New technique for dispersion compensation in compact femtosecond lasers

Summary form only given. Femtosecond laser sources usually require negative group delay dispersion (GDD) to compensate for the positive dispersion of the gain material and possibly other cavity elements. Various techniques to generate negative GDD from wavelength-dependent refraction at Brewster interfaces have been demonstrated. The resulting negative GDD is usually the product of a purely material-dependent factor and the distance L between the Brewster interface and the X-point, which we define as the point where the axes of the resonator modes with different frequencies intersect. Therefore, large values of negative GDD normally require large distances L, which may be a problem, e.g. when a short cavity length is required. We demonstrate that, for a given value of L, both the angular dispersion and the negative group delay dispersion generated by a Brewster interface can be strongly enhanced by the influence of a focusing action, which may arise from a curved surface or a thermal lens in the gain medium. We verified our theoretical prediction in a diode-pumped high-power Nd:glass laser.