Optical Parametric Amplifiers using Chirped Quasi-Phase-Matching Gratings


Several scientific and technological applications require short optical pulses with large peak power. Optical amplifiers are critical components of these high-power ultrafast laser systems. Conventionally, this role is played by solid-state optical amplifiers, but increasingly, optical parametric amplifiers (OPAs) are being used. The amplification of femtosecond pulses with OPAs often requires an effort to broaden the amplification bandwidth. The approach explored in this work consists in using chirped quasi-phase-matching (QPM) gratings.

We first describe a 1-D model, which, in the case of linearly chirped QPM gratings, predicts a flat gain over a wide bandwidth. We also explore more general phase-matching profiles, such as apodized profiles for reducing the gain and phase ripple, periodically modulated profiles for selective frequency amplification and a tandem grating design for simultaneous gain and group delay control.

We carried out a simple experiment to verify these predictions. The experiment revealed two unexpected effects: the presence of stronger parametric fluorescence than anticipated from the 1-D model and different behavior associated with positive and negative chirp rates. These phenomena can be explained by a 2-D model which includes non-collinear interactions and transverse localization of the gain due to the finite diameter of the pump beam. This model reveals the existence of localized growing modes, which can be amplified over the entire length of the grating and offer a gain much larger than expected in a 1-D model.

The existence of large-gain noncollinear growing modes imposes constraints to the design of chirped-QPM grating OPAs. We formulate design guidelines to suppress those undesired effects.


June, 2007