There are several important factors determine the optical fiber’s capability to collect light and transmit it along the fiber. These factors include optical fiber’s core size, its mode field diameter and its numerical aperture.
What is optical fiber’s core size?
Core size is the physical dimension of the fiber core. The core’s size has a big impact on coupling light into a fiber. The core’s size has to be at least as big as the light source in order to collect light efficiently. If the light source is larger than the fiber core, much of its power goes into the fiber cladding and escape quickly.
Multimode fibers comes in a variety of core sizes between 7um and 3mm, of which the most usual are 50um, 62.5um, 100um and 200um. The industry standard for data communications is now 50um and 62.5um multimode using silica glass fibers.
Single mode silica glass fibers typical has a 8.3um core size. Telecommunications uses exclusively single mode glass fibers, though data communications also shows a slow but steady adoption of single mode glass fibers.
For plastic optical fibers, the fiber core size ranges from 0.25mm to 3mm of which 1mm is the most popular.
What is optical fiber’s mode field diameter?
Although most light travels inside an optical fiber’s core, the light actually spreads through a slightly larger volume including the inner edge of the fiber cladding. This effective area is called the fiber’s mode field diameter or MFD.
Mode field diameter is a measure of the spatial extend of the fundamental mode and it is only important for single mode fibers. Its effect is so small in multimode fibers that it really doesn’t matter any more.
Mode field diameter plays an important role in estimating splice losses, source to fiber coupler losses, macro bending and micro bending losses, etc. For single mode fibers manufacturing, MFD is used as a rather more important parameter than fiber’s core size.
What is optical fiber’s numerical aperture?
Optical fiber’s capability to collect light is not only determined by fiber core size, but also by its acceptance angle. Acceptance angle is the range of angles over which a light ray can enter the fiber and be trapped in its core.
The acceptance angle and numerical aperture are figures of merit used to describe the angles associated with light propagation in optical fibers. The sine of the half-angle of the acceptance angle is known as the numerical aperture – NA. These parameters can be related to the refractive indices of the fiber materials by using Snell’s law and a bit of geometry.
The aforementioned formula can only be used for step-index fibers and numerical aperture is not calculated the same way in graded-index fibers. Actually NA varies across the core with the refractive index.
However, you can measure NA for graded-index fibers by monitoring the divergence angle of light leaving a fiber core as show below. The light emerging from a multimode fiber spreads over an angle equal to its acceptance angle.
But for practical measurements of multimode fiber numerical apertures, we have to remove the modes guided along the fiber cladding from the calculation. So the edge of multimode fiber’s acceptance angle is defined as where intensity drops to 5% of that in the core center.
Typically, for 50um graded-index multimode fibers, the numerical aperture is 0.20. Numerical aperture is 0.28 for 62.5um graded-index multimode fibers.
For single mode glass fibers, their core is so small that diffraction takes control of how light spreads out from the fiber. And because of that, numerical aperture is not as important for single mode fibers than it is for multimode fibers.