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.