Investigation of the Feasibility of Mode-Locked Semiconductor Devices as Excitation Sources for Two-Photon Fluorescence

Abstract

<p> The potential of a mode-locked semiconductor laser oscillator as a short pulse source for two-photon fluorescence microscopy is explored. Amplification of the 1075 nrn laser is performed with a single pass semiconductor optical amplifier or a ytterbium-doped fibre amplifier. The mode-locked diode oscillator amplified by the Yb-doped fibre amplifier has been shown to produce uncompressed pulses of 4-10 ps with an average power of up to ~0.8 W. Compression with a single pass modified grating pair compressor reduces the pulse duration to as short as 860 fs. The output power level can be easily scaled to higher values. </p> <p> The ability to tightly focus the Yb-doped fibre amplifier beam and semiconductor optical amplifier beam for the purpose of microscopy is studied. Results indicate that the fibre performs close to an ideal Gaussian laser beam source. The semiconductor optical amplifier beam does not focus as well. Measurements suggest that regions of the beam, when focused, do not significantly contribute to the generation of two-photon fluorescence. </p> <p> The efficiency of two-photon fluorescence generation of the two amplifiers is compared to that of the conventional two-photon excitation source: the mode-locked titanium sapphire laser. Results illustrate the need to improve certain operating parameters of the laser oscillator and two amplifiers to be considered practical for two-photon fluorescence microscopy. The mode-locked semiconductor laser oscillator amplified by the Yb-doped fibre amplifier is deemed to be close to being ready for two-photon imaging applications. </p>