Fabrication of homogeneously emitting optical fiber diffusors using fslaser ablation

摘要

The photodynamic therapy (PDT) enables selective treatment of cancerous tissue with light inside the human body. Optical fibers deliver the light into the cancerous tissue and diffusors that are attached to the ends of the fibers scatter the light inside the tumor. In order to destroy the entire cancerous tissue, the emission profile should be uniform along the fiber. Such fiber diffusors can be fabricated directly at the fiber end by ablating light-scattering cavities with fs-pulses around the tip using a combination of scanner and rotation axis. These cavities must be designed such that the light distribution is uniform along the diffusor. For a deeper understanding of the light propagation inside the fiber diffusor, the light transmission is measured during laser processing. The influence of the number of repetitions and of the number of ablated cavities is investigated. In addition, the transmission and the transmission change along the fiber is simulated as a function of the number, distance and size of defects using the Monte Carlo method. The results suggest that as soon as the cavities penetrate into the fiber core the total internal reflection is inhibited and light scatters at the roughened surface. Both simulation and experiment show that the number of emitted photons per cavity remains constant along the fiber until a photon is reflected a second time within the diffusor. Then the number of emitted photons decreases exponentially along the diffusor. In order to achieve a uniform emission profile, first the decoupling probability of the cavities should be constant until a photon is reflected a second time, then the decoupling probability should be increased approximately linear to compensate the attenuation along the diffusor.