Showing papers on "Femtosecond pulse shaping published in 1997"

Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating

Abstract: We summarize the problem of measuring an ultrashort laser pulse and describe in detail a technique that completely characterizes a pulse in time: frequency-resolved optical gating. Emphasis is placed on the choice of experimental beam geometry and the implementation of the iterative phase-retrieval algorithm that together yield an accurate measurement of the pulse time-dependent intensity and phase over a wide range of circumstances. We compare several commonly used beam geometries, displaying sample traces for each and showing where each is appropriate, and we give a detailed description of the pulse-retrieval algorithm for each of these cases.

Laser ablation and micromachining with ultrashort laser pulses

Abstract: The mechanisms of ultrashort-pulse laser ablation of materials are discussed, and the differences to that of long laser pulses are emphasized. Ultrashort laser pulses offer both high laser intensity and precise laser-induced breakdown threshold with reduced laser fluence. The ablation of materials with ultrashort pulses has a very limited heat-affected volume. The advantages of ultrashort laser pulses are applied in precision micromachining of various materials. Some femtosecond laser pulse micromachining results, including comparison with long pulses, are presented. Ultrashort-pulse laser micromachining may have a wide range of applications where micrometer and submicrometer feature sizes are required.

Ablation of metals by ultrashort laser pulses

Abstract: Ablation of metal targets by Ti:sapphire laser radiation is studied. The ablation depth per pulse is measured for laser pulse durations between 150 fs and 30 ps and fluences ranging from the ablation threshold ∼0.1 J/cm2 up to 10 J/cm2. Two different ablation regimes are observed for the first time. In both cases the ablation depth per pulse depends logarithmically on the laser fluence. A simple theoretical model for a qualitative description of the experimental results is presented.

Femtosecond pulse shaping by an evolutionary algorithm with feedback

Abstract: We report on computer controlled compression of femtosecond laser pulses using a programmable liquid crys- tal spatial light modulator which is feedback-controlled by an evolutionary algorithm. This algorithm generates the opti- mal laser field on the basis of feedback from the experiment by optimizing the laser pulse iteratively. Without knowledge of the (chirped) input pulses, the experimental signal (second harmonic light=SHG) is maximized by the algorithm, thus re- sulting in fully compressed pulses. This method only makes use of the experiment's response (SHG signal) on the formed pulses. No other parameters need to be considered. This ap- proach leads to many experimental applications in all fields of optics and ultrafast spectroscopy where particularly shaped pulses are advantageous.

Moving focus in the propagation of ultrashort laser pulses in air

Abstract: The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.

Adaptive femtosecond pulse compression.

Abstract: A practical adaptive method for femtosecond optical pulse compression is demonstrated experimentally for the first time to our knowledge. The method is robust and capable of handling the general case of pulse compression, in which the input pulses are completely uncharacterized or partially characterized.

Optical pulse compression to 5 fs at a 1-MHz repetition rate.

Abstract: We report on the characterization and compression of the white-light continuum produced by injection of a 13-fs pulse from a cavity-dumped self-mode-locked Ti:sapphire laser into a single-mode fiber. Pulses as short as 5 fs were generated at repetition rates up to 1 MHz.

Quasi-phase-matched parametric chirped pulse amplification systems

Abstract: Use of quasi-phase-matched (QPM) materials for parametric chirped pulse amplification (PCPA) substantially reduces the required pump peak power and pump brightness, allowing exploitation of spatially-multimode and long duration pump pulses. It also removes restrictions on pump wavelength and amplification bandwidth. This allows substantial simplification in pump laser design for a high-energy PCPA system and, consequently, the construction of compact diode-pumped sources of high-energy ultrashort optical pulses. Also, this allows elimination of gain-narrowing and phase-distortion limitations on minimum pulse duration, which typically arise in a chirped pulse amplification system. One example of a compact source of high-energy ultrashort pulses is a multimode-core fiber based PCPA system. Limitations on pulse energy due to the limited core size for single-mode fibers are circumvented by using large multimode core. Limitations on pulse duration and beam quality due to multimode core are circumvented by using a PCPA scheme. Additionally, the large core of the multimode fiber facilitates cladding-pumping by inexpensive and high-power multiple-mode laser diodes.

High-resolution acousto-optic shaping of unamplified and amplified femtosecond laser pulses

Abstract: We focus theoretically and experimentally on the fundamental limitations of spectral pulse shaping using an acousto-optic modulator. We analyze the role of the relative thickness of the interaction region as dictated by the acousto-optic Q parameter and show that varying Q allows flexibility in choosing between diffraction efficiency and pixels of resolution. We model and experimentally demonstrate the effects of potential nonidealities such as nonlinear acoustic attenuation. In addition, we derive a simple and intuitive expression to predict the magnitude of the distortions in the spatial profile generated by an acousto-optic-modulator spectral light modulator. Finally, we demonstrate amplification of acousto-optic-modulator-generated shaped pulses for the first time.

Pulse compression during second-harmonic generation in aperiodic quasi-phase-matching gratings

Abstract: We propose a simple means for compressing optical pulses with second-harmonic generation. Aperiodic quasi-phase-matching gratings impart a frequency-dependent phase shift on the second-harmonic pulse relative to the fundamental pulse and can be engineered to correct for arbitrary phase distortions. The mechanism is discussed, and a detailed analysis of the compression of quadratic phase (linear frequency) chirped pulses is presented.

All-solid-state cavity-dumped sub-5-fs laser

Abstract: We discuss in detail a compact all-solid-state laser delivering sub-5-fs, 2-MW pulses at repetition rates up to 1 MHz. The shortest pulse generated thus far measures only 4.6 fs. The laser system employed is based on a cavity-dumped Ti:sapphire oscillator whose output is chirped in a single-mode fiber. The resulting white-light continuum is compressed in a novel high-throughput prism chirped-mirror Gires–Tournois interferometer pulse compressor. The temporal and spectral phase of the sub-5-fs pulses are deduced from the collinear fringe-resolved autocorrelation and optical spectrum. The derived pulse shape agrees well with the one retrieved from the measured group delay of the continuum and calculated characteristics of the pulse compressor.

Transient-grating frequency-resolved optical gating

Abstract: We introduce a transient-grating beam geometry for frequency-resolved optical-gating measurements of ultrashort laser pulses and show that it offers significant advantages over currently used geometries. Background free and phase matched over a long interaction length, it is the most sensitive third-order pulse-measurement geometry. In addition, for pulses greater than ∼300 fs in length and ∼1 µJ in energy, the nonlinear medium can be removed and the nonlinearity of air can be used to measure the pulse.

Femtosecond pulse imaging: ultrafast optical oscilloscope

Abstract: A nonlinear optical processor that is capable of real-time conversion of a femtosecond pulse sequence into its spatial image is introduced, analyzed, and experimentally characterized. The method employs nonlinear spectral domain three-wave mixing in a crystal of LiB3O5, where spectral decomposition waves of a shaped femtosecond pulse are mixed with those of a transform-limited pulse to generate a quasi-monochromatic second-harmonic field. By means of this nonlinear process, the temporal-frequency content of the shaped pulse is directly encoded onto the spatial-frequency content of the second-harmonic field, producing a spatial image of the temporal shaped pulse. We show that, unlike the commonly used autocorrelator, such time-to-space conversion carries both amplitude and phase information on the shape of the femtosecond pulses.

Simultaneous measurement of two ultrashort laser pulses from a single spectrogram in a single shot

Abstract: Frequency-resolved optical gating (FROG) is a technique that produces a spectrogram of an ultrashort laser pulse. The intensity and phase of the ultrashort laser pulse can be determined through solving for the phase of the spectrogram with an iterative, phase-retrieval algorithm. This work presents a new phase-retrieval algorithm that retrieves both the probe and the gate pulses independently by converting the FROG phase-retrieval problem to an eigenvector problem. The new algorithm is robust and general. It is tested theoretically by use of synthetic data sets and experimentally by use of single-shot, polarization-gate FROG. We independently and simultaneously characterize the electric field amplitude and phase of a pulse (probe) that was passed though 200 mm of BK7 glass and the amplitude of an unchanged pulse (gate) from an amplified Ti:sapphire laser. When the effect of the 200 mm of BK7 glass was removed mathematically from the probe, there was good agreement between the measured gate and the calculated, prechirped probe.

Noise of the stretched pulse fiber laser. I. Theory

Abstract: A perturbation formalism is developed for the passively mode-locked stretched pulse fiber ring laser analogous to that of the fiber ring soliton laser. It is applied to determine the amplitude fluctuations, carrier frequency noise, and the pulse to pulse jitter due to the amplifier spontaneous emission noise.

All-optical switching in long-period fiber gratings.

Abstract: Nonlinear pulse propagation in long-period fiber gratings is studied with a mode-locked Q-switched laser pulse approximately 80 ps in duration at a wavelength of 1.05 µm. Optical switching, pulse reshaping, and optical limiting are found at intensities in the range of 1–20 GW/cm2.

High-resolution, ultrafast laser pulse shaping and its applications

Abstract: Publisher Summary It is now possible to generate laser pulses, with extremely sophisticated phase and amplitude modulation, or to generate complex laser pulse sequences, with well-defined phase relationships, between the pulses. This has been proved useful in a wide range of spectroscopic applications, such as nuclear magnetic resonance (NMR), which has evolved into an extraordinarily important technique for monitoring structure and dynamics and a diagnostic tool of outstanding clinical value and it is now possible to create staggeringly complex excitation sequences. Useful sequences do not require anything approaching state-of-the-art radiofrequency (rf) capabilities; commercially available arbitrary waveform generators (AWGs) can give possible voltage waveform with ≈1 ns rise time, covering the radiofrequency spectrum at one time. The nanosecond switching time of common digital components is far faster than NMR relaxation times, but far slower than optical relaxation times. The most commonly used laser pulse sequences work, because they do not require specific pulse shapes or bandwidths and are unaffected by phase shifting any individual pulse in the sequence. The rapid rise times of ultrafast laser pulses implies a far greater bandwidth than is conceivable at microwave frequencies. With the addition of a laser and a few optical components a commercial NMR spectrometer could shape femtosecond laser pulses. In this chapter, it is shown that this approach has advantages of rapid waveform update rates (100 kHz–1MHz), commercially available components, simplicity (the phase and amplitude modulation is encoded with a single rf pulse), and extremely high resolution pulse shaping ( ≈ 1000 independently adjustable amplitudes and phases). A variety of applications in quantum control and optical communications have been discussed in the chapter. The chapter discusses various optical modulation techniques, characterization of shaped laser pulses, effective resolution limits, with acousto-optic pulse shaping, and optimal design for maximum resolution without distortions.

Time-resolved structures of macromolecules at the ESRF: Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis

Abstract: We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photocycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps pulses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Laue data from cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Laue patterns are sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We propose the use of microcrystals in the range 25–50 μm for efficient photolysis with femto and picosecond pulses. The performance of circular storage rings is compared with the predicted performance of an X-ray free electron laser (XFEL). The combination of micro beams, a gain of 105 photons per pulse and an ultrashort pulse length of 100 fs is likely to improve pulsed diffraction data very substantially. It may be used to image coherent nuclear motion at atomic resolution in ultrafast uni-molecular reactions.

Ultrashort pulse reflection from fiber gratings: a numerical investigation

Abstract: We consider the linear reflection of ultrashort broadband pulses by uniform and nonuniform narrowband fiber gratings. We examine the effects of grating characteristics, including peak reflectivity, bandwidth, phase response (dispersion and chirp), and apodization on the reflection of such pulses from various gratings. A symmetric transform-limited 1-ps Gaussian pulse is assumed as the ultrashort broadband input to the gratings; the reflected pulses take on significantly different shapes and vary in duration. The prominent features observed are qualitatively explained in order to gain physical insight into the nature of the ultrashort pulse response and corresponding interaction. The results of this study indicate that there is the potential for a new class of applications, including temporal pulse shaping and novel devices for optical communications systems, by combining ultrashort broadband pulses with narrowband fiber gratings.

Space-time profiles of an ultrashort pulsed Gaussian beam

Abstract: By using a different initial value from the previous treatments, we reveal that the pulsed Gaussian beam in free space can be expressed as a simple wave packet along with the complex time, which gives all the information about the spatial and temporal behaviors of the pulse. Then the space-time profiles of a space-time Gaussian pulse are studied. It is shown that even when the pulse propagates in free space, there exist couplings among the beam parameters in space and time. The spot-size-related couplings enlarge the temporal domain (TD) spatial size of the pulse and lessen the carrier frequency of the pulse of the paraxial points, and the wavefront-related couplings bend the transverse spatial shape of the pulse with the wavefront.

Femtosecond pulse shaping by dynamic holograms in photorefractive multiple quantum wells

Abstract: Femtosecond pulses can be shaped in the time domain by diffraction from dynamic holograms in a photorefractive multiple quantum well placed inside a Fourier pulse shaper. We present several examples of shaped pulses obtained by controlling the amplitude or the phase of the hologram writing beams, which modifies the complex spectrum of the femtosecond output.

Apparatus and method for femtosecond pulse compression based on selective attenuation of a portion of an input power spectrum

Abstract: An apparatus for, and method of, decreasing a temporal duration of an optical input pulse having an input power spectrum associated therewith, the apparatus and method operative on at least unchirped optical input pulses. The apparatus comprises: (1) a zero-dispersion pulse shaper for receiving the optical input pulse and spectrally spreading the input pulse to produce a spectrally-spread pulse and (2) a modulator array interposed within the zero-dispersion pulse shaper for receiving the spectrally-spread pulse and selectively attenuating a portion of the power spectrum of the spectrally-spread pulse by a predetermined amount to produce a selectively-attenuated spectrally-spread pulse, the zero-dispersion pulse shaper focusing and recombining the selectively-attenuated spectrally-spread pulse to produce an output pulse having a broader power spectrum than the input pulse, the output pulse further having a temporal duration less than the input pulse, regardless of whether the input pulse is chirped.

Use of Chirped Quasi-phase-matched materials in chirped pulse amplification systems

Abstract: The limitations on maximum pulse energies from a fiber-grating pulse compressor are circumvented by placing a chirped-period quasi-phase-matched (QPM) crystal after the fiber-grating pulse compressor. The crystal accomplishes second-harmonic generation and stretched-pulse compression at the second-harmonic in a single device. This hybrid compressor configuration enables a substantial increase in ultrashort pulse energies obtainable with a compact all-fiber chirped pulse amplification system. Furthermore, with such a QPM crystal the adjustable compensation of both linear and nonlinear frequency chirp in second-harmonic pulses is possible. This property makes a variety of compact, robust and simple ultrashort-pulse fiber amplifier designs possible. It also allows for certain tolerances in the design and manufacturing of a pulse amplification system. Capability to compensate an arbitrary frequency chirp allows nonlinear spectral-broadening techniques for achieving shorter second-harmonic pulse durations. Also, by employing chirped QPM crystals maximum energy throughput and good second-harmonic pulse quality can be achieved.

Fourth-order-dispersion limitations of aberration-free chirped-pulse amplification systems

Abstract: To obtain shorter pulses in chirped-pulse-amplification lasers, researchers have recently proposed several designs for aberration-free pulse stretchers. We examine the limitations of two aberration-free chirped-pulse-amplification systems and show that comparable results can be obtained with simpler, conventional pulse stretchers. In addition, we present a simple, quintic-phase-limited, aberration-free chirped-pulse-amplification system that can support ultrashort, high-contrast pulses.

Propagation of half-cycle far infrared pulses

Abstract: We have studied the propagation of half-cycle (i.e., unipolar) electromagnetic pulses centered at terahertz frequencies, in free space, through apertures, and through focusing optics. The temporal pulse shape of an apertured half-cycle pulse is significantly altered during propagation, but retains much of its unipolar character after traveling more than 20 times the aperture dimension. When focused by an achromatic lens, a half-cycle pulse evolves into a single-cycle pulse at the focal waist and an inverted half-cycle pulse in the far field.

High-intensity pulse propagation in uniform gratings and grating superstructures

Abstract: We briefly review our recent nonlinear propagation experiments in optical fiber Bragg gratings (FBGs). These experiments demonstrate nonlinear pulse compression, and pulse shaping, and also show how a train of pulses may be generated from a single input pulse. Our results also demonstrate the existence of Bragg grating solitons, solitons which exist by balancing of the nonlinearity of the glass and the dispersion of the grating.

Ultrafast optical pulse digitization with unary spectrally encoded cross-phase modulation

Abstract: A method to digitize the intensity of ultrashort laser pulses for high-speed optical signal processing is described. This digitization was based on the spectral broadening of a weak probe (carrier) pulse by a more intense pump (signal) pulse through the nonlinear optical process of cross-phase modulation (XPM). The signal pulse intensity was varied to generate different spectral widths that can be encoded into digital form. Using a 50-ps time-divided multiplexing pulse train with a waveguide splitter, combiner, and an array of fibers with variable lengths, a unary XPM encoding approach is demonstrated. The spectral encoding scheme can be used to achieve a 5-GHz sampling rate at a 16-level accuracy.

Nonlinear fibre-optic receiver for ultrashort pulse code division multiple access communications

Abstract: The authors present experimental results on a nonlinear optical receiver For ultrashort pulse code division multiple access communications which uses nonlinear frequency shift effects in optical fibres to perform intensity discrimination with femtosecond response times. The receiver exhibits a contrast ratio of nearly 1000 for received pulse energies of /spl sim/1 pJ.