On-off laser delivery into a precise tissue area using smart tissue-activated fiber probes

A novel and simple concept for on-off switching laser radiation delivery into a precise tissue area using tissue-activated optical fiber probes is demonstrated. The authors present the operating principle and general optical features of the fiber-optic-based delivery technique. The basic idea includes the use of a single delivery fiber with a specially shaped angled tip. Because of the frustrated-total-internal reflectance caused by the refractive-index change of the surrounding medium, the angled fiber tip acts as a smart tissue-activated probe. It provides a safe way for laser delivery that includes only two states of tissue illumination: 1) off-state (no tissue illumination), when the fiber tip is out of the tissue area and the laser emission is backreflected due to total-internal-reflection and 2) on-state (maximum tissue illumination), when the fiber tip is on the absorbing tissue area and becomes "transparent" because of the frustrated-total-internal reflectance. Here, optical properties of tissue-activated fiber probes used for precise laser delivery are investigated both experimentally and theoretically by analyzing the backreflectance signal power. Optical fibers working in the visible and mid-infrared spectral regions with various geometrical parameters are used and a spatial resolution of 2 μm is achieved when the fiber tip is moved toward the absorption tissue surface.