Nonlinear optical generation and detection of ultrashort electrical pulses in transmission lines

The nonlinear optical generation and detection of subpicosecond electrical pulses on coplanar transmission lines is demonstrated. The electrical pulses are generated by optical rectification of ultrashort optical pulses and detected by electro-optic sampling. Both processes are the result of a second-order nonlinear optical response that occurs in the same poled polymer medium. A bipolar temporal waveform with a FWHM duration of 180 fs for the positive lobe that was measured after a propagation distance of 125 mum was observed. Pulse broadening was minimized by careful attention to the device structure.     

光脉冲在标准单模光纤中演化形成孤子的实验研究

利用二次谐波频率分辨光学门脉冲分析仪从实验上分析研究了具有啁啾的10 GHz光脉冲在不同输入功率下演化形成孤子的规律和特点。实验发现:输入脉冲在光纤中传输3.5个色散长度时,其时间宽度、频率啁啾和时间带宽积都随着输入功率的增加而减小。当输入功率大于一阶孤子功率的理论值时,光脉冲能够演化形成孤子;脉冲在随后传输过程中其宽度基本保持不变,波形、频率啁啾和时间带宽积仍随着传输距离的变化而变化;输入功率越高,形成的一阶孤子脉冲的宽度越窄。当输入功率小于一阶孤子功率的理论值时,输出脉冲的时间宽度随着传输距离的增加而增加,频率啁啾随着传输距离的增加而减小,光脉冲不能演化形成孤子。     

Linear optical bullets

We describe a class of spatio-temporal optical pulses that spread neither spatially nor temporarily in linear dispersive media over long propagation distances. These new spatio-temporal pulses, referred to as linear optical bullets, can have any spatio-temporal profile and temporal width, and they carry a finite amount of energy. We also discuss in detail a technique for the experimental realization of linear optical bullets.     

Temporal and spectral properties of contra-propagating picosecond optical pulses in SOA

A useful analysis of contra-propagating optical pulses in semiconductor optical amplifier (SOA) is presented. Pulse temporal and spectral evolutions are investigated by resolving coupled equations describing pulse field propagation and SOA gain dynamics. With reference to the case of single pulse propagation, collision between pulses tends to maintain a good time-bandwidth product of amplified pulse and could provide a temporal compression by about 10% compared to pulse’s initial width.     

用时域ABCD矩阵元表达的时间菲涅耳数

利用时域ABCD矩阵引入过程中给出的与ABCD矩阵的类比关系,给出了对光脉冲传输的基本时间特征(脉冲形状、宽度和啁啾行为)可作出定性或半定量分析的时间菲涅耳数,引入了有效传输时间的概念,讨论了几种特例。     

高斯光脉冲在非线性增益介质中的孤子特性

本文从理论上分析了高斯光脉冲在非线性增益介质中的传播特性,从而解释高斯光脉冲的压缩及展宽和形成光孤子的机理。用计算机模拟了不同参数的高斯光脉冲在非线性增益介质中的传播过程,并获得了脉宽沿传播方向变化的定量关系。结果表明,以不同能量向增益介质中注入高斯脉冲,有可能产生各阶光孤子,但由于增益的存在,光孤子的传输距离受到限制。     

Nonlinear pulse propagation: a time-transformation approach

We present a time-transformation approach for studying the propagation of optical pulses inside a nonlinear medium. Unlike the conventional way of solving for the slowly varying amplitude of an optical pulse, our new approach maps directly the input electric field to the output one, without making the slowly varying envelope approximation. Conceptually, the time-transformation approach shows that the effect of propagation through a nonlinear medium is to change the relative spacing and duration of various temporal slices of the pulse. These temporal changes manifest as self-phase modulation in the spectral domain and self-steepening in the temporal domain. Our approach agrees with the generalized nonlinear Schr?dinger equation for 100 fs pulses and the finite-difference time-domain solution of Maxwell's equations for two-cycle pulses, while producing results 20 and 50 times faster, respectively.     

Effects of initial frequency chirp on the linear propagation characteristics of the exponential optical pulse

In this paper, the linear propagation characteristics of the exponential optical pulse with initial linear and nonlinear frequency chirp are numerically studied in a single mode fibre for \be₂<0. It can be found that the temporal full width at half maximum and time-bandwidth product of exponential pulse monotonically increase with the increase of propagation distance and decrease with the increase of linear chirp C for C<0.5, go through an initial decreasing stage near \zeta=1, then increase with the increase of propagation distance and linear chirp C for C\geq0.5. The broadening of pulses with negative chirp is faster than that with positive chirp. The exponential pulse with linear chirp gradually evolves into a near-Gaussian pulse. The effect of nonlinear chirp on waveform of the pulse is much greater than that of linear chirp. The temporal waveform breaking of exponential pulse with nonlinear chirp is first observed in linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the exponential optical pulse with the frequency chirp are given by use of the numerical analysis method.     

Transient sum-frequency mixing with temporally tailored laser pulses

We discuss the possibility of using temporally tailored pump laser pulses to control the temporal width and shape of optical pulses generated in a process of transient sum-frequency mixing in crystals with second-order nonlinearity. Specific calculations performed in a model crystal in the case of fifth-harmonic generation are presented.     

Propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium

We investigate the propagation dynamics of nonlinear chirped optical laser pulses in a two-level medium. For certain chirp strength and chirp width, an incident 2π nonlinear chirped pulse will split into optical precursors and a stable self-induced transparency soliton. This is caused by the particular Fourier spectrum that includes not only central resonant frequency components but also high-frequency and low-frequency sidebands. Moreover, the effects of chirp parameters on the evolution of nonlinear chirped pulses are also discussed.     

Plasma induced pulse breaking in filamentary self-compression

A plasma induced temporal break-up in filamentary propagation has recently been identified as one of the key events in the temporal self-compression of femtosecond laser pulses. An analysis of the Nonlinear Schrödinger Equation coupled to a noninstantaneous plasma response yields a set of stationary states. This analysis clearly indicates that the emergence of double-hump, characteristically asymmetric temporal on-axis intensity profiles in regimes where plasma defocusing saturates the optical collapse caused by Kerr self-focusing is an inherent property of the underlying dynamical model.     

Ultraslow propagation of squeezed vacuum pulses with electromagnetically induced transparency

We have succeeded in observing ultraslow propagation of squeezed vacuum pulses with electromagnetically induced transparency. Squeezed vacuum pulses (probe lights) were incident on a laser-cooled 87Rb gas together with an intense coherent light (control light). A homodyne method sensitive to the vacuum state was employed for detecting the probe pulse passing through the gas. A delay of 3.1 micros was observed for the probe pulse having a temporal width of 10 micros.     

Demonstration of the Bessel-X pulse propagating with strong lateral and longitudinal localization in a dispersive medium

We report experiments on ultrashort pulses that maintain their strong lateral and longitudinal localization in a bulk linear highly dispersive medium. The diameter of the central peak and the temporal width of the field autocorrelation function of the pulses were 20mum and 210 fs, respectively, and the spatiotemporal structure was preserved in the course of 7-cm propagation in the sample. The pulses were obtained with a computer hologram designed for generating the Bessel beam and can be applied in femtosecond laser optics.     

空间光孤子脉冲在平面光波导中的传输

分析研究了空间光孤子脉冲化后在反常群速色散的克尔非线性平面光波导中的传输特性和稳 定性.当空间光孤子脉冲的时间宽度等于某一临界值时，脉冲发生自陷，小于该值时，脉冲 发生扩散，大于该值时，脉冲发生塌陷.空间光孤子的阶数越高，这个临界值越小.当空间光 孤子脉冲的时间宽度等于某一特定值时，脉冲塌陷最快，与这个特定值相差越大，塌陷越慢 .空间光孤子的阶数越高，这个特定值越小.脉冲自陷后的峰值光强、时间和空间宽度与输入 时的值有明显的不同. 关键词： 脉冲自陷 脉冲塌陷 光孤子     

Propagation of bright femtosecond pulses in a nonlinear optical fibre with the third- and fourth-order dispersions

We solve the generalized nonlinear Schr\"{o}dinger equation describing the propagation of femtosecond pulses in a nonlinear optical fibre with higher-order dispersions by using the direct approach to perturbation for bright solitons, and discuss the combined effects of the third- and fourth-order dispersions on velocity, temporal intensity distribution and peak intensity of femtosecond pulses. It is noticeable that the combined effects of the third- and fourth-order dispersions on an initial propagated soliton can partially compensate each other, which seems to be significant for the stability controlling of soliton propagation features.     

Analytic study of continuum spectrum pulse dynamics in optical waveguides

It is demonstrated that for femtosecond supercontinuum generation in optical waveguides with nonresonant electronic nonlinearity the squared root-mean-square pulse duration varies near-parabolically with distance, irrespectively of spectrum ultra-broadening scenarios. The derived formulas for parabolic coefficients are applied to find the areas of spectral and energetic parameters of few-cycle pulses propagating in hollow capillaries filled with noble gases, where temporal pulse broadening, compression, or quasi-stationary propagation occur. Each regime is illustrated by the typical changes in the pulse electrical field and spectrum. PACS 42.65.Re     

Slow-light optical bullets in arrays of nonlinear Bragg-grating waveguides

We demonstrate that propagation direction and velocity of optical pulses can be controlled independently in the structures with multiscale modulation of the refractive index in transverse and longitudinal directions. We reveal that, in arrays of waveguides with phase-shifted Bragg gratings, the refraction angle does not depend on the speed of light, allowing for efficient spatial steering of slow light. In this system, both spatial diffraction and temporal dispersion can be designed independently, and we identify the possibility for self-collimation of slow light when spatial diffraction is suppressed for certain propagation directions. We also show that broadening of pulses in space and time can be eliminated in nonlinear media, supporting the formation of slow-light optical bullets that remain localized irrespective of propagation direction.     

Transform-limited spectral compression due to self-phase modulation in fibers

We demonstrate near-transform-limited pulse generation through spectral compression arising from nonlinear propagation of negatively chirped pulses in optical fiber. The output pulse intensity and phase were quantified by use of second-harmonic generation frequency-resolved optical gating. Spectral compression from 8.4 to 2.4 nm was obtained. Furthermore, the phase of the spectrally compressed pulse was found to be constant over the spectral and temporal envelopes, which is indicative of a transform-limited pulse. Good agreement was found between the experimental results and numerical pulse-propagation studies.     

Measurements of dynamics of nondegenerate optical nonlinearity in ZnS with pulses from optical parameter generation

We expand the degenerate PO pump-probe technique to nondegenerate field and use it to investigate optical nonlinear dynamics in ZnS single crystal. Excited by 532-nm laser pulses with 21-ps duration, the temporal response of nondegenerate nonlinear absorption and nondegenerate nonlinear refraction are probed by laser pulses from optical parameter generation (OPG) at 600 and 680 nm with pulse width of 10 ps. Based on the theory of free-carrier optical nonlinearity, we study the pure free-carrier refraction in ZnS. By numerically fitting based on the nondegenerate pump-probe theory, the nondegenerate two-photon absorption coefficient, the free-carrier lifetime, the free-carrier absorptive cross section and refractive coefficient at both probe wavelengths are determined respectively. The dispersion of the free-carrier refractive coefficients is discussed.     

Spectral broadening of a multimode continuous-wave optical field propagating in the normal dispersion regime of a fiber

We investigate experimentally and theoretically the broadening of the optical spectrum of a multimode cw field propagating in the normal dispersion regime of a single-mode fiber. The width of the optical spectrum is not a monotonic function of propagation length. This behavior arising from the interplay between the Kerr effect and group-velocity dispersion contrasts with spectral broadening of mode-locked pulses.