Optical characteristics of laser medical instrument with side-firing fiber under complete bevel angle range.

The side-firing instrument is studied under the complete bevel angle range. The fiber core and cladding are 0.6 mm and 0.66 mm, respectively, the fiber core refractive index is 1.457, and the fiber cladding refractive index is from 1.409 to 1.452 corresponding to the numerical aperture from 0.37 to 0.12. The bevel angle range is subdivided by ten crucial angles, whose relationship changes as the fiber cladding refractive index reaches 1.418. The beam's divergence angle and coverage increase as the bevel angle deviates from being equal and close to π/4 rad, respectively. When all rays achieve total internal reflection, with numerical aperture being 0.37, the divergence angle and coverage would increase by 28.29% and 44.74%, respectively. The required emission opening size has a minimum under the bevel angle being close to π/4 rad, whose expression is obtained. It increases sharply as the bevel angle reaches a certain value.