Chirp control of a single-mode, good beam quality, zigzag dye laser

We report a substantial reduction of frequency chirp of a single-mode laser-pumped zigzag dye laser. A linear optical cavity using counterpropagating orthogonally polarized waves was injection-seeded at 568 mn and operated with a laser output of about 1 J. The chirp was controlled by an intracavity Pockels cell that was configured to add optical density at a rate which counterbalanced the decrease in optical density due to dye-solvent heating during the ~1-μs laser pulse. Heterodyne measurements were used to determine that the bandwidth was near the transform limit and chirp rate of ~1 MHz/μs. The beam quality of the laser was measured at 10 Hz as 1.7 XDL     

利用块状介质进行飞秒强激光脉冲的腔外压缩

高强度飞秒激光脉冲的腔外压缩是获得高次谐波阿秒脉冲驱动源的必要手段.实验研究了超强超短飞秒激光脉冲在经过块状介质后的光谱展宽和色散补偿压缩现象.单脉冲能量0.26 mJ,脉宽50 fs的激光脉冲经透镜在空气中聚焦后再入射到块状材料上,出射脉冲光谱被展宽到接近40 nm.由于在块状材料中的自聚焦效应,出射光束质量变好并保持较小的空间啁啾.利用熔融石英棱镜对补偿带有正色散的出射脉冲,最后得到＞0.1 mJ,19 fs的压缩脉冲.利用SPIDER装置测量了出射脉冲的脉宽和光谱相位.整个系统的能量效率大约为35%,压缩后的激光脉冲具有很好的空间分布和平滑的时域包络.实验结果实现了利用块状材料对飞秒激光脉冲的腔外压缩,这种方法将适用于对更高能量飞秒脉冲的压缩.     

A 170 J electron beam pumped XeF(C→A) laser

A pulse output energy of 170 J has been achieved from an XeF(C→A) laser system, pumped by a pair of counterpropagating, three-meter-long electron beams. This represents a record for all types of pumping, for this excimer system. Energy was extracted from a volume of ~100 L, using a free-running stable oscillator. No evidence of laser oscillations on the competing XeF(B→X) transition was observed. Within the extraction volume the laser gas was pumped at a rate of 140 kW/cm³ (time average value), for a period of 1.7 μs. The optical cavity was folded, giving a gain length of 6 m. The optical pulse duration was 0.8 μs (full width at half maximum), and the observed flux buildup time of ~1 μs was consistent with modeling and a measurement of the net gain. The specific output energy was 1.7 J/L which is comparable to that achieved in previous, small scale experiments at somewhat higher pump rate. The results confirm the volumetric scalability of the electron beam pumped XeF(C→A) laser system to high output energy per pulse, and the feasibility of operating this system at a low electron beam pump rate which relaxes constraints on the design of the electron gun and pulse power subsystems in a high output energy device. Means for extending the laser pulse duration and increasing the output energy of the specific test device are discussed. An output energy of ~1000 J is projected for an optimized gas cell width, for full size resonator mirrors, and with injection     

五阶非线性对光纤中高斯型脉冲传输特性的影响

从含修正项的非线性薛定谔方程出发,采用变分法,导出了五阶非线性情况下光纤中高斯型脉冲参数随传输距离的演化方程组,求出了振幅与脉宽、频率与啁啾、脉宽与啁啾之间的三个解析约束关系,得出了脉宽随传输距离演化的解析解,讨论了五阶非线性对光纤中高斯型脉冲传输特性的影响.并且描绘了五阶非线性系数和啁啾对脉宽的影响、啁啾与初始啁啾对脉宽的影响、脉宽随传输距离以及五阶非线性系数、脉宽随传输距离以及初始啁啾四种情况下的三维图形.     

光纤中皮秒高斯脉冲的传输特性研究

对描述具有初始频率啁啾的皮秒高斯脉冲传输的微分方程组用龙格-库塔算法进行了求解,得到了高斯脉冲各参数随距离演化的数值解并据此画出了高斯脉冲包络的演化。数值结果表明：二阶色散与光纤非线性的比值为0.72时,脉冲的展宽可以被光纤非线性有效地补偿,且脉冲的频率抖动和啁啾也可以得到较好的抑制,但是过强的三阶色散和初始啁啾将导致脉冲能量的迅速衰减和波形的快速畸变。     

Analysis of laser pulse chirping in mode-locked vertical cavitysurface-emitting lasers

Mode-locked vertical cavity lasers have a large cross-sectional area and consequently a large saturation energy and large peak powers. The authors analyze excess optical bandwidth generation in these lasers and find that self-phase modulation due to optical pumping and gain saturation is the dominant factor in inducing laser pulse chirping. The large magnitude of the chirp makes intracavity prism-pair compensation difficult. Adjustment of the cavity length has a major impact on the pulse chirping, as observed experimentally. Proper adjustment can result in a large linear frequency chirp which can be compensated using external pulse compression techniques     

半导体光放大器中SPM效应对光脉冲传输性能的影响

为了研究半导体光放大器（SOA）中自相位调制（SPM）效应对光信号传输性能的影响,从SOA中光脉冲的传输方程出发,采用数学推导的方法,分析了SPM 效应的产生机理,得到了光脉冲经SOA 放大后的时域和频域变化特性,并针对无啁啾和带啁啾光脉冲两种情况进行了仿真研究。结果表明:SPM 导致了光脉冲时域波形的倾斜和频谱的红移,并且随增益的变大倾斜和红移的程度增强;啁啾系数的符号决定了频谱是红移还是蓝移。     

超高斯光脉冲在单模光纤中的传输特性

从光脉冲在光纤中传输演变所满足的基本方程出发,研究了超高斯光脉冲在普通单模光纤中的传输特性。数值模拟的结果表明,超高斯光脉冲入射后脉冲波形、瞬时啁啾以及峰值强度的演变规律与高斯脉冲入射时不同,脉冲波形不再总保持单峰结构,通常将经历一个从近平顶、多峰、最后到单峰的演变过程;从脉冲的前沿到后沿,瞬时啁啾也不再为线性,而是具有多个极值;峰值强度随传输距离的变化趋势也不再为单调递减,而是先增加后减小。这些变化规律还与入射超高斯脉冲的边沿陡峭程度和初始频率啁啾有关。     

自相似抛物线脉冲的噪声扰动分析

为了研究自相似脉冲系统中的噪声扰动，我们从非线性薛定谔方程出发，推导出了时间抖动方差和能量抖动方差表达式。数值研究结果表明噪声使得自相似抛物线脉冲形状，频谱和啁啾的畸变非常严重。噪声扰动导致了脉冲振荡和频谱抖动。虽然脉冲仍具有线性啁啾，但却在前后沿出现起伏波动。在初始阶段，噪声扰动很小，可以忽略，但随着传输距离增大，噪声扰动的影响将迅速增大。不同的正色散和增益对时间抖动和能量抖动方差影响不同。     

基于DDF的光脉冲绝热压缩和高阶孤子压缩

采用分步傅里叶数值计算方法研究了不同光纤长度、不同色散剖面曲线的色散渐减光纤(DDF)中光脉冲压缩的规律,最后以线性DDF为例,研究了光纤长度和孤子阶数对DDF中高阶孤子压缩性能的影响.数值计算结果表明,基于DDF的光脉冲压缩可以获得低脉座和无频率啁啾的高质量超短光脉冲.特别地给出了光纤长度和色散剖面对脉冲压缩性能如压缩比、峰值功率和脉座等的影响,对DDF光脉冲压缩有实际的指导意义.     

Investigation of pulse pedestal and dynamic chirp formation on picosecond pulses after propagation through an SOA

The authors investigate the propagation of picosecond pulses through semiconductor optical amplifiers using the measurement technique of frequency resolved optical gating. The work shows the generation of significant pulse pedestals and frequency chirp across the optical pulses, which initially have a duration of 2 ps. As the input peak power of the optical pulses is increased from 2.4 to 80 mW, the pulse pedestals increased by 20 dB and the chirp became significantly more nonlinear. The generated pedestals and the nonlinear output chirp may cause serious degradation in high-speed communications systems employing wavelength-division-multiplexing and optical time-division-multiplexing techniques.     

Locking bandwidth of actively mode-locked semiconductor lasersusing fiber-grating external cavities

We demonstrate numerically that actively mode-locked semiconductor lasers employing a linearly-chirped fiber grating in an external cavity can exhibit multiple drive frequency ranges of stable pulse generation, and thus give a large locking bandwidth. The locking bandwidth defines the range of RF drive frequencies over which the laser will generate pulses with low timing jitter. The stable optical pulses are generated with leading or trailing subpulses due to optical energy circulating within the fiber grating. The multiple stable ranges merge if loss is introduced into the fiber-grating region. We also show that the locking bandwidth can be improved if chirp is introduced into the grating. Suprisingly, however, chirp of either sign improves the locking bandwidth     

啁啾指数脉冲的非线性传输研究

数值研究了初始线性啁啾指数光脉冲在单模光纤反常色散区的非线性传输特性,并将这种特性与双曲正割脉冲的相关特性作了比较.脉冲振幅A=1时,负啁啾对脉冲时域宽度展宽的影响比正啁啾大;指数脉冲受啁啾影响比双曲正割脉冲更敏感.A＞1.2时,脉宽在演化过程中呈周期性衰减振荡,振荡周期和振幅随啁啾|C|(|C|≤2)的增大而增大,最后能演化成光孤子.脉冲演化成光孤子所需的最小振幅A比啁啾双曲正割脉冲的大.     

Statistical properties of the spectrum of light pulses in fastpseudorandom word modulation of a single-mode semiconductor laser

The spectrum of single-mode laser pulses generated by fast pseudorandom word modulation is studied numerically for return-to-zero and nonreturn-to-zero control signals. We analyze both statistics and the worst cases for the frequency chirp during each optical pulse, and we study the connection between these frequency chirps and the turn-on times. We show that patterns in the modulation signal sequences contribute to chirp noise. The worst case values of the turn-on time and the chirp range are very similar in the two modulation schemes, hence, the optimum choice depends mainly on the characteristics of the decision circuit and on the driver and detector bandwidths     

三阶色散对光纤中高斯型脉冲传输特性的影响

从含修正项的非线性薛定谔方程出发,采用变分法,导出了在三阶色散情况下光学高斯型脉冲参数随传输距离的演化方程组;在此基础上,求出了振幅与脉宽、频率与啁啾、脉宽与啁啾之间的三个重要约束关系;并进一步得出了脉宽随传输距离演化的解析解;讨论了三阶色散对光纤中高斯型脉冲传输特性的影响.     

Chirped RF Pulse Generation Based on Optical Spectral Shaping and Wavelength-to-Time Mapping Using a Nonlinearly Chirped Fiber Bragg Grating

Chirped radio-frequency (RF) pulse generation based on optical spectral shaping and nonlinear wavelength-to-time mapping in a nonlinearly chirped fiber Bragg grating (NLCFBG) is investigated. In the proposed approach, the spectrum of a femtosecond pulse generated by a mode-locked fiber laser is shaped by an optical filter that has a sinusoidal frequency response. The spectrum-shaped optical pulse is sent to the NLCFBG, to implement nonlinear wavelength-to-time mapping. A chirped electrical pulse with the central frequency and chirp rate determined respectively by the first- and second-order dispersions of the NLCFBG is then obtained at the output of a high-speed photodetector. An approximate model that describes the chirped RF pulse generation is derived, which is verified by numerical simulations. Chirped pulse generation with a pulse compression ratio as high as 450 is demonstrated. The key device in the chirped RF pulse generation system is the NLCFBG, which is investigated in detail with an emphasis on the influence of its group delay ripples on the performance of the pulse generation system. Techniques to design and fabricate the NLCFBG are also discussed. The proposed approach provides a potential solution for the generation of chirped RF pulse with a high central frequency and large chirp rate for applications in pulse compression radar systems.     

Rabi oscillations for subpicosecond pulses in quantum-well optical amplifiers: interplay of carrier heating, nonlinear, and spectral effects

Using the Foreman effective mass Hamiltonian for strained In/sub x/Ga/sub 1-x/As--In/sub y/Ga/sub 1-y/As/sub z/P/sub 1-z/ quantum wells, the propagation of subpicosecond pulses in a 1.55-/spl mu/m optical amplifier was calculated. The multisubband carrier dynamics as well as the polarization dynamics were taken into account. Carrier heating and coherent light-carrier interactions as well as the interplay of these nonlinear processes and the amplifier dispersion are studied. Strong Rabi oscillations occur in the optical field of a propagated pulse, its frequency chirp, as well as in the carrier density and temperature. While the Rabi oscillation imposes negative frequency chirp and hence red-shifts the pulse spectrum, positive frequency chirp can occur due to the local gain dispersion, where the higher frequency components of the pulse have larger gain. Due to the Rabi oscillation, the spectrum of the amplified pulse is considerably distorted and sidebands emerge. For a linearly chirped input pulse, the spectrum of the output pulse can be either red-shifted or blue-shifted with respect to its center frequency, depending on its initial chirp. For strong pulse propagation, a pronounced pulse break up occurs when a 175-fs pulse propagates in the gain regime, while a significant pulse compression occurs when the pulse propagates at the transparency point.     

A new theory for the treatment of a pulsed beam propagating througha grating pair

A new theoretical model, without the first-order approximation of grating diffraction, has been developed for studying the propagation of a pulsed beam through a grating pair. By using our model, the astigmatic aberration of the grating pair has been analyzed in detail. It was shown that the grating pair may be a good optical element for astigmatic compensation of monochromatic optical beams. But for grating pulse compression, the astigmatic aberration affects the pulse fronts and the effects of finite beam size (FBS). These effects on the compression of ultra-broadband pulses have been studied. It was found that, due to these effects, the space-time shapes of the compressed pulse are severely distorted. The magnitude of this distortion depends not only on the astigmatic aberration, but also on the spectral bandwidth and spatial divergence of the pulse. When the input pulsed beam is collimated, the waveform distortion due to the effects of FBS can be eliminated, but the pulse front distortion remains. In addition, the spatial and temporal properties of the compressed pulse for a single-pass compressor have been studied. An analytical expression without the well-collimated condition was obtained to describe the effect of lateral frequency shift. Also, the effect of third-order dispersion on a single-pass compressor is discussed     

初始啁啾对单模光纤中超高斯脉冲传输特性的影响

从非线性Schr(o)dinger(NLS)方程出发,采用变分法,导出了单模光纤中超高斯脉冲参数随传输距离的演化方程组,求出了振幅与脉宽、频率与啁啾、脉宽与啁啾之间的三个解析约束关系,得出了脉宽随传输距离演化的解析解,用龙格-库塔法进行数值求解描绘了初始啁啾和脉冲前后沿锐度对单模光纤中超高斯脉冲脉宽的影响.结果表明:初始啁啾和脉冲前后沿锐度对超高斯脉冲的振幅、脉宽、啁啾和相位有直接影响,超高斯脉冲传输过程中会产生啁啾,但脉冲中心的等效频率保持为常数.     

A neural network based technique for vibration characterization using Gaussian laser beams

This paper presents a neural network technique combined with an optical measurement system for the characterization of mechanical vibrations in industrial machinery. In the proposed system, the Gaussian beam of a laser source illuminates on an array of photodetectors. If either the laser source or the photodetector array is coupled with a vibrating system, then the optical powers detected by the photodetectors will vary accordingly, and are expected to reflect the magnitude and frequency of the X–Y planar vibrations of the monitored system. The time-varying optical powers are input to an artificial neural network-based vibration monitoring system which maps the power distributions to the X–Y position of the laser beam center. An experimental setup of the system is built and used for training and testing purposes. The obtained experimental results demonstrate the adequacy of combining optical techniques with neural networks to estimate the vibration frequency and magnitude. Estimated frequencies were within 1% of the actual ones, and the estimated magnitudes were within 29% of the actual magnitudes when using a chirp signal in the training phase. The magnitude estimation percentage error was further reduced below 12% when the neural network was trained with a decaying chirp signal.