MULTIPHOTON SPECTROSCOPY THROUGH OPTICAL SINGLE MODE FIBERS

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Autofluorescence in tissues can be used as a diagnostic tool in photodynamic therapy (e.g. for cancer). Multiphoton excitation; i.e. near infrared wavelength range; can be used for autofluorescence imaging where biological tissues are most transparent. However, the penetration depth is still limited (~200 ?m) and even more problematic for autofluorescence detection. Using single-mode fiber for both excitation and detection simultaneously (mutliphoton endoscopy) offers an excellent solution. Therefore, preliminary studies on the broadening effects of femtosecond laser pulses (810-980 nm) and the intensity-dependence of the fiber coupling efficiency has been carried out. Furthermore, the detected fluorescence (of fluorescein, ~200 ?M; dissolved in water) through the same fiber has been resolved in the frequency domain. Power-dependent measurements reveal a genuine two-photon fluorescence.