We have now demonstrated and characterized gas-filled hollow-core fiber lasers based on population inversion from
acetylene (<sup>12</sup>C<sub>2</sub>H<sub>2</sub>) and HCN gas contained within the core of a kagome-structured hollow-core photonic crystal fiber.
The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns pulses from an
optical parametric amplifier. Transitions from the pumped overtone modes to fundamental C-H stretching modes in both
molecules create narrow-band laser emissions near 3 μm. High gain resulting from tight confinement of the pump and
laser light together with the active gas permits us to operate these lasers in a single pass configuration, without the use of
any external resonator structure. A delay between the emitted laser pulse and the incident pump pulse has been observed
and is shown to vary with pump pulse energy and gas pressure. Furthermore, we have demonstrated lasing beyond 4 μm
from CO and CO<sub>2</sub> using silver-coated glass capillaries, since fused silica based fibers do not transmit in this spectral
region and chalcogenide fibers are not yet readily available. Studies of the laser pulse energy as functions of the pump
pulse energy and gas pressure were performed. Efficiencies reaching ~ 20% are observed for both acetylene and CO<sub>2</sub>.
An optically pumped overtone HBr laser is investigated experimentally and theoretically. The frequency tuning and
stabilization of the Nd:YAG pump laser is described. Results of HBr laser emission are presented. The simulation shows
promising features of both pulsed and cw pumped systems concerning efficiency, frequency tuning and heat dissipation.
Laser induced breakdown of a high-quality mirror consisting of alternating (lambda) /4 layers of Ta<SUB>2</SUB>O<SUB>5</SUB> and SiO<SUB>2</SUB> and a single 500-nm thin film of Ta<SUB>2</SUB>O<SUB>5</SUB> were studied with amplified and unamplified femtosecond pulses. The experimental data can be fitted with a model taking into account multiphoton absorption, impact ionization, and local intensity enhancements due to interference effects in the films. The results indicate that state of the art, high- quality thin films show a damage behavior that is similar to bulk materials. Defects and impurities play a negligible role. Incubation effects are found to reduce the damage threshold when the coatings are damaged with multiple pulses from a femtosecond oscillator. Time-resolved pump-probe reflection and transmission experiments indicate a decay of the excited electron plasma with characteristic time constants of 4 ps, 60 ps, and 700 ps.