Backscatter reduction using combined spatial, temporal, and polarization beam smoothing in a long-scale-length laser plasma.

Spatial, temporal, and polarization smoothing schemes are combined for the first time to reduce to a few percent the total stimulated backscatter of a NIF-like probe laser beam (2x10(15) W/cm (2), 351 nm, f/8) in a long-scale-length laser plasma. Combining temporal and polarization smoothing reduces simulated Brillouin scattering and simulated Raman scattering (SRS) up to an order of magnitude although neither smoothing scheme by itself is uniformly effective. The results agree with trends observed in simulations performed with the laser-plasma interaction code F3D simulations [R. L. Berger et al., Phys. Plasma 6, 1043 (1999)].

Ultrafast picket fence pulse trains to enhance frequency conversion of shaped inertial confinement fusion laser pulses.

A high-frequency train of 5-100-ps pulses (picket fence) is proposed to improve significantly the third-harmonic frequency conversion of Nd:glass lasers that are used to generate high-contrast-shaped pulses for inertial confinement fusion (ICF) targets. High conversion efficiency of the low-power foot of a shaped ICF pulse is obtained by use of a low duty cycle, multi-gigahertz train of approximately 20-ps pulses with high peak power. Even with less than 10% duty cycle, continuous illumination is maintained on the target by a combination of temporal broadening schemes. The picket fence approach is analyzed, and the practical limits are identified as applied to the National Ignition Facility laser. It is found that the higher conversion efficiency allows approximately 40% more third-harmonic energy to be delivered to the target, potentially enabling the larger drive needed for high-yield ICF target designs. In addition, the frequency conversion efficiency of these short pulses saturates much more readily, which reduces the transfer of fluctuations at the fundamental and thus greatly improves the power stability of the third harmonic.

Complete all-optical switching of visible picosecond pulses in birefringent fiber.

Switching of visible picosecond pulses with a 50:1 contrast ratio is demonstrated in a time-division interferometry arrangement. The high-contrast switching of the signal is accomplished by uniform cross-phase modulation by an orthogonally polarized control pulse that walks through the signal during propagation in a strongly birefringent 10-m fiber.

Observation of the formation of dark-soliton trains in optical fibers.

We present observations that demonstrate a technique for the generation of trains of dark solitons. The technique consists of the nonlinear copropagation of two delayed visible pulses in a long optical fiber. Single-shot streak-camera temporal images depict the formation of trains of dark solitons with picosecond widths. Single-shot spectra are also observed and depict ringing around spectral holes, which is characteristic of dark solitons on a chirped background pulse. Good agreement is found between theory and both temporal and spectral measurements.

Pulse splitting during self-focusing in normally dispersive media.

The self-focusing of femtosecond optical pulses in a normally dispersive medium is studied numerically. This situation represents a general problem that may be modeled by a 3 + 1-dimensional nonlinear Schrödinger equation, where two dimensions are self-focusing and the third is self-defocusing. The numerical simulations show that the dispersion causes the splitting of a pulse before it self-focuses into two temporally separated pulses, which then continue to self-focus and compress rapidly. The calculated behavior results in periodic modulation of the generated continuum spectrum, as was recently observed in continuum generation by focused femtosecond pulses in gases.

Observation of the buildup of modulational instability from wave breaking.

Orthogonally polarized pulses copropagating in birefringent fiber are observed to develop coherent terahertz intensity oscillations owing to modulational instability. Rather than forming spontaneously from noise with random phase, the modulational instability is coherently initiated from wave-breaking oscillations. This results in stable terahertz oscillations that have little timing jitter from pulse to pulse. Experimentally this is evidenced by the terahertz modulation observed in cross correlations that are obtained by averaged sampling over many pulses. Numerical integrations of coupled nonlinear Schrödinger equations verify this description.

Colliding visible picosecond pulses in optical fibers.

The observation of the nonlinear interaction of two visible picosecond pulses, separated by a small time interval, while propagating in an optical fiber is reported. It is found that as propagation proceeds, a region of pulse collision develops in which intensity oscillations form that are similar to those recently observed in optical wave breaking. The frequency of these oscillations increases for an increasing time separation between the input pulses and is found to be as large as 5 THz. For small time intervals between the input pulses no oscillations formed. This observed behavior is found to agree well with numerical integrations of the nonlinear Schrödinger equation.

Intrafiber visible pulse compression by cross-phase modulation in a birefringent optical fiber.

Detailed subpicosecond observations of the time dependence of a propagating cross-phase-modulated pulse are presented. It is observed that propagation of a weak visible pulse in one polarization mode of a birefringent optical fiber, so that it is cross-phase modulated by the edge of a strong pulse in the other mode, can result in significant compression of the weak pulse, for normal dispersion, without external compensation. The compression efficiency is found to be enhanced by utilizing the group-velocity mismatch between the two modes to synchronize the weak pulse with the broadening edge of the strong pulse. This is the first observation, to the author's knowledge, of intrafiber pulse compression for normal group-velocity dispersion, and a compressed pulse width of 0.3 psec for a 2-psec input pulse is observed.

Observation of the transient expansion of heated surfaces by picosecond photothermal deflection spectroscopy.

The observation of the picosecond thermal expansion of an absorbing surface after a short heating pulse is reported for the first time to my knowledge. The measurement was performed on Si by adapting the method of photothermal deflection to a pump-probe technique using 1-psec laser pulses. This was achieved by separating out a novel deflection effect due to the gradient in the near-surface optical constants. The results are compared to a thermo-elastic model, and good agreement is found.

Measurement of optical phase with subpicosecond resolution by time-domain interferometry.

A new and extremely general interferometric technique has been experimentally demonstrated that permits the direct measurement of optical phase on a subpicosecond time scale. The intensity is characterized by cross correlation, and thus the optical field is completely determined.

Observation of a 1.9-psec polarization beat.

We have observed polarization beats by simultaneously exciting both the D(1) (589.6-nm) and the D(2) (589-nm) lines of sodium in a vapor with a 0.4-psec resonant laser pulse. This impact excitation of both D lines produces two oscillating macroscopic polarizations, which interfere with each other and procedure a beat note at their difference frequency in the intensity of the light emitted from the cell. The beat period of 1.9 psec is the fastest ever observed optically.

High-resolution extreme-ultraviolet spectroscopy of potassium using anti-Stokes radiation.

We report the use of a new extreme-ultraviolet radiation source based on spontaneous anti-Stokes scattering for high-resolution absorption spectroscopy of transitions originating from the 3p(6) shell of potassium. The region from 546.6 to 536.8 A is scanned at a resolution of about 1.2 cm(-1). Within this region, four previously unreported lines are observed.