Complete characterization of a self-mode-locked ti:sapphire laser in the vicinity of zero group-delay dispersion by frequency-resolved optical gating

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.     

Spatial beam shaping of ultrashort laser pulses: theory and experiment

We studied the spatial intensity profile of an ultrashort laser pulse passing through a laser beam shaping system, which uses diffractive optical elements to reshape a Gaussian beam profile into a flat-topped distribution. Both dispersion and nonlinear self-phase modulation are included in the theoretical model. Our calculation shows that this system works well for ultrashort pulses (approximately 100 fs) when the pulse peak intensity is less than 5 x 10(11) W/cm2. Experimental results are presented for 136 fs pulses at 800 nm wavelength from a Ti:sapphire laser with a 6 nJ pulse energy. We also studied the effects of lateral misalignment, beam-size deviation, and defocusing on the energy fluence profile.     

Measurement of group delay dispersion of high numerical aperture objective lenses using two-photon excited fluorescence

We determined the group-delay dispersion (GDD) of five microscope objectives by measuring the second-order autocorrelation at the focal points of the objectives with two-photon excited fluorescence as the power square sensor. We found that typical microscope lens systems introduce significant GDD (2000-6500 fs(2)). The third-order dispersion determined for these objectives limits the minimum obtainable pulse width at the focal point of an objective to 20-30 fs if not compensated. No significant chromatic aberration or higher-order dispersion effects were found for any of the optical components measured within the wavelength range of 700-780 nm and for pulse widths greater than 50-60 fs.     

Efficient type I second-harmonic generation of subpicosecond laser pulses with a series of alternating nonlinear and delay crystals

We propose a novel configuration of efficient type I second-harmonic generation (SHG) with ultrashort laser pulses by group-velocity compensation. The configuration is composed of a type I SHG crystal and a series of alternating time-delay and type I SHG crystals. The numerical calculations show that the conversion efficiency can be increased to almost 100% by using crystal pairs in series, and the duration of the second-harmonic pulse is almost the same as that of the fundamental pulse.     

Third-order dispersion in a pair of prisms

The general equation for third-order group velocity dispersion is derived in this paper, which contains all the terms of equation (11) in Arissian and Diels [1], as well as one extra term, which we have verified is not negligible. To verify our equation we have modeled the sum of Gaussian modulated frequencies. We show the electric field of bandwidth limited incident ultrashort pulses with a time duration of: 5 fs, 10 fs, 15 fs, 20 fs, 30 fs and 50 fs after propagating through a pair of identical isosceles prisms for a central frequency of 0.8 µm and 0.620 µm. The model is applied by using (a) the finite phase, (b) the phase approximated up to the second-order dispersion and (c) up to the third-order dispersion for each frequency component of the pulse. Graphs are presented for prisms made with fused silica and SF14.     

Effects of primary spherical aberration, coma, astigmatism, and field curvature on the focusing of ultrashort pulses: Gaussian illumination and experiment

We analyze the spatiotemporal intensity of Gaussian temporal envelope pulses with initial durations of 200 fs and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens for a well-collimated incoming pulse beam by using the Seidel aberration theory for thin lenses with the stop at the lens. We analyze the effect of these aberrations in the focusing of ultrashort pulses for Gaussian illumination and present experimental results for 200 fs pulses focused by a near-IR achromatic doublet.     

Diffraction characteristics of 10 fs laser pulses passing through an aperture

Results are presented on experimental and theoretical work performed to compare diffraction phenomena for ultrashort 10 fs pulses and continuous-wave propagation modes illuminating different-sized pinholes and slits. Results demonstrate that 10 fs pulses do not produce high-frequency diffraction like that produced with continuous-wave illumination. The diffraction through a 1 mm pinhole of temporally stretched pulses obtained by using fused silica plates whose frequency spectrum remains the same is compared with those of 10 fs pulses. The overall diffraction intensity profiles are, however, nearly identical in this case. The simulations of diffraction patterns for 100 fs, 10 fs, and 1 fs incident pulse were compared theoretically for different aperture sizes and frequencies. Calculations indicate that the lack of high-frequency diffraction for the mode-locked case is due to the broadband nature of the ultrashort laser pulses; i.e., the distribution of the frequency contained in the pulse ends up washing out when objects are illuminated with pulses of broad frequency content. The results of this work have important application in biomedical imaging and remote imaging applications, to name only a few.     

70-femtosecond Gaussian pulse generation in a dispersion-managed erbium-doped fibre laser

We experimentally demonstrated the generation of transform-limit Gaussian ultrashort pulses as short as 70 fs from a dispersion-managed mode-locked erbium-doped fibre (EDF) laser. The output spectrum is close to the Gaussian profile with a full-width half-maximum (FWHM) output of 49?nm, and the measured autocorrelation trace exhibits the Gaussian profile. The shortest pulse duration of 70 fs was observed after external recompression. The time-band product is 0.44, showing the best transform limit pulse.     

Effects of primary spherical aberration, coma, astigmatism and field curvature on the focusing of ultrashort pulses: homogenous illumination

We analyze the spatiotemporal intensity of pulses with durations of 20 fs and shorter and a carrier wavelength of 810 nm at the paraxial focal plane of an achromatic doublet lens. The incident pulse is well-collimated, and we use the Seidel aberration theory for thin lenses to evaluate the phase change due to the aberrations of the lens. In a set of cemented thin lenses with the stop at the lens, there is only spherical aberration, coma, astigmatism and field curvature, whereas the distortion aberration in the phase front is zero. We analyze the effect of these aberrations in the focusing of ultrashort pulses for homogenous illumination. We will show that the temporal spreading introduced by these aberrations in pulses shorter than 20 fs at 810 nm is very small but the spatial spreading is not, which reduces the intensity of the pulse considerably.     

Programmable ultrashort optical pulse delay using an acousto-optic deflector

We present an optical pulse delay (OPD) for delaying ultrashort optical pulses that uses an acousto-optic deflector as an active component. The OPD is designed to correct for chromatic dispersion caused by the significant color spectrum of ultrashort pulses. It is intended to be used as one of the components in a three-dimensional memory system based on pulse-collision addressing in two-photon materials. Calculations show that the OPD should be able to provide 65 arbitrary delays with a random access time of ? 1 μs for 100-fs pulses. The power efficiency of the OPD can be as high as 85% and hence permits two units to be cascaded to provide more than 4000 distinct delays. The number of delays and the access time can be optimized such that a fewer number of delays are obtained with a shorter access time, which favors high-speed operations. We provide experimental results that use a Michelson interferometer to measure three different delays, approximately 1 mm apart (equivalent to ?3-ps time delay), obtained with 130-fs pulses. In addition we include an analysis of the performance of acousto-optic devices for both monochromatic light and ultrashort pulsed lasers. Finally, we provide the design of the optical pulse-delay system for a three-dimensional memory application.     

Two-photon excitation of atoms by ultrashort electromagnetic pulses in a discrete spectrum

Abstract

The paper is devoted to the theoretical investigation of two-photon excitation of atom in a discrete energy spectrum by ultrashort electromagnetic pulses of femto- and subfemtosecond ranges of durations. An analytical expression for the total probability of the process is derived. Numerical simulations are made for hydrogen and sodium atoms. It is shown that the total probability of the process is nonlinear function of pulse duration and character of this function depends strongly on the frequency detuning of pulse carrier frequency from two-photon resonance.     

Feasibility of depth profiling of animal tissue by ultrashort pulse laser ablation

Experiments were performed to examine the feasibility of mass spectrometry (MS) depth profiling of animal tissue by ~75 fs, 800 nm laser pulses to expose underlying layers of tissue for subsequent MS analysis. Matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) was used to analyze phospholipids and proteins from both intact bovine eye lens tissue and tissue ablated by ultrashort laser pulses. Laser desorption postionization mass spectrometry (LDPI-MS) with 10.5 eV single photon ionization was also used to analyze cholesterol and other small molecules in the tissue before and after laser ablation. Scanning electron microscopy was applied to examine the ablation patterns in the tissue and estimate the depth of the ablation craters. Ultrashort pulse laser ablation was found to be able to remove a layer of several tens of micrometers from the surface of eye lens tissue while leaving the underlying tissue relatively undamaged for subsequent MS analysis. MS analysis of cholesterol, phospholipids, peptides, and various unidentified species did not reveal any chemical damage caused by ultrashort pulse laser ablation for analytes smaller than ~6 kDa. However, a drop in intensity of larger protein ions was detected by MALDI-MS following laser ablation. An additional advantage was that ablated tissue displayed up to an order of magnitude higher signal intensities than intact tissue when subsequently analyzed by MS. These results support the use of ultrashort pulse laser ablation in combination with MS analysis to permit depth profiling of animal tissue.     

Two-photon absorption of ultrashort electromagnetic pulses by negative halogen ions

Abstract

Within the framework of the perturbation theory, two-photon absorption of ultrashort electromagnetic pulses of a corrected Gaussian shape by negative halogen ions during the action of a pulse is calculated and analysed. Photoabsorption with excitation of an electron to a state with a specified energy is also considered. Particular attention is given to the dependence of the probability of these processes on the pulse duration. The features of two-photon absorption that are characteristic of sufficiently short pulses and do not take place in the monochromatic limit are revealed.     

Design of an extreme-ultraviolet monochromator free from temporal stretching

High-order harmonic generation in gases by use of femtosecond lasers is a source of ultrashort pulses in the extreme-ultraviolet (XUV). For many applications it is necessary to select radiation of only one specific harmonic order without affecting the duration of the ultrashort pulse. A three-grating monochromator that meets this demand has been designed and modeled by ray tracing as well as by wave-optical simulations. The only remaining temporal stretching of an XUV pulse is due to distortion of the pulse front on the gratings and is predicted to be approximately 1 fs. The design has been successfully tested in the near infrared. Finally, the monochromator is also capable of eliminating any existing linear chirp in the harmonic pulses, thus compressing them to shorter durations.     

Optimal control of high harmonics for the generation of double attosecond pulses by using a chirped laser pulse

An efficient scheme for the optimization of ultrashort femtosecond pulse shapes interacting with an atom to control high harmonics spectrum and double attosecond pulse generation is presented. The time-dependent Schrödinger equation of one-dimensional hydrogen atom is solved numerically to obtain electric field emission. The genetic algorithm optimization method is used to control the phase and amplitude of ultrashort excitation laser pulses to generate the desired attosecond-shaped pulses. An appropriate cost function is introduced for genetic algorithm optimization of double attosecond pulse generation. It is shown that the relative intensity of two generated pulses, their delay time and duration can be controlled in this approach. Finally, the parameters of the optimized emitted attosecond pulse are compared with those of desired pulses, and the underlying physical mechanisms are discussed in detail.     

Effects of sample temperature in femtosecond single-pulse laser-induced breakdown spectroscopy

As much as tenfold atomic emission enhancements have been observed in experiments combining nanosecond (ns) and femtosecond (fs) laser pulses in an orthogonal dual-pulse configuration for laser-induced breakdown spectroscopy (ns-fs orthogonal dual-pulse LIBS). In the examination of one of several potential sources of these atomic emission enhancements (sample heating by a ns air spark), minor reductions in atomic emission and as much as 15-fold improvements in mass removal have been observed for fs single-pulse LIBS of heated brass and aluminum samples. These results suggest that, although material removal with a high-powered, ultrashort fs pulse is temperature dependent, sample heating by the ns air spark is not the source of the atomic emission enhancements observed in ns-fs orthogonal dual-pulse LIBS.     

Effect of material dispersion on the focusing of isodiffracting ultrashort pulses by a lens

Abstract

Based on the Fourier transform, the focusing of isodiffracting ultrashort pulses by a lens is studied, where the material dispersion of first, second and higher order is taken into account, respectively. Numerical calculation results for spatial and temporal intensity distributions, photon flux and energy density of focused isodiffracting ultrashort pulses are given and illustrated. It is shown, compared to the dispersion-free case, that the first-order dispersion leads to a broadening of the pulse form, photon flux and energy density, and a decrease of their peak values. The second-order dispersion results in a further broadening of the pulse form and photon flux, and a further decrease of their peak values, whereas the higher-order dispersion plays a relatively minor role.     

Sub-wavelength ripples in fused silica after irradiation of the solid/liquid interface with ultrashort laser pulses

Laser-induced backside wet etching (LIBWE) is performed using ultrashort 248?nm laser pulses with a pulse duration of 600?fs to obtain sub-wavelength laser-induced periodic surface structures (LIPSS) on the back surface of fused silica which is in contact with a 0.5?mol?l(-1) solution of pyrene in toluene. The LIPSS are strictly one-dimensional patterns, oriented parallel to the polarization of the laser radiation, and have a constant period of about 140?nm at all applied laser fluences (0.33-0.84?J?cm(-2)) and pulse numbers (50-1000 pulses). The LIPSS amplitude varies due to the inhomogeneous fluence in the laser spot. The LIPSS are examined with scanning electron microscopy (SEM) and atomic force microscopy (AFM). Their power spectral density (PSD) distribution is analysed at a measured area of 10?μm × 10?μm. The good agreement of the measured and calculated LIPSS periods strongly supports a mechanism based on the interference of surface-scattered and incident waves.     

Analysis of spatial-temporal converters for all-optical communication links

We analyze parallel-to-serial transmitters and serial-to-parallel receivers that use ultrashort optical pulses to increase the bandwidth of a fiber-optic communication link. This method relies on real-time holographic material for conversion of information between spatial and temporal frequencies. The analysis reveals that the temporal output of the pulses will consist of chirped pulses, which has been verified experimentally. When the signal pulses are transmitted along with a reference pulse, the distortions of the received signal, caused by dispersion and other factors in the fiber, are canceled because of the phase-conjugation property of the receiver. This self-referencing scheme simplifies the receiver structure and ensures perfect timing for the serial-to-parallel conversion.     

Generation of high-repetition-rate femtosecond pulses from 8 to 18 microm

We generated subpicosecond pulses from 8 to 18 mum by difference-frequency mixing in a 1-mm-thick AgGaSe(2) crystal, the 130- and 180-fs output pulses (1.45 < lambda < 1.85 mum) from an 84-MHz-repetition-rate optical parametric oscillator. Numerical simulations show that intrapulse and interpulse group velocity dispersion determine minimum pulse duration above and below 15 mum, respectively. By cross correlation (upconversion) of 10.5-mum pulses with 90-fs, 810-nm pulses in AgGaS(2), the pulse length was measured to be 310 fs in good agreement with simulations.