频率分辨光学开关法测量飞秒脉冲

阐述了频率分辨光学开关法测量飞秒脉冲的原理,详细分析了模式尺寸效应和非线性效应对飞秒脉冲测量的影响.构建了一台用于飞秒脉冲测量的二次谐波-频率分辨光学开关装置,利用该装置对谐振腔输出的飞秒脉冲及压缩后的脉冲进行了测量.得到了飞秒脉冲的时间宽度及光谱宽度、电场及其相位在时域和频域的详细信息.谐振腔直接输出脉冲的时间宽度为56 fs,光谱宽度为27 nm,时间带宽积为0.686,算法中的最小误差为0.001792.脉冲压缩后的测量结果为27 fs,光谱宽度为92 m,时间带宽积为1.27,算法误差为0.0093289.     

超短光脉冲的频率分辨光学开关法测量研究

为了对二次谐波型和偏振开关型频率分辨光学开关法测量超短光脉冲的研究,利用矩阵的方法对实验系统中几种常见超短光脉冲的二次谐波-频率分辨光学开关和偏振开关-频率分辨光学开关光谱图进行了数值模拟,并采用基于矩阵的主元素广义投影算法从数值模拟的二次谐波-频率分辨光学开关光谱图中恢复了脉冲的振幅和相位,误差达到收敛的标准(G-4)。结果表明,频率分辨光学开关能够精确地测量超短光脉冲。     

短脉冲在色散平坦光纤中传输前后波形、相位和啁啾测量的实验研究

二次谐波频率分辨光学门(SHG-FROG)是能够准确测量短脉冲多项特性参量的新技术。利用二次谐波频率分辨光学门脉冲分析仪对在色散平坦光纤中传输前后的短脉冲进行了测量,得到了待测光脉冲的频率分辨光学门(FROG)图、自相关曲线、自相关频谱曲线、波形和相位曲线以及脉宽、谱宽、啁啾等反映短脉冲特性的信息,对实验结果进行了分析,并与高斯脉冲在单模光纤中的线性传输理论进行了比较。结果表明,激光器输出的短脉冲是具有负线性啁啾的近变换极限高斯脉冲,经过12.7 km色散平坦光纤传输后仍然为具有负线性啁啾的高斯脉冲,其谱宽在传输过程中基本保持不变,脉宽展宽了3.1倍,啁啾增大了4倍。实验测量结果和理论预期一致。     

Complete characterization of ultrashort pulse sources at 1550 nm

This paper reviews the use of frequency-resolved optical gating (FROG) to characterize mode-locked lasers producing ultrashort pulses suitable for high-capacity optical communications systems at wavelengths around 1550 nm, Second harmonic generation (SHG) FROG is used to characterize pulses from a passively mode-locked erbium-doped fiber laser, and both single-mode and dual-mode gain-switched semiconductor lasers. The compression of gain-switched pulses in dispersion compensating fiber is also studied using SHG-FROG, allowing optimal compression conditions to be determined without a priori assumptions about pulse characteristics. We also describe a fiber-based FROG geometry exploiting cross-phase modulation and show that it is ideally suited to pulse characterization at optical communications wavelengths. This technique has been used to characterize picosecond pulses with energy as low as 24 pJ, giving results in excellent agreement with SHG-FROG characterization, and without any temporal ambiguity in the retrieved pulse     

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.     

Frequency-resolved optical gating measurement of ultrashort pulsespassing through a high numerical aperture objective

We investigate using collinear type II second harmonic generation frequency-resolved optical gating (SHG FROG) to measure the pulse intensity and phase at the focus of high numerical aperture (NA) oil objectives. Because of the strong focusing for such objectives, it is not clear theoretically that such a measurement should work. Such objectives can produce severe distortions of the pulse as a function of radius in the objective. In addition, the standard SHG FROG algorithms are based on the assumption that the fundamental and second harmonic fields are plane waves that can be described by the paraxial approximation, and for high NA objectives, such assumptions are suspect. We show that such measurements work remarkably well. The tight focus, while a theoretical difficulty, eliminates many of the problems traditionally associated with SHG FROG including the difficulty of phase matching and walkoff of different polarizations in the crystal. Specifically, we use collinear type II SHG FROG to measure the intensity and phase at the focus of a Zeiss CP-Achromat 100x, 1.25 NA, infinity-corrected oil objective, and accurately retrieve 20 fs pulses     

40-GHz picosecond-pulse second-harmonic generation in an MgO-doped PPLN waveguide

Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40-GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data.     

Recent progress toward real-time measurement of ultrashort laserpulses

Frequency-resolved optical gating (FROG) is a technique that produces a spectrogram of an ultrashort laser pulse optically. While a great deal of information about the pulse can be gleaned from its FROG trace, often it is desirable to obtain of the pulse information immediately, in real time. Quantitative information about the guise is not readily obtainable from its spectrogram without the use of a two-dimensional phase retrieval algorithm. While current algorithms are quite robust, retrieval of all the pulse information can be slow. In this paper, I describe a recently developed FROG trace inversion algorithm called Principal Component Generalized Projects that is fast, robust, and can invert FROG traces in real time. A femtosecond oscilloscope based on second-harmonic generation FROG is also described that uses this new algorithm to rapidly (up to 2.3 Hz) and continuously display the intensity and phase of ultrashort laser pulses     

Microcavity-enhanced surface-emitted second-harmonic generation forultrafast all-optical signal processing

By incorporating an integrated microcavity into an optical waveguide structure with vertical quasi-phase-matching, we have realized surface-emitted second-harmonic generation devices that significantly enhance the conversion efficiency for optical pulses in the picosecond and sub-picosecond regimes. We demonstrate both theoretically and experimentally that nonlinear interactions involving short optical pulses can be enhanced by a microcavity, even when the resonance width is substantially narrower than the spectral content of the pulse. The resulting enhancement enables practical signal processing functions such as ultrafast optical time-division demultiplexing at 1.55 μm in multilayer AlGaAs structures     

达曼类滤波器的飞秒脉冲整形技术

为了研究飞秒激光时空变换整形技术,采用理论分析与计算机模拟结合讨论的方法,分析了以达曼类滤波器为模板的4f 系统的飞秒脉冲整形技术。以梯度算法优化设计出达曼类滤波器,包括等间距、等强度和不等间距、不等强度达曼光栅,并模拟讨论了产生的超快时域多脉冲与光谱平面上调制周期的重复数、元件间距不等效应和波长效应的影响。结果表明,以达曼类滤波器为模板的飞秒脉冲整形中,模板周期重复数越多,输出脉冲的质量越高;元件间距不等和波长效应使输出脉冲的平均度和衍射效率都有所降低。     

Dynamic electrooptic frequency shifter for pulsed light signals

We report continuous frequency shifting of a train of mode-locked pulses of light by up to /spl plusmn/400 GHz using electrooptic frequency shifting (EOFS). Dynamic, continuous, and accurate shifting of the optical pulses is achieved by controlling the phase and power of a single frequency microwave signal in a traveling wave phase modulator structure. Operation of the device under different microwave power and optical pulse length settings was investigated in order to study spectral distortion predicted to occur at high levels of frequency shifting and when using long pulses. Possible techniques for future device improvement were proposed and preliminary tests were performed. The use of a second-harmonic component of the microwave drive signal leads to a longer region of constant amplitude gradient and results in reduced distortion.     

光谱相位相干技术测量飞秒脉冲的倍频误差

从理论上推导了第Ⅱ类相位匹配下宽带飞秒脉冲的二次谐波光场,分析输入飞秒脉冲的非共线相位匹配方式、脉冲带宽引起相位失配与群速失配对测量的影响。结果表明,为了消除飞秒脉冲的带宽影响,需要对测量记录的光强乘以一个调制因子;测量相位误差与非共线相位匹配的夹角和晶体长度成正比;相位失配与群速失配产生相位测量误差,且第Ⅱ类相位匹配方式下脉冲附加相位值较大;强度和相位误差需要在脉冲重建结果中补偿。     

Generation of picosecond optical pulses with highly RF modulated AlGaAs DH laser

AlGaAs DH lasers with strong RF modulation superimposed on the relatively low dc bias below the oscillation threshold were demonstrated to be feasible for generation of a train of approximately 30 ps optical pulses at a repetitive frequency of sub-GHz range. The pulse envelope width was measured by three different methods: a fast response photodetector, the second-harmonic generation (SHG) correlation method, and an ultrafast streak camera. The results of the SHG correlation and the streak camera agreed fairly well. In order to explain the generation mechanism and the characteristics of these ultrashort optical pulses in a highly RF modulated semiconductor diode laser, the rate equation analysis was performed and the results were generally in good agreement with the experiment. Furthermore, from the computer simulation for the analysis of the SHG correlation traces, it was inferred that an individual ultrashort optical pulse has internal substructures made of fluctuating fields whose spike widths were of the order of subpicoseconds, due to the randomness of the phases among lasing modes.     

不等间距达曼型滤波器的飞秒脉冲时空变换整形与识别技术

研究了以不等间距达曼(Dammann)型滤波器为模板的飞秒激光时空变换与识别技术。优化设计出不等间距多脉冲达曼型滤波器,并在高斯光谱分布的模拟中讨论产生的超快时域多脉冲平均度和光谱平面上调制周期重复数的相关性。给出了频率分辨光开关法(FROG)技术识别超快时域两脉冲的实验结果。     

飞秒脉冲测量技术的新进展

本文综述了超快激光领域中测量技术的最新发展，特别详细地论述了目前国际上公认的测量脉冲位相的2种标准方法；FROG和SPIDER的基本原理、实验构成和性能比较。     

Practical issues in ultrashort-laser-pulse measurement usingfrequency-resolved optical gating

We explore several practical experimental issues in measuring ultrashort laser pulses using the technique of frequency-resolved optical gating (FROG). We present a simple method for checking the consistency of experimentally measured FROG data with the independently measured spectrum and autocorrelation of the pulse. This method is a powerful way of discovering systematic errors in FROG experiments. We show how to determine the optimum sampling rate for FROG and show that this satisfies the Nyquist criterion for the laser pulse. We explore the low- and high-power limits to FROG and determine that femtojoule operation should be possible, while the effects of self-phase modulation limit the highest signal efficiency in FROG to 1%. We also show quantitatively that the temporal blurring due to a finite-thickness medium in single-shot geometries does not strongly limit the FROG technique. We explore the limiting time-bandwidth values that can be represented on a FROG trace of a given size. Finally, we report on a new measure of the FROG error that improves convergence in the presence of noise     

Complete characterization of terahertz pulse trains generated fromnonlinear processes in optical fibers

The measurement technique of frequency-resolved optical gating (FROG) is used to characterize the intensity and phase of terahertz pulse trains generated from nonlinear and dispersive interactions in optical fibers. We show that existing FROG retrieval algorithms are easily adapted to allow the retrieval of periodic pulse characteristics and, using synthetic pulse trains generated from numerical simulations, we demonstrate how FROG can differentiate between periodic pulse trains with fundamentally different intensity and phase characteristics, yet qualitatively similar autocorrelation functions and spectra. Experimental results are presented for the FROG characterization of a 0.3-THz sinusoidal beat signal from a dual wavelength laser source, a 2.5-THz train of dark solitons generated in a high-birefringence fiber, and a 0.6-THz bright polarization domain wall soliton train generated in an ultra-low birefringence fiber. These results are shown to be in good agreement with nonlinear Schrodinger equation simulations     

Generation of picosecond blue light pulse by frequency doubling ofgain-switched GaAlAs laser diode having saturable absorber

Picosecond pulse generation of blue light by frequency doubling of a GaAlAs laser diode is reported. High power pulse generation is realized by incorporating gain switching of a laser diode with a saturable absorber and frequency doubling in a proton-exchanged MgO:LiNbO₃ waveguide. The laser diode with a longer saturable absorber can produce optical pulses with higher peak power and narrower pulse width. The spectral bandwidth of second-harmonic generation for the waveguide is evaluated at about 20 nm. This is wide enough to frequency-double all the multilongitudinal modes of the gain-switched laser diode. A blue light pulse of 7.88-mW maximum peak power and 28.7-ps pulsewidth is obtained for a 1.23-W peak pulse of the laser diode     

Second-harmonic generation by picosecond optical pulses

The generation of second-harmonic radiation in a non-linear dielectric by an arbitrary broad-band optical signal is analyzed and application of the theory is made to harmonic generation by a short pulse. The spectrum and time dependence of the harmonic radiation is calculated in terms of the corresponding quantities for the fundamental pulse. Several types of pulses, including a frequency-swept pulse, are treated. Experimental results on the second-harmonic spectrum are presented and compared with the theory, as are previously reported results on the time dependence.     

基于光学参量振荡器的高分辨率CARS光谱研究

提出了一种基于三硼酸锂（LBO）晶体的飞秒绿色泵浦光学参量振荡器（OPO）,其采用飞秒525 nm绿光光纤激光器作为泵浦源。OPO产生的信号光其可调谐的光谱范围由780～940 nm,光功率可达到250 mW以上;闲频光可调谐范围从1630～1190 nm,光功率可达到300 mW以上。利用本系统可在波数为1330 cm－1和6650 cm－1之间进行相干反斯托克斯拉曼散射（CARS）光谱测量。由于脉冲啁啾,光谱分辨率达到100 cm－1,与时间带宽限制脉冲激励源相比,分辨率达到后者的2.5倍。