基于瞬态光栅频率分辨光学开关法测量飞秒脉冲的研究

超宽光谱的飞秒脉冲测量一直是超快激光领域的重要研究方向之一.常规的飞秒脉冲自相关方法是通过测量自相关倍频信号来获得,而倍频信号具有波长选择性,不同中心波长的飞秒脉冲测量需要更换不同的倍频晶体,十分不方便.因此,提出了一种改进型的瞬态光栅频率分辨光学开关(TG-FROG)方法用于测量飞秒脉冲.该方法结合四波混频和频率分辨光学开关方法,其基本过程是将待测脉冲分为三束,其中两束脉冲经过精密的延时控制并聚焦在光学介质上达到时空重合,利用三阶非线性效应产生稳定的瞬态光栅作为开关光;另一束脉冲作为探测光与产生的瞬态光栅进行相互作用产生一个信号光,使用光谱仪对该信号光的光谱与延迟时间进行测量,并通过反演迭代算法处理而获取待测飞秒脉冲的光谱与电场信息.该方法只需要待测光的功率密度达到三阶非线性效应就可以实现测量,因此可以应用于任意中心波长的飞秒脉冲测量.利用该方法对中心波长分别为800 nm, 400 nm的飞秒脉冲,以及超连续亚10 fs的周期量级超宽光谱飞秒脉冲进行了测量,并与常规的干涉自相关仪器测量结果进行了比较,所得测量结果基本一致.实验结果表明,建立的基于TG-FROG方法对不同中心波长,不同脉冲宽度的飞秒脉冲测量是十分有效的.     

Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric-field reconstruction

We demonstrate spectral phase interferometry for direct electric-field reconstruction (SPIDER) as a novel method to characterize sub-6-fs pulses with nanojoule pulse energy. SPIDER reconstructs pulse phase and amplitude from a measurement of only two optical spectra by use of a fast noniterative algorithm. SPIDER is well suited to the measurement of ultrabroadband pulses because it is quite insensitive to crystal phase-matching bandwidth and to unknown detector spectral responsivity. Moreover, it combines highly accurate pulse-shape measurement with the potential for online laser system diagnostics at video refresh rates.     

Simultaneous temporal characterization of telecommunication optical pulses and modulators by use of spectrograms

We demonstrate a simple technique for simultaneous and complete characterization of the optical pulses and temporal modulators commonly used in telecommunication. The electric field of a pulse and the response of a modulator are obtained from the analysis of the two-dimensional spectrogram of the pulse gated by the modulator. The measurement sensitivity is greatly improved compared with the conventional nonlinear optical techniques. Trains of picosecond pulses as weak as 10(-17)J are accurately characterized with an electroabsorption modulator as the temporal gate. The time-resolved transmission and phase of the modulator are also presented.     

阿秒脉冲测量的研究进展

对于发生在原子范围内的电子动力学过程的观测需要阿秒量级的时间分辨率.理论和实验研究都已证明,用周期量级超短脉冲直接泵浦的高次谐波过程可以产生阿秒脉冲序列甚至单个的阿秒脉冲,将阿秒脉冲用于测量超快动力学过程之前先要对阿秒脉冲本身性质做出描述,传统的自相关方法和互相关方法不能直接推广到阿秒量级超短脉冲的测量.文章详细介绍了近几年发展起来的阿秒脉冲测量方法,分析了它们的分辨极限和局限性.     

无干涉条纹的光谱位相相干直接电场重构法

提出一种新的诊断飞秒脉冲振幅与位相的高精度测量方法.该方法发展了传统的SPIDER方法，保留了原方法对脉冲信息可以实时、有效和全面地测取的优点，并能克服其不能测量脉宽较大或位相信息复杂的脉冲的缺点.在新方法中和频后的脉冲对没有相对延时，形成无干涉条纹的剪切干涉图.在调节一个小量延时于某些特定值，可去掉剪切干涉图的歧义性.给出数值模拟结果，证明此方法无需经过傅里叶变换滤波，可直接由干涉图唯一地提取出脉冲的振幅与位相信息. 关键词： 光谱位相相干直接电场重构法 飞秒脉冲测量 超快信息光学     

脉冲激光测距回波信号时间选通处理技术

为了降低高重频脉冲激光测距回波信号的误识别率,提高测距性能,对高重频脉冲激光测距回波信号调理技术进行深入研究。采用FPGA作为主控芯片,产生激光调制脉冲,并根据被测距离3.33 μs～33.33 μs时间选通方波信号,驱动开关芯片产生与量程关联的时间波门,有效滤除脉冲回波中的干扰脉冲,该方法改进了常规脉冲激光测距信号处理系统的自动增益控制环节。测试实验结果表明:在脉冲回波信号60 dB的动态范围内,可有效滤除回波信号中引入的干扰脉冲,极大地降低了干扰脉冲误识别造成粗大误差的可能性。该方法可推广应用于脉冲激光测距信号处理系统,使系统测距精度提高12.6%。     

Simulations and experiments of mode-locking of semiconductor lasers: pulse evolution,frequency detuning,and bias dependence

Simulations of a mode-locked diode laser based on a travelling-wave rate-equation model have been compared with experiments. The pulse measurement technique involved a conventional intensity autocorrelator together with an internally generated second-harmonic emission measurement set-up. The latter is ideal for systematic relative measurements. For the first time, pulse evolution as a function of the number of round-trips was measured. Short optical pulses were obtained after approximately 40 round-trips. The experimental and simulated detuning range was about 1 MHz and the d.c. bias dependence was investigated.On leave from Physics Institute of the Lithuanian Academy of Sciences, 232 600 Vilnius, Lithuania.     

Cross-validation of theoretically quantified fiber continuum generation and absolute pulse measurement by MIIPS for a broadband coherently controlled optical source

The predicted spectral phase of a fiber continuum pulsed source rigorously quantified by the scalar generalized nonlinear Schrödinger equation is found to be in excellent agreement with that measured by multiphoton intrapulse interference phase scan (MIIPS) with background subtraction. This cross-validation confirms the absolute pulse measurement by MIIPS and the transform-limited compression of the fiber continuum pulses by the pulse shaper performing the MIIPS measurement, and permits the subsequent coherent control on the fiber continuum pulses by this pulse shaper. The combination of the fiber continuum source with the MIIPS-integrated pulse shaper produces compressed transform-limited 9.6 fs (FWHM) pulses or arbitrarily shaped pulses at a central wavelength of 1020 nm, an average power over 100 mW, and a repetition rate of 76 MHz. In comparison to the 229-fs pump laser pulses that generate the fiber continuum, the compressed pulses reflect a compression ratio of 24.     

Single-shot multiple-delay crossed-beam spectral interferometry for measuring extremely complex pulses

We demonstrate a single-shot measurement technique based on spectral interferometry (SI) for measuring the complete intensity and phase vs. time of extremely complex ultrashort laser pulses. Ordinarily, such a method would require an extremely-high-resolution spectrometer, but, by temporally interleaving many SI measurements, each using a different reference-pulse delay, our method overcomes this need. It involves introducing a transverse time delay into the reference pulse by tilting its pulse front transversely to the spectrometer dispersion plane. The tilted reference pulse then gates the unknown pulse by interfering with it at the image plane of a low-resolution imaging spectrometer, yielding an effective increase in the delay range and spectral resolution—by a factor of 30 in our proof-of-principle implementation. Our device achieved a temporal resolution of ~ 130 fs and a temporal range of 120 ps. This simple device has the potential to measure even longer and more complex pulses.     

Measurement of fiber chromatic dispersion using spectral interferometry with modulation of dispersed laser pulses

We propose and experimentally demonstrate a method for fiber dispersion measurement based on the modulation of laser pulses stretched by the fiber under test. The measured spectrum of the modulated pulses is the result of the interference between the stretched pulse spectra shifted by the modulation harmonics. The interference pattern is processed as in Fourier transform spectral interferometry. Unlike to conventional spectral interferometry, environmental conditions do not affect the interferogram due to the lack of any interferometer; additionally, large dispersions can be characterized by the method proposed. Its high accuracy is demonstrated in experimental comparison with the widely used phase shift technique.     

研制超宽带全光取样示波器设备

全光取样示波器是研究与开发超高速光通信系统和光子交换网络的关键性测试仪器设备. 本文简介了我们自行设计和研制出的超宽带全光取样示波器设备的实验样机系统, 并报道了我们已取得的初步实验结果. 采用自主研发的高稳定性被动锁模飞秒光纤激光器作为该光学示波器的光脉冲取样源, 我们通过利用高度非线性光纤中的四波混频效应, 成功地实现了对脉宽为1.8ps、重复频率分别为10GHz和40GHz的光脉冲信号的全光取样. 然后通过数字信号处理和计算机图形处理, 得到了再现后的超短光脉冲信号波形, 并测出了其脉冲宽度值为2.3ps. 借助于该光学取样示波器实验样机, 我们还成功地完成了对脉宽为1.8ps、经过伪随机数据序列调制后的10Gbit/s和40Gbit/s光数据信号眼图的精确测量. 这是我国首次报道有关超宽带全光取样示波器设备的实际研制工作及其相应的实验测试结果. 所得到的有关超短光脉冲信号波形的测试结果也与用70GHz宽带电子示波器和超快光电探测器组成的常规光电测量系统所获得的结果进行了比较, 清楚地表明了我们研制出的全光取样示波器实验样机比后者具有更高的时间分辨率和更大的测量带宽.     

Numerical simulation for characterizing femtosecond optical pulses with the SPIDER algorithm

In this article based on the spectral phase interferometry for direct electric-field reconstruction (SPIDER), the femtosecond pulses with various phase characters are numerically simulated. The spectral phases and amplitudes of the transform-limited pulse, the linear chirped pulse, the cubic dispersion pulse, the quartic dispersion pulse, the self-phase modulation pulse and the pulses with the combination of different chirped characters are retrieved. These characterized pulses are applicable to the real-time measurement as samples for diagnosing the chirped characters of pulses quickly.     

优化重聚脉冲提高梯度场核磁共振信号强度

缩短射频脉冲宽度, 有助于解决脉冲电力消耗大、样品吸收率高、信噪比低等极端条件核磁共振探测的关键问题. 本文首先分析射频脉冲角度对核磁共振自旋回波信号强度的影响机理, 基于Bloch方程推导了回波信号幅度与扳转角、重聚角的关系. 在特制核磁共振分析仪上采用变脉冲角度技术, 分别在均匀磁场和梯度磁场条件下实现对扳转角和重聚角与回波信号强度关系的数值模拟和实验测量. 结果表明, 梯度场中, 扳转角为90°、重聚角为140°的射频脉冲组合获得最大首波信号强度, 比180°脉冲对应的回波幅值提高13%, 能耗降低至78%. 选用该重聚角(140°) 优化设计饱和恢复脉冲序列探测流体的纵向弛豫时间T₁特性, 准确获得 T₁分布.该结果对于低电力供应、且对信噪比有较高要求的核磁共振测量, 如随钻核磁共振测井和在线核磁共振快速检测等, 具有重要意义. 关键词： 核磁共振 信号强度 重聚脉冲角度 Bloch方程     

Parabolic pulse generation in a dispersion-decreasing solid-core photonic bandgap Bragg fiber

We analyze the interplay of nonlinearity and dispersion in a dispersion-decreasing photonic bandgap Bragg fiber as a new platform for generating parabolic pulses. A suitably designed linearly tapered, low-index-contrast, solid-core Bragg fiber - amenable to fabrication by conventional modified chemical vapor deposition technology - is shown to yield stable parabolic pulses. The fiber design was optimized through a simple and accurate transfer-matrix formalism and pulse evolution was studied by the well-known split-step Fourier method. Our study revealed feasibility of generating parabolic pulses in such a dispersion-decreasing Bragg fiber of length as short as 1 m. We have also studied the effect of third order dispersion on generated parabolic pulse, which is an important deteriorating factor in such applications. The effective single-mode operation of the proposed device is achieved through appropriate tailoring of the outer cladding layers.     

Performance of a CW pumped Nd:YVO4 amplifier with kHz pulses

Until recently, simple and reliable high repetition-rate laser sources with nanosecond pulses much shorter than from conventional A-O Q-switch lasers were not available. However over the past 2 years we have developed such lasers based on proprietary fast E-O switching technology, which allows designs delivering 1 ns pulses and subnanosecond jitter for good synchronisation. The technology provides pulses with multi-kW peak power and repetition-rates to >100 kHz.Most recently, the performance of these short pulse lasers has been developed further by implementing oscillator/amplifier (master oscillator and power amplifier, MOPA) technology which increases the output to >1 W average power. Here we report on a simple model that has been used to predict the performance of the CW pumped Nd:YVO₄ amplifier used in the MOPA laser. The model is based on the well-known expressions for the saturated gain applying to laser pulses, but more usually applied to pulse-excited amplifiers. The model is shown to allow a good interpretation of the amplifier behaviour for kHz pulses and to be a useful tool for predicting the performance of the MOPA laser.     

Zero-additional-phase SPIDER: full characterization of visible and sub-20-fs ultraviolet pulses

We demonstrate a novel spectral-shearing interferometry setup for characterizing the temporal amplitude and phase of ultrashort optical pulses over an extremely wide wavelength region. By the mixing of two strongly chirped auxiliary pulses with the pulse to be characterized, two spectrally sheared replicas are generated, and their spectral interference is evaluated. We fully characterize 10-fs pulses in the visible region by sum-frequency mixing and 19-fs pulses in the ultraviolet region by difference-frequency mixing. The scheme is self-referencing and highly sensitive. The zero-additional-phase scheme does not alter the unknown pulses and yields the pulse shape at the interaction point of a spectroscopic experiment.     

三角形光脉冲在正色散光纤中产生的实验研究

利用一种无源非线性脉冲整形方法, 实现了在普通正色散(ND)光纤中产生三角形光脉冲, 此方法依赖于脉冲预啁啾和脉冲在ND光纤中传输时群速度色散与自相位调制的相互作用. 实验研究表明, 在较宽的脉冲预啁啾值范围内, 通过优化脉冲输入功率和脉冲传输的ND光纤长度, 均可得到典型的三角形光脉冲: 脉冲时域形状前后沿的变化率接近恒定、整个脉冲具有线性频率啁啾. 另外, 在不同的脉冲预啁啾下, 要得到高质量的三角形光脉冲, 均需要较高的脉冲输入功率; 并且脉冲预啁啾较大时, 三角形脉冲的形成对ND光纤长度和脉冲输入功率有较大的容差, 易获得三角形光脉冲.     

A precision analysis and determination of the technical requirements of an atom interferometer for gravity measurement

The influence of the wave-front curvature of Raman pulses on the measurement precision of gravitational acceleration in atom interferometry is analysed by the method of a transmission matrix. It is shown that the measurement precision of gravitational acceleration is largely dependent on the spot size of the Raman pulse, the temporal interval between Raman pulses and the optical path difference of the two counter-propagating Raman pulses. Moreover, the influence of Doppler frequency shift on the precision is discussed. In order to get a certain measurement precision, the requirement for the accuracy of frequency scanning of the Raman pulse to compensate for the Doppler frequency shift is obtained.      

Multiphoton intrapulse interference. IV. Ultrashort laser pulse spectral phase characterization and compensation

We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.     

Control of molecular rotation with a chiral train of ultrashort pulses

Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. In this work we introduce a chiral pulse train--a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers.