Ultrahigh-sensitivity ultralow-noise optical fiber endoscope

Optical fiber probes are generating a large interest to develop portable fluorescence spectrometers. However, the sensitivity of conventional fibers is severely limited by the high luminescence background generated in the glass and the low collection efficiency of fibers.

We solve this issue in a recent Optics Express publication by using a hollow core photonic crystal fiber probe combined with a polystyrene microsphere. Thanks to the hollow-core photonic crystal fiber, the background noise is reduced by two orders of magnitude. Thanks to the microsphere, the excitation beam is further focused and the fluorescence collection efficiency is improved.  As compared to the previous state-of-the-art, we report a 200x improvement of the signal-to-noise ratio for single molecules detection events, together with a 1000x gain on the minimum detectable concentration.

Our approach takes advantage of a polystyrene microsphere directly set at the fiber end-face to focus the fiber mode down to a spot of 540 nm FHWM. These results and their application for fluorescence imaging and direct laser writing have been reported in Applied Optics.

We believe that this device offers new opportunities for remote or in vivo optical characterization together with a miniaturization of bulky microscope setups. Applications include inspection of semiconductor wafers, photolithography, laser surgery, and fluorescence sensing.