Femtosecond pulse width control in microscopy by two-photon absorption autocorrelation

A technique for both the measurement and the control of the pulse width at the focal point of a high-NA lens, based on two-photon absorption interferometric autocorrelation, is presented. The technique is applied to measuring the pulse broadening induced on a pulse propagating through a high-NA lens system for several objectives. It is known that the pulse width may increase by up to ?50% of its original value by propagation through an objective which has wide field compensation for spherical and chromatic aberrations. The two-photon absorption autocorrelation technique allows adjustment of the actual pulse width in the focus of a high-NA lens through pre-chirp compensation. The pulse width is shown to be almost independent of penetration depth into the sample, while the amplitude of the autocorrelation signal shows a strong decrease with depth. The ability to both measure and control the actual pulse width under strong focusing conditions is of direct importance to, among others, two-photon absorption imaging approaches.