Page 154 - Optical Communications Essentials

Passive Optical Components

144 Chapter Nine

■ Combine or separate independent signals at different wavelengths: WDM
device

The passive components described in this chapter include optical couplers,
isolators, circulators, filters, gratings, and wavelength multiplexers. Some of
these passive devices also can be configured as active devices, which Chap. 10
addresses. Other passive devices used for wavelength division multiplexing
(WDM) are described in Chap. 12. These include arrayed waveguide gratings
(AWGs), bulk diffraction gratings, and interleavers.

9.1. Optical Couplers
The concept of a coupler encompasses a variety of functions, including splitting
a light signal into two or more streams, combining two or more light streams,
tapping off a small portion of optical power for monitoring purposes, or trans-
ferring a selective range of optical power from one fiber to another. When one
is discussing couplers, it is customary to refer to them in terms of the number
of input ports and output ports on the device. For example, a device with two
inputs and two outputs is called a 2 2 coupler. In general, an N M coupler
has N 2 input ports and M 2 output ports. The coupling devices can be fab-
ricated either from optical fibers or by means of planar optical waveguides using
material such as lithium niobate (LiNbO 3 ) or InP.
The couplers described in this section include 2 2 couplers, tap couplers,
star couplers, and the Mach-Zehnder interferometer.

9.1.1. Basic 2 2 coupler
The 2 2 coupler is a simple fundamental device that we will use here to
demonstrate the operational principles of optical couplers. These are known as
directional couplers. A common construction is the fused-fiber coupler illus-
trated in Fig. 9.1. This is fabricated by twisting together, melting, and pulling
two single-mode fibers so they get fused together over a uniform section of
length W. Each input and output fiber has a long tapered section of length L,
since the transverse dimensions are reduced gradually down to that of the cou-
pling region when the fibers are pulled during the fusion process. This device is
known as a fused biconical tapered coupler. As shown in Fig. 9.1, P 0 is the input
power on the top fiber (which we will take as the primary fiber in a link), P 1 is
the throughput power, and P 2 is the power coupled into the second fiber.
Parameters P 3 and P 4 are extremely low optical signal levels ( 50 to 70dB or,
equivalently, factors of 10 5 to 10 7 below the input power level). These result
from backward reflections and scattering due to packaging effects and bending
in the device.
To understand how power is coupled from one fiber to the other, recall Fig. 4.3.
This figure shows that the power distributions of any given mode are not confined

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