DISPERSION PRE-COMPENSATION OF 15 FEMTOSECOND OPTICAL PULSES FOR HIGH-NUMERICAL-APERTURE OBJECTIVES

The excitation efficiency in two-photon absorption (TPA) microscopy de pends strongly - owing to the square dependence of the TPA fluorescenc e on the excitation intensity - on the temporal width of the excitatio n pulse. Because of their inherently large frequency bandwidth, ultras hort optical pulses tend to broaden substantially because of dispersio n from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of micro scope objectives are investigated. It is shown that the induced disper sion can be pre-compensated in all cases for pulses as short as 15 fs, Because of the excellent agreement between the results from theoretic al modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well a s 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 maximi zes the excitation efficiency - provides optimal image contrast in TPA microscopy.