Fiber beam shaping and ophthalmic applications

In recent years laser technology has undergone a tremendous development in the field of ophthalmology. New laser sources such as solid state (Er:YAG, Ho:YAG) and diode lasers have become available and exhibit promising characteristics in view of routine therapeutic procedures. Therapeutic effects are now attempted to be obtained inside the eye by making use of optical fibers having a plane distal end surface. Under these conditions the laser beam emitted from the fiber diverges according to the numerical aperture of the fiber (typically from 0.2 to 0.4). Because the therapeutic effect obtained depends primarily on the power density on the tissue to be treated, optical components can be utilized for refocusing the laser beam. For this purpose, small reshaping micro optics set at the distal end of the fiber have been extensively presented. So far, they have been developed to fit the diameter of endoscopic working channels, i.e., typically about 2.2 mm. However, this diameter should be further reduced for intraocular applications because standardized surgical ports are only 0.9 mm in diameter (20 gauge). After a short review of axial systems based on the refocusing of the beam transmitted from the fiber, a more extended presentation of systems capable of deflecting this beam is given. Designs have been evaluated with ray patterns, spot dimensions, and irradiance distribution. Ray patterns have been computed using ray-tracing algorithms on the basis of the Snell's law of refraction while irradiances have been derived with the 3D optical analysis program ASAP (BRO, USA). Rays were homogeneously distributed, spatially across the fiber core as well as angularly across its numerical aperture. Foci have been defined as the location where the cross-section of the beam is the smallest: all the rays are taken into account regardless of their intensity, so that no irradiation of the targeted tissue occurs outside this area.