The optimized Gires Tournois mirrors used for a femtosecond mode locked Ti:sapphire laser is reported. The electric field distribution as a function of wavelength inside the optimized Gires Tournois mirrors is calculated. The penetration depths in these mirrors change with wavelength. The penetration depth increases within the wavelength range from 720 nm to 9000 nm. So they can give larger time delay at longer wavelength range. They can be used as intracavity dispersion compensation devices instead of traditional prism pairs. With these mirrors and a semiconductor saturable absorber mirror, self starting 56 fs pulses was obtained in a mode locked Ti:sapphire laser.