Scientists at the University of Michigan in Ann Arbor have performed real-time, two-photon fluorescence measurements in vitro using a single optical fiber. Employing an 830-nm, 80-fs pulsed laser cycling at 80 MHz as an excitation source, they coupled a beam into an optical fiber that was inserted into a sample. They used the same fiber to collect the fluorescence and a dichroic mirror to distinguish fluorescence from the excitation radiation. They tested the method by measuring the uptake of a targeted drug delivery agent into cancer cells.As reported in the Aug. 15 issue of Optics Letters, the method produces low background noise, permits local detection at specific sites and uses infrared radiation that causes less photodamage than methods employing UV radiation.