The applicability of the self-modelocking technique to a range of broadband gain media is reviewed with particular emphasis on lasers that incorporate Ti:Al₂O₃ or Cr:LiSrAlF₆ or NaCl:OH^- color-center crystals. Frequency-conversion schemes, most notably involving optical parametric oscillation and difference-frequency mixing, are also briefly discussed within the general context of femtosecond pulse generation in near/mid-infrared spectral regions. A cavity frequency referenced, self-modelocked laser configuration that affords reduced pulse-timing jitter (i.e., phase noise) is described and preliminary details of schemes which offer compatibility with diode-based optical pumping of Ti:Al₂O₃ and Cr:LiSAF lasers are also included.

It is now well established that Kerr lensing is most efficient at creating short pulses when the cavity is operated near the edges of the geometrical stability zones. In this paper, we describe the effects of the thermal aberration due to laser pumping on the beam shape and on the initiation of the self-mode-locked regime. A simple 1-D analysis of the thermal aberration shows that the phase delay is parabolic near the center of the beam while it is linear in the wings. We show that such an aberration produces the break up of the beam profile when the laser is operated near the edges of the geometrical stability zones. Kerr lensing present in the self-mode-locked regime tends to reduce the beam size in such a way that the non-parabolic contribution of the thermal aberration is not seen by the beam; hence, self-mode locking effectively suppresses beam break up.

We analyze the high power operation of self-mode-locked solid-state lasers. Excessive Kerr lensing is shown to bring the laser cavity out of the limits of geometrical stability; this situation results in temporal fluctuations, and pulse-break-up into many pulses. Kerr lensing can also reduce the size of the beam to such a point where higher-order transverse modes start to oscillate. Sudden changes in beam size are also examined. Experimental results obtained with a Ti:sapphire laser indicates that two-pulse emission can be more stable than one-pulse emission.

We discuss the physical mechanisms involved in kinematic mode locking. The issues of what determines a velocity range for the motion of a cavity mirror in order to get the laser mode locked and why a slight cavity-length mismatch is always required for coupled-cavity systems are addressed.

The emergence of new, efficient, nonlinear crystals and the development of passively mode- locked, solid state lasers, such as the Kerr-lens mode-locked Ti:sapphire laser, has rekindled interest in optical parametric oscillators (OPOs) while opening up new regions of the spectrum to high repetition rate, high average power femtosecond pulses. With synchronous pumping techniques it is now possible to generate pulses as short as 40 fs for (nominally) 1 < (lambda) < 4 micrometers , at repetition rates of 10⁸ Hz, and with average powers measured in 100s of mW. Tuning can be achieved in critically or non-critically phase-matched KTP or LBO via crystal angle, temperature or pump wavelength tuning each of which has its merits in terms of tuning range, ease of use, noise properties, etc.

We present results on the first application of upconversion dynamics for short pulse generation in visible solid state lasers. We also describe prospects for passive mode-locking mechanisms based on cooperative nonlinearities and instabilities.

We show that mode-locking based on the formation of a fundamental soliton can be accomplished with a nonlinear twin-core fiber added to a homogeneously broadened gain medium within a laser cavity. The transient state towards equilibrium in such a device is characterized by an increase in the spectrum width of the pulse until a steady-state value is reached which is exceeding the gain bandwidth. At steady-state, the linear coupling properties of the twin-core fiber is continuously allowing for the radiated power from the perturbed soliton to exit the laser cavity. The corresponding energy gain per round trip eventually becomes a small perturbation to the soliton which recovers its shape and phase profile within each round trip.

The combination of broadly tunable solid-state lasers and the technique of chirped pulse amplification has made it possible to produce energetic, femtosecond pulses capable of focused intensities > 10¹⁸ W/cm². In addition, these lasers, which have multi-terawatt peak power capability, can be designed in very compact and robust configurations. Further, the average power capability of solid-state chirped pulse amplification sources can now exceed 1 W, as compared to the 10 mW average power limitation of previous femtosecond lasers.

We have developed a cw Krypton ion laser-pumped Cr³⁺:LiSrAlF₆ regenerative amplifier for femtosecond pulses operating at a 5 kHz repetition rate. After recompression, we obtained 3.6 (mu) J, 170 fs pulses at 825 nm. By focusing these pulses into a cell of water, a white-light continuum was generated.

A fiberless 1-TW all-Nd:glass chirped-pulse amplification laser system is described in this paper. Starting from high-contrast 1-ps pulses produced directly from a Nd:glass feedback- controlled oscillator, this system employs a fiberless, gratings-only expansion/compression scheme, and produces clean (5 X 10⁷ prepulse contrast) 1-J, 1.2-ps recompressed pulses without added pulse-cleaning. The same system can also be configured to produce up to 5-J uncompressed 410-ps pulses. A novel subpicosecond cross-correlation technique is also described.

We report experimental results concerning the propagation at high intensity of Laguerre- Gaussian beams in a nonlinear Kerr medium. We show that such patterns are remarkably stable and allow the beam to transport several times the critical power for self focusing before critical breakdown. We study the influence of the azimuthal order of the beam on the spatial and temporal averaging of the nonlinear phase shifts.

We present in this paper the characterization of the temporal shape of a picosecond chirped pulse diffracted by a transient grating by four-wave mixing experiments in a bulk sample of polydiacetylenes. We show that the coupled measurements of the amplitude correlation and the spectral intensity allow us to determine, in our simple experimental case, all the parameters for the material and the incident pulse included in the model for the diffracted pulse.

We present a detailed study of the spatial properties of spectral continuum generation in glasses. We show that the conical emission which accompanies filamentation in the glasses is a Cerenkov-based effect resulting from self-focusing. A theoretical analysis of the angular dependence of the emission based on a moving focal point is in good agreement with the experimental data.

Experiments on supercontinuum generation in high pressure CO₂ (1 - 40 atm) by 1 ps pulses at 593 nm are described. While changing the gas pressure and the laser power we observed self-focusing and optical breakdown in the gas. Their influence on the supercontinuum spectral width evolution is experimentally demonstrated and discussed. Threshold powers for optical breakdown and supercontinuum generation have been measured under different focusing conditions. The relation between these phenomena is analyzed.

The multiphoton ionization of liquid water and methanol and the dynamics of electron solvation in those media are studied with femtosecond time-resolved spectroscopy using a 2- eV laser light pump beam. The formation of the solvated electron in methanol is probed for the first time with femtosecond time resolution. The absorbance A is measured as function of the laser irradiance E for both water and methanol. The power law A varies direct as E^x obtained for water (x equals 1.8) and for methanol (x equals 1.9) differs markedly from that expected from energetic considerations. This is explained in terms of a resonant contribution of intermediate electronic states in the multiphotonic ionization.

The ultrafast optoelectronic technique of coherent microwave transient spectroscopy (COMITS) is applied to the measurement of photonic band structure phenomena in 2-D periodic dielectric arrays. The phase sensitivity of COMITS is exploited to measure the dispersion relation of electromagnetic radiation propagating in regular arrays of alumina ceramic rods. An attenuated-total-reflection configuration is used to demonstrate the coupling of microwave radiation to electromagnetic surface modes which exist at the interfaces of suitably terminated photonic crystals.

Pulsed laser spectroscopy has been applied to determine details of the excited state structure of the N-V center in diamond. Excited state splittings and dephasing behavior are reported.

Semiconductors and in particular AlGaAs operated at photon energies below half the band gap have proven over the last few years to be optimum materials for studying nonlinear guided phenomena, including ultrafast all-optical switching. Here we report experimental results on a range of characterization measurements and implementations of all-optical switching devices.

Coherent radiative control is a quantum-interference based approach to controlling molecular processes by the use of coherent radiation. A method of carrying out such control in the presence of thermal broadening effects and laser jitter is described and applied to the photodissociation of Na₂.

We examine two consequences of the unique behavior of molecular ions in intense laser fields. First, laser-induced crossings can be created with femtosecond laser pulses resulting in trapping of the molecular ion in high-field induced potential wells. We demonstrate this by numerical solutions of the time-dependent Schroedinger equation for H₂⁺. Second we show results of numerical calculation with vibrationally excited H₂⁺ of harmonic generation (HG), which can be enhanced by the trapping mechanism. We show next that in molecular ions, two plateaus appear in the harmonic generation spectrum, one of a molecular nature and the other atom like. Finally, we present an example of phase control of HG which can be explained in terms of a classical model.

Femtosecond laser pulses have been used to monitor chemical dynamics with the finest time- resolved details. They also offer the possibility of actually steering the molecular dynamics of a reactive system by controlling the laser pulses' parameters. Explorations of this possibility follow two main avenues: laser coherent control exploits phase coherence in weak-field excitation. Optimal control theory is being used to derive pulse shape functions which optimally favor certain objectives to be reached by the laser-induced molecular dynamics. Ideally, a complete non-perturbative characterization of the molecular response to a laser irradiation as an explicit functional of the laser pulse shape would allow for control scenarios to be inferred in a transparent manner. This complete characterization is now accessible for a number of simple molecular models with are reviewed in this communication.

Studies of multiphoton molecular resonant absorption (MPA) processes in polyatomic molecules induced by intense picosecond infrared laser radiation provide a successful method to investigate the dynamics of nonlinear molecular excitation and laser-controlled photophysical and photochemical reactions. We present the 10 micrometers picosecond pulse laser technique and the results of experimental study of the multiphoton absorption and THG in SF₆ and C₂H₄.

A review is given (intended for the non-expert) of the field of ultrashort laser pulses at ultra high intensities and their interactions with plasmas. The review covers progress and basic concepts for theory, modelling and experiments, with emphasis on the background aspects, the basic experimental considerations and some possible applications.

We present a comparative study of the x-ray emission produced by laser plasma sources in short ((tau) _L equals 0.6 ps) and long ((tau) _L equals 0.6 ns) pulse regimes for copper and tantalum targets. The experiments at (tau) _L equals 0.6 ps show that the x-ray conversion efficiency (eta) is still increasing with laser intensity between 2 X 10¹⁵ W/cm² and 5 X 10¹⁶ W/cm² for both sub-keV (0.1 - 0.75 keV) and keV (0.75 - 2 keV) ranges. In addition, we found that the optimum values (eta) at (tau) _L equals 0.6 ps are at least 2 - 4 times lower than those at (tau) _L equals 0.6 ns.

The ratio of isoelectronic lines was used to measure the electron temperature of subnanosecond and picosecond plasmas of a range of two-element materials: NaF, Mg-Al alloy, and KCl. Modelling shows that although populations may be far from steady-state in picosecond plasmas, the ratio of isoelectronic lines may be nearly steady-state, simplifying interpretation. Contour plots of the ratios of a number of isoelectronic helium-like line-pairs, suitable for steady-state electron-temperature interpretation, are provided.

Interaction of super strong as er fie1dE~E at with plasma is considered taking into account i ndi vi dual processes ionization of atoms and i onsl as we 1 1 as collective ones interaction with electrons and ions in hydrodynamic approach) In both cases laser field may result in new effects that can be displayed in specific acoustical and optical features of pl as ma. As for i ndi vi dual interaction, effect of ocal i oni zat ion suppression LI SJ e. stabilization of atom Cionl i n re - gard to tunnel ionization. is predicted At the same time collective plasma-superstrong laser field interaction may lead to non-steady state generation of unusual C supersonic and supersharpl plasma density profiles due to the ponderomotive forces of focused laser radiation.

The conversion efficiency of a Raman cell at high pump energies has been increased with an astigmatic focus. Experimental and numerical results show that the increased conversion is due to the reduction of cascade second order Stokes. For our experimental setup, it is shown that other effects, namely Brillouin scattering, anti-Stokes generation, and ground-state depletion, are negligible. Another important advantage of an astigmatic focus is the reduced risk of optical breakdown due to the lower intensity at the focus with only a small gain reduction. Good agreement was found between experimental and numerical results. In particular, the pump energy where the conversion for an ordinary focus is identical to an astigmatic focus is reproduced by the calculations.

We describe the performance of a Nd:YAG laser which produces 4.1 mJ, 12 ns FWHM Q- switched pulses when side-pumped with 180 W, 250 microsecond(s) pulses from a stacked AlGaAs laser diode array (SDL-3230 TZB). We have studied the effects of varying gain and cavity length on the Nd:YAG laser performance. We have found that the properties of diode lasers require design compromises different than for lamp-pumped lasers.

We present an efficient all-fiber intensity modulator designed for repetitive Q-switching of rare-earth doped fiber lasers. This device shows many advantages over standard bulk modulation technique like immunity to mechanical vibrations and lower excess losses. The principle of operation is simple and relies on the evanescent field coupling of the fiber mode to an overlay medium plate which is rapidly moved away from the surface of a standard polished coupler using a fast piezoelectric translator. A spliceless Q-switched erbium fiber laser incorporating this modulator was also tested.

Continuous wave (cw) and Q-switched fiber lasers were implemented using highly doped erbium fiber pumped with a 980-nm pigtailed laser diode. Thresholds and slope efficiencies were characterized for various reflector combinations to achieve optimal cw operation. An output coupling of 90% was found to produce maximum output power of 30 mW at 1560 nm with 78 mW of launched pump power. Pulsed operation was achieved with low loss on the first diffracted order of a bulk acousto-optic Q-switch. Output coupling from the fiber laser was obtained through an intrinsic fiber grating. Available pump power levels of 78 mW produced 20-ns pulses of 250 W peak power and a repetition rate of up to 3.5 kHz. The effects of repetition rate, cavity losses and pump power on the pulsewidth, peak power and lasing spectrum were characterized.

Unstable resonators can improve the beam quality of solid-state lasers. Various configurations have been devised in order to reduce the sensitivity of the resonator to variations of the thermal lensing taking place within the pumped gain medium. A simple geometrical analysis allows a first order comparison of these configurations, in terms of their sensitivity to thermal lensing variations as well as to misalignments.

A diffractive optical system consisting of one single binary CGH with high numerical aperture (N.A. > 0.7) for collimating or focusing laser-diode radiation is presented. Taking into account the exact phase distribution for a certain injection current of the laser-diode all aberrations of the phase can be corrected: A theoretically circular symmetric and diffraction limited spot can be obtained without astigmatism. High diffraction efficiencies can be expected by using high frequent binary reflection holograms where higher diffraction orders become evanescent. The zero order can be minimized by selecting an adequate relief thickness. Thus local DEs of the designed CGH can achieve DEs of theoretically 100%.

Two-wavelength interferometry is a well known technique to enlarge the ambiguity range of conventional interferometric systems without loss of resolution. Two laser emission lines with a spectral distance of a few nm form a so-called synthetic wavelength, which is used for absolute ranging. Due to the lack of an inexpensive and compact light source providing for a tunable synthetic wavelength, two-wavelength interferometry has been without technical importance up to now. We present a new light source for absolute ranging two-wavelength interferometry based on stabilized semiconductor lasers. Two semiconductor lasers operating at different wavelengths are prestabilized concerning their chip temperatures and their injection currents. Then the generated synthetic wavelength is stabilized directly by a Fabry Perot resonator. The stability and the performance of the synthetic wavelength control is shown.

A Nd:GGG(Ca,Mg,Zr) and a Nd:YAG laser were compared when end-pumped by a continuous-wave laser diode. With a 30-mW single-stripe laser diode, 2.4 mW and 4.6 mW of output power were obtained for the Nd:GGG(Ca,Mg,Zr) and the Nd:YAG laser, respectively. With a 1-watt laser-diode array, the output power increased to 223 mW for the Nd:GGG(Ca,Mg,Zr) and 297 mW for the Nd:YAG laser. The Nd:GGG(Ca,Mg,Zr) was also pumped by a 19-watt quasi-continuous-wave laser-diode bar. Side and end pumping were compared. With the appropriate optics the end-pumped geometry was more efficient and gave more power, 47% slope efficiency with 4.3 watts of output power, compared to side-pumped geometry, 16% slope efficiency with 2.6 watts.

There is a growing need for compact efficient and eye-safe lasers. Diode pumping is normally used to produce compact and efficient lasers when the required pumping wavelength and power are available. This paper presents the characteristics of an eye-safe Yb:Er:glass laser at 1.54 micrometers pumped by a Ti:Sapphire laser tuned between 905 and 980 nm.

Tunable, single longitudinal mode output has been obtained from a grazing-incidence grating- tuned Nd:YVO₄ slab laser pumped by a quasi-cw diode laser bar. In order to compensate for the very high output coupling losses, a side pumping scheme using a single high angle of incidence reflection at the flat pump face is used to produce very high gain in the laser material. A pulse energy of up to 1 mJ in 200 microsecond(s) has been obtained for a 12 mJ pump pulse. The almost TEM₀₀ output remained single longitudinal mode for periods of several minutes without active stabilization and could be tuned over a wavelength range of greater than 1 nm.

The propagation of symmetrical laser pulses in a Kerr medium produces a broadening of the pulse spectrum while that of nonsymmetrical pulses leads also to a spectral shift. Such a spectral reshaping can be used to mode lock a laser by inserting two spectral filters with different transmission profiles at the ends of a laser cavity including a Kerr material; it can be shown that short pulses travelling in such a laser experience a higher feedback per round trip than do lower power multimode signals. Numerical simulations tend to indicate that the pulses so produced have a nonsymmetrical shape; their spectrum is shifted with respect to that of multimode oscillations. Different considerations suggest that the method may not be self- starting and that another mode-locking technique (active or passive) may be needed to initiate mode locking.

We first describe an interferometric method suitable for the mode-locking of diode lasers, where the amplifying medium provides both the gain and the phase nonlinearity. The amplifying medium is placed in interferometric reflectors used as nonlinear mirrors. Numerical simulations have shown that, in the presence of synchronous pumping, conditions can be found where low power oscillations are quenched and where an intense short pulse is stable. We next propose and analyze numerically an interferometric scheme for the reshaping and amplification of a mode-locked train of short laser pulses, based on a ring cavity containing a semiconductor laser amplifier. Pulse compression and amplification are predicted over a wide range of values of the linear phase shift between the cavity and pulse train. Best reshaping is achieved with the use of a low cavity finesse operated near laser threshold.

External cavities terminated by a phase conjugate mirror have been used to provide a weak feedback to a synchronously pumped picosecond dye laser, a mode-locked argon ion laser and a continuous-wave argon ion laser. With the phase conjugate mirror, the external cavity is self-aligned and the reinjected field is best adapted to the intracavity field. Significant intensity noise reduction has been observed between 5 kHz and 1 GHz. For mode-locked lasers, it was possible to control the pulse length. These effects occur for wide ranges of cavity detuning and feedback level.

In the present paper we report about experimental investigation of acousto-optical mode locking in various types of lasers: gas, solid-state and dye. The theoretical description which takes into account the amplifying process of short pulses in resonant active medium had been carried out. Experimentally the pulse location inside the temporal mode-locker `window' and its shape as a function of both active medium gain and the resonator length detuning have been done. The pulse shape comprises satellite(s) following the main pulse. The number of the satellites with their relative amplitude and spacing from the main pulse was also one of the subjects of investigation.

.It is well known that ciode-pumped solid-state lasers exibit a high efficiency at low and moderate powers elny pumped by single laser diode aioig optical axis of the resonator. The futher lncreaslng c)f the power s being achieved now by diode array side--pumpiiig of neodilriiulrL rods. But the efficiency of this type of pumping iS somewhat smaller because of nonideal overlapping of the inversion profile and the desirable iasnn rnode(TEM )."i.. This drawback could be 0 0 avoided at moderate powers by the use of the end-pumping by diode-arrays or several diodes that results in a highly efficient fundamental-mode operation/2/. Nevertheless this kind of the end-pumping leads to significant heating of the active elelrient/3/, because the pumped region of the crystal should be sufficiently small to select TEM mode. These are the reasons 00 which limits the output power in the standard end-pumping scheme, although the use of the moving slabs and rotating disks could overcome the some of the above difficulties //3/ The alternative approach is to refuse from Gaussian beams paradigm and to select spatially periodic modes using the phenomenon of the seif-imaging/4/.

In this paper we report results of experimental investigation of spectra-time structure of backward SRS in crystal calcite with nano- and sub-nanosecond pulse duration and phenomenological treatment of this structure.

The link of a XeCl excimer laser, a hydrogen Raman cell, and a Ti:Al₂O₃ laser oscillator to form an integrated laser system is proposed to offer wavelength diversity for laboratory uses. The performance of the Raman conversion and the tunable output of Ti:Al₂O₃ laser is presented in detail.

Nonradiative relaxation from the ⁴G_7/2 level of Nd³⁺ in YAG was investigated by the luminescence light quenching method. This method is described in detail. Luminescence quenching from the ⁴F_3/2 level is a result of stimulated emission under simultaneous 532 and 1064 nm pulse excitation. The experimental data is compared with theoretical data. The relaxation rate from ⁴G_7/2 level in YAG was estimated to be about 10⁹ sec^-1.

Laser action around 2 micrometers on the ⁴F_9/2 yields ⁴I_11/2 transition of Er³⁺ ion in BaYb₂F₈-Er crystals has been investigated. The lasing spectra of the crystal in selective and nonselective cavities have been determined. Laser energy parameters have been measured.

An analysis of the lateral-mode behavior of broad-area semiconductor lasers operating with a simple external-cavity configuration is presented. A numerical approach is employed for propagating the optical field around the external-cavity resonator, as well as for the calculation of realistic carrier-density profiles inside the active region of the broad-area laser. By slightly detuning the position of the laser relative to the collimation lens, it is shown that the lateral- mode selectivity can be significantly enhanced compared to that of a free-running broad-area laser. The lateral-mode selectivity and the shape of the far field radiation pattern emitted from the external mirror are found to depend heavily upon the separation between the external mirror and the collimation lens. The calculations also revealed that output powers of a few hundreds mW can be emitted in a single-lateral-mode output beam for configurations maximizing the lateral-mode selectivity.

1.5 MW, < 10 nsec long gain-switched laser pulses were obtained from a Nd⁺³ laser-pumped Cr⁺⁴:YAG laser with an efficiency of 16%. Tuning was achieved from 1.44 to 1.51 micrometers . Repetition rate of 30 pps was demonstrated at approximately equals 10 mJ/pulse.