Measurement of femtosecond pulses in the focal point of a high-numerical-aperture lens by two-photon absorption

We demonstrate a method for the measurement of femtosecond optical pulses in the focal point of a high-NA lens, using interferometric autocorrelation through two-photon absorption. A chirp-free input pulse of 47 fs is found to broaden by ≈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 (almost) its initial value. The unique value of the presented two-photon autocorrelation technique is its capability of measuring the actual pulse width at the focal point of a high-NA lens, an aspect that is of direct importance to two-photon imaging approaches, for example.