Apodization
All chirped QPM devices exhibit a spectral ripple unless apodization techniques are employed. This ripple originates from the abrupt turn-on of the nonlinear susceptibility at the edge of the crystal. With apodization, this spectral ripple is removed by turning the coupling between the three fields (idler, signal and pump) on and off at the edges of the device.
The two primary techniques for QPM apodization are the duty cycle apodization (where the QPM duty cycle is changed from 50% to 0%) and nonlinear chirp apodization (where the chirp rate is rapidly and smoothly increased). These techniques have very recently been studied in detail external page[1] (by us and the group of Prof. Fejer at Stanford University).
Our group has been performed the first experimental demonstrations of the critical importance of apodization in APPLN based OPCPA DownloadRef. [333] (PDF, 1021 KB)DownloadRef. [339] (PDF, 863 KB). In accordance with the mentioned theory, our experiments (Fig. 1 and 2) showed that nonlinear chirp apodization offers the best performance and the greatest flexibility.
References
[1] C. R. Phillips, C. Langrock, D. Chang, Y. W. Lin, L. Gallmann, and M. M. Fejer,
external page“Apodization of chirped quasi-phasematching devices,”
J. Opt. Soc. Am. B, Vol. 30, Issue 6, pp. 1551-1568 (2013)
[333] C. Heese, C. R. Phillips, L. Gallmann, M. M. Fejer, U. Keller
Download“The role of apodization in optical parametric amplifiers based on aperiodic quasi-phasematching gratings” (PDF, 1021 KB)
Optics Express, vol. 20, pp. 18066-18071, 2012
[339] C. Heese, C. R. Phillips, B. W. Mayer, L. Gallmann, M. M. Fejer, U. Keller
Download“75 MW few-cycle mid-infrared pulses from a collinear apodized APPLN-based OPCPA” (PDF, 863 KB)
Optics Express, vol. 20, No. 24, pp. 26888-26894, 2012