Acceptance Angle in Fiber Optics
The acceptance angle of an optical fiber is defined based on a purely geometrical consideration (ray optics): it is the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core. The sine of that acceptable angle (assuming an incident ray in air or vacuum) is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum:
For larger incidence angles, there is no total internal reflection, and much of the incident light will not be reflected at the core–cladding boundary. It will thus get into the cladding and will then usually experience strong propagation losses particularly at the outer part of the cladding.
For a strongly multimode waveguide, the acceptance angle can be used to estimate the maximum input angle of a laser beam for which a high launch efficiency of the waveguide can be achieved. For single-mode fibers, however, this rule does not hold, as explained in the following.
The concept of ray optics (geometrical optics) is not fully appropriate for describing the operation details of optical fibers, because wave aspects are important – particularly for fibers with small core such as single-mode fibers, while the approximation is more appropriate for large-core multimode fibers. A real light beam (for example, a laser beam) is not well resembled by a ray, since it inevitably has both a finite beam radius and a finite beam divergence. Therefore, there is in reality not a well-defined transition between guidance and non-guidance, when a beam angle is varied; the launch efficiency varies gradually. Only in the limit of a highly multimode waveguide, such estimates based on geometrical optics become reasonably accurate.
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