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.

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