A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation in photonic crystal fiber

The application of chirped-pulse amplification to short-pulse lasers has led to a dramatic increase in the number of high-power subpicosecond laser systems. Accordingly, knowing the short-pulse damage thresholds of optical components and scaling the damage thresholds with pulse width have become increasingly important.

Laser-induced damage in dielectrics with nanosecond-to-subpicosecond pulses

The absorption and emission cross sections of the transition between the ground spin-orbit multiplet and the lowest excited multiplet were measured for Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/ ions in a variety of crystalline hosts. The materials that were investigated include LiYF/sub 4/, BaY/sub 2/F/sub 8/, Y/sub 3/Al/sub 5/O/sub 12/, LaF/sub 3/, KCaF/sub 3/, YAlO/sub 3/, and La/sub 2/Be/sub 2/O/sub 5/. The absolute magnitudes of the emission cross sections were determined from the absorption spectra, with the aid of the principle of reciprocity. The calculated radiative emission lifetimes derived from these measured cross sections agree well with the measured emission decay times for most materials. The potential use of these rare-earth-doped materials in pulsed laser applications requires that the ground state exhibit adequate splitting to minimize the detrimental effects of the ground state thermal population, and also that the emission cross section be sufficiently large to permit efficient extraction energy. The systems based on Ho/sup 3+/ in the eightfold coordinated sites of LiYF/sub 4/, BaY/sub 2/F/sub 8/, and Y/sub 3/Al/sub 5/O/sub 12/ appear to be the most promising. >

Infrared cross-section measurements for crystals doped with Er/sup 3+/, Tm/sup 3+/, and Ho/sup 3+/

Nonlinear Optical Crystals: A Complete Survey

Laser-induced damage threshold measurements were performed on homogeneous and multilayer dielectrics and gold-coated optics at 1053 and 526 nm for pulse durations τ ranging from 140 fs to 1 ns. Gold coatings were found, both experimentally and theoretically, to be limited to 0.6 J/cm2 in the subpicosecond range for 1053-nm pulses. In dielectrics, we find qualitative differences in the morphology of damage and a departure from the diffusion-dominated τ1/2 scaling that indicate that damage results from plasma formation and ablation for τ ≤ 10 ps and from conventional heating and melting for τ > 50 ps. A theoretical model based on electron production by multiphoton ionization, joule heating, and collisional (avalanche) ionization is in quantitative agreement with both the pulse-width and the wavelength scaling of experimental results.

Optical ablation by high-power short-pulse lasers

The literature describes more than 30 measurements, at wavelengths
between 249 and 1550 nm, of the absolute value of the nonlinear
refractive-index coefficient of fused silica. Results of these
experiments were assessed and best currently available values were
selected for the wavelengths of 351, 527, and 1053 nm. The best
values are (3.6 ± 0.64) × 10-16
cm2/W at 351 nm, (3.0 ± 0.35) ×
10-16 cm2/W at 527 nm, and (2.74 ±
0.17) × 10-16 cm2/W at 1053
nm.

Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica.

Mechanically polished fused silica surfaces were heated with continuous-wave CO(2) laser radiation. Laser-damage thresholds of the surfaces were measured with 1064-nm 9-nsec pulses focused to small spots and with large-spot, 1064-nm, 1-nsec irradiation. A sharp transition from laser-damage-prone to highly laser-damage-resistant took place over a small range in CO(2) laser power. The transition to high damage resistance occurred at a silica surface temperature where material softening began to take place as evidenced by the onset of residual strain in the CO(2) laser-processed part. The small-spot damage measurements show that some CO(2) laser-treated surfaces have a local damage threshold as high as the bulk damage threshold of SiO(2). On some CO(2) laser-treated surfaces, large-spot damage thresholds were increased by a factor of 3-4 over thresholds of the original mechanically polished surface. These treated parts show no obvious change in surface appearance as seen in bright-field, Nomarski, or total internal reflection microscopy. They also show little change in transmissive figure. Further, antireflection films deposited on CO(2) laser-treated surfaces have thresholds greater than the thresholds of antireflection films on mechanically polished surfaces.

Carbon dioxide laser polishing of fused silica surfaces for increased laser-damage resistance at 1064 nm

In this work, we used 248-nm 20-nsec laser pulses to measure laser-damage thresholds for halfwave-thick single layers of fifteen potential UV coating materials, for highly reflective coatings made of thirteen combinations of these materials, and for antireflective coatings made using five combinations of the materials. Refractive index, absorption, position of the UV absorption edge, stress, and environmental stability were measured for the halfwave-thick single layers. The first three of these parameters are closely related and were generally correlated with damage thresholds of the single-layer coatings. Thresholds of HR coatings were correlated with absorption in the high-index materials used in the coatings and with refractive index of the low-index materials. Thresholds of AR films were not well correlated with properties of single-layer coatings.

Materials for optical coatings in the ultraviolet.

Results from experiments on laser-induced damage to surfaces of optically polished glass and fused-silica and thin-film coatings are presented. Measurements were made under proper conditions to characterize actual laser components. Data are given for distributions of thresholds for 1.06 μm, 1 ns pulses, the influence of coating materials and designs, and effects of surface preparation on damage threshold, variations of threshold with duration of the laser pulse, and thresholds for selected visible and ultraviolet wavelength pulses.

Laser-induced surface and coating damage

We have obtained an increase (factors of 1.5–3.0) in the threshold fluence for laser‐induced inclusion damage in crystals of potassium dihydrogen phosphate (KDP) by irradiating the crystals at a fluence below that necessary to cause damage with a single laser pulse (1064‐nm, 1‐ns). The threshold increase was greatest for less damage resistant samples.

David Milam

Papers

Review and assessment of measured values of the nonlinear refractive-index coefficient of fused silica.

Carbon dioxide laser polishing of fused silica surfaces for increased laser-damage resistance at 1064 nm

Materials for optical coatings in the ultraviolet.

Laser-induced surface and coating damage

Improving the bulk laser damage resistance of potassium dihydrogen phosphate crystals by pulsed laser irradiation