Including the nonlinear medium´s dispersion in frequency-resolved optical gating

When we measure something, we unavoidably change it, and ultrashort laser pulse measurement is a perfect example. When we measure a pulse, the measurement itself affects the pulse´s shape. All ultrashort pulse measurement techniques use a nonlinear medium, and this medium´s dispersion changes the shape of the pulse as it propagates. For many years, pulse measurers worried obsessively about group-velocity mismatch (GVM), which limited crystal thickness to as little as 5 microns. New frequency-resolved-optical-gating (FROG) variations, such as GRENOUILLE and crystal-angle dithering, either take advantage of GVM effects or avoid them completely, allowing the use of a nonlinear medium more than an order of magnitude thicker than that allowed by GVM considerations. Because nonlinear-optical efficiency scales with thickness, these techniques are considerably more sensitive. On the other hand, the use of such thick crystals allows a usually smaller dispersion effect, group-velocity dispersion (GVD), formerly negligible in all cases, to potentially yield pulse distortions. Fortunately, as we show here, in FROG, we can take advantage of our knowledge of the dispersion and the generality and versatility of the FROG algorithm to precisely remove these adverse effects in angle-dithered FROG and GRENOUILLE devices. This will allow these convenient techniques to measure ever shorter and ever weaker pulses.