Ray-tracing model for the stretching calculation of the new type aberration-free stretcher

Chirped pulse amplification(CPA) has become a common technique for the generation of ultrashort and high peak power femtosecond(fs) optical pulses. One key point of this technique is the reasonable design for the pulse stretcher. We improved the aberration-free stretcher replacing the two spherical mirrors with two concentric rectangular focused cylindrical ones. The first mirror is convex, and the second one concave. Their radius of curvature ratio is two and of opposite sign. This combination cancels spherical aberration and astigmatism, and has no on-axis coma and exhibits no chromatic aberration. We calculated the group velocity dispersion(GVD) of the stretcher by ray-tracing model. Supposing that the distance between the grating and the center of curvature is a variable l, we obtained the variety of the group delay and the GVD of the stretcher with the parameter l and the incidence wavelength λ. For the input pulse, let pulsewidth t₀ = 9.8 fs, the initial chirp C=0.75, the distance l=0.1m, we gained GVD was 2.5fs²/μm for the center wavelength 790nm, the stretching factor was 90197.4, that is, the seed pulse(9.8fs in duration) can be stretched to 884 ps.