In optical fiber, total internal reflection occurs at which interface?

• A. All the core-to-cladding interface
• B. Between core and air interface
• C. Along the fiber length
• D. At the cladding boundary

Answer: (A)

Light is confined in an optical fiber by total internal reflection at the boundary where light moves from a higher refractive index medium to a lower refractive index one. In a fiber, the core has the higher index and is surrounded by cladding with a lower index. When light traveling in the core hits the core–cladding boundary at angles steeper than the critical angle, it cannot refract into the cladding and is completely reflected back into the core. This reflection happens at all around-the-core interfaces as the light travels, keeping the light guided along the fiber. The core–air boundary or other boundary descriptions don’t apply to the typical fiber design, since the cladding is what provides the lower index and the confinement mechanism.