GRENOUILLE

Grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields (GRENOUILLE) is an ultrashort pulse measurement technique based on frequency-resolved optical gating (FROG). The acronym was chosen because of the technique's relationship to FROG; grenouille is French for frog.

Because most FROG techniques have an , they also have the sensitive alignment issues that come with it. In addition, most FROGs use a thin second harmonic generation (SHG) crystal and a spectrometer, adding signal strength requirements as well as additional alignment issues. GRENOUILLE is a simple device based on the SHG FROG, replacing the beam splitter, delay line and beam recombination components of the autocorrelator with a prism, and replacing the spectrometer and thin SHG crystal combination with a thick SHG crystal. The effect of these replacements is to eliminate all sensitive alignment parameters while increasing the signal strength. These changes also reduce the complexity and cost of this type of system. However like the previous systems, GRENOUILLE still determines the full phase and intensity data of a pulse, and produces traces identical in form to those from SHG FROG.

A typical GRENOUILLE setup used with a theoretical square input beam can be seen above. The first element, a horizontal cylindrical lens, is used to tightly focus the incoming signal beam into a horizontal stripe at the thick SHG crystal in order to yield a range of crystal incidence angles (more on this below). While being focused, the beam is passed through a Fresnel biprism with an apex angle close to 180°. The Fresnel biprism is essentially two thin prisms joined at their base. The effect of this element is to split the beam into two sources and superimpose the two at the focus point in the SHG crystal, thus mapping delay to horizontal position. This replaces the function of the autocorrelator in the original FROG designs. However, unlike the autocorrelator, the beams from the Fresnel biprism are automatically aligned in time and space, eliminating a number of sensitive alignment parameters.

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