An overview of the basic principles, major theoretical aspects and technological issues of self- mode-locked solid state lasers is presented, with particular emphasis on recent advances in femtosecond Ti:sapphire lasers.

Optical thin film structures exhibiting high reflectivity and nearly constant negative group- delay dispersions, or optionally superimposed higher-order dispersions over frequency ranges beyond 80 THz are presented. This attractive combination makes these special laser mirrors well suited for intra- or extracavity dispersion control in broadband femtosecond laser systems. We address design issues and the principle of operation of these novel devices. Spectrally resolved white-light interferometry with high time and spectral resolution was used to measure the dispersion of the deposited mirrors. Experimental results on the operation of femtosecond laser systems utilizing dispersion compensating mirrors are given.

The development of low-cost and potentially compact femtosecond laser oscillators and amplifiers is discussed. The first all-solid-state femtosecond Cr:LiSAF laser is described. A tunable femtosecond solid-state amplifier system pumped by only 3 W of 488 nm argon ion radiation has been demonstrated to deliver microjoule pulses at repetition rates up to approximately 20 kHz, with a maximum pulse energy of 14 (mu) J obtained at 5 kHz. The first all-solid-state, tunable, diode-pumped Cr:LiSAF regenerative amplifier, which amplifies femtosecond pulses to energies exceeding 1 (mu) J at up to 16 kHz repetition rate, is also reported.

We are developing a concept where the ultrashort pulse laser operation can be described and understood in terms of spatio-temporal dynamics of light pulses in equivalent continuous media. This concept is based on studying the evolution of a seeding pulse with the duration less than the laser round trip time and treats the mode-locking process as a temporal squeezing of the pulse in the equivalent non-linear medium. The set-up of the steady-state lasing corresponds to the formation of a stable localized wave packet, a so-called autosoliton, starting from arbitrary initial field distributions. In this report, we classify the autosolitons with respect to key ultrashort lasing parameters such as gain and absorption saturation, gain band width and the proximity of the group-velocity dispersion to zero. We distinguish between solitons with weak absorption saturation, solitons with strong absorption saturation, and superluminous solitons. The scenarios of autosoliton formation are investigated by analytical and computational methods and an attempt is made to put the investigated soliton processes in accordance with the known regimes of femtosecond laser operation.

We review the application of time-frequency concepts in the field of ultrashort laser science. We consider the Wigner distribution, the spectrogram, and the sonogram, and show how each of these distributions are used in ultrafast laser science. We also plot four example pulses in each of the three representations.

Recent developments in frequency-resolved optical gating include an analysis of the iterative algorithm's performance in the presence of noise and the development of a direct, rapid retrieval method using a computational neural network.

We present a discussion of several different types of joint time-frequency distributions of optical pulses. Particular attention is paid to the Wigner distribution W(t,(omega) ), as it is the fundamental distribution from which all others can be derived. We elucidate the relationship between the Wigner distribution and other spectrograms of current interest, such as that obtained from frequency-resolved optical gating (FROG).

We review the design of an LBO optical parametric oscillator (OPO) synchronously pumped by a cw mode-locked Ti:sapphire laser. The synchronously pumped OPO produces 100 fsec pulses at 1.3 and 1.5 micrometers as well as near 600 nm using frequency doubling. We also demonstrate the synchronization of the pump, signal and idler pulses on the 100 fsec time scale. We then review applications that utilize these capabilities. These applications include studies of: InGaAs absorbers, GaAs quantum wells, electro-optic modulators, soliton-soliton interactions, energy transfer in photosynthetic bacteria, uncaging of biological molecules using confocal microscopy, electronic distributions and two photon fluorescence in dyes.

We have designed and experimentally demonstrated a broadband femtosecond optical parametric amplifier of white-light continuum producing ultrashort light pulses. By using different nonlinear processes femtosecond pulses are generated from 2 micrometers down to 250 nm. Frequency-resolved-optical-gating technique is used to characterize the compressed pulses.

Femtosecond pulse generation based on Kerr-type and carrier-type modelocking of Nd³⁺ doped fiber lasers is discussed. Special emphasis is to be payed to the startup conditions and the methods used to initiate the modelocking process.

We present a detailed description of a passive harmonically mode-locked laser. Experimental results are consistent with the suggestion of a passive self-stabilization effect driven by transverse acoustic wave excitation due to electrostriction. We also demonstrate some applications of the laser.

Compact sources of high energy ultrashort pulses are described. Femtosecond and picosecond optical pulses with microjoule energies are obtained using chirped-pulse fiber amplifiers. Mode-locked fiber lasers and fast-tuned laser diodes are used to generate initial pulses for amplification. Efficient frequency conversion of amplified pulses is demonstrated and microjoule second-harmonic pulses are produced. The first all-fiber chirped pulse amplification circuit is demonstrated. It uses in-fiber chirped Bragg gratings, which replaces conventional diffraction-grating compressors and stretchers.

We have demonstrated a temporal imaging system with a novel time lens which magnifies 100 Gb/s optical data by a factor of twelve. The function of a time lens is to impart a quadratic phase modulation or linear frequency sweep to the waveform under study. Our approach to achieving time lens action is to up-convert the waveform under study using a linearly swept pump, thus imparting a linear frequency sweep to the waveform. This technique allows for much greater frequency sweep rates and hence shorter focal times than can be obtained with electro-optic modulators. Additionally, the increased bandwidth that can be obtained optically instead of electro-optically should result in higher resolution in a temporal imaging system.

We present results of frequency resolved optical gating (FROG) measurements on the superconducting accelerator (SCA) mid-IR free-electron laser (FEL) at Stanford University. FROG retrieves complete amplitude and phase content of an optical pulse. First, we review the properties of FELs including the ability to tune wavelength and pulse length. In addition, the electron beam driving the FEL often affects the optical pulse shape. The SCA mid-IR FEL currently operates at wavelengths between 4 micrometers and 10 micrometers and its pulse length can be varied from 700 fs to 2 ps. We then describe details of the experimental layout and procedures particular to FELs and to the mid-IR. Finally, we show FROG measurements on the FEL including examples of nearly transform limited pulses, frequency chirped pulses, and pulses distorted by atmospheric water vapor absorption.

Microjoule pulse energies are achieved from a single stage upconversion fiber amplifier for the first time in this demonstration of chirped pulse amplification using a multimode Tm:ZBLAN fiber. A Ti:sapphire laser system provides the seed pulse for the upconversion fiber amplifier which produces subpicosecond pulse trains with energies as great as 16 (mu) J at repetition rate of 4.4 kHz. The compressed pulse peak power is more than 1 MW, and the pulse is characterized both temporally and spatially.

The single-mode fibers with chromatic dispersion varying along the length are a novel medium for non-linear optics. For optical soliton, a small dispersion variation perturbs soliton in the same way as an amplification or loss. The fibers with varying dispersion can have a lot of application in the soliton propagation control. Such fibers allow us to realize both the regime of effective amplification and the effective compression of optical solitons. This method needs the precise control of the varying fiber core diameter and for the first time has been implemented.

We discuss the method of time-resolved spectral phase measurement (TRSPM) as a technique for complete characterization of the electric field of a short optical pulse. Several phase retrieval algorithms, both deterministic and iterative, are compared.

Using an inverse propagation algorithm it is possible to determine the phase and the amplitude of ultrashort optical pulses in a single laser shot by analysis of the pulse spectrum after passage through a medium of known nonlinear response. Demonstration of the potential of the method is given for a Ti:Sapphire oscillator-amplifier chain.

In analogy to Young's double slit interference, the phase of the temporal beat between two oscillators with different frequencies is precisely the spectral phase difference between those oscillators. We present a technique that directly measures the spectral phase of femtosecond optical pulses using this double slit interference principle. A pair of narrow slits in a thin opaque sheet select two spectral frequencies from the femtosecond pulse spectrum in a zero dispersion pulse stretcher. Measurement of the temporal phase of a family of beat frequencies obtained over a range of slit spacings yields the desired spectral phase directly. We demonstrate this technique by accurately measuring the quadratic phase added to 80 femtosecond optical pulses by a 6.5 cm block of BK-7 glass.

The intensity and phase of the approximately 10 femtosecond pulse from a self-modelocked Ti:sapphire laser was reconstructed using frequency-resolved optical gating (FROG). Our results verify recent models which show that uncompensated higher-order dispersion in the laser cavity is the main limitation on pulse duration.

Recent advances in the shaping of ultrafast optical waveforms using liquid crystal (LC) spatial light modulators (SLM) are presented. Two LC SLMs are used in a novel arrangement to produce programmable waveforms with specified time-dependent amplitude and temporal phase profiles with the greatest fidelity and complexity to date. The apparatus is also used to demonstrate the generation of an ultrafast waveform with a programmable time-dependent polarization profile. A general theoretical result that describes the space-time electric field profile of waveforms shaped by the spectral filtering of spatially separated frequency components is also presented. The main result is that diffraction gives rise to a translational spatial shift in the electric field profile that varies linearly with time along the shaped waveform.

We demonstrate the shaping of ultrabroad bandwidth femtosecond optical pulses with temporal resolution approaching 10 fsec and explore the limitations of femtosecond ultrabroad bandwidth pulse shaping. Using a commercially available liquid crystal spatial light modulator within a reflective optics pulse shaping apparatus, we synthesize a variety of complex waveforms and show that it is possible to compensate for large amounts of high order phase dispersion by appropriate cubic and quartic phase modulation of the pulse. Finally, we theoretically examine inherent limitations of pulse shaping. Surprisingly, Fourier transform pulse shaping techniques are robust and can be used for pulses with 5 fsec durations, beyond the current capabilities of ultrafast laser systems.

We have used the direct optical spectral phase measurement (DOSPM) technique to characterize the cubic phase tuning ability of our pulse stretcher. We have compared the measured phase to the phase determined from cross-correlation measurements.

The Trident Nd:glass ICF laser at Los Alamos National Lab may be operated in a mode that produces high energy ultrashort pulses by the chirp/compression method. The 125-ps pulses from a standard mode-locked, Nd:YLF oscillator are first frequency-broadened to 3-nm bandwidth, chirped in a quartz fiber, and then compressed with a grating pair to 1.5 ps. A second quartz fiber then provides nonlinear polarization rotation for background and satellite suppression and to further broaden the spectrum to 4.5 nm. Pulses are chirped again to 500 ps width with a second grating pair and amplified in a Ti:sapphire regenerative amplifier pumped by frequency-doubled Nd:YAG. Millijoule-level output is then amplified through the existing phosphate glass Trident amplifier chain before compression to < 400 fs. Energy up to 1.5 J with excellent beam quality and contrast ratio is routinely produced by compressing after three rod amplifier stages. Higher energies are possible by compression further along the amplifier chain. Simultaneous use of long (approximately 1 ns) pulses for plasma formation is also possible.

We have developed a chirped pulse amplification system capable of producing femtosecond pulses with energy above one joule. This is accomplished by using a large aperture, flashlamp pumped Cr:LiSrAlF₆ (Cr:LiSAF) amplifier. Optimum design of the 19 mm diameter amplifier results in a single pass gain of 5 with good beam quality. This amplifier produces 1.05 J pulses after compression with a width of < 125 fs at a repetition rate of 0.05 Hz.

Phase and amplitude control during multiterawatt, ultrashort-pulse amplification is discussed. Methods for efficient energy extraction and scaling to 100-TW peak powers are outlined.

We have demonstrated a simple multipass amplifier system that generates pulses as short as 26 fs in duration, with an energy of > 60 mJ per pulse. The design minimizes higher-order dispersion and spectral distortions, and results in a near-transform limited 2 TW peak power output.

We describe a simple method for compensating large amounts of second- and third-order material dispersion, and we present two compact and robust stretcher-compressor systems for microjoule-and millijoule-level chirped pulse-amplification.

The chirped pulse amplification technique combined with the use of different shifted narrow band amplifying media allows us to produce sub-100 TW pulses with duration in the 250 - 300 fs range. By focusing these pulses on target we have obtained peak intensities above 2.10¹⁹ W/cm² with a contrast ratio of 10^-7 - 10^-6 for the nanosecond background.

We report laser-induced damage threshold measurements on pure and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations, (tau) , ranging from 140 fs to 1 ns. Damage thresholds of gold coatings are limited to 500 mJ/cm² in the subpicosecond range from 1053-nm pulses. In dielectrics, qualitative differences in the morphology of damage and a departure from the diffusion-dominated (tau) ^1/2 scaling indicate that damage results from plasma formation and ablation for (tau) <EQ 10 ps and from conventional melting and boiling for (tau) > 50 ps. A theoretical model based on electron production via multiphoton ionization, Joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulsewidth and wavelength scaling of experimental results.

We report focused intensities of > 10¹⁹ Wcm^-2, with subpicosecond operation of the VULCAN Nd:glass laser at the Rutherford Appleton Laboratory using chirped pulse amplification (CPA) techniques. This paper describes novel aspects of the system including: picosecond and subpicosecond diode pumped oscillators; the use of a regenerative amplifier and system optimization. The ultrashort pulse generated from an additive pulse modelocked LMA oscillator was stretched from 0.5 ps to approximately 200 ps in a double pass grating system and amplified from 1 nJ to 50 J, in phosphate glass amplifiers with a final beam aperture of 150 mm diameter. The stretched pulse was recompressed using a pair of gratings (300 X 150 mm, 1740 lines per mm) and focused using an off-axis parabola to avoid nonlinear effects from transmissive optics. The compressed pulse was monitored using a suite of diagnostics to determine the focusability, pulsewidth, and spectrum. We also describe the current system development program, which is in progress and designed to achieve intensities of approximately 10²⁰ Wcm^-2 to target.

Efficient second harmonic conversion (70 - 80%) using Type II and Type I crystals is demonstrated with 400-fs, 1.053-micrometers laser pulses at intensities up to several hundreds of GW/cm². The experimental results generally agree with the predictions of the code MIXER. For the Type II predelay scheme, evidence is obtained of pulse shortening down to approximately 100 fs.

High power femtosecond pulses in the vacuum ultraviolet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. In xenon, four wave mixing processes lead to VUV pulses at 147 nm and 108 nm with pulse energies in the 10 (mu) J range. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. To characterize the pulse width of the VUV pulses at 147 nm and 83 nm, techniques based on plasma-induced defocusing and spectral blue shifting are demonstrated.

The decoupling of the counterpropagating pulses in a passively mode-locked ring laser makes possible the measurement of non-reciprocal phase differences of 10^-6 and index differences of 10^-10, with temporal resolution in the femtosecond range (limited by the pulse width). Active stabilization increases this sensitivity by three orders of magnitude. Applications are found in the measurement of fast electrical transients and small magnetic field splittings.