Pulse duration spectra and measurements for laser scanning microscope systems

Computer-controlled, repeatable ultra-short pulse lasers and a broad wavelength range autocorrelator have opened up practical spectral investigations of the group delay dispersion in laser scanning microscope systems. The laser output pulse duration was measured via intensity-based (two-photon absorption) autocorrelation and the laser spectral bandwidth was measured via spinning spectrometer. The separate measurements provided the ∼0.39 time-bandwidth product for two different Coherent ultra-short pulse lasers. The laser-scanning system pulse durations were measured at the sample plane with high numerical aperture objective lenses. The pulse broadening of ultra-short laser pulses through a laser-scanning multi-photon system has been characterised experimentally from 720nm to 950nm. The pulse spreading of individual laser-scanning system components was estimated from Gaussian pulse dispersion with data acquired using a standard, characterized objective lens and an external GaAs:P diode signal. The objective lens was found to produce the most pulse broadening, and most apochromatic objective lenses showed two-fold increases in the magnitude of dispersion values. All spectra of measured components followed the same shape and trend, as expected for normal dispersion. During this study, the average power incident on the Carpe autocorrelator was kept below ∼50mW (130fs and 800nm), but this limitation has been removed in current versions of the autocorrelator. Dielectric broadband reflector mirrors introduce significant dispersion, which changes sign with wavelength; this can reduce or increase the system pulse spreading, but will certainly make component spectral measurements challenging.