5.1 Source-to-Fiber Power Coupling
219
5.1.4 Equilibrium Numerical Aperture
As noted earlier, a light source may be supplied with a short (typically 1 m long)
fiber flylead attached to it in order to facilitate coupling the source to a system fiber.
To achieve a low coupling loss, this flylead should be connected to a system fiber that
has a nominally identical NA and core diameter. A certain amount of optical power
(ranging from 0.1 to 1 dB) is lost at this junction, the exact loss depending on the
connecting mechanism and on the fiber type; this is discussed in Sect. 5.3. In addition
to the coupling loss, an excess power loss will occur in the first few tens of meters
of a multimode system fiber. This excess loss is a result of nonpropagating modes
scattering out of the fiber as the launched modes come to an equilibrium condition.
This loss is of particular importance for surface-emitting LEDs, which tend to launch
power into all modes of the fiber. Fiber-coupled lasers are less prone to this effect
because they tend to excite fewer nonpropagating fiber modes.
The excess power loss must be analyzed carefully in any system design as it can
be significantly higher for some types of fibers than for others [5]. An example of
the excess power loss is shown in Fig. 5.4 in terms of the fiber numerical aperture.
At the input end of the fiber, the light acceptance is described in terms of the launch
numerical aperture NA in . If the light-emitting area of the LED is less than the crosssectional area of the fiber core, then at this point the power coupled into the fiber is
given by Eq. (5.7), where NA = NA in .
However, when the optical power is measured in long multimode fibers after the
launched modes have come to equilibrium (which is often taken to occur at 50 m), the
effect of the equilibrium numerical aperture NA eq becomes apparent. At this point,
the optical power in the fiber scales as
P eq = P 50
N A eq
N A in
2
(5.13)
Fig. 5.4 Example of the
change in NA as a function
of distance along a
multimode fiber
219
5.1.4 Equilibrium Numerical Aperture
As noted earlier, a light source may be supplied with a short (typically 1 m long)
fiber flylead attached to it in order to facilitate coupling the source to a system fiber.
To achieve a low coupling loss, this flylead should be connected to a system fiber that
has a nominally identical NA and core diameter. A certain amount of optical power
(ranging from 0.1 to 1 dB) is lost at this junction, the exact loss depending on the
connecting mechanism and on the fiber type; this is discussed in Sect. 5.3. In addition
to the coupling loss, an excess power loss will occur in the first few tens of meters
of a multimode system fiber. This excess loss is a result of nonpropagating modes
scattering out of the fiber as the launched modes come to an equilibrium condition.
This loss is of particular importance for surface-emitting LEDs, which tend to launch
power into all modes of the fiber. Fiber-coupled lasers are less prone to this effect
because they tend to excite fewer nonpropagating fiber modes.
The excess power loss must be analyzed carefully in any system design as it can
be significantly higher for some types of fibers than for others [5]. An example of
the excess power loss is shown in Fig. 5.4 in terms of the fiber numerical aperture.
At the input end of the fiber, the light acceptance is described in terms of the launch
numerical aperture NA in . If the light-emitting area of the LED is less than the crosssectional area of the fiber core, then at this point the power coupled into the fiber is
given by Eq. (5.7), where NA = NA in .
However, when the optical power is measured in long multimode fibers after the
launched modes have come to equilibrium (which is often taken to occur at 50 m), the
effect of the equilibrium numerical aperture NA eq becomes apparent. At this point,
the optical power in the fiber scales as
P eq = P 50
N A eq
N A in
2
(5.13)
Fig. 5.4 Example of the
change in NA as a function
of distance along a
multimode fiber