Single mode fibers used for telecommunication networks are designed carry optical signals for long distance with low transmission loss. One of the prime design criteria is the design of the Core-Clad interface of the fiber. Based on the doping in core and cladding, Single mode fibers can be classified as Step index and Graded index.
Step index single mode fibers can have a Matched Clad design or Depressed Clad design. Graded index single mode fibers are typically used in sensor applications while step index single mode fibers find their application in telecommunications.
Matched-cladding means that the fiber cladding consists of a single homogeneous (matching) layer of glass (dielectric material). The cladding region has only one refractive index profile.
Depressed-cladding means that the fiber cladding consists of two regions: the inner and outer cladding regions. The cladding region has two distinct refractive index profiles.
Refractive index profiles of matched-clad and depressed-clad single mode step-index fibers are different and both have unique refractive index profiles. Both type of fibers can be identified by their refractive index profiles. Depressed-clad fibers have advantage over Matched-clad fibers in terms of bending loss performance. Due to the depression inside the cladding, a significant portion of the light that might try to escape through cladding when a fiber is bent, will be forced to return back to the core-cladding region.
A matched-clad single mode step-index fiber has a core with a constant refractive index. It has a cladding of slightly lower refractive index surrounding the core. The below diagram shows the refractive index profile for the matched-clad single mode fiber.
Clad Core Clad
Matched-clad refractive index profile
On the other hand, a depressed-clad single mode step-index fiber has a core with a constant refractive index. A cladding, made of two regions, surrounds the core. An inner cladding region surrounds the core of the fiber and has a refractive index of n2. The refractive index of inner cladding is lower than the core’s refractive index. An outer cladding region surrounds the inner cladding region and has a higher refractive index than the inner cladding region. However, the outer cladding refractive index is lower than the core’s refractive index. The below diagram shows the refractive index profile for the depressed-clad single mode fiber.
Outer clad > Inner clad < Core > Inner clad < Outer clad
Depressed-clad refractive index profile
Irrespective of the design of refractive index profile, a Single mode step-index fiber allows only one mode. This means that both matched-clad step index single mode fiber and depressed-clad step index single mode fiber allows the propagation of fundamental mode only, due to the the core-clad geometrical relations. For both depressed-clad and matched-clad single mode fibers, the normalized frequency value is between 0 and 2.405. The value of V should remain near the 2.405 for a single mode fiber. A single mode fiber will carry majority of light in the cladding region of fiber if the normalized frequency value, that is V is less than 1. Light traveling through the cladding region will try to escape and cause higher losses at bends and splice points.