MEASUREMENT OF FEMTOSECOND PULSES IN THE FOCAL POINT OF A HIGH-NUMERICAL-APERTURE LENS BY 2-PHOTON ABSORPTION

We demonstrate a method for the measurement of femtosecond optical pul ses in the focal point of a high-NA lens, using interferometric autoco rrelation through two-photo absorption. A chirp-free input pulse of 47 fs is found to broaden by approximate to 50% after focusing by a well -compensated objective. With proper prechirp compensation, the actual pulse width in the focus of such a lens system can be restored to (alm ost) its initial value. The unique value of the presented two-photon a utocorrelation technique is its capability of measuring the actual pul se width at the focal point of a high-NA lens, an aspect that is of di rect importance to two-photon imaging approaches, for example.