For the first time nearly bandwidth-limited 10-fs laser pulses centered at 790 nm are generated in a Ti:sapphire ring resonator. Intracavity dispersion control is provided by dispersive dielectric mirrors together with the insertion of small amounts of intracavity glass path. Mode- locking is self-starting without the use of an aperture.

`Real-world' applications of femtosecond pulses require laser sources that are reliable, compact, and easy-to-use. Diode-pumped lasers are one key step in this direction, and as a next key step we have developed and demonstrated a simple technique using semiconductor saturable absorbers to passively start and stabilize mode-locked lasers. Because the saturable absorber stabilizes soliton modelocking, we achieve self-starting modelocking over a wide cavity stability range, in contrast to KLM, which tends to require critical cavity alignment and is usually not self-starting. We discuss different saturable absorber designs. An A-FPSA is used in a diode-pumped Nd:glass laser (130 fs, 100 mW avg output power) and a Cr:LiSAF laser (45 fs, 80 mW output). A thin saturable absorber design provides self-starting mode- locking over a wavelength range of 30 nm, and a low-loss design supports a record mode- locked output power of 120 mW from a Cr:LiSAF laser. A dispersive saturable absorber mirror design combines both negative dispersion compensation and saturable absorption within one semiconductor device and produces a compact mode-locked Cr:LiSAF laser (160 fs, 25 mW) without the need of prisms for dispersion compensation. Finally, we present an optimized diode-pumped cavity layout for Cr:LiSAF which allows for higher output powers (> 1 W cw).

We studied both experimentally and theoretically, the conversion efficiency of 150-fs-duration Ti:sapphire laser pulses to the second, third and fourth harmonic. The laser pulses at the wavelength of 780 nm have a spectral width of 10 nm and were amplified to 0.8 mJ energy in a regenerative amplifier. With a repetition rate of 1 kHz intensity of up to 150 GW/cm² could be reached by focusing the beam with a lens. KDP, BBO and LilO₃ crystals of different length were used for the conversion process. The experimentally observed conversion efficiency to the second, third and fourth harmonic does not exceed 50%, 15% and 8%, correspondingly. It is shown theoretically that one possible process limiting the conversion efficiency of the femtosecond pulses at low as well as high intensity is the modulation of the phase of the fundamental wave.

We report what is to our knowledge the first generation of femtosecond VUV radiation tunable in the spectral range from 102 - 124 nm generated by two photon-photon resonant and near resonant four-wave difference-frequency mixing ((omega) _DF equals 2(omega) _P - (omega) ₁) in krypton and argon. The FWDFM process was pumped by intense 250 fs, 193 nm pulses ((omega) _P) derived from the 80 fs output pulses of a 0.1 TW Ti:sapphire laser system by cascaded fourth harmonic generation in three BBO crystals and subsequent amplification in an ArF gain module. The synchronized injection pulses ((omega) ₁) were generated in an travelling wave optical parametric system driven by the second harmonic of the Ti:sapphire output. Frequency up-conversion of the pulses at (omega) ₁ allows the extension of the tuning range also to lower frequencies at (omega) _D.

A three stage optical parametric amplifier is pumped by a 250-kHz (mu) J level Ti:sapphire regenerative amplifier system. Broad-bandwidth pulses are produced by tuning each stage of the OPA system to amplify adjacent frequency components of a white-light continuum source. These broad-band pulses may be compressed with a standard prism compressor to pulse- widths of less than 30 fs.

We demonstrate a femtosecond laser chain that generates 20 TW at 10 Hz in less than 50 fs with focused intensities higher than 3 X 10¹⁹ W/cm². This system is optimized for a broad transmission bandwidth and includes an aberration-free stretcher compressor.

We report on a Ti:sapphire amplifier system which produces 21 fs, 1 mJ pulses at 1 kHz. A nanojoule-level seed pulse makes 8 passes through a highly doped Ti:sapphire crystal for a total gain of approximately 10⁶. The crystal is pumped with 12 mJ of 527 nm light which is converted into amplified pulse energy with an efficiency exceeding 10%. We have used this laser system to generate ultra-short UV light pulses through third harmonic generation. By focusing the 21 fs pulses in argon and other gases (including air), we obtain UV pulses with energy 1 (mu) J and wavelength 267 nm. The duration of the UV pulses was measured to be approximately 16 fs, representing the shortest pulses in this wavelength range measured to date.

An ultrafast modelocked semiconductor laser diode system has been used to seed a flashlamp pumped Cr:LiSAF regenerative amplifier system, producing subpicosecond pulses with millijoule output pulse energy. This system has the potential to eliminate argon ion pumped based ultrafast laser systems.

A diode-pumped Nd:fluorophosphate regenerative amplifier was developed. Chirped seed pulses were amplified from between 54 (mu) J at 100 Hz to 5 (mu) J at 10 kHz and compressed to 850 femtoseconds. The effects of continuous-wave pumping on the pulse energy at high repetition rates are also presented.

Techniques for the production of multiterawatt, sub-20-fs, optical pulses via chirped pulse amplification are discussed. Regenerative pulse shaping is used to control gain narrowing during amplification and an optimized, quintic-phase-limited, dispersion compensation scheme is used to control higher order phase distortions over a bandwidth of approximately 100 nm. Transform-limited, 18-fs pulses of 4.4-TW peak power have been produced in a Ti:sapphire- based, chirped pulse amplification system at a repetition rate of 50 Hz. Extensions to shorter durations and peak powers approaching 100 TW are also described.

The availability of ultrashort (100 fs or shorter) high intensity lasers capable of producing focused intensities in excess of 10¹⁸ W/cm² provides many new opportunities in studying the interaction of radiation with matter. In this paper we describe and compare three methods for generating these high intensity pulses into the 100 TW power range. In addition, we describe in detail the use of Cr:LiSAF as an amplifying medium for this type of laser system. This work also includes descriptions of the factors that affect the performance of short pulse laser systems, especially with regard to Cr:LiSAF. Techniques of how to overcome the obstacles created by these factors are also discussed together with experimental data regarding the bandwidth limits of an existing regenerative amplifier.

We report on the development of a Chirped Pulse Amplification system in which the final amplifier is a large Ti:Sapphire disk pumped by a frequency doubled Nd:Glass laser. Using this amplifier we have produced near diffraction limited 120 fs pulses with energies in excess of 1 J. Focusing of these pulses results in peak irradiances exceeding 5 X 10¹⁹ W/cm². The scalability of this amplifier to the 10 - 20 J level is also discussed.

We extend the Frequency-Resolved Optical Gating (FROG) technique to the measurement of the time-dependent intensity and phase of arbitrary ultraweak ultrashort pulses. We accomplish this by combining FROG with spectral interferometry. Spectral interferometry (SI) yields the phase difference between an unknown and a known pulse (the latter characterized with FROG). SI is a linear-optical method and hence is very sensitive. We demonstrate the combination of these two techniques by measuring the intensity and phase of a train of pulses 42 zeptojoules in energy (1/5 of a photon per pulse).

We have developed a spectral phase measurement technique that provides a direct, sensitive, fast, and discontinuous phase measurement. We demonstrated this technique by measuring the quadratic, cubic, and 0-(pi) phase distortions.

We show theoretically and experimentally that a simple modification of the dynamic spectrogram may be used to characterize a stochastic ensemble of ultrashort optical pulses. The method makes use of the temporal analysis of spectral components to measure an intuitive spectrogram of the ensemble using a low order nonlinear technique. Pulse-shape reconstruction is demonstrated via an iterative inversion algorithm for an ensemble of identical input pulses. For pulses that are not identical, we measure for the first time a representation of the two-time correlation function of the pulsed field, using a non-iterative decorrelation procedure.

We describe an optical detection system for simultaneous time- and frequency-resolved measurements: the Balanced-Homodyne Chronocyclic Spectrometer (chrono equals time; cyclic equals frequency). This system uses balanced, optical homodyne detection, with a wavelength- tunable, pulsed local-oscillator (LO) field to time resolve the spectrum of weak light pulses. The LO field defines the time and frequency window in which the signal field is sampled. The method time resolves the photon statistics as well as the mean intensity. Measurement examples are given for: (1) Temporal oscillations of laser pulses transmitted through a semiconductor quantum well in an optical microcavity and (2) The time-frequency profile of a linearly chirped ultrashort laser pulse.

We consider interferometric techniques for capturing ultra-fast pulsed images. We analyze the signal-to-noise performance and information capacity of pulsed image detection systems and we briefly discuss the possibility of improving detection systems using spectral holographic image capture.

We present results of second harmonic generation (SHG) frequency-resolved optical gating (FROG) measurements on the mid-IR free-electron laser (FEL) at the Stanford Picosecond FEL Center. These are the first SHG FROG measurements performed in the mid-IR or on an FEL. The observed pulses have an optical wavelength near 5 micrometers , and the field profiles reconstructed from the FROG trace exhibit narrow-line absorption and free-induction decay caused by atmospheric water vapor. The SHG FROG signal is easier to isolate than for the polarization gate geometry; hence the SHG traces are not corrupted by a residual background. The experiment used only 10% of the full laser power, and the spectrum and autocorrelation were quickly calculated from the FROG trace, demonstrating the feasibility of using SHG FROG as a real-time diagnostic for the FEL facility.

Frequency-resolved optical gating (FROG) is a method for measuring the intensity and phase of an ultrashort laser pulse. The technique involves producing a `FROG trace,' which is a type of spectrogram of the pulse, and an iterative phase-retrieval algorithm that determines the intensity and phase of the laser pulse. Although the iterative FROG algorithm performs well, it requires a minute or more to converge for complex pulse shapes. For many applications more rapid retrieval is important, and it is therefore desirable to have a direct, i.e., non- iterative, computational method capable of inverting the highly non-linear and complex function that relates the pulse intensity and phase to its experimental FROG trace. In previous work we showed that a neural network can retrieve simple pulses rapidly and directly. Unfortunately, this approach involved feature extraction by computing the integral moments of the FROG trace, making it particularly sensitive to the presence of additive noise. Using parallel-processing hardware, we are now able to use FROG traces of limited size without any feature extraction as input for a neural net. This gives us the opportunity to compare the network performance using raw data with other representations of the FROG traces. Particularly interesting seemed a representation of the traces in terms of their wavelet coefficients, because the wavelet transform is known for its noise-insensitivity and the compression of most information about a given signal in only a small number of wavelet coefficients, making it therefore appealing for feature extraction in signal processing applications. We found, however, that a representation of the FROG trace in terms of its Lemarie-wavelet coefficients did not yield convergence despite many attempts, and hence is not suitable as an input signal for simple neural networks. Fortunately,--and surprisingly--use of no feature extraction appears quite promising.

Full characterization of the electric field amplitude and shape of an ultrashort optical pulse is important for a number of fields of research, such as high-field atomic and plasma physics, and the generation of ultrashort-pulse XUV and soft x-ray radiation. We have used the technique of frequency-resolved optical gating (FROG) to characterize low-energy pulses as short as 13 fs duration, and high peak-power pulses as short as 25 fs. We discuss experimental considerations for implementing FROG for very short pulses, and the limitations of the technique.

We show that a pair of grisms (transmission gratings written onto prisms) exhibits second- and third-order dispersion opposite to that of an optical fiber. Using a fiber stretcher and a grism- pair compressor, we demonstrate chirped-pulse amplification and transform-limited compression of 135-fs pulses. This scheme can be used to produce 50-fs, microjoule pulses in a compact and simple system.

We propose a method for compensating phase dispersion in chirped pulse amplifier systems which is based on adaptive optics. By placing a feedback-controlled arbitrarily deformable mirror in the stretcher where frequency components are highly dispersed, arbitrarily high- order phase dispersion within the amplifier can be canceled by introducing the appropriate phase delay. To demonstrate this concept, we model a 10 fsec chirped pulse amplifier and show that transform limited pulses are obtainable. We also show that deleterious effects such as angular beam divergence and spatially dependent broadening are negligible. Finally, we discuss design considerations for a 1D adaptive optic mirror.

Regenerative pulse shaping is used to overcome gain narrowing during ultrashort pulse amplification. We have demonstrated multiple spectral filters for broadening the amplified spectrum. We have produced amplified pulses with an energy of approximately 5 mJ and bandwidths of approximately 100 nm, or nearly 3 times wider than the gain narrowing limit of Ti:sapphire.

We propose a dielectric-based, multistaged, laser-driven electron linear accelerometer microstructure operating in a vacuum that is capable of accelerating electrons to 1 TeV in one kilometer. Our study shows that a GeV/m gradient is achievable using two 100 fsec focused crossed-laser-beams, repeated every 300 micrometers , operated at a peak power of 0.2 GW and an energy density of less than 2J/cm² on the accelerator structure.

An extension to dilute systems of the measurement of the non-linear refractive index by spectral analysis after non-linear propagation is presented. We show that one can deduce the magnitude and temporal characteristics of the induced nonlinearity in air and other gases. Our results obtained with femtosecond pulses are compared to those measured using much longer pulses.

The change in phase of ultrashort optical pulses induced by travelling through nonlinear media is used to determine the nonlinear index of refraction and to study the dynamics of femtosecond optical-field-ionization of noble gases.

We have studied nonlinear interactions of optical pulses comprising a small number of optical cycles with quantum particle systems. For model intraatomic potentials we have monitored nonlinear medium responses with characteristic features for a variety of processes, including Kerr effect, harmonic generation and blue-shifting, atom excitation and ionization, and subfemtosecond XUV burst production. The efficiency and the hierarchy of these processes have been investigated.

We propose a method for producing sub-femtosecond VUV pulses by compressing the high order harmonic radiation emitted from a nonlinear medium driven by an intense, ultrafast laser pulse. We present single atom calculations of the harmonic amplitudes and phases as well as simulations of a VUV compressor. Single harmonics from a 27 fs driving pulse can be compressed to less than 2 fs duration, and several adjacent harmonics with similar phase structure can be combined within the same compressor to produce an attosecond pulse.

A novel method to suppress the pedestal of high-power ultrashort laser pulses by use of self- focusing is proposed and demonstrated. Both the steady-state self-focusing and the transient self-focusing are explored. Numerical results show that the intensity contrast ratio of better than 10⁸ can be obtained with this method.

The nonlinear effects on chirped-pulse amplification and recompression are modeled for different levels of amplifier saturation. It is shown that gain saturation and self-phase modulation act together to significantly distort the recompressed pulse after amplification, and a large linear chirp of input pulse contributes to suppress the pedestal of recompressed pulse induced by self-phase modulation in gain saturation regime. Spectral distortion due to gain saturation also has a stronger effect on the low intensity pedestal of recompressed pulse.

The possibility for compression of self-modulated pulses with Joule energy and duration about 1 psec to durations less than 100 fsec is shown. Variation of the compression degree and of contrast of such pulses was studied in dependence on their shape, parameters of the medium, type of applied compressor, and limitations by competing nonlinear processes. The effect of supercompression of self-modulated and modulated pulses by means of a compressor of a special type was found. The possibility of SBS compression of laser pulses with super-high power was found.

One consider the amplification of pulses of coherent light in the resonant three-level active media that is excited by short power pulse of coherent light according to the common-upper- level double resonance scheme. The radiation with major frequency (pump) is supposed to be stronger than one with minor frequency (signal). The self-induced transparency is supposed to be established in the pump channel. The exponential growth of stationary shape short signal pulses is studied mainly. One shows that when any described here conditions for initial signal pulse shape are meeting the stationary gained signal pulse of complicated form can be formed. The gains of these pulses are less than ones for signal pulses of quasi-hyperbolic-secant form. Therefore in the case of incomplete condition meeting the competition between the exponentially growing signal pulses with different gains and shapes leads finally to the overwhelming of signal pulse with quasi-hyperbolic-secant-shape form. The effect of inhomogeneous broadening of spectral transition lines on the parameters of signal pulses was studied too. Evaluations that were carried out for any atomic gases show that described in this paper phenomena can be produced at realistic levels of power and duration of pump and signal pulses.