Dispersion pre-compensation of 15 femtosecond optical pulses for high-numerical-aperture objectives

The excitation efficiency in two-photon absorption (TPA) microscopy depends strongly - owing to the square dependence of the TPA fluorescence on the excitation intensity - on the temporal width of the excitation pulse. Because of their inherently large frequency bandwidth, ultrashort optical pulses tend to broaden substantially because of dispersion from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of microscope objectives are investigated. It is shown that the induced dispersion can be pre-compensated in all cases for pulses as short as 15 fs. Because of the excellent agreement between the results from theoretical modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well as for even shorter pulses, can be inferred. Since for TPA imaging the background due to single photon absorption processes and scattering is independent of the pulse width, proper dispersion pre-compensation - which minimizes the pulse duration at the focal point and hence maximizes the excitation efficiency - provides optimal image contrast in TPA microscopy.