Pulse Stretcher Research Articles

Spatiotemporal distortions of attosecond pulses

In this article, we report that usual multilayer mirror configurations to focus attosecond pulses generate geometric aberrations and can significantly stretch pulses. The numerical simulations show that the effects can be strong enough to delay some parts of the pulses of an attosecond pulse train and make them interfere with the next pulses of the train. The influence of the numerical aperture on the pulse duration is also studied, showing that such effects can occur even with very low numerical apertures.

Effect of pulse temporal shape on optical trapping and impulse transfer using ultrashort pulsed lasers

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscope's temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

Pulse stretcher with variable pulse length for excimer laser applications

Excimer laser systems are often combined with pulse stretchers, e.g., in lithography and Raman spectroscopy to reduce pulse peak intensities. We developed a pulse stretcher which is suited especially for pulse length dependent applications, e.g., in material research. This pulse stretcher is based on imaging and provides identical beam profiles and divergences at different pulse lengths in combination with the possibility of switching between four pulse lengths. Therefore, beam splitters are mounted onto motorized translation stages. These features are important for the characterization and development of improved optical materials for deep-ultraviolet applications. Here, two 193 nm single tube excimer lasers (Lambda Physik: LPX 240i, Cymer: ELS 5600) were evaluated concerning their combinability with an attached pulse stretcher. The wave-front radii of the ELS 5600 are dependent on repetition rate or pulse energy and are changing during warming-up. Without adapted pulse shaping, this can cause destruction of stretcher optics due to moving foci. We found that the LPX 240i could be combined easily with a pulse stretcher using beam shaping with only one cylindrical lens. We could demonstrate that stretched 193 nm laser pulses retard aging of CaF(2) crystals.

A stretcher fiber for use in fs chirped pulse Yb amplifiers

A newly developed fiber for use in pulse stretchers for chirped pulse amplifiers working in the 1 mum wavelength range of Yb fiber amplifiers is reported. The fiber has a record high numerical third order to second order dispersion beta(3)/beta(2) ratio of -7.7 fs. The fiber has very good dispersion match to a grating compressor for second, third, and fourth order dispersion. By combining the stretcher fiber with an anomalous dispersion fiber working in a higher order mode, even higher beta(3)/beta(2) ratio of -16.8 fs is demonstrated. The combined module shows very good dispersion match to a grating compressor.

超短脉冲在光子晶体中传播时的色散参量

Using the least-squares spline approximation to analyze the photonic band structure,the dispersion parameters of a two-dimensional photonic crystal are quantitatively calculated.Considering the large bandwidth of an ultrashort pulse,average group velocity and average group velocity dispersion are achieved by introducing a weighting factor,which reveals the share of different frequency components.So the pulse evolution can be predicted theoretically.Propagation of ultrashort pulses through the photonic crystals is simulated by using the finite-difference time-domain method.The results of numerical simulations are well consistent with the theoretical predictions.This process for achieving dispersion parameters is valuable for designing photonic-crystal dispersion devices such as dispersion compensator,pulse compressor and pulse stretcher.

Cross-correlation technique for dispersion characterization of chirped volume Bragg gratings

We propose using cross-correlation frequency-resolved optical gating for dispersion characterization of optical elements with high dispersion, such as ultrashort pulse stretchers and compressors. The technique is based on spectrally resolved second-order cross correlation (sum frequency generation) of a stretched pulse with a reference short pulse. Dispersion of optical elements with a high pulse stretching ratio can be completely characterized using this method, even with moderate resolution of spectral measurements of the cross-correlation signal. The proposed technique is used to measure dispersion of a chirped Bragg grating recorded in photo-thermo-refractive glass. It was found that dispersion of these gratings is almost linear with wavelength and has approximately the same slope with opposite signs for two orientations of the grating with opposite faces of the grating used as the input face. Analysis of higher-order dispersion of the grating shows some variations of dispersion across the aperture of the grating, mostly in the amount of third-order dispersion contribution.

Amplified dispersive delay generator using angular dispersion amplification

Angular dispersion amplification can be used to design compact dispersive delay generators with delay amplification. A specific design is presented that can be used to generate 1 ns delay for two wavelengths separated by 1 nm. The delay generator can be easily switched from a positive to a negative group velocity delay and can be used as a pulse compressor, a pulse stretcher, or a pulse shaper in chirped pulse amplification, high-resolution time-gated spectroscopy as well as other applications.

Stretch pulses and cardiac arrhythmias in rat hearts: is it a good model?

Stretch pulses and cardiac arrhythmias in rat hearts: is it a good model?

A novel confocal optical pulse stretcher for laser pulses

We present an analytical and experimental study of a novel confocal optical pulse stretcher (COPS). The simple and passive pulse stretcher consists of two concave mirrors and a scraper beam-splitter and its optical configuration ensures a perfect spatial overlapping of laser pulses at the beam-splitter. The pulse stretcher is compact and suitable for laser pulses of large divergences. The confocal optical pulse stretcher is demonstrated on a pulsed copper vapour laser to convert a 40 ns (1/ e 2 % points) pulse into a 55 ns with reduction of peak power by a factor of 1.375 without loss of pulse energy.

Scanning all-fiber-optic endomicroscopy system for 3D nonlinear optical imaging of biological tissues

An extremely compact all-fiber-optic scanning endomicroscopy system was developed for two-photon fluorescence (TPF) and second harmonic generation (SHG) imaging of biological samples. A conventional double-clad fiber (DCF) was employed in the endomicroscope for single-mode femtosecond pulse delivery, multimode nonlinear optical signals collection and fast two-dimensional scanning. A single photonic bandgap fiber (PBF) with negative group velocity dispersion at two-photon excitation wavelength (i.e. approximately 810 nm) was used for pulse prechirping in replacement of a bulky grating/lens-based pulse stretcher. The combined use of DCF and PBF in the endomicroscopy system made the endomicroscope basically a plug-and-play unit. The excellent imaging ability of the extremely compact all-fiber-optic nonlinear optical endomicroscopy system was demonstrated by SHG imaging of rat tail tendon and depth-resolved TPF imaging of epithelial tissues stained with acridine orange. The preliminary results suggested the promising potential of this extremely compact all-fiber-optic endomicroscopy system for real-time assessment of both epithelial and stromal structures in luminal organs.

Tunable all-optical pulse compression and stretching via doublet Brillouin gain lines in an optical fiber

We demonstrate tunable all-optical pulse compression and stretching via doublet Brillouin gain lines in an optical fiber. Tunable pulse compression or a stretching ratio of 1.47-0.43 accompanying a time delay is achieved by controlling the separation between two gain lines, for an input pulse train with a 40 ns width and a repetition rate of 5 MHz, in a 4 km silica fiber with a fixed pump power of 88.1 mW. The limitation of this pulse compressor or stretcher is also discussed.

Analysis of the adjustment precision of the stretcher in the petawatt laser facility

Effects of stretcher adjustment aberration on the residual dispersions of the laser system is analyzed making use of the ray tracing method. It is shown that the adjustment aberrations affect the pulse contrast and we must select an appropriate adjustment precision for the stretcher in the petawatt laser system. And as an example, a proper adjustment precision of the pulse stretcher is also proposed for the SHENGUANG (SG) II laser facility.

Spatial non-uniformity of excitation-contraction coupling can enhance arrhythmogenic-delayed afterdepolarizations in rat cardiac muscle

We examined whether non-uniform muscle contraction affects delayed afterdepolarizations (DADs) by dissociating Ca(2+) from myofilaments within the border zone (BZ) between contracting and stretched regions. Force, sarcomere length (SL), membrane potential, and [Ca(2+)](i) dynamics were measured in 31 ventricular trabeculae from rat hearts. Non-uniform muscle contraction was produced by exposing a restricted region of muscle to a jet of solution containing 20 mmol/L 2,3-butanedione monoxime (BDM). DADs were induced by 7.5 s-2 Hz stimulus trains at an SL of 2.0 microm (24 degrees C, [Ca(2+)](o) 2.0 mmol/L). The BDM jet enhanced DADs (n = 6, P < 0.05) and aftercontractions (n = 6, P < 0.05) with or without 100 micromol/L streptomycin and occasionally elicited an action potential. A stretch pulse from an SL of 2.0 microm to 2.1 or 2.2 microm during the last stimulated twitch of the trains accelerated Ca(2+) waves in proportion to the increment of force by the stretch (P < 0.01) with or without streptomycin. In the presence of 1 mmol/L caffeine, rapid shortening of the muscle after the stretch pulse increased [Ca(2+)](i) within the BZ, whose amplitude correlated with the increment of force by the stretch (n = 15, P < 0.01). These results suggest that non-uniform muscle contraction can enhance DADs by dissociating Ca(2+) from myofilaments within the BZ and thereby cause triggered arrhythmias.

EXtreme Chirped Pulse Oscillator Operating in the Nanosecond Stretched Pulse Regime

An eXtreme Chirped Pulse Oscillator (XCPO) implemented with a Theta cavity and based on a semiconductor optical amplifier (SOA) is presented for generating 10 ns frequency-swept pulses and 3.6 ps compressed pulses directly from the oscillator. In this experiment, we show the two distinct characteristics of the XCPO which are the scalability of the output energy and the mode-locked spectrum. By using these characteristics, we obtain a pulse energy of 58.4 pJ from the stretched pulse and a mode-locked optical bandwidth of 14.6 nm (10 dB) directly from the oscillator. The laser cavity design allows for low repetition rate operation <100 MHz, as well. The cavity, significantly, reduces nonlinear carrier dynamics, integrated self phase modulation (SPM), and fast gain recovery in an SOA. Due to the laser's ability to generate directly frequency-swept pulses from the oscillator, this oscillator can be used for high speed frequency-swept optical coherence tomography (OCT) and time-stretched photonic analog to digital converters (P-ADC).

Dependence of the stimulated Raman scattering threshold in droplets on the temporal stretching of a nanosecond laser pulse

AbstractFor linear Raman spectroscopy in micro‐droplets the excitation laser energy and therefore the signal strength is limited either by the threshold for the excitation of nonlinear processes in the droplets or by the generation of a plasma (gas breakdown) in the vicinity of the droplets. These processes are due to the enhancement of the excitation field by morphology‐dependent resonances (MDRs) which are built up in the cavity synthesised by the droplets. As these stimulated signals disturb the detection of the linear signal it is of high interest for different applications to suppress the stimulated Raman scattering (SRS) or to use an excitation intensity below the SRS threshold with a sufficiently large integrated laser energy to obtain an appropriate signal‐to‐noise ratio for the linear Raman process. In this paper the effect of temporal stretching of the incoming laser pulse with the help of an optical pulse stretcher on the SRS threshold is described and discussed. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Development of a Terawatt Class Sub-10 fs Laser System

We have developed an amplifier chain of Ti:Sapphire laser with a spectral width of more than 200 nm, which potentially generates a terawatt sub-10 fs pulse. This laser system consists of a Ti:Sapphire oscillator followed by a pulse stretcher, dispersion compensator with an SLM and the amplifier chain. The first amplifier is a regenerative one pumped by a green laser with energy of 11.5 mJ at 1 kHz. We could successfully compensate for the gain narrowing during regenerative amplification by using high-damage-threshold chirped mirrors and specially designed partial mirrors in the cavity, resulting in pulse energy of 73 µJ. The output pulse is sent to a beam-slicer to reduce the repetition rate to 10 Hz. After 5-pass amplification in a multi-pass amplifier pumped by the second harmonic of a YAG laser with an energy of 180 mJ, we obtained an amplified pulse energy of 30 mJ.

Stretcher Design for the SGII Petawatt Upgrade Laser Facility

To ensure high contrast ratio for the SHENGUANG (SG) II petawatt upgrade laser facility, many factors such as pulse spectral clipping, chromatic aberration and manufacture errors of the optical elements are considered carefully in the design of the pulse stretcher. An eight pass Offner configuration is selected for the laser system and the final design of the stretcher is also given out.

Independence of Maximum Single Molecule Fluorescence Count Rate on the Temporal and Spectral Laser Pulse Width in Two-Photon FCS

We investigate the fluorescence emission characteristics of standard dye tetramethylrhodamine (TMR) in two-photon fluorescence correlation spectroscopy for different temporal and spectral properties of the femtosecond excitation pulses. After determining the second-order dispersion of our setup, including the microscope objective, a pulse stretcher was employed to control the temporal width at the location of the specimen. As expected, the fluorescence per molecule and therefore the signal-to-noise ratio of an FCS-measurement can be improved at constant average excitation power by altering either the temporal or spectral width of the excitation pulses. We found however, that the maximum achievable molecular brightness is largely independent of the temporal and spectral width in the regime analyzed. This observation confirms the current working hypothesis for two-photon fluorescence correlation spectroscopy that bleaching and saturation, and thus, the inherent properties of the dye system, are the dominant effects limiting the quality of measurements. As a practical consequence, elaborate optimization of temporal and spectral laser pulse width, e.g. by introducing pulse stretchers in the beam path, is less critical than previously expected.

Efficient angular dispersion compensation in holographic generation of intense ultrashort paraxial beam modes

We experimentally demonstrate that small misalignments of the pulse stretcher or compressor of our chirped-pulse-amplification laser can precompensate for angular chirp when producing ultrashort paraxial beam modes with holographic gratings. Using this approach we can eliminate one of the two gratings needed in our 2f-2f setup [Mariyenko, Opt. Express 13, 7599 (2005)]. This allows for up to an order of magnitude more output power. We see our method as the next step in the production of intense exotic forms of ultrashort pulses, which can be used in the investigation of intense laser-matter interactions. In addition, we produce the first femtosecond (helical-)Ince-Gaussian beams.

Short pulse laser train for laser plasma interaction experiments

A multiframe, high-time resolution pump-probe diagnostic consisting of a consecutive train of ultrashort laser pulses (approximately ps) has been developed for use with a chirped pulse amplification (CPA) system. A system of high quality windows is used to create a series of 1054 nm picosecond-laser pulses which are injected into the CPA system before the pulse stretcher and amplifiers. By adding or removing windows in the pulse train forming optics, the number of pulses can be varied. By varying the distance and thickness of the respective optical elements, the time in between the pulses, i.e., the time in between frames, can be set. In our example application, the CPA pulse train is converted to 527 nm using a KDP crystal and focused into a preformed plasma and the reflected laser light due to stimulated Raman scattering is measured. Each pulse samples different plasma conditions as the plasma evolves in time, producing more data on each laser shot than with a single short pulse probe. This novel technique could potentially be implemented to obtain multiple high-time resolution measurements of the dynamics of physical processes over hundreds of picoseconds or even nanoseconds with picosecond resolution on a single shot.