Cu (II) ions suggested that the hydrogels can be used as an adsorbent for removal of Cu (II) heavy metal ions from dilute aqueous solutions.     

New trend for optical signal-to-noise ratio of disturbed Raman fiber amplifier

In a distributed Raman fiber amplifier (DRFA), Raman amplification allows a lower signal launch powers to transverse the span above the noise floor while still increasing the optical signal-to-noise ratio (OSNR). It improves the noise figure and reduces the nonlinear penalty of fiber systems. In this paper, we demonstrate a new trend of OSNR at different pump configurations: forward, backward and bidirectional pumping for DRFAs as a function of fiber length. We also present the variation of OSNR with both input pump power and input signal power. It is found that forward pumping provides the highest OSNR, reaching its maximum value of 37 dB. However, backward pumping provides the smallest OSNR that has its maximum of 22 dB and the bidirectional pumping provides the moderate OSNR between the others having its peak of 26 dB.     

Influence of Er-doped superfluorescent fiber source's spectrum-stability on the SNR of fiber optic gyroscopes

The output spectrum-stability of Er-doped superfluorescent fiber source (SFS) was measured by changing the temperature around the excited Er-doped fiber (EDF). With the experiment results, the influences of the SFS's spectrum-stability on the signal-to-noise ratio (SNR) of fiber optic gyroscope (FOG) were simulated. The simulated results demonstrate that the SNR of FOG will change no matter the power, mean wavelength or the spectral bandwidth of the light output from the Er-doped SFS is changing. The results show that the changing of SNR caused by the output power's variation is three orders of magnitude larger than that caused by the variation of mean wavelength or bandwidth. And the main factor influencing the stability of FOG's SNR is the temperature-stability of the output power of Er-doped SFS.     

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ITER测试平台无功补偿及谐波抑制系统包含一套由RLC滤波支路和晶闸管控制电抗器(TCR)组成的无功补偿装置,当接入电网时,为防止系统甩负荷对电网的安全性产生影响,通过计算解析研究分析了系统甩负荷时在110kV母线上产生的过电压。结果表明,当因系统故障进行甩负荷时,母线电压上确实会产生一个较高的过电压,其数值已超出国家标准允许范围,可能会对电网产生一定影响,需采取防护措施,即加入了一条由反并联二极管和电阻组成的旁通支路来释放主回路电流以减少过电压。MATLAB的仿真结果也验证了上述结果的正确性。     

Vertical flow apparatus for enhancement and efficient collection of Raman signal

To improve the signal‐to‐noise (S/N) ratio of Raman spectra, we developed a sample flow system in a vertical flow apparatus (VFA). The VFA consists of a brass plate with a drilled pinhole, cover slip on the brass plate, circulation pump, and reservoir. The sample solution flows through a gap between the brass plate and cover slip, spouts from the pinhole, and forms a laminar flow column. The sample column and surrounding air, respectively, serve as the core and cladding of an optical fiber that confines both excitation beam and scattered light owing to the total reflection at the core–cladding interface. The VFA enabled the enhancement and efficient collection of the Raman signal. The VFA with a 0.2‐mm pinhole produced a 12.0‐fold enhancement of the non‐resonant Raman signal of H₂O compared to that measured using a capillary, and the S/N ratio exhibited a 4.1‐fold enhancement. The signal count was inversely proportional to the square of the pinhole diameter (φ), possibly owing to the dependence on the density of the electromagnetic energy of the excitation beam in the sample column. The S/N ratio was inversely proportional to φ because the signal enhancement was accompanied by an increase in the photon shot noise. The VFA also produced a 4.9‐fold enhancement of the resonance Raman signal of Congo red, although the enhancement factor was smaller than that in the non‐resonant Raman measurement. Copyright © 2014 John Wiley & Sons, Ltd.     

LED显示屏高光效高画面填充比光学设计与研究

针对传统LED显示屏光能利用率和图像画面填充比低的缺点,基于非成像光学理论,提出了一种提高光能利用率和画面填充比的全彩LED显示模块结构系统和设计方法。利用复合抛物面集光器CPC对LED管芯发出光线的发散角进行变换压缩,从而避免了外表面全反射损耗,大幅度提高了系统的光能利用率。利用积分方腔匀光原理和散射元件对光能的二次分配,提高了显示屏的画面填充比、单位像素均匀度及基色复用面积。作为实例,根据上述方法设计了一个P10 mm全彩LED显示模块,利用光学设计软件LIGHTTOOLS对该显示模块系统进行了仿真建模和光线追迹,并对设计结果进行了分析。结果表明,系统光能利用率大于70%,画面填充比接近100%,单位像素区域内均匀度好于85%。显示模块具有能量利用率高、高画面填充比、显示效果均匀柔和、易于生产和装调的优点。