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Conference on Lasers and Electro-Optics

Optically induced birefringence in TI:Al2O3

Roshan L. Aggarwal, Kevin F. Wall, R. E. Fahey, A. J. Strauss

Published Apr 25, 1988• 0 citations
OpenAlex

Abstract

We have observed optically induced birefringence1 in Ti:AI2O3 at signal wavelengths of 632.8 and 790.7 nm during the course of small-signal gain measurements2 on a Ti:AI2O3 amplifier pumped with 532-nm radiation from a frequency-doubled Q-switched (10-ns pulse) Nd:YAG laser. The changes in intensity of the signal beam due to induced birefringence were found to decay with a time constant characteristic of the fluorescence lifetime of Ti3+ ions. We, therefore, attribute the induced birefringence to the Ti3+ excited-state (2Eg) population.

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Japanese Journal of Applied Physics•Dec 1, 1997•1 citation
We have observed optically induced birefringence1 in Ti:AI2O3 at signal wavelengths of 632.8 and 790.7 nm during the course of small-signal gain measurements2 on a Ti:AI2O3 amplifier pumped with 532-nm radiation from a frequency-doubled Q-switched (10-ns pulse) Nd:YAG laser. The changes in intensity of the signal beam due to induced birefringence were found to decay with a time constant characteristic of the fluorescence lifetime of Ti3+ ions. We, therefore, attribute the induced birefringence to the Ti3+ excited-state (2Eg) population.
In this experiment, Li2 is excited from ground electronic state to the first excited state by 1 μJ, 75 s pulses from an amplified CPM laser at about 620 nm. The birefringence created by the pump pulses is measured at a delayed time by using a white-light-continuum probe (10 nm width, center at 770 nm and 800 nm). Unlike similar experiments, we are pumping in the resonance regime (the Rabi period here is 1-10 ps). This experiment is laying the groundwork for the use of shaped ultrafast pulses to develop a birefringence time history by design. We have computed a birefringence and dichroism signal based upon spectroscopy parameters. Most of the features have been confirmed experimentally. Experimentally, a birefringence step is observed after the initial pump pulse; this is due to a population transfer that results from the pumping process. Peaks that appear later are believed to be due to rotational coherences in the ground and first excited states.
We studied both experimentally and theoretically the transmission and polarization characteristics of a Cr4+ : YAG saturable absorber crystal as a function of the intensity and polarization state of an incident laser beam. We found that a birefringent absorption behaviour is induced and we show that the experimental results are well described by a full numerical model which includes excited-state absorption as well as a ground-state cross-absorption term. By using pump-probe measurements we show that a finite cross-saturation of the absorption is achieved along the crystal axes orthogonal to the polarization of the optical radiation. No induced refractive index birefringence is observed.
The mechanism for refractive-index changes that accompany changes in population during pumping and amplification of an optical beam is studied in a Nd:YAG laser crystal by both resonant four-wave mixing and interferometric techniques. It is found that these refractive-index changes (connected with the difference in polarizability of excited and unexcited Nd3+ ions) have two components: a small noninertial component that accompanies an amplified optical pulse and the major changes that are intensified with a delay time of ∼3 µs. The latter effect is explained by population (or depopulation) of a higher-lying energy level 2(F2)5/2 of the 4f electron shell of Nd3+ ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the inverted Nd:YAG caused by the refractive-index changes is also discussed.
A polarising interferometer was used to investigate changes in the refractive index of an Nd:YAG laser crystal associated with changes in the populations of the electronic energy levels characterised by different polarisabilities. The study was carried out on diode-pumped crystals as well as on crystals which were flashlamp-pumped under conditions of saturation of the gain experienced by the optical beam. The changes in the refractive index depended nonlinearly on the pump power. The ratio of the real and imaginary parts of the nonlinear susceptibility of an Nd:YAG laser amplifier was determined: this ratio rose from 0.2 to 1.2 when the gain increased from 0.2 to 0.45 cm -1 .
Femtosecond free induction decay (FID) beats were observed for the 4F3/2 - 4I9/2 transition of Nd3+ ions doped in Y3Al5O12 (YAG) crystal at room temperature using a mode-locked Ti3+:Al2O3 laser. This is the first observation of room-temperature dephasing processes in the time domain for rare-earth ion doped materials. The FID beat signal shows a dip structure produced by the interference between the pump pulse and the FID signal, and a quantum beat. The FID signal became large and comparable to the pump pulse as the crystal length was increased. The FID intensity evolution due to the pulse propagation is explained well by phenomenological equations.