Abstract

In a widely used technique for the characterisation of second-order nonlinear gratings the measuring of the quasi-phase-matching (QPM) curve for second-harmonic generation (SHG) is achieved by continuously varying either sample temperature or fundamental (F) wavelength. This curve reveals the information concerning the length and the regularity of the nonlinear grating. This technique exhibit some drawbacks, especially for long gratings. A temperature gradient, practically unavoidable over a long grating, leads to a chirp in the period or the need for very fine scan step in the tuning of the fundamental wavelength are some of these drawbacks. To overcome these constraints, we propose a method that does not need tuning of any parameter and involves short laser pulses and relatively long nonlinear gratings. The method is applied to the case of SHG of ultra-short pulses in QPM structures without depletion of the pump. Following ref. [1], SH is treated as resulting from the filter response of the nonlinear medium to an input F pulse in the presence Group Velocity Dispersion (GVD). Two different cases can be distinguished, according to the ratio of grating length L to the temporal walk-off length L_wo. When l< L_wo the spectrum of SH is completely defined by the spectrum of F and no useful information on the grating is obtained. When L> L_wo the SH spectrum is directly related to the filter response of the grating and contain important information on the quality of the grating. We tested our method on D-shaped 7.5cm long QPM fibres, obtained by thermal poling [2]. The case L< L_wo is represented in Fig. 1a), where the QPM curve obtained with 2 ns pulses leads to a completely wrong grating length (> 7.5 cm). On the contrary, the spectrum obtained with 100 fs (L> L_wo) reveals the homogeneity of the grating over its whole length (Fig. 1 b)).

© 2000 IEEE