Lifetime prediction for 405-nm single-mode delivery systems for therapeutic laser applications

Near-UV laser light is used for soft tissue treatment for several years now. In first applications the light was delivered directly from the laser, but for in vivo treatment more flexibility was needed. Multi-mode fibers can be used to achieve a high output power coupled from multi-mode lasers. If fiber bundles are used the power can be increased additionally. But the power density on the treated tissue does not rise proportionally, because of the larger spot. A better ablation can be achieved with a Gaussian beam profile coming from a single-mode fiber. Higher beam quality and higher intensity from a small single-mode core produce power densities in the order of kW/cm2 in a focus spot smaller than 100 μm. If the laser therapy is used with the scanning fiber endoscope, treatment in between imaging spirals can be employed and only a single fiber is required. 405 nm laser-induced fluorescence may be able to produce both wide-field fluorescence imaging and laser therapy in a single laser. However additional wavelengths combiners and dual-clad couplers are necessary for multi-wavelength reflectance imaging requiring increased input power to compensate for the losses of these devices. This leads to very high intensities at the fiber coupler and damage will occur at this interface. Differences in damage rate due to differently treated fiber end-faces will be discussed. We suggest a new loss mechanism which is basal for the end-face damage and show miscellaneous methods to reduce the occurring damage and enhance the system lifetime.