Page 155 - Optical Communications Essentials

Passive Optical Components

Passive Optical Components 145

Throughput
Input power
power
Optical power
P 0 P 1
coupling
P 4

P 3 P 2
Crosstalk Coupled
L W L power
Tapered region Coupling region Tapered region
Figure 9.1. Cross-sectional view of a fused-fiber coupler having a coupling
region W and two tapered regions of length L. The total span 2L W is the
coupler draw length.

completely to the fiber core, but instead extend partially into the cladding.
Therefore if two fiber cores are brought close together, the tail of the power
distribution in one fiber will extend into the adjacent fiber core. Consequently,
some of the optical power will transfer to the adjacent fiber through evanescent
coupling. The amount of optical power coupled from one fiber to another can be
varied by changing the coupling length W or the distance between the two fiber
cores.

9.1.2. Coupler performance
In specifying the performance of an optical coupler, one usually indicates the
percentage division of optical power between the output ports by means of the
splitting ratio or coupling ratio. Referring to Fig. 9.1, with P 0 being the input
power and P 1 and P 2 the output powers, we have

Coupling ratio = P 2 × 100% (9.1)
P 1 + P 2

By adjusting the parameters so that power is divided evenly, with one-half of
the input power going to each output, one creates a 3-dB coupler. A coupler could
also be made in which almost all the optical power at 1550nm goes to one port
and almost all the energy around 1310nm goes to the other port. Such couplers
also are used to combine 980- or 1480-nm laser pump powers along with a
C-band signal in erbium-doped fiber amplifiers, which Chap. 11 describes in
greater detail.

Loss Categories In the above analysis, we have assumed for simplicity that the
device is lossless. However, in any practical coupler there is always some light that is
lost when a signal goes through it. The two basic losses are excess loss and insertion
loss. The excess loss is defined as the ratio of the input power to the total output power.

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