Pulse compensation for dispersive optical elements. Ultrafast lasers necessarily support a frequency spread. The transform-limited pulse duration, or shortest pulse for a given frequency spread, is determined by the uncertainty principle. However utilizing short pulses (<100 fs) introduces an experimental problem; dispersion of the wavelength band by microscope components can be considerable. In most optical elements bluer frequencies travel slower than redder ones (positive dispersion). Linear dispersion can be compensated by pre-chirping the pulse, that is by using prisms or spectral gratings to introduce a frequency dependent optical path difference designed to effect negative dispersion. With an appropriate pre-chirp, red lags blue before the microscope but catches up to form an approximately transform-limited pulse within the sample.

Using the apparatus diagrammed at the bottom left, in which a lab-built Michelson interferometer is introduced to produce an oscillating pulse delay for autocorrelation pulse-width measurements at the sample, we have characterized the group delay dispersion (GDD) of various microscope lens systems (Guild et al., 1997). GDD is found to be primarily derived from the objectives and for complex lenses it can be significant (1000-6000 fs 2 for the range of objectives studied).

The plot at the bottom right shows the measured interferometric autocorrelation trace of Ti:S laser pulses (730 nm) at the focus of a complex objective (1.4 NA oil Plan Neo-Fluar) a) without compensation and b) with compensation. Without compensation, the ~5-fold spread in the pulse due to dispersion corresponds to a GDD of 5500 fs 2 and results in a 5 times decrease in the measured 2PE signal.

Reference:

Guild, J.B., C. Xu and W.W. Webb, "Measurement of group delay dispersion of high numerical aperture objective lenses using two-photon excited fluorescence," Applied Optics 36(1), 397-401, 1997