Transmission of LED-light through optical fibers for optical tissue diagnostics

Background: High brightness light emitting diodes (LEDs) have become available that appear suitable to replace light sources currently used for illumination through thin light guides, e.g. in endoscopy. We investigated the essential characteristics of a series of commercial single LEDs relevant for direct coupling to a single optical multimode fiber. Methods: LED fiber coupling efficiency was assessed experimentally and theoretically by using a ray tracing software. Results: Surface emitting LEDs proved suitable to be coupled directly into plastic optical fibers (POFs). We have successfully applied a 1 mm core POF-fiber (outer diameter 1.01 mm) in contact with a OSTAR LED (Osram Opto Semiconductors, Regensburg, Germany) to achieve a coupling efficiency of 10-20%, which gave 42 mW, 23.7 mW and 27 mW for blue, green and red LED respectively. Ray tracing simulation revealed a considerable part of photons travelling "out of axis" in spirals along the core-clad interface (non-meridional beams). They account for approximately 30% of the transmitted power.