This paper describes recent developments in modelocking techniques for ultrashort pulse generation in solid stat lasers. Studies in Ti:Al2O3 provide a model system for examining different modelocking approaches. Special emphasis is placed on the use of additive pulse modelocking for achieving passive modelocking using a fast saturable absorber-like mechanism. Different models of APM are described and its extensions to other solid state laser materials are discussed.
Proc. SPIE 1413, Generation of ultrashort high-average-power passively mode-locked pulses from a Nd:YLF laser with a nonlinear external-coupled cavity at high-repetition rates, 0000 (1 June 1991); doi: 10.1117/12.46131
A general consideration on the cavity detuning ranges and stability requirements for obtaining high pulse repetition rate in an additive-pulse passively mode-locked laser are presented. Without modifying the main cavity length, up to a sixfold increase over the original repetition rate of 76 MHz has been obtained in a coupled-cavity passively mode-locked Nd:YLF with an average power of 3W.
The design, test and optimization of a picosecond CO2 pulse-forming system are presented. The system switches a semiconductor's optical characteristics at 10 micrometers under the control of a synchronized 1.06-micrometers Nd:YAG picosecond laser pulse. An energy-efficient version of such a system using collimated beams is described. A simple, semi-empirical approach is used to simulate the switching process, specifically including the spatial distributions of the laser energy and phase, which are relevant for experiments in laser-driven electron acceleration.
The pedestal (pre-pulse and post-pulse) associated with a chirped-pulse-amplification (CPA) laser is modeled. In this model, the power gain can be treated as a function of instantaneous frequency due to the large chirp of this type of laser. It is found that self-phase-modulation (SPM) can play an important role determining the final shape of the compressed pulse, even at relatively low values of the cumulative B-integral, B less than approximately 2.
The plasma mirror is a self-induced, plasma-based optical element which can be inserted into existing experiments to reduce prepulse energy without significant degradation of ultrashort pulse laser light. We have directly observed the nonlinear reflectivity of the plasma mirror as well as the spatial and temporal characteristics of the reflected pulse. The initial measurements indicate that the incident pulse reflects specularly from a high density, highly reflective plasma. The reflected pulse has a smoothed spatial profile and reduced pulsewidth. We outline future work to characterize both the plasma mirror technique of prepulse suppression and its reflected pulse.
A short pulse will develop temporal sub-structures as it self- focuses. We show how these substructures lead to both the catastrophic expansion of the spectrum and the remarkable beam stability against self-focusing.
Design studies for recombination x-ray lasers based on plasmas ionized by high intensity, short pulse optical lasers are presented. Transient lasing on n=3 to n=2 transitions in Lithium-like Neon allows for moderately short wavelengths (less than or equal to 100 angstroms) without requiring ionizing intensities associated with relativistic electron quiver energies. The electron energy distribution following the ionizing pulse affects directly the predicted gains for this resonance transition. Efficiencies of 10-6 or greater are found for plasma temperatures in the Vicinity of 40 eV. Simulation studies of parametric heating phenomena relating to stimulated Raman and Compton scattering are presented. For electron densities less than about 2.5 x1020 cm-3 and peak driver intensity of 2x1017 W/cm2 at 0.25 μm with pulse length of 100 fsec, the amount of electron heating is found to be marginally significant. For Lithium-like Aluminum, the required relativistic ionizing intensity gives excessive electron heating and reduced efficiency, thereby rendering this scheme impractical for generating shorter wavelength lasing (less than or equal to 50 angstroms) in the transient case. Following the transient lasing phase, a slow hydrodynamic expansion into the surrounding cool plasma is accompanied by quasi-static gain on the n = 4 to n = 3 transition in Lithium-like Neon. Parametric heating effects on gain optimization in this regime are also discussed.
Lateral and axial thermal energy transport in laser-plasma interactions has been studied using a short (12 psec) pre-pulse free KrF pumped Raman laser pulse (λ = 268 nm) of low beam divergence. A new method of producing narrow line foci using random phase plates was employed to study the plasma conditions required to generate high gain, short wavelength recombination X-ray lasers. In addition, the laser energy was focused into a small focal spot (8 μm) for higher intensity (1-2 x 1017 cm-2) interaction studies.
In these proceedings, we report on a time-resolved investigation of the hydrodynamics of a laser-produced plasma. A sub-100ps pulse is focused into a chamber filled with xenon for various pulse energies and pressures. This pulse (the pump pulse) forms a plasma, which is probed by a second pulse (the probe pulse) with a variable delay of up to 2.5 ns. The gradients in the plasma density profile produce a lensing effect on the probe pulse. The beam transmitted through the plasma is viewed with a CCD camera. The diffraction pattern of the probe pulse can be seen by subtracting the image of the first pulse beam from the image produced by the two-pulse beam.
The ultra-fast ionization of argon, krypton and xenon by intense ultra-short laser pulses is studied. Intensities in the range 1014 to 1016 W/cm2 at a wavelength of 616 nm are provided by a colliding pulse mode-locked dye laser system. To date, ion yields have been monitored using a 1 meter time.of flight spectrometer. Intensity dependent photoelectron signals are deduced from ion data. In an ultra-fast strong field environment, ionization times can be less than an optical period, affording a better distinction between a field ionization picture and a multiphoton ionization picture. For most cases that we study, Keldysh parameters are below 1, indicating that we are in a field ionization regime. As a laser system under development progresses towards the design goal of 1 joule per 100 femtosecond pulse, we will extend these investigations to peak intensities of order 1018 to 1019 Watts/cm2.
A subroutine which calculates the absorption of short pulse electromagnetic radiation in a material has been installed into the laser fusion modeling program called LASNEX. Calculational results show the necessity for NLTE physics to account for ionization, the development of non-exponential density profiles for the expanding plasma and movement of the critical point toward the surface which results in Doppler shifts of the reflected light. Comparison of calculations of local scale lengths with experiments shows not only good agreement but the correct scaling with intensity.
The problem of interaction of short laser pulse (light frequency ω0, pulse duration, τ < ls/VTi; ls, skin depth, VTi, ion velocity) with dense (ω0 << ωpe) semi-infinite plasma was solved. We formulated the self-consistent problem of obtaining the electron distribution function and space dependence of electric field in skin layer, and solved the problem for the case of absence of the energy losses from the skin layer. We found self-similar nonstationary electron distribution function and space dependence of electric field in this case, and basing on these solutions, have calculated mean electron energy , absorption coefficient, bremsstrahlung radiation, time dependent skin depth. Finally we discussed the limitations of our theory.
We present results from a 1-D plasma dynamics calculation, describing the evolution of strongly heated material in the vicinity of a solid-vacuum interface. We find that the radiation emitted by the hot material in the range hν > kTe, where Te is the initial peak plasma temperature, comes primarily from the region of the original step function interface. This emission is dominated by recombination radiation. The emitted radiation pulse is extremely short; the cooling at the interface is dominated by expansion. It is seen that thermal conduction minimally affects the radiation pulse intensity and duration.
The kilo-electron-volt x-ray emission from picosecond laser plasma interactions is investigated using filtered x-ray PIN diodes and a time-integrated crystal spectrograph. Different laser pulse shapes are used. The conditions required to generate high-power, short-pulse duration x-ray line emission are analyzed. The plasma must be hot enough to ionize to the appropriate charge state and then cool rapidly to quench the emission.
The density profile evolution of picosecond laser-plasmas has been inferred from the angular dependence of the specular reflectivity of a probe picosecond pulse. Time-resolved density profiles, critical density scale-lengths as short as 3 nm and values for the plasma collisionality are presented for a gold target irradiated by a 580 nm, 0.8 ps FWHM laser pulse with a peak intensity of up to 2x1013 Wcm-2. The results are best fitted at early times (<10 ps) by the self-similar isothermal exponential density profile and a critical density collision frequency of 0.3ω, in good agreement with 1-D hydrodynamic simulations.
We report on the stimulated Raman scattering (SRS) that occurs when short (~ 50-100 femtoseconds), intense (I~2 x 1017 W/cm2) laser pulses are focused into a plasma, or a neutral gas which quickly becomes a plasma due to multiphoton ionization. Review of the usual SRS growth rates will be given, followed by a discussion on how this theory is modified for short pulses. It is found that for some recombination x-ray laser schemes employing short pulses, the Raman instability can potentially heat the electrons to levels that would greatly reduce the efficiency of such a device. However, there are areas in parameter space where the temperature of heated electrons can be kept to acceptable levels. One dimensional, relativistic, particle-in-cell (PlC) simulations are presented.
Measurements of reflectivity of stimulated Brillouin scattering, from an underdense, homogeneous plasma, irradiated by a 10 ps-1.06μm laser pulse, show an increase from 10-4 to 10-2, as the laser intensity is varied between 1013 to 1015 W/cm2. Numerical simulations have been developed to interpret these data. At low laser intensity, (<1014 W/cm2), the low reflectivity is well explained as being due to saturation of the instability in the convective regime, and good agreement with numerical simulations has been obtained. At high laser intensity, the model predicts higher reflectivities than measured in the experiment, although non-linear effects in ion sound waves evolution have been observed.