Inhaltszusammenfassung

The duration of ultrashort laser pulses in the femtosecond regime can only be measured by optical autocorrelation. Therefore, single laser pulses are divided by a 50:50 beam splitter and conducted over two different paths with approximately the same optical length, before they are both focused on a nonlinear optical material that performs second harmonic generation (SHG). By making one of the path lengths adjustable the two pulse halves can be temporally superimposed. Measuring the second har...The duration of ultrashort laser pulses in the femtosecond regime can only be measured by optical autocorrelation. Therefore, single laser pulses are divided by a 50:50 beam splitter and conducted over two different paths with approximately the same optical length, before they are both focused on a nonlinear optical material that performs second harmonic generation (SHG). By making one of the path lengths adjustable the two pulse halves can be temporally superimposed. Measuring the second harmonic signal intensity for adjustments of spatial and temporal overlap in comparison to the left and right-side non-overlap (i.e. using one path to scan the other one), the pulse duration can be calculated from the full width at half maximum (FWHM). This procedure can be seen as the underlying method of several ultrashort pulse measurement techniques like e.g. FROG. The standard nonlinear material for autocorrelators is monocrystalline beta barium borate (BBO), which is used in transmission for non-collinear phase matching. The advantage is to be able to use low pulse energies as well as to measure background free. However, BBO crystals are brittle and expensive. Since fiber-based high energy femtosecond laser systems become more and more achievable, it may not be necessary to use BBO. For generating second harmonic signals we chose a fairly uncommon approach by using nonlinear light-scattering media in diffuse reflection instead of monocrystalline BBO in transmission. Determining the nonlinear optical properties of scattering materials like powders has previously been of interest, but to our knowledge scattered second harmonic signals have not been used for pulse duration measurements by now. We compared the applicability of several light-scattering nonlinear media such as aluminum nitride (AlN), pressed ammonium dihydrogen phosphate (ADP), zinc oxide (ZnO) powder, and ZnO crystals grown by irradiating zinc spray with femtosecond laser pulses. Allowing for measurement inaccuracies, calculated pulse durations are comparable. The scattered signal is typically symmetric and reveals a smooth shape as expected. Although SHG signals for AlN are weaker by a factor of roughly 104 this does not turn out to be problematic for ultrashort laser pulses in the microjoule range at pulse repetition rates of some kilohertz. In addition, homogeneous scattering media do not need complicated adjustment unlike monocrystalline BBO. In conclusion we present a simple and low-cost method to measure the duration of ultrashort pulses in the microjoule range. » weiterlesen» einklappen

Autoren

Klassifikation

DFG Fachgebiet:
Optik, Quantenoptik und Physik der Atome, Moleküle und Plasmen

DDC Sachgruppe:
Physik