Index-of-refraction change in optically pumped solid-state laser materials

Kinetics and wavelength dependence of thermal and excited-state population on lens effect induced in a Nd:YAG rod amplifier

Using a wavefront sensor, we have measured the temporal evolution of the lens induced in a Nd:YAG rod amplifier under side pumping by laser diode bars centered around 808 nm in a quasi-continuous wave regime. The evolution of the induced lens is drastically different when measured with a probe pulse centered at 532 nm or 1064.5 nm. To explain this evolution, we developed a model that accounts for both the excited state population of the Nd³⁺ ions and the thermal contribution to the refractive index of the amplifier. This model, which takes into account amplification and wavelength shift of the probe pulse at 1064.5 nm, makes it possible to quantitatively describe the spectral and temporal evolution of the amplifier focal length. It also shows that the excited state population contribution is more important around 1064.5 nm and can partly compensate for the thermal lens induced in the amplifier.

Impact of pump light spatial modulation on the generation of gain-switched sub-nanosecond Ti:Sapphire laser

The Ti: Sapphire laser generation was investigated under the pumping by the spots with a different distribution of the pump fluence. We found that the characteristics obtained under pumping by the small-scale spatially modulated 532 nm light are indicative of intracavity light scattering. This scattering can be accounted for by pump light-induced optical nonuniformities in the Ti:Sapphire crystal. The scattering increases the laser angular divergence, but, at the same time, produces a coupling, required for the synchronization of the generation over the laser aperture. The appearance of the nonuniformities under the pumping is explained by well-known RIC-effect.

Mechanisms of laser-induced damage in absorbing glasses with nanosecond pulses

The propagation of 355-nm, nanosecond pulses in absorbing glasses is investigated for the specific case examples of the broadband absorbing glass SuperGrey and the Ce³⁺-doped silica glass. The study involves different laser irradiation conditions and material characterization methods to capture the transient material behaviors leading to laser-induced damage. Two damage-initiation mechanisms were identified: (1) melting of the surface as a result of increased temperature; and (2) self-focusing caused by a transient change in the index of refraction. Population of excited states greatly affects both mechanisms by increasing the transient absorption cross section via excited-state absorption and introducing a change of the refractive index to support the formation of graded-index lensing and self-focusing of the beam inside the material. The governing damage-initiation mechanism depends on the thermodynamic properties of the host glass, the electronic structure characteristics of the doped ion, and the laser-spot size.

Modeling population and thermal lenses in the presence of Auger Upconversion for Nd(3+) doped materials

We present a theoretical model to thermal (TL) and population (PL) lenses effects in the presence of Auger upconversion (AU) for analysis of Nd(3+) doped materials. The model distinguishes and quantifies the contributions from TL and PL. From the experimental and theoretical results, the AU cannot be neglected because it plays an important role on the excited state population and therefore on the temperature and polarizability difference between excited and ground states. Considering the extensive use of these techniques, the model presented here could be useful for the investigation of materials and also to avoid misleading analysis of lenses transients.

Multi-octave supercontinuum generation from mid-infrared filamentation in a bulk crystal

In supercontinuum generation, various propagation effects combine to produce a dramatic spectral broadening of intense ultrashort optical pulses. With a host of applications, supercontinuum sources are often required to possess a range of properties such as spectral coverage from the ultraviolet across the visible and into the infrared, shot-to-shot repeatability, high spectral energy density and an absence of complicated pulse splitting. Here we present an all-in-one solution, the first supercontinuum in a bulk homogeneous material extending from 450 nm into the mid-infrared. The spectrum spans 3.3 octaves and carries high spectral energy density (2 pJ nm⁻¹–10 nJ nm⁻¹), and the generation process has high shot-to-shot reproducibility and preserves the carrier-to-envelope phase. Our method, based on filamentation of femtosecond mid-infrared pulses in the anomalous dispersion regime, allows for compact new supercontinuum sources.

Broadband coherent light sources are crucial for numerous applications, such as imaging and spectroscopy. Using filamentation of mid-infrared laser pulses in bulk crystals, Silva et al. generate supercontinuum spectra over three octaves, from 4.5 μm to 450 nm, with carrier-envelope phase stability.

Detailed characterization of pump-induced refractive index changes observed in Nd:YVO(4), Nd:GdVO(4) and Nd:KGW

The refractive index changes which can be induced in the Nd:YVO(4), Nd:GdVO(4) and Nd:KGW high gain laser crystals, when their Nd(3+) laser active ions are pumped from their ground- to excited- energy levels, have been carefully measured and characterized. By using two complementary optical techniques based on pump-probe interferometry and transient diffraction grating, the electronic and thermal contributions to the observed refractive index variations have been accurately determined and successfully exploited to derive various parameters such as polarizability changes, thermo-optic coefficients and thermal diffusivities.

Nonlinear electronic line shape determination in Yb3+-doped phosphate glass

Nonlinear refraction spectroscopy has been performed in Yb3+-doped phosphate glass to determinate the line shape of real and imaginary parts of n2 (n2' and n2"). The n2' spectrum presented an asymmetric feature due to the interference of resonant and nonresonant contributions, where the nonresonant term arises from the polarizability difference between excited and ground states (delta alpha). The measurements were performed in the transient regime to determine population dynamics and the pump saturation intensity at 975 nm (peak of the absorption spectrum). Because of the small quantum defect of Yb3+, we estimated that the magnitude of the thermal lens effect is approximately 20 times smaller than the population lens effect, caused by n2.

Observation of a 100-MHz frequency variation across the output of a frequency-doubled injection-seeded unstable-resonator Q-switched Nd:YAG laser

We report high-resolution measurements of the spatial variation of the optical frequency of an injection-seeded unstable-resonator Q-switched Nd:YAG laser. Images of the second harmonic taken through a molecular-iodine notch filter show frequency variations of as much as 100 MHz (second harmonic) between the center and the edge of the beam.

Laser linewidth narrowing induced by amplifier gain saturation

We demonstrate that temporal broadening of transform-limited nanosecond pulses during gain saturation in a Nd:YAG amplifier can produce substantial linewidth narrowing. Measurements of linewidths with two-color Doppler-free saturated spectroscopy in I(2) show reductions in linewidth by more than a factor of 2.5 from 58 to 20 MHz when a double-pass amplifier is used. The observed temporal broadening is described well by a simple gain saturation theory.