US Patent 7956317 Fibre optic probe

A bidirectional fiber optic probe comprises an optical in/out coupler and a single fiber or a bundle of fibers, each fiber having a proximal end and a distal end and a numerical aperture NA=sin θ. The numerical aperture NA describes the range of angles over which the optical fiber's proximal end can accept or emit light. The numerical aperture depends on the refractive index n of the fiber core and is given by NA=n sin* θ. θ is the acceptance angle being defined as the half angle of the acceptance cone of the fiber at its proximal end.