掺铒光纤中方波信号高次谐波的快慢光特性

光纤中方波信号的慢光技术在全光通信和光纤传感等领域具有重要的应用前景. 提出了谐波相对延时量的度量方法, 分别采用速率方程和相干布居振荡理论, 对掺铒光纤中方波信号的基波和高次谐波的快慢光特性进行了研究. 在无抽运光输入情况下, 改变入射光功率, 入射探测光的基波最大相对延时量能达到20%, 且存在实现最大相对延时量的入射光功率为8 mW; 在有抽运光输入的情况下, 改变信号光增益, 入射探测光的基波相对超前量同样能达到-20%, 且随着信号光增益的增大而增加. N次谐波(N=1, 3, 5, 7, …)在频率f/N(f为基波信号最大延时量对应的调制频率)处有最大相对延时量, 且它们的最大延时量相同, 频率处于相干布居振荡引起的光谱烧孔带宽内.     

Modulation-frequency-controlled change from sub- to superluminal regime in highly doped erbium fibers

We report a change from sub- to superluminal propagation upon increasing the modulation frequency of an amplitude-modulated 1,550 nm signal when propagating through highly doped erbium fibers pumped at 980 nm. We show that the interplay between the pump absorption and the pump-power broadening of the spectral hole induced by coherent population oscillations may drastically affect the fractional advancement or delay of the signal for the considered fibers.     

Nonlinear joint fractional Fourier transform correlation for target detection in hyperspectral image

In this paper, we present a spectral joint fractional Fourier transform correlation (SJFRTC) for detecting and identifying very small target with only a few pixels using reflectance spectral information in the hyperspectral imagery. In this technique, fractional correlation (FC) between input spectral signature and known reference spectral signature is performed digitally or optically. Binary differential technique is utilized to enhance the performance of the SJFRTC, resulting in delta-function-like correlation peaks without dc interference. Distinctive and distinguishable correlation performance is obtained using peak-to-clutter ratio (PCR) instead of absolute correlation peak value, which makes the proposed technique independent from the intensity variation of the spectral signature. Simulation results using real-life hyperspectral images show that the binary SJFRTC can detect both single or/and multiple targets successfully with excellent detection capability, as compared to the other traditional hyperspectral detection algorithms.     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

Necessity analyses of phase masks in joint fractional Fourier transform correlator

In this paper, the mathematical expressions and the particularities of the joint fractional Fourier transform correlator (JFRTC) are detailedly analyzed. In a JFRTC, the distance between the two fractional correlation peaks can be adjusted flexibly by shifting the fractional orders. However, the intensity of the fractional correlation peaks decreases rapidly as the first fractional order p₁ drops. The former property is very useful in a practical recognition system, but the latter one seriously reduces the recognition sensitivity of a JFRTC. For this reason, phase masks should be added at the input plane of the JFRTC to enhance the intensities of the fractional correlation peaks and effectively overcome the default. Compared with the existing JFRTC with single phase modulation, a JFRTC with double phase modulation is suggested to more flexibly improve the performance of a JFRTC.     

Slowdown of Group Velocity of Light Using Coherent Population Oscillation

We investigate the reduction of the group velocity propagation resulting from the steep change of the refractive index by the coherent population oscillation in erbium-doped optical fibre. The largest delay is measured to be about 8.75ms corresponding to a group index of 1.312×10^6. The time delay or advancement depends on the pump intensity. Influences of the ion density on the fractional delay and the group velocity reduction of light propagation are studied. Based on our discussion the optimization parameters should be selected in order to obtain more appropriate time delay and the slowdown of group velocity.     

A saturable absorber, coherent population oscillations, and slow light

The paper presents a critical analysis of publications on one of the methods of creating so-called slow light (light with an anomalously low group velocity) arising due to a high steepness of the refractive index dispersion curve. The method employs, for this purpose, the effect of coherent population oscillations accompanied by burning of a narrow spectral hole in the homogeneously broadened absorption spectrum. The interpretation of the experimental data in the studies under consideration is based on the analysis of the response of a nonlinear medium to a low-frequency intensity modulation of the propagating light beam. We show that all the observations of these papers can be easily interpreted in the framework of the simplest model of a saturable absorber and have nothing to do with the hole burning effect or group velocity reduction.     

Expansion of the relative time delay by switching between slow and fast light using coherent population oscillation with semiconductors

The slow-light effect based on free-carrier population oscillation in semiconductors is analyzed using generalized macroscopic Bloch equations. Using this model, we are able to investigate the relation between the fractional time delay and the intensity of control laser beams for semiconductors. It is found that although increasing the control-beam intensity can enlarge the fractional time delay, the maximum delay that can be achieved is limited and determined by a critical value of the separation between the quasi-Fermi levels of electrons and holes. We also show that there is an intensity threshold for the control beam above which the probe signal becomes gain-assisted fast light rather than slow light. We therefore suggest a method to expand the relative time delay by switching between the fast-light region and the slow-light region through changing the intensity of the control laser beams. PACS 42.25.Bs; 42.70.Nq; 42.79.-e     

腔体约束LIBS结合机器学习对土壤重金属元素的定量分析

土壤重金属元素含量检测及防治,对我国农业、生态环境修复具有重大意义。利用外加腔体约束结合激光诱导击穿光谱技术（LIBS）获得土壤光谱数据,采用机器学习对土壤中重金属元素Ni和Ba含量进行分析。实验设置延迟时间为0.5～5 μs,选择Ni Ⅱ 221.648 nm和Ba Ⅱ 495.709 nm作为目标研究特征谱线,计算两种LIBS条件下延迟时间对信噪比、光谱强度及增强因子的影响。结果表明,腔体约束LIBS（CC-LIBS）可以增大光谱强度及目标元素信噪比,同时随着采集延迟时间增长,等离子体数目变少,光谱强度及信噪比逐渐减小并趋于稳定;当延迟时间设置为1 μs时,CC-LIBS条件下Ni和Ba元素特征谱线信噪比达到最优,确定此时为LIBS最优实验条件。通过最优条件获取9种含Ni和Ba元素土壤样品的光谱数据,由于采集到的每组光谱信息有12 248个数据点,利用主成分分析（PCA）对CC-LIBS条件下的光谱数据降维,在保留95%以上的土壤原始信息后,选择9个主成分作为定量分析模型的输入变量,以提高模型的运算速度。采用机器学习中的Lasso,AdaBoost和Random Forest模型,对PCA降维后的光谱数据进行建模及预测,实现土壤重金属元素Ni和Ba的定量分析。结果表明,与Lasso和AdaBoost模型相比,Random Forest模型在训练集和测试集中表现出的预测性能最优。Random Forest模型下Ni元素在测试集中的R2为0.937,RMSEP为3.037;Ba元素在测试集中的相关系数R2为0.886,均方根误差RMSEP为90.515。基于腔体约束LIBS技术结合机器学习,为土壤重金属元素的高精度检测提供了技术指导。     

激光聚焦位置对半球腔约束等离子体光谱增强特性的影响

光谱信号增强是提高激光诱导击穿光谱技术分析性能的重要手段之一,对等离子体进行空间约束由于装置简单且约束效果好而常被采用,等离子体的特性会直接影响空间约束的效果,而等离子体的特性与实验系统中激光的聚焦情况密切相关,为研究激发光源的聚焦情况对半球形空腔约束等离子体光谱增强特性的影响,通过控制透镜到样品之间的距离(LTSD)来改变激光的聚焦位置,分别在无约束和有半球形空腔约束两种实验条件下,烧蚀合金钢产生等离子体,采集15个不同LTSD位置时等离子体的时间演变光谱,得到谱线强度和增强倍数随着LTSD和采集延时的二维空间分布图。研究结果发现：无约束情况下,谱线强度分别在LTSD为94和102 mm时出现峰值,在采集延时小于8 μs时,谱线强度的最大值在LTSD为94 mm的位置,采集延时大于8 μs后,谱线强度的最大值出现在LTSD为102 mm的位置;当用半球空腔约束等离子体,谱线强度先后在采集延时范围为4~10和12~15 μs出现第一次增强和第二次增强。谱线强度出现第二次增强的主要原因是被半球腔内壁反射的冲击波与等离子体相互作用后会继续向前传播,遇到另一侧的腔壁再次被反射,进而对等离子体产生二次压缩。分析增强倍数随LTSD和采集延时的二维变化关系发现,第一次增强的最大增强倍数随LTSD的变化没有明显规律,增强倍数在2~6之间波动;谱线第二次增强时的增强倍数相对较高,最大增强倍数随着LTSD变化呈现出先增大再减小,然后再小幅增加后降低的变化规律,在LTSD为96 mm时达到最大值,两条谱线的最大增强倍数约为6倍。分析出现最大增强倍数对应的延迟时间发现,第一次增强出现的最优延迟时间在6~9 μs之间变化,当LTSD在85~93 mm范围时,最优延迟时间保持不变,当LTSD在94~105 mm时,出现先降低再增大的变化规律;第二次增强出现的延迟时间主要在14~15 μs,随着LTSD的变化没有明显的变化规律。     

Simple and highly sensitive optical pulse-characterization method based on electro-optic spectral signal differentiation

A very simple self-referenced, linear pulse-characterization technique based on spectral phase reconstruction by frequency-domain signal differentiation is introduced. This technique can be implemented using electro-optic intensity modulation of the pulse under test with a synchronized RF sinusoid. The pulse spectral phase profile can be accurately and unambiguously reconstructed from only two measured energy spectra, i.e., at the input and at the output of the modulator, using a direct analytic equation. The method is experimentally demonstrated by precisely characterizing microwatt-power picosecond pulses after linear dispersion through short sections (50-700 m) of conventional single-mode fiber.     

Optimization of grating size in chirped-pulse-amplification. laser system

We performed calculations for a chirped-pulse-amplification laser system with a sub-100-fs pulse duration including the finite size of gratings in an aberration-free stretcher/compressor design. Both the higher-order phase dispersion and the spectral clipping, which is due to the finite size of the optics in the stretcher and the compressor, are taken into account in the calculation. We discuss the influence of both phase dispersion and spectral clipping on the important parameters of the recompressed pulse, such as pulse duration, pulse contrast, and peak intensity, employing different phase-dispersion compensation methods. The calculations show that the use of a small compressor grating reduces the higher-order phase errors and smoothes the sharp spectral cut-off caused by spectral clipping in the stretcher, which consequently improves the pulse contrast ratio of the main pulse to the pedestal. Received: 1 April 2000 / Published online: 8 November 2000     

基于布居振荡效应实现掺铥光纤中光波群速度减慢传输

从稳态条件下铥离子光纤的速率方程出发,得到掺铥光纤中光速减慢传输的时间延迟和相对调制衰减的数值解析表达式,利用数值求解法分别模拟计算了在大功率信号和小功率信号条件下的光速减慢传输。相对于小功率信号,大功率信号情况下的相对时延、时间延迟和群折射率都比较大,同时最大相对时延也向高频率处移动。     

Ultraslow light propagation in an inhomogeneously broadened rare-earth ion-doped crystal

We show that coherent population oscillations effect allows us to burn a narrow spectral hole (26 Hz) within the homogeneous absorption line of the optical transition of an erbium ion-doped crystal. The large dispersion of the index of refraction associated with this hole permits us to achieve a group velocity as low as 2.7 m/s with a transmission of 40%. We especially benefit from the inhomogeneous absorption broadening of the ions to tune both the transmission coefficient, from 40% to 90%, and the light group velocity from 2.7 m/s to 100 m/s.     

光子晶体光纤中调制不稳定现象与超连续光谱的产生

研究了光子晶体光纤中调制不稳定性效应.从非线性薛定谔方程出发，计算和分析了光子晶体光纤中反常色散区以及正常色散区内的调制不稳定性现象，详细讨论了超短脉冲的脉宽、峰值功率、高阶色散和高阶非线性效应（如脉冲内喇曼散射、自陡峭效应）对调制不稳定性产生的影响.结果表明：二阶色散对调制不稳定性的影响要远大于三阶色散，同时也发现随着初始脉冲宽度的减小，调制不稳定性旁瓣增大但是强度有所降低.另外还发现高阶非线效应如自陡峭和喇曼效应会在不同程度上抑制调制不稳定性.     

光谱位相相干电场法的参数分析及测量

从光谱位相相干电场重构法（ＳＰＩＤＥＲ）的原理出发,实现了算法、讨论时间延迟、光谱剪切量、滤波窗口宽度、色散量等几个方面的优化选取。当展宽器色散和脉冲宽度一定时,脉冲对的时间延迟存在一个最佳取值范围。相对光谱剪切量在５％～１５％间,滤波窗口宽度为／３,重构出的位相误差最小。对干涉图取平均来减小噪声的影响。用ＳＰＩＤＥＲ算法还原了脉冲的电场和位相信息,由ＳＰＩＤＥＲ测量的脉冲宽度为１７.７ fs。同时为了比较,用自相关法测量了同一脉冲,由自相关曲线可估算出脉冲宽度为１６.８ fs,与ＳＰＩＤＥＲ 的误差比为５．１％,说明了实验的有效性。     

Dynamics of the spectral width of intense laser pulses consisting of several field oscillations in optical waveguides

The dependence of the rms spectral width of a light pulse consisting of several light-field oscillations on the distance passed in an optical waveguide with arbitrary dispersion and nonresonant electronic nonlinearity has been derived. This dependence allows one to rapidly predict the scenarios of the initial evolution of the spectrum (broadening, distance independence, or compression) by using the input pulse parameters and waveguide characteristics. It is shown that the pulse spectral width increases when the enrichment of the spectrum due to the generation of multiple harmonics is taken into account. In this case, for pulses with the spectrum in the region of the anomalous group dispersion of the waveguide, there is the intensity range for which the self-narrowing of the main spectral peak around the central radiation frequency is characteristic.     

Pulse shortening and quality improvement based on spectral filtering in a stretched-pulse mode-locked fiber laser with large third-order dispersion

We demonstrate a passively mode-locked ring fiber laser operating in the stretched-pulse regime. The ring cavity model is constructed with standard single-mode fiber and dispersion delay line for dispersion management. Here, all-solid Yb-doped photonic bandgap fiber is used as the dispersion delay line and provides laser gain simultaneously. As the all-solid Yb-doped photonic bandgap fiber has large third-order dispersion, it is necessary to study its impact on the stretched-pulse operation. The presence of the third-order dispersion reduces pulse quality because of the relatively small group velocity dispersion. To improve the pulse quality, a spectral filter is employed in the model. The simulation results show that spectral filtering could improve the pulse quality and shorten the pulse duration. Meanwhile, the optimum filter bandwidth is studied.     

Group velocity control of femtosecond pulse in folded dielectric axes structures by electro-optic effect

Theoretical investigation on the group velocity control of ultra-fast pulses by transverse electro-optic effect as well as its cascading and cubic nonlinearity is presented. Numerical simulation shows that the group velocity can be tuned via conveniently modulating the external electric field strength or the intensity of input pulse. The response of group velocity modulation is in proportion to these two factors, and the advancement or delay can reach the magnitude of 1–2 fractional pulses, which could be a potential scheme for controlling the velocity of pulse in future high speed and large-capacity communication networks.     

分数傅里叶变换计算全息

在计算全息和分数傅里叶变换的基础上提出了不对称分数傅里叶变换计算全息和双随机相位不对称分数傅里叶变换计算全息。在这种方法中,首先用一随机相位函数乘以输入图像信息,然后沿x方向实施α级次的一维分数傅里叶变换,再乘以第二个随机相位函数,最后,沿y方向实施β级次的一维分数傅里叶变换。采用迂回位相编码法对变换后的结果编码,绘出计算全息图。为了恢复原始图像,需要知道变换级次和随机相位函数。利用这种方法进行图像加密,使加密图像的密钥由原来两重增加到四重,从而提高了系统的保密性能。