Amplitude and phase characterization of 4.5-fs pulses by frequency-resolved optical gating

The technique of second-harmonic generation frequency-resolved optical gating is applied to measure the intensity and the phase of 4.5-fs pulses resulting from the fiber-compressed output of a cavity-dumped Ti:sapphire laser. Characterization of even shorter optical pulses by this method should also be feasible.     

Shaping, propagation and characterization of ultrafast pulses in optical fibers

The characterization of medium- to low-energy shaped pulses at 1.55 7m through frequency-resolved optical gating (FROG) is illustrated. This capability enables the study of ultrafast pulse propagation through optical fibers. The phase dynamics detected furnishes insight on pulse evolution, specifically on soliton formation - a subject of great importance for telecommunication applications. The combination of shaping and propagation of ultrafast pulses in fibers is examined theoretically using an adaptive pulse-shaping model, based on genetic algorithms, that furnishes optimized pulse shapes for fiber propagation.     

Multipulse interferometric frequency-resolved optical gating: real-time phase-sensitive imaging of ultrafast dynamics

We demonstrate a powerful new tool for real-time single-shot imaging of ultrafast phase shifts based on multipulse interferometric frequency-resolved optical gating that can directly measure and display ultrafast-time-scale phase shifts without computation. In addition, this technique can, with the application of interferogram analysis and iterative phase-retrievial techniques, recover the intensity and phase of three pulses in a single shot and exhibits a linear sensitivity to the pulse field in the wings.     

Frequency-resolved optical gating of femtosecond pulses in the extreme ultraviolet

Femtosecond extreme ultraviolet (XUV) pulses were fully characterized for the first time by using a newly developed cross-correlation frequency-resolved optical gating (FROG) technique in the XUV region. This method utilizes laser-assisted two-photon ionization as a nonlinear optical process. Near-infrared pulses characterized by FROG were used as a reference. The amplitude and phase of XUV pulses with a pulse duration of 10 fs were found to be in good agreement with a model analysis, taking into account phase modulation by ionization, self-phase modulation, and the atomic dipole phase.     

Time-resolved study of the spectral characteristics of supercontinuum pulses propagating in scattering media

The spectral and temporal characteristics of supercontinuum pulses propagating through monodisperse scattering media consisting of sub-lambda-sized particles have been measured with a broad-bandwidth cross-correlated frequency-resolved optical gating (XFROG) technique. The results show that the ballistic component of the supercontinuum preserves a phase relationship among its spectral components, which acquire a temporal shift in relation to propagation in a non-scattering medium.     

Full-field characterization of femtosecond pulses by spectrum and cross-correlation measurements

We present a practical and accurate technique for retrieving the amplitude and the phase of ultrashort pulses from a nonlinear (second-order) intensity cross correlation and the spectrum that overcomes shortcomings of previous attempts. We apply the algorithm to theoretical and experimental data and compare it with frequency-resolved optical gating.     

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.     

Polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating

We demonstrate polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating (FROG) measurements with a fiber-pigtailed, aperiodically poled lithium niobate waveguide. By scrambling the polarization much faster than the measurement integration time, we eliminate the impairment that frequency-independent random polarization fluctuations induce in FROG measurements. As a result we are able to retrieve intensity and phase profiles of few hundred femtosecond optical pulses with 50 MHz repetition rates at 5.2 nW coupled average power without control of the input polarization.     

Amplitude and phase measurement of mid-infrared femtosecond pulses by using cross-correlation frequency-resolved optical gating

We describe a cross-correlation-based frequency-resolved optical gating (XFROG) technique for simultaneously measuring the amplitude and phase of two ultrashort pulses that have different wavelengths but are derived from a common mode-locked oscillator. A measurement is presented in which 4.0-mum mid-IR pulses from a synchronously pumped femtosecond optical parametric oscillator (OPO) are characterized by mixing with the 770-nm OPO pump pulses. Details of the pulse-retrieval algorithm are included, together with examples of pulse data retrieved from the experimentally measured XFROG trace.     

Collinear type II second-harmonic-generation frequency-resolved optical gating for use with high-numerical-aperture objectives

Ultrashort-pulse lasers are now commonly used for multiphoton microscopy, and optimizing the performance of such systems requires careful characterization of the pulses at the tight focus of the microscope objective. We solve this problem by use of a collinear geometry in frequency-resolved optical gating that uses type II second-harmonic generation and that allows the full N.A. of the microscope objective to be used. We then demonstrate the technique by measuring the intensity and the phase of a 22-fs pulse focused by a 20x, 0.4-N.A. air objective.     

飞秒激光脉冲的谐波频率分辨光学开关法测量研究

建立了一台谐波频率分辨光学开关法(FROG)飞秒脉冲测量装置，利用该装置进行了掺钛蓝宝石飞秒激光脉冲的测量研究.在二次谐波自相关测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息，所得脉宽与干涉测量的结果基本一致. 关键词： 频率分辨光学开关法(FROG) 迭代计算 飞秒激光 自相关     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Complete pulse characterization at 1.5 mum by cross-phase modulation in optical fibers

Cross-phase modulation in optical fibers has been used for complete characterization of ultrashort pulses by a modified frequency-resolved optical gating (FROG) measurement technique. This technique has been used for characterization of picosecond pulses at 1.5mum with energy as low as 24 pJ, and the results are in excellent agreement with second-harmonic generation (SHG)-FROG characterization. The use of an optical waveguide gives measurement sensitivity comparable with that of SHG-FROG but without any temporal ambiguity in the retrieved pulse.     

Generation of a 160-GHz transform-limited pedestal-free pulse train through multiwave mixing compression of a dual-frequency beat signal

We report the experimental generation of a 160-GHz picosecond pulse train at 1550 nm, using multiple four-wave mixing temporal compression of an initial dual-frequency beat signal in the anomalous-dispersion regime of a nonzero dispersion-shifted fiber. Complete intensity and phase characterizations of the pulse train were carried out by means of a frequency-resolved optical gating technique, showing that 1.27-ps transform-limited pedestal-free Gaussian pulses were generated.     

Measurement of the intensity-dependent atomic dipole phase of a high harmonic by frequency-resolved optical gating

The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.     

Evolving FROGS: phase retrieval from frequency-resolved optical gating measurements by use of genetic algorithms

We present a new technique based on genetic algorithms for retrieving the electric field and the phase of ultrashort pulses from frequency-resolved optical gating (FROG) traces. We have successfully applied a very basic genetic algorithm to the two most common beam geometries: polarization gate and second-harmonic generation (SHG). In the case of SHG FROG, the genetic algorithm returns a lower error than the standard iterative composite algorithm.     

Collinear type II second-harmonic-generation frequency-resolved optical gating for the characterization of sub-10-fs optical pulses

For the first time to our knowledge, we demonstrate a collinear frequency-resolved optical gating (FROG) technique that is suitable for the characterization of sub-10-fs pulses. This FROG variant does not suffer from geometrical blurring effects, and a temporal resolution of 1 fs can be achieved without the need for additional aperturing. The apparatus is suitable for subnanojoule pulse energies. We apply this technique for the full characterization of pulses from a Kerr-lens mode-locked Ti:sapphire laser.     

Ultrasensitive second-harmonic generation frequency-resolved optical gating by aperiodically poled LiNbO3 waveguides at 1.5 microm

We retrieve intensity and phase profiles of 280 fs, 50 MHz optical pulses with 124 aJ coupled pulse energy (960 photons) by second-harmonic generation (SHG) frequency-resolved optical gating, using aperiodically poled LiNbO3 waveguides. The strong nonlinear interaction that is due to confinement within the micrometer-sized waveguide structure and the linearly chirped poling period contribute, respectively, to high SHG efficiency and broad phase-matching bandwidth. The achieved sensitivity is 2.7 x 10(-6) mW2, improving on the previous record for self-referenced complete pulse characterization by 5 orders of magnitude.     

Designer femtosecond pulse shaping using grating-engineered quasi-phase-matching in lithium niobate

The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically poled lithium niobate crystal in the undepleted pump regime. Second-harmonic pulse shapes, including Gaussian, stepped, square, and multiple pulses have been characterized using cross-correlation frequency-resolved optical gating and have been shown to agree well with theory.     

Observation of chirped soliton dynamics at lambda = 1.55 microm in a single-mode optical fiber with frequency-resolved optical gating

We present an experimental observation of the dynamics of an initially chirped optical soliton at 1.55microm that is propagating through a single-mode optical fiber, using frequency-resolved optical gating (FROG). FROG permits observation of both the amplitude and the phase profiles of ultrashort pulses, providing complete information on the pulse evolution. The features that are detected, which include what is believed to be the first experimental observation of phase slips, are in quantitative agreement with numerical simulations that employ the nonlinear Schr?dinger equation.