Ida Zsuzsanna Kozma, Peter Baum, Uli Schmidhammer, Stefan Lochbrunner

The tunable visible and near IR output of NOPAs are routinely used in femtosecond spectroscopy. There is a need for instrumentation that makes characterization of the temporal profiles of these pulses fast and reliable. Secondharmonic autocorrelation is the most straightforward approach by which to measure the time dependent intensity of an ultrashort pulse. More advanced method - like FROG and SPIDER - are capable of complete electric field characterization, however, they require complex setups and elaborate mathematical retrieving procedures. Intensity autocorrelations, on the other hand, provide sufficient information for the determination and optimization of the NOPA output for practical applications. This method allows one to determine the pulse duration with reasonable accuracy and, with the help of the pulse spectrum, to estimate the magnitude of the chirp.   The following requirements were postulated for a new autocorrelator design: functionality in the full NOPA tuning range with minimal modification, high sensitivity, real-time pulse duration evaluation for sources with 100 Hz-10 kHz repetition rate, simple in construction, dispersion-free operation. The most demanding requirement for precise autocorrelation of extremely short pulses is dispersion-free operation. The scheme depicts the optical arrangement of our dispersion-free all-reflective autocorrelator that utilizes a novel concept (U.S. Patent No. 6,671,053, International Patent No. WO 00/77482 A1).   The pulse enters through an iris and two replicas of it are reflected off the half-mirror pair. One half mirror is fixed on an adjustable mirror mounting and the other on a piezoceramic translator (PZT), which scans the optical delay. An off-axis parabolic mirror focuses the two replicas into one common spot in a thin BBO crystal under a small angle. When there is temporal overlap between the two replicas a sum frequency optical signal is generated in between the two frequency doubled signals. A second iris behind the BBO crystal blocks the fundamental light beams and the frequency doubled beams, while a color filter placed before the detector ensures spectral filtering of the remaining fundamental light.     By selecting the appropriate bandpass filter, and a BBO crystal with the proper orientation and cut angle, the autocorrelator can be operated in the different wavelength regions. The PZT is operated in open-loop mode. The integrating photodiode module and the data processing algorithm makes online evaluation of the autocorrelation possible.

"Convenient pulse length measurement of sub-20-fs pulses down to the deep UV via two-photon absorption in bulk material" C. Homann, N. Krebs and E. Riedle Appl. Phys. B 104, 783 - 791 (2011). Details "Compact autocorrelator for the online measurement of tunable 10-femtosecond pulses" I. Z. Kozma, P. Baum, U. Schmidhammer, S. Lochbrunner, and E. Riedle Rev. Sci. Instrum. 75, 2323 - 2327 (2004) Details