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.

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. %.

Variable birefringent beam splitter.

A sapphire disk is used as a variable birefringent beam splitter for the linearly polarized pump beam in an oscillator-amplifier dye laser system. Rotation of the disk varies the extent of retardation. The induced elliptical polarization results in a variable back surface reflectance between approximately 0.02-0.15 for the green spectral region. Reflectances are measured on two disks with different thicknesses and compared with an approximate model describing the reflectance as a function of angle of rotation of the disk.

Spin-lattice relaxation in the excited (4)S(3/2) state of Er(3+) in LaF(3).

We have employed a purely optical method to measure spin-lattice relaxation in the lower excited (4)S(3/2) state of Er(3+) in LaF(3) at helium temperatures. Above 3 K the relaxation is dominated by an Orbach process through the upper (4)S(3/2) state. Below 3 K the relaxation is caused by some residual mechanism that obscures the Kramers-Van Vleck direct process. By relating the Orbach relaxation measurements to the homogeneous broadening of the upper (4)S(3/2) level, we determine values of the transition rates between the upper and lower (4)S(3/2) states.