Investigation of two-photon absorption and optical parametric oscillation.

摘要

In the first part of this dissertation, we have systematically studied two-photon absorption (TPA) in some important optical materials, i.e. GaSe and GaAs. Specifically, we have measured the spectra of two-photon absorption coefficients for these two materials based on z-scan technique. We have compared our results with theoretical values as well as the previously-measured values. We conclude that for both crystals the measured and theoretical values are in fair agreement. In the second part, we show that intensity-dependent absorption strongly affects the performance of optical parametric oscillators (OPO's), in particular, limit the output powers of the signal wave and tuning ranges. However, in the past there are few quantitative studies on OPO's in the presence of strong TPA. We have shown that TPA strongly affects backward and forward OPO's in certain wavelength ranges. After introducing a normalized coefficient of TPA, we have obtained a simple expression for the threshold intensities in the presence of TPA. This expression is shown to be the same for both configurations of OPO's. The threshold intensities increase as the TPA coefficient increases. At the same time, the conversion efficiencies decrease. We have shown that even if the threshold intensity is just increased by a very small factor due to TPA the conversion efficiency can be reduced considerably. We have considered several nonlinear optical materials as examples to illustrate the trade-off among different parameters such as the two-photon absorption coefficient vs. pump wavelength, and, the pulse energy density vs. crystal length. Our results are important for implementing optical parametric oscillators in the extended wavelength ranges, such as visible, mid-IR, far-IR, mm and THz. Finally, we have demonstrated that the threshold for a mid-IR OPO based on periodically-poled LiNbO3 is significantly reduced after the introduction of an optical amplifier inside the cavity. The thresholds as low as 8 mW and the tuning range of 3.0 to 4.2 mum have been attained for the stable single-mode operation.