Recent advances in fluorescence microscopy add to the versatility of this optical technique and intensify its significance as an indispensable tool in biological research. Especially the use of multiphoton excitation offers the microscopist many advantages like inherent optical sectioning, reduced out-of-focus bleaching and higher penetration depths into the sample. In this regard, the commercial availability of pulsed solid-state lightsources like the Ti:Sapphire laser, that provide short pulses needed in the nonlinear multiphoton process, have paved the way for the routine implementation of multiphoton microscopy in the biologists laboratory. Although the spectral range of the commonly used Ti:Sapphire laser allows the application of two-photon fluorescence microscopy on chromophores that absorb in the range of 350-450 nm, a lasersource that enables the two-photon excitation of molecular probes at even shorter wavelengths (<350nm) would be highly beneficial.

In this contribution we present a visible femtosecond optical parametric oscillator (OPO, Figure 1) that is ideally suited to excite molecular species at 285-335 nm by means of a two-photon process. Femtosecond pulses with durations as short as 30 fs can be generated within a tuning range from 570 to 670 nm. A cavity dumper incorporated in the laser cavity provides variable pulse repetition rates (single shot to 82 MHz).