Pulse Compression

Knopp et al. ͓J. Raman Spectrosc. 31, 51 ͑2000͔͒ have recently used resonant femtosecond coherent anti-Stokes Raman spectroscopy ͑CARS͒ to prepare and probe highly excited vibrational wave packets on the ground electronic potential surface of molecular iodine. The experiment uses a sequence of three resonant femtosecond pulses with two independently variable time delays. The first two pulses act as a pump and dump sequence to create a predefined, highly excited wave packet on the ground electronic state, whose amplitude is optimized by selecting the proper pump-dump ͑Raman͒ frequency difference and varying the time delay. The third pulse promotes the pump-dump wave packet to an excited electronic state, resulting in subsequent coherent emission of light at the anti-Stokes frequency. This fully-resonant CARS signal, measured as a function of time delay between the second and third pulses, oscillates at a frequency characteristic of the pump-dump wave packet. Due to anharmonicity, this frequency is a sensitive measure of the amount of vibrational excitation. Knopp et al. observed that under certain conditions the signal exhibits pronounced beating between the pump-dump wave packet frequency and the frequency characteristic of the bottom of the ground state well. In this paper we show that these beats arise only when the final pump-dump-pump wave packet is above the excited state dissociation threshold of the molecule. We derive analytical expressions showing that under these conditions, where the polarization is short-lived, there may be strong interferences between the contributions from molecules originally in different vibrational states of the thermal ensemble. In contrast, the CARS polarization in the below threshold case is long-lived, and these interferences cancel. Numerical evaluation of the CARS signal through vibrational wave packet propagation confirms the predictions of the analytical theory and reproduces the distinctive beating pattern observed in the experiments. Additional experiments and simulations demonstrate that these interferences can be turned on or off by carefully selecting the pulse frequencies. The experiments can also be viewed from a different perspective, as an extension of the pump-dump mechanism for selective bond breaking on the ground electronic state, to a pump-dump-pump sequence for selective bond breaking on the excited electronic state.

We demonstrate generation of 3.8-fs pulses with energies of up to 15 mJ from a supercontinuum produced in two cascaded hollow fibers. Ultrabroadband dispersion compensation was achieved through a closed-loop combination of a spatial light modulator for adaptive pulse compression and spectral-phase interferometry for direct electric-field reconstruction (SPIDER) measurements as feedback signal.

A pulsed time-of-flight laser range finder with a 1 GHz avalanche photo diode ͑APD͒ receiver and a laser pulser with ϳ35 ps pulse width has been developed and tested. The receiver channel is constructed using a silicon ASIC chip and a commercially available silicon APD placed on a hybrid ceramic susbstrate. The laser pulser utilizes a single heterostructure laser operating in Q-switching mode. It is shown that the single-shot precision of the complete laser range finder is ϳ2.1 mm ͑ value͒ at best. The nonaccuracy in the distance range of 0.5-34.5 m was ϳϮ2 mm excluding errors caused by the statistical variations and long-term instability. The single-shot precision is clearly better than the single-shot precision of the earlier laser range finders with ϳ100-200 MHz bandwidths. Also, two types of optics, coaxial and paraxial, were tested. The linearity of the coaxial optics was better, especially with a long ͑4 m͒ receiver fiber. Some possible applications of the laser range finder utilizing ps level pulses are, for example, fast three-dimensional vision in industrial environments and structure analysis of materials.

Optical ultra-short pulse compression and amplification using semiconductor optical amplifier (SOA) is presented. Using pump-probe pulse configuration, we present an SOA model which includes the nonlinear effects such as, spectral hole burning (SHB), carrier heating (CH), two-photon absorption (TPA) and group velocity dispersion (GVD) taking into account gain spectrum effect. Then by adjusting time delay between the pump pulse and probe pulse we use the model for simultaneous compression and amplification of probe pulse. We also analyze the four wave mixing (FWM) signal during pulse compression process. It is shown that dispersive effect of GVD on output probe pulse becomes more important for larger cavity length and probe-pump pulses relative time delays.

Stretching and compressing of laser pulses is demonstrated with a single-grating apparatus. A laser pulse of 110 fs is stretched to 250 ps and then recompressed to 115 fs. The apparatus exploits a two-level structure: one level for stretching and the other for compressing. This single-grating configuration shows significant simplification in structure and alignment over existing multiple-grating systems. Such a stretcher-compressor is particularly suitable for use with chirped-pulse amplification in which laser wavelength tuning is desirable. Only one rotational adjustment is rquired to restore the alignment of the entire stretcher and compressor when the laser wavelength is changed.

We use the real (Kerr) and imaginary (two photon absorption) parts of a third order optical nonlinearity to tune the long (1.6μm) and short wavelength (1.3μm) band edges of a stop gap in a two-dimensional silicon photonic crystal. From pump-probe reflectivity experiments using 130fs pulses, we observe that a 2μm pulse induces optical tuning of the 1.3μm edge via the Kerr effect whereas a 1.76μm pulse induces tuning of the 1.6μm band edge via both Kerr and Drude effects with the latter related to two-photon induced generation of free carriers with a lifetime of ˜900ps .

A low-cost ultrawideband (UWB), 1.926-4.069 GHz, phased array radar system is developed that requires only one exciter and digital receiver that is time-division-multiplexed (TDM) across 8 receive elements and 13 transmit elements, synthesizing a fully populated 2.24 m long (λ/2 element-toelement spacing) linear phased array. A 2.24 m linear phased array with a 3 GHz center frequency would require 44 antenna elements but this system requires only 21 elements and time to acquire bi-static pulses across a subset of element combinations. This radar system beamforms in the near field, where the target scene of interest is located 3-70 m down range. It utilizes digital beamforming, computed using the range migration synthetic aperture radar (SAR) algorithm. The phased array antenna is fed by transmit and receive fan-out switch matrices that are connected to a UWB LFM pulse compressed radar operating in stretch mode. The peak transmit power is 1 mW and the transmitted LFM pulses are long in time duration (2.5-10 ms), requiring the radar to transmit and receive simultaneously. It will be shown through simulation and measurement that the bi-static antenna pairs are nearly equivalent to 44 elements spaced λ/2 across a linear array. This result is due to the fact that the phase center position errors relative to a uniform λ/2 element spacing are negligible. This radar is capable of imaging free-space target scenes made up of objects as small as 15.24 cm tall rods and 3.2 cm tall metal nails at a 0.5 Hz rate. Applications for this radar system include short-range near-real-time imaging of unknown targets through a lossy dielectric slab and radar cross section (RCS) measurements.

The white-light continuum in optical bulk material has the potential to be used as a seed
pulse in optical parametric amplifiers, which entails the compression of seed pulses to
temporally overlap with pump pulses. An experimental study of the compression of the
white-light continuum is reported in this article. Both chirped mirror compression and prism
pair compression methods are used. The 280 fs pulses centered at 1053 nm are employed
to generate a white-light continuum in a bulk sapphire plate; the spectrum of a white-light
continuum spans from 650 nm to 1000 nm and its second harmonic generation (SHG) is
measured. The scanning SHG frequency resolved optical gating technique was used to
characterize the compression of the white-light continuum. Prism pair only compression of
the (650–1000) nm spectra yielded 190 fs, with chirp mirrors added after leading to 150 fs.
Spectral masking of the spectrum below 850 nm inside the prism compressor resulted in a pulse duration of ~120 fs for the 850–1000 nm spectral portion of the pulses.

The optical fiber based supercontinuum source has recently become a significant scientific and commercial success, with applications ranging from frequency comb production to advanced medical imaging. This one-of-a-kind book explains the theory of fiber supercontinuum broadening, describes the diverse operational regimes and indicates principal areas of applications, making it a very important guide for researchers and graduate students. With contributions from major figures and groups who have pioneered ...

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: ͑1͒ plasma resonance heating and current drive; ͑2͒ rf acceleration of charged particles; ͑3͒ radar and communications systems; and ͑4͒ high-peak power sources for weapons-effect simulation and exploratory development.

Modern radar systems often have the conflicting requirements of a large phased array antenna for increased power-aperture product and/or angular resolution versus wide instantaneous signal bandwidth for fine range resolution. Pulse-to-pulse diversity may also be desired as a countermeasure against coherent repeater jammers. Using the multicarrier nature of orthogonal frequency-division multiplexing to encode wide instantaneous bandwidth pulses we propose a novel digital phased array architecture which mitigates dispersion effects experienced when the array main beam is electronically steered off broadside. The result is the ability to transmit and receive wideband pseudo-random phase coded pulses with a phased array antenna. Simulation results are used to quantify the benefit of the architecture. An overview of digital orthogonal frequency-division multiplexing waveform generation is also provided.

Pulsed noise jamming is a common anti-radar jamming technique. It creates a noise pulse when radar signal is received, thus concealing any aircraft flying behind it with a block of noise. Modern Linear Frequency Modulated Pulse Compression (LFM-PC) radar, which is characterized by its high processing gain, is considered as one of the challenges to jammer systems. In this paper, the performance of such radar is evaluated analytically, which has not been exploited in any other literature before, under the effect of pulsed noise jamming. Mathematical models of the LFM-PC matched filter response in clear environment as well as pulsed noise jamming are derived. Receiver Operating Characteristic (ROC) is derived and used as a performance measurer. The Derived analytical results agreed with simulation results.

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broadband optical pulses to rotate electron spins and provide the clock signal to the system. Nonlocal two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical simulations of this scheme demonstrate high-fidelity single-qubit and two-qubit gates with operation times comparable to the inverse Zeeman frequency.

A novel switched excitation method for linear frequency modulated excitation of ultrasonic transducers in pulse compression systems is presented that is simple to realise, yet provides reduced signal sidelobes at the output of the matched filter compared to bipolar pseudochirp excitation. Pulse compression signal sidelobes are reduced through the use of simple amplitude tapering at the beginning and end of the excitation duration. Amplitude tapering using switched excitation is realised through the use of intermediate voltage switching levels, half that of the main excitation voltages. In total five excitation voltages are used creating a quinary excitation system. The absence of analogue signal generation and power amplifiers renders the excitation method attractive for applications with requirements such as a high channel count or low cost per channel. A systematic study of switched linear frequency modulated excitation methods with simulated and laboratory based experimental verification is presented for 2.25 MHz non-destructive testing immersion transducers. The signal to sidelobe noise level of compressed waveforms generated using quinary and bipolar pseudo-chirp excitation are investigated for transmission through a 0.5 m water and kaolin slurry channel. Quinary linear frequency modulated excitation consistently reduces signal sidelobe power compared to bipolar excitation methods. Experimental results for transmission between two 2.25 MHz transducers separated by a 0.5 m channel of water and 5% kaolin suspension shows improvements in signal to sidelobe noise power in the order of 7-8 dB. The reported quinary switched method for linear frequency modulated excitation provides improved performance compared to pseudo-chirp excitation without the need for high performance excitation amplifiers.

In this paper, equations for the calculation of the self-and mutual inductances of the modular toroidal coil (MTC) applicable to Tokamak reactors are presented. The MTC is composed of several solenoidal coils (SCs) connected in series and distributed in the toroidal and symmetrical forms. These equations are based on Biot-Savart's and Neumann's equations, respectively. The numerical analysis of the integrations resulting from these equations is solved using the extended three-point Gaussian algorithm. Comparing the results obtained from the numerical simulation with the experimental and the empirical results confirms the presented equations. Furthermore, the comparison of the behavior of these inductances, when the geometrical parameters of the MTC are changed, with the experimental results shows an error of less than 0.5%. The behavior of the inductance of the coil indicates that the optimum structure of this coil, with the stored magnetic energy as the optimization function, is obtained when the SCs are located on the toroidal planes.

Multiple-input-multiple-output (MIMO) radar is attractive for target detection, parameter identification, and target classification due to diversity of waveform and perspective. However, the mutual interference among the waveforms may lead to performance degradation in resolving spatially close returns. In this paper, we consider the use of space-time coding (STC) to mitigate the waveform cross-correlation effects in MIMO radar. First, it turns out that a joint waveform optimization problem can be decoupled into a set of individual waveform design problems. Second, a number of monostatic waveforms can be directly used in a MIMO radar system, which offers flexibility in waveform selection. We provide conditions for the elimination of waveform cross correlation, and discuss four kinds of space time codes. In addition, we also extend the model to partial waveform cross-correlation removal based on waveform set division. Numerical results demonstrate the effectiveness of STC in MIMO radar for waveform decorrelation.

Pulse compression is used in radar systems to improve range resolution while maintaining a high duty cycle. In addition to practical implementation constraints, the key issues for the selection of a pulse-compression waveform are mismatch loss, peak / integrated range sidelobes, and Doppler tolerance. While much progress has been made in the design of nonlinear frequency modulated (FM) chirp waveforms satisfying these requirements, the corresponding performance for binary phase-coded waveforms is often inadequate. In order to improve the range sidelobes achieved with phase-coded waveforms, specially designed mismatched pulse compression filters can be used. Several such approaches have been described in the literature since 1959. This paper review these techniques and highlight a particular approach using infinite impulse response (IIR) filters, which has received little attention in the past. Using this technique the performance for a number of binary phase codes of different length have been determined and their Doppler tolerance is investigated.

Linear frequency modulated (LFM) excitation combined with pulse compression provides an increase in signal to noise ratio (SNR) at the receiver. LFM signals are of longer duration than pulsed signals of the same bandwidth. Consequently, in many practical situations, maintaining temporal separation between echoes is not possible. Where analysis is performed on individual LFM signals, a separation technique is required. Time windowing is unable to separate signals overlapping in time. Frequency domain filtering is unable to separate signals with overlapping spectra. This paper describes a method to separate time overlapping LFM signals through the application of the fractional Fourier transform (FrFT), a transform operating in both time and frequency domains. A short introduction to the FrFT, its operation and calculation are presented. The proposed signal separation method is illustrated by application to a simulated ultrasound signal, created by the summation of multiple time overlapping LFM signals and the component signals recovered with ±0.6% spectral error. The results of an experimental investigation are presented where the proposed separation method is applied to time overlapping LFM signals, created by the transmission of a LFM signal through a stainless steel plate and water-filled pipe.

We review the field of femtosecond pulse shaping, in which Fourier synthesis methods are used to generate nearly arbitrarily shaped ultrafast optical wave forms according to user specification. An emphasis is placed on programmable pulse shaping methods based on the use of spatial light modulators. After outlining the fundamental principles of pulse shaping, we then present a detailed discussion of pulse shaping using several different types of spatial light modulators. Finally, new research directions in pulse shaping, and applications of pulse shaping to optical communications, biomedical optical imaging, high power laser amplifiers, quantum control, and laser-electron beam interactions are reviewed.

Pulse compression theory and techniques were developed originally for radar systems but could be carefully adopted for ultrasound-specific problems. In past ten years a considerable amount of research has been done in the area of mismatched filtering for the purpose of its application in ultrasound. However, in a similar way to radar systems for distributed targets, pulse compression has not achieved a success of penetration in commercially available systems.

The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density (Ͻ2ϫ10 16 cm Ϫ3 ) with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz ͑far-infrared͒ pulses of several hundred femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model (6540 cm 2 V Ϫ1 s Ϫ1 at room temperature with Nϭ1.6ϫ10 16 cm Ϫ3 , and 13600 cm 2 V Ϫ1 s Ϫ1 at 70 K with Nϭ1.5ϫ10 16 cm Ϫ3 ) are in good agreement with literature values. There are, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the ⌫ valley due to LO-phonon-assisted intravalley absorption. At long delay times ͑5-900 ps͒, the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is 8.5ϫ10 5 cm/s.

Digital Signal Processing is a curriculum closely integrated by theory, implementation and application. With the development of microelectronics technology in recent years, the emergence of variety of chips makes digital signal processing widely used in various fields. Therefore, almost all of the electronic and computer engineering departments are now offering the digital signal processing courses. The main content of this course is abstract signal processing and transforming, involving a large number of mathematical knowledge and basic theory. The Matlab software can make the signal be presented in the form of visualized graphic image. Besides, FPGA, DSP and other high-performance chips can make digital signal processing technology widely used in various fields. Leading Matlab and FPGA into the teaching of digital signal processing is useful to improve the students' mastery of the knowledge points and enhance interest in learning. In this paper, for the purpose of radar real-time processing system application, methods of using Matlab and FPGA for teaching knowledge points such as bandpass sampling, polyphase filters, FFT processing, etc. are introduced.

A high resolution direction-of-arrival algorithm for narrow-band coherent and incoherent sources," &EE Trans. Acoust., Speech, Signal Processing, vol. 36, pp. 965-979* July 1988. S . Mayrargue, "On the common structure of several well-known methods for harmonic analysis and direction-of-arrival estimation induced by a new version of ESPRIT," in Abstract-This correspondence derives the Cramer-Rao lower bound for the variances of the estimated parameters of complex signals with constant amplitude and polynomial phase, measured in additive Gaussian noise. Signals of this type appear in many radar applications, and the CRLB formulas have practical importance in these applica-

We design As 2 Se 3 and As 2 S 3 chalcogenide photonic nanowires to optimize the soliton self-compression with short distances and ultralow input pulse energy. We numerically demonstrate the generation of single optical cycle in an As 2 S 3 photonic nanowire: a 5.07 fs compressed pulse is obtained starting from 250 fs input pulse with 50 pJ in 0.84 mm-long As 2 S 3 nanowire. Taking into account the high two photon absorption (TPA) coefficient in the As 2 Se 3 glass, accurate modeling shows the compression of 250 fs down to 25.4 fs in 2.1 mm-long nanowire and with 10 pJ input pulse energy.

We demonstrate the generation of optical vortices with radial or azimuthal polarization using a space variant polarization converter, fabricated by femtosecond laser writing of self-assembled nanostructures in silica glass. Manipulation of the induced form birefringence is achieved by controlling writing parameters, in particular, the polarization azimuth of the writing beam. The fabricated converter allows switching from radial to azimuthal polarization by controlling the handedness of incident circular polarization.

Dichroism experiments with 150 fs time resolution on anthracene in benzyl alcohol are presented as a function of viscosity from 14.4 cP ͑274 K͒ to 2.7 cP ͑329 K͒. These measurements test a qualitative prediction of the viscoelastic picture of liquid dynamics, specifically that earlier ''inertial'' dynamics have a viscosity independent rate, whereas later ''diffusive'' dynamics have a rate directly proportional to viscosity. This paper focuses on two components of the dichroism decay that are assigned to rotational motion. A third component is assigned to electronic-state solvation and is analyzed in a companion paper ͓J. Chem. Phys. 115, 4231 ͑2001͔͒. The longest component is due to rotational diffusion and is very well described by a hydrodynamic model with slip boundary conditions. A fast decay component in the subpicosecond region is found and shown to have a viscosity-independent rate. It is assigned to inertial rotation by comparison to the computer simulations of Jas et al. ͓J. Chem. Phys. 107, 8800 ͑1997͔͒. Inertial rotation extends out to at least 1 ps, longer than the range commonly assumed for inertial dynamics. Over much of this range, the inertial rotation is not free-rotor-like, but is strongly modified by interaction with the solvent. The inertial rotation also accounts for the ''missing'' anisotropy found when the rotational diffusion fits are extrapolated to zero time.

Stimulated Brillouin Scattering (SBS) imposes a limitation on the transmitted power in radioover-fiber (RoF) distribution systems. By employing an optical filter at the transmitter that converts the optical microwave signal to Single Side Band (SSB) format, in order to overcome dispersive fading, it is also possible to reduce the effect of SBS on system performance. In this paper the authors compare the cases of filtering the optical microwave signal at the transmitter or the remote site of a radio-over-fibre system, in terms of the effect of SBS on system performance. The authors also investigate and demonstrate how the modulation depth of the optical-microwave data signal influences the Brillouin threshold in RoF distribution systems.

Pulse compression technique is most widely used in radar and communication areas. Its implementation requires an optimized and dedicated hardware. The real time implementation places several constraints such as area occupied, power consumption , etc. The good design needs optimization of these constraints. This paper concentrates on the design of optimized model which can reduce these. In the proposed architecture a single chip is used for generating the pulse compression sequence like BPSk, QPSk, 6-PSK and other Polyphase codes. The VLSI architecture is implemented on the Field Programm-able Gate Array (FPGA) as it provides the flexibility of reconfigurability and reprogrammability .It was found that the proposed architecture has generated the pulse compression sequences efficiently while improving some of the parameters like area, power consumption and delay when compared to previous methods.

Numerical results for the second-and third-order dispersion in PW prism-sequence (double prism-pair) with arbitrary apexangle by ray-tracing method are presented. The in¯uence of the prism separation distance and apex-angle on second-and thirdorder dispersion is discussed. In certain conditions, the PW prism-sequence with small apex-angle (208) is a better choice to reduce third-order dispersion than single prism-pair without requiring excessive length. As a less dispersion material, quartz is suitable to be used with the smallest possible value of third-order dispersion. This result could be the basis of dispersion compensation and pulse compression. 7

In this paper a comparative analysis of sidelobe levels at matched and mismatched filters depending on input signal-to-noise ratio is presented. The analysis was performed for various types of sequences. The sidelobe level of the mismatched filter output is lower than the sidelobe level of the matched filter output up to some threshold level of the input signal SNR value. Below that level, the sidelobe level is determined by the noise level and the difference between filters elapses.

Amplified dispersive Fourier transformation ͑ADFT͒ is a powerful technique that maps the spectrum of an optical pulse into a time-domain waveform using group-velocity dispersion ͑GVD͒ and simultaneously amplifies it in the optical domain. It replaces a diffraction grating and detector array with a dispersive fiber and single photodetector, greatly simplifying the system and, more importantly, enabling ultrafast real-time spectroscopic measurements. Here we present a theory of ADFT by deriving the general equation and spectral resolution for ADFT and studying the evolution of the pulse spectrum into time, the effect of GVD coefficients on ADFT, and the requirement for dispersion. This theory is expected to lend valuable insights into the process and implementation of ADFT.

This article reviews the state-of-the-art in high-power microwave source research. It begins with a discussion of the concepts involved in coherent microwave generation. The main varieties of microwave tubes are classified into three groups, according to the fundamental radiation mechanism involved: Cherenkov, transition, or bremsstrahlung radiation. This is followed by a brief discussion of some of the technical fundamentals of high-power microwave sources, including power supplies and electron guns. Finally, the history and recent developments of both high-peak power and high-average power sources are reviewed in the context of four main areas of application: ͑1͒ plasma resonance heating and current drive; ͑2͒ rf acceleration of charged particles; ͑3͒ radar and communications systems; and ͑4͒ high-peak power sources for weapons-effect simulation and exploratory development.

In this paper, a system analysis of needs and possible application of DIRLS mismatched filter in portable surveillance radar PR-15 is presented. Transmitting pulses are binary phase intra-pulse modulated. Because of limited peak power, the usage of longer radar pulses is necessary in long distance mode of operation. Applied binary M sequences have insufficient good autocorrelation properties so a method of side-lobs suppression became needed. We designed and verified the signal processing unit for this kind of compressors on a digital radar signal synthesis and processing platform based on DDS and FPGA technology.

This letter presents a method aimed at quantifying the dimensions of the heat-affected zone ͑HAZ͒, produced during nanosecond and femtosecond laser-matter interactions. According to this method, 0.1 m thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes ͑i.e., 5 and 2 J/cm 2 , respectively͒. The grain size in the samples was observed near the microholes. The main conclusion is that a 40 m wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 m, which is our observation limit.

In this paper a novel method for a potential range resolution improvement is proposed, based on the received signal oversampling and the mismatched filter design. In the synthesis of the mismatched filter with improved resolution an algorithm for maximal sidelobe suppression has been used. This idea can be applied in high resolution radar and sonar design.

We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7-300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

When ultrafast noncritical cascaded second-harmonic generation of energetic femtosecond pulses occur in a bulk lithium niobate crystal optical Cherenkov waves are formed in the near-to mid-IR. Numerical simulations show that the few-cycle solitons radiate Cherenkov (dispersive) waves in the λ = 2.2 − 4.5 µm range when pumping at λ 1 = 1.2 − 1.8 µm.