Structural changes in fused silica after exposure to focused femtosecond laser pulses.

Using in situ Raman scattering in a confocal microscopy setup, we have observed changes in the network structure of fused silica after modifying regions inside the glass with tightly focused 800-nm 130-fs laser pulses at fluences of 5-200 J cm(-2). The Raman spectra show a large increase in the peaks at 490 and 605cm(-1), owing to 4- and 3-membered ring structures in the silica network, indicating that densification occurs after exposure to the femtosecond laser pulses. The results are consistent with the formation of a localized plasma by the laser pulse and a subsequent microexplosion inside the glass.

4.0-4.5-mum lasing of Fe:ZnSe below 180 K, a new mid-infrared laser material.

Lasing of Fe:ZnSe is demonstrated, for the first time to the authors' knowledge, for temperatures ranging from 15 to 180 K. The output wavelength of the Fe:ZnSe laser was observed to tune with temperature from 3.98mum at 15 K to 4.54mum at 180 K. With an Er:YAG laser operating at 2.698mum as the pump source, a maximum energy per pulse of 12muJ at 130 K was produced. Laser slope efficiencies of 3.2% at 19 K and 8.2% at 150 K were determined for an output coupling of 0.6%. A laser emission linewidth of 0.007mum at 3.98mum was measured at 15 K. Absorption and emission spectra and emission lifetimes for Fe:ZnSe are also discussed.

Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber.

Permanent photoinduced optical attenuation has been observed in Tm(3+)-doped aluminosilicate glass fibers on exposure to near-resonance mode-locked 1064-nm radiation at 300 K. The rate of this darkening was observed to follow a 4.7 +/- 0.4 power dependence on the 1064-nm intensity. The result shows that absorption of infrared light of moderate intensity can lead to photoionization in rare-earth-doped glass, provided that a multiphoton stepwise excitation channel exists.

Observation of resonant enhancement of photoinduced second-harmonic generation in Tm-doped aluminosilicate glass fibers.

We report a 10(3) increase in efficiency of second-harmonic generation in Tm(3+)-doped aluminosilicate glass fibers when the wavelength of the fundamental beam is tuned to the maximum of the (3)H(6) ? (3)F(4) absorption of Tm(3+) at 785 nm. This result shows the role of resonance enhancement in the nonlinear optical process that is responsible for the formation of the X((2)) grating.

Experimental study of second-harmonic-generation saturation in fibers by phase modulation between the fundamental and second-harmonic writing beams.

We study saturation of photoinduced second-harmonic generation in a Ge-doped silica fiber by modulating the relative phase between the fundamental and second-harmonic writing beams. We shift the phase by pi and present measurements that identify saturation that is due to site depletion and saturation that is due to a backfield.

Seeded second-harmonic generation in optical fibers: the effect of phase fluctuations.

Phase fluctuations between the infrared and green beams during seeding of second-harmonic generation (SHG) in optical fibers lead to a drastic reduction of the initial SHG intensity and to a strong time dependence of the SHG after seeding. Experiments using mode-locked lasers with a stable phase relation between the infrared and green writing beams lead to reliable, reproducible results. Under these conditions we do not observe a significant difference in SH efficiency between fibers with Ge concentrations of 3 and 14 mol. %.

Photoinduced second-harmonic generation in rare-earth-doped aluminosilicate optical fibers.

The presence of defect states in the band gap is an essential ingredient of recent models of photoinduced second-harmonic generation (SHG) in fibers. We have created such states by doping aluminosilicate glass fibers that do not contain Ge with Ce or Eu and observed SHG from 1.06-microm light after preparation of the fibers with 0.532- and 1.06-microm light. In an aluminosilicate fiber doped with 0.008 wt. % Ce, the SHG conversion efficiency is as high as 1.5% at infrared (1.06-microm) peak powers of 200 W.