Determination of the duration of UV femtosecond pulses
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Pulse duration
Ti:sapphire laser
Pulse duration
Prism compressor
Chirped pulse amplification
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Chirped pulse amplification
Frequency-resolved optical gating
Optical parametric amplifier
Chirp
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We use an acousto-optic modulator at the input to a two-dimensional, Fourier-domain pulse shaper to achieve built-in characterization of the output pulses with spectral interferometry. The device is capable of rapid switching between pulse shapes.
Prism compressor
Characterization
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Until recently, experiments in ultrashort pulse science have involved measuring the spectrum and autocorrelation of the input pulse(s) and only measuring the integrated energy or perhaps time-resolved energy of the output signal. These experiments ignored the information contained in the input and output pulse phases and intensity profiles. New pulse measurement techniques such as frequency-resolved optical grating, when combined with older techniques such as spectral interferometry, now allow the complete characterization of the pulses. These techniques allow measurements of the intensity, phase, and polarization state of ultrashort pulses as functions of time (or frequency) and space. These techniques work for wavelengths from the UV to the IR and for extremely weak pulses and very high power pulses. They also allow entirely new classes of experiments for measuring ultrafast phenomena. Now the phases and temporal profiles of the input pulses may be measured and controlled, and the intensity and phase of the output pulses can also be measured. These new measurement techniques have thus greatly increased the obtainable information in ultrafast experiments. This paper reviews current pulse measurement methods including frequency-resolved optical grating and spectral interferometry and describes how they are changing the way that ultrashort pulse experiments are performed.
Frequency-resolved optical gating
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The temporal pulse shape is given by Fourier transform of spatial pattern transferred from the phase and amplitude masks onto the spectrum. By changing the spectrum and the phase of an ultrafast pulse in space, the recombined laser pulse has controllable temporal shape in time. The shaped ultrafast pulse generation and characterization have been applied in the studies of ultrashort pulse code division multiple access (CDMA) communications, image processing in biomedicine, femtosecond chemistry etc. Some methods of femtosecond pulse shaping for time-to-space conversion of ultrafast optical waveforms are summarized.
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We combine two nominal 100 fs pulses into a pulse train using an adaptive holographic quantum-well film as an adaptive pulse combiner in a two-wave mixing geometry. The two pulses in the combined pulse train are phase-locked and are immune to drifting optical path differences or delay times between the two input pulses. The phase is controlled by the choice of center wavelength. The spectrum of the pulse train is equivalent to the spectral interferogram between two ultrafast pulses.
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Summary form only given. The need for precise control of phase and intensity profiles of ultrashort laser pulses is becoming increasingly apparent. Applications include coherent control and control of pulse propagation in optical fibers. Additional applications will arise as the technology for control of ultrashort optical pulses becomes more common and easier to implement. Typical coherent control and propagation experiments optimize some parameter such as second harmonic intensity or product yield using a genetic algorithm which can take several hours. Unfortunately, if laser alignment changes over time, additional time consuming optimizations may be required unless the optimal ultrafast laser pulse shape can be set directly, which requires real-time closed loop control over the pulse shape. In a step toward the goal of real-time closed loop control of ultrashort laser pulse shapes, we combine pulse shape control with real time pulse measurement.
Frequency-resolved optical gating
Coherent control
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Using the ultrashort pulse and highly nonlinear fibers, we can obtain large nonlinear optical effects effectively. The group of author has been working on the highly functional optical control using ultrafast nonlinear optical effects induced by ultrashort pulse. We have demonstrated 1.55-2.0 mum and 1.0-1.7 mum widely wavelength tunable ultrashort pulse generation system by use of ultrashort pulse fiber laser and optical fibers. We have also demonstrated 1.0-2.0 mum ultrawideband super continuum generation in all fiber system. Recently, low noise and highly coherent high quality super continuum generation has been demonstrated for the first time. The octave spanning high quality super continuum is also generated using high energy soliton pulse. The group of author has discovered the two novel pulse trapping phenomena; pulse trapping across the zero-dispersion wavelength and trapped pulse amplification. Using the pulse trapping, we can tune the wavelength, output time, and temporal shape of trapped pulse. We can also trap the only arbitrary one pulse in the pulse train selectively using this technique. For the trapped pulse amplification, we can amplify the trapped pulse through the Raman gain of control pulse. The highly functional ~1 THz ultrafast all optical switching is also demonstrated by use of these techniques.
Prism compressor
Pulse duration
Zero-dispersion wavelength
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Using the ultrashort pulse and such highly nonlinear fibers, we can obtain large nonlinear optical effects effectively. The group of author has been working on the highly functional optical control using ultrafast nonlinear optical effects induced by ultrashort pulse. We have demonstrated 1.55 - 2.0 μm and 1.0 - 1.7 μm widely wavelength tunable ultrashort pulse generation system by use of ultrashort pulse fiber laser and optical fibers. We have also demonstrated 1.0 - 2.0 μm ultrawideband super continuum generation using ultrashort pulse fiber laser and highly nonlinear fibers. Recently, low noise and highly coherent high quality super continuum generation has been demonstrated for the first time. When the optical pulses overlap in the optical fibers, the pulse trapping occurs and they copropagate along the fibers under some conditions. The author has discovered the two novel pulse trapping phenomena. One of them is the pulse trapping across the zero-dispersion wavelength. In this phenomenon, the optical pulse at normal dispersion region is trapped by the ultrashort soliton pulse at anomalous dispersion region. We can tune the wavelength, output time, and temporal shape of trapped pulse. We can also trap the only one pulse in the pulse train selectively using this technique. The other pulse trapping phenomenon is between the orthogonally polarized ultrashort pulses in polarization maintaining fibers. We can also amplify the trapped pulse using this phenomenon. The highly functional ~1 THz ultrafast all optical switching was also demonstrated by use of these techniques.
Ultrafast optics
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With the development of ultrafast science, free-electron lasers (FELs) with ultrashort pulses have become a state-of-the-art tool in ultrafast phenomena studies. In an externally seeded FEL, the output pulse duration is usually determined both by the seed laser pulse duration and FEL amplification process, which can hardly reach the timescale of a few femtoseconds. In this study, through a simple method of changing the relative time delay and correspondingly the pulse energy of the two seed lasers employed in a seeded FEL, we demonstrated the possibility of generating few-femtosecond soft X-ray pulses and controlling the final FEL pulse durations. Based on theoretical calculations and practical experiments, we conducted a detailed study on the capabilities and limitations to this method with the parameters of the Shanghai Soft X-ray FEL Facility. Start-to-end simulations indicate that we can achieve ultrashort soft X-ray FEL pulses with the pulse duration down to 5.2 fs, and the final pulse durations can also be controlled in terms of relative time delays.
Pulse duration
Ultrashort pulse laser
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