Effects of walk-off on cross-phase modulation induced modulation instability in an optical fibre with high-order dispersion

This paper investigates the effects of walk-off among optical pulses on cross-phase modulation induced modulation instability in the normal dispersion region of an optical fibre with high-order dispersion. The results indicate that, in the case of high-order dispersion, the walk-off effect takes on new characteristics and will influence considerably the shape, position and especially the number of the spectral regions of the gain spectra of modulation instability. Not only the group-velocity mismatch, but also the difference of the third-order dispersion of two optical waves will alter the gain spectra of modulation instability but in different ways. Depending on the values of the walk-off parameters, the number of the spectral regions may increase from two to at most four, and the spectral shape and position may change too.     

Error analysis on dihedral angle of corner-cube reflectors in Fourier transform spectrometer北大核心CSCD

Fourier transform spectrometer is an important spectral analysis instrument. To research the influence of the dihedral angle error of corner-cube reflectors on interference quality of oscillating Fourier transform spectrometer, the theoretical analysis and computer simulation were carried out. According to the oscillating Fourier transform spectrometer model, the mathematical expressions of the interference intensity and modulation depth with dihedral angle error were deduced. The tolerance of dihedral angle was determined by the modulation depth criterion. The optical design software Zemax was used to establish the model for simulation and verify the results of theoretical derivation. According to the modulation depth criterion, the tolerance of dihedral angle obtained by the theoretical model is 2.52″, and that obtained by the Zemax simulation is 2.38″. The error is 0.14″, which is acceptable. The analysis results show that the established theoretical model is reasonable, which has certain reference value for the design and installation of oscillating Fourier transform spectrometers. © 2022 Editorial office of Journal of Applied Optics. All rights reserved.     

Implementation of the direct demodulation method for natural gamma ray spectral logging

Spectrum analysis of natural gamma ray spectral logging (SGR) data is a critical part of surface information processing systems. Due to the low resolution, which is an inherent weakness of SGR, and the low signal-to-noise ratio problem of logging measurements, SGR is usually treated with a low confidence level. The Direct Demodulation (DD) method is an advanced technique to solve modulation equations interactively under physical constraints. It has higher sensitivity and spatial resolution than the traditional methods and can effectively suppress the logging noise. Based on standard count rate spectral data obtained from the China Offshore Oil Logging Company SGR Calibration Facility, this paper presents the application of the DD method to gamma-ray logging. The results are compared with four traditional algorithmic methods, showing that the DD method is a credible choice, with higher sensitivity and higher spatial resolution in gamma-ray log interpretation. The Point-Spread-Function of the Shengli Oil Logging Company's natural gamma ray spectroscopy instrument is obtained for the first time. The quantities of various radionuclides in their calibration pits are also obtained. The DD method was applied successfully to gamma-ray logging, offering a new option for SGR logging algorithm selection.     

Solitons in optical lattices with nonlocal nonlinearity

The propagation of spatial solitons is systematically investigated in nonlocal nonlinear media with an imprinted transverse periodic modulation of the refractive index. Based on the variational principle and the infinitesimal approximation of Maclaurin series expansion, we obtain an analytical solution of such nonlocal spatial solitons and an interesting result that the critical power for such solitons propagation is smaller than that in uniform nonlocal self-focusing media. It is found that there exist thresholds in modulation period and lattice depth for such solitons. A stable spatial soliton propagation is maintained with proper adjustment of the modulation period and the lattice depth.     

Artificial spectral filtering in dissipative soliton fiber lasers with invisible bandpass filters

We numerically study the artificial spectral-filtering effect in dissipative soliton fiber lasers without intracavity spectral filters. It is found that in dissipative soliton lasers with real saturable absorbers (SAs), the dynamic spectral filtering of the real SAs serves as an artificial spectral filter and contributes to the pulse shaping. While in the dissi- pative soliton lasers with artificial SAs, such as nonlinear polarization rotation, the spectral filtering introduced by the intracavity polarization-dependent components acts as an artificial spectral filter and shapes the pulses to obtain mode-locking. An investigation of the artificial spectral-filtering effect reveals the operating mechanisms of the dissipative soliton fiber lasers without visible bandpass filters.     

Dynamic behavior and complexity of modulated optical micro ring resonator

The dynamic behavior of an optical micro ring resonator (OMRR) with an amplitude modulator positioned in the micro ring is investigated quantitatively by adopting a recently introduced quantifier, the permutation entropy (PE). The effects of modulation depth are focused on, and the roles of input power are considered. The two-dimensional (2D) maps of PE showing dependence on both modulation depth and input power are presented as well. PE values nearly increase with modulation depth. On the other hand, the optimal value of input power is achieved when the PE reaches its maximum. Thus, PE can successfully quantify the dynamics of modulated OMRR. Selecting the parameters in the region with high PE values would contribute to the complexity-enhanced OMRR-based chaotic communication systems.     

Solving the atmospheric scattering optical transfer function using the multi-coupled single scattering method

The atmospheric scattering optical transfer function （OTF） is solved by applying the multi-coupled single scattering （MCSS） method to the three-dimensional radiative transfer equation （RTE） under the periodic ground condition. This approach is a direct hit to the atmospheric scattering OTF using the same original context of modulation transfer function （MTF） measurement, i.e., images of sinusoidal grating at different spatial frequencies. Both the amplitude and phase shift of the OTF at various zenith and azimuth angles can be obtained at an arbitrary spatial frequency.     

Synchronization of chaotic VCSELs by external chaotic signal parameter modulation

Synchronization of chaotic vertical-cavity surface-emitting lasers （VCSELs） is achieved by external chaotic signal modulation successfully. Simulation indicates that we can get chaos synchronization if the intensity of external chaotic signal is large enough. First of all, we use direct current modulation to achieve the chaos of VCSELs, and determine the laser＇s chaotic state by analyzing time series of the output and the corresponding power spectrum. And then we achieve synchronization of the two chaotic systems by external chaotic signal parameter modulation. We also find that the larger the modulation intensity is, the easier it is to achieve synchronization for chaotic VCSELs. This approach can also be applied to systems with a number of modulated lasers.     

Research on stimulated Brillouin scattering suppression based on multi-frequency phase modulation

The method of generating equal-amplitude spectral lines by multi-frequency phase modulation is used in stimulated Brillouin scattering （SBS） suppression. The spectra of three, five, seven, and eleven equalamplitude spectral lines are obtained in experiment with flatnesses less than 0.3 dB. Theoretical research on SBS suppression shows that the threshold power after modulation is in reverse proportion to the maximum square of amplitude moduli of fundamental frequency and the nth harmonic wave. The threshold powers of three, five, seven, and eleven equal-amplitude spectral lines are improved by 5.21, 8.36, 9.39, and 10.76 dB, respectively.     

Temperature independent 80 Gb/s transmission system using spectral phase modulation-based TDC

A temperature independent 80-Gb/s 100-km transmission system is demonstrated with the use of spectral phase modulation-based tunable dispersion compensator (TDC). The principle of dispersion compensation based on spectral phase modulation as well as the relationship between spectral phase modulation function and group velocity dispersion (GVD) are theoretically studied. TDC based on spectral phase modulation is implemented. The performance of 80-Gb/s transmission system is experimentally evaluated. The nonlinear relationship between temperature and temperature-induced dispersion fluctuations is demonstrated through the asymmetric temperature-induced power penalty without dispersion compensation. With respect to the low temperature area, the temperature-induced dispersion fluctuations are smaller than those in the high temperature area. By using the proposed TDC, temperature independent 80-Gb/s transmission is successfully demonstrated under a temperature range of -20 - 60 ?C with a power penalty of less than 0.8 dB.     

Suppression for the Low spatial frequency Rings and Ripples of Laser Near field Intensity Distribution in the Spatial Filter Used in ICF Laser DriveSupported by the key project of the National Science Foundation, National Hi Tech Foundation.

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高功率固体激光近场强度功率谱密度特性分析

高功率固体激光驱动器的负载能力已经成为限制驱动器发展的瓶颈问题,负载能力研究的主要是激光束近场和光学元件的相互作用过程。采用功率谱密度(Power spectral density,PSD)方法分析了多程放大构型高功率固体激光驱动器实验输出近场强度的频谱在激光强度提升过程中的演变规律,分析了引起近场强度调制的主要因素。结果表明,高强度下,激光驱动器的输出近场存在明显的频谱特征,二维功率谱密度近似满足各向同性,近场调制主要源于小尺度自聚焦效应,PSD曲线显示获得增长的频率主要是各级空间滤波器截止频率所限定的频段。     

A two-dimensional diffraction pattern sampling system for determining the optimum parameters of a matched spatial filter

The synthesis of a matched spatial filter based on holographic methods requires that the spatial frequency band in which the holographic fringe pattern with the best modulation is formed be tuned to the appropriate weighting at the Fourier transform plane of the spatial frequencies of the input object to be studied. For this purpose both the spectral intensity distribution of an input object at the Fourier transform plane and the intensity distribution of a reference beam at that plane must be accurately known beforehand. A two-dimensional diffraction pattern sampling system has been constructed which enables the Fourier spectral intensity distribution of the input object and the reference beam intensity distribution to be faithfully recorded. A preliminary experimental study shows the usefulness of this system.     

杆箍缩二极管X射线焦斑的成像法测量

 X射线源的焦斑尺寸是反映杆箍缩二极管射线源成像性能的重要参数。利用针孔成像法对MeV级脉冲X射线源的焦斑进行了2维图像测量。厚针孔采用直孔段加单锥体结构,直孔段孔径为0.2 mm。对于0.5 MeV的X射线,5倍成像倍率下调制传递函数值为0.5时空间分辨达到2.0 lp·mm^-1。图像采集系统由闪烁体、物镜和CCD相机组成,物镜的成像倍率约0.34。实验结果经过模糊校正后,得到了焦斑的图像和调制传递函数。根据调制传递函数值为0.5时对应的空间频率值,给出X射线源焦斑的尺寸。阳极杆直径为1.2 mm时,X射线源焦斑的高斯分布等效直径为0.86 mm。     

Evolution of low-frequency noise passing through a spatial filter in a high power laser system

The theoretical model of spatial noise passing through a spatial filter is established in high power laser system under the small signal approximation. The transmission characteristic for a noise signal passing through spatial filters with different magnifications is analyzed by numerical simulation, according to the actual structure of the high power laser system. The results show that the spatial modulation period of low-frequency noise getting through the pinhole will be proportional to the magnification...     

无外加氟施主产生KrF准分子发射谱实验研究

We report a new technique to suppress the low-spatial-frequency rings and ripples (LSFRs) of laser near-field intensity distribution in the spatial filter (SF) used in ICF laser drivers. This technique employs a spatial filter with Gaussian transmission, instead of the hard-edged pinhole, at the confocal plane of a Newtonian telescope. The resulting beam has a super-Gaussian transverse intensity profile without the LSFRs that certainly appear when a hard-pinhole was used as a filter in the conventional technique.     

强激光束在柱面镜空间滤波器中的传输特性

为了解决空间滤波器滤波针孔堵孔问题,提出了一种新型柱面镜空间滤波器,新型柱面镜空间滤波器由柱面镜和狭缝构成。利用线性传输理论,对高功率激光束在新型柱面镜空间滤波器中的传输特性进行了数值模拟。结果表明:新型柱面镜空间滤波器的焦斑面积比传统滤波器大,而焦斑功率密度较小;新型柱面镜空间滤波器的滤波效果与传统滤波器相同,近场均匀性也相同。因此新型柱面镜空间滤波器能够抑制空间滤波器堵孔效应,可以缩短空间滤波器系统透镜焦距,从而提高功率装置的整体性价比。由此,采用新型柱面镜空间滤波器替换传统球面镜空间滤波器具有可行性。     

针孔相机全场X射线荧光成像的模拟研究

为解决全场X射线荧光(XRF)成像中针孔形状和尺寸的选取问题,采用Geant4软件,模拟了6种不同类型针孔和4种不同的针孔孔径,分析了这些参数对点扩散函数和调制传递函数的影响;模拟了不同能量X射线荧光平面源的成像过程,并用均值滤波和Richardson迭代法对图像进行处理,分析了图像处理的效果。模拟结果表明:对于能量小于20 keV的荧光X射线,双锥孔结合直孔模型的点扩散函数尖锐性和等晕性更好,调制传递函数的截止频率更大,空间分辨更好,更适合做全场XRF成像的针孔;均值滤波结合Richardson迭代法的图像处理算法对全场XRF图像处理的效果较好。     

编码栅在光信息处理中的应用

本文从编码栅对光源、物函数、谱函数的调制原理出发,从三个方面分析了编码栅在光信息处理中的作用及应用实例。     

基于神经网络的大孔径厚针孔成像复原算法研究

为了更好地获取低强度辐射源空间分布图像,提出一种使用神经网络算法将大孔径厚针孔退化图像复原的方法。建立了孔径5 mm、10 mm、15 mm的厚针孔模型,获得了3600个汉字形状辐射源的厚针孔退化图像集。基于DnCNN神经网络模型,建立了大孔径厚针孔退化图像复原神经网络,并与维纳滤波、Lucy-Richardson这些传统算法进行了比较。在考虑噪声影响后,利用迁移学习理论,对原神经网络模型进行迁移训练,再对含噪大孔径厚针孔退化图像进行复原。神经网络算法复原的RMSE明显低于传统方法,迁移学习显著减小了噪声的影响。证明了神经网络算法在大孔径厚针孔退化图像复原领域的优越性,并验证了神经网络方法复原含噪大孔径厚针孔退化图像的可行性。