Page 173 - Optical Communications Essentials

Active Optical Components

Active Optical Components 163

Chromatic dispersion compensators optically restore signals that have become
degraded by chromatic dispersion, thereby significantly reducing bit error
rates at the receiving end of a fiber span. This chapter describes the con-
struction and operation of one representative device type. Chapter 15 illus-
trates further applications.
Optical performance monitors track optical power, wavelength, and optical sig-
nal-to-noise ratio to check operational performance trends of a large number
of optical channels and to identify impending failures. Chapter 18 looks at
these devices in greater detail within the discipline of system maintenance
and control.
Optical switches that work completely in the optical domain have a variety of
applications in optical networks, including optical add/drop multiplexing,
optical cross-connects, dynamic traffic capacity provisioning, and test equip-
ment. Chapter 17 looks at switching applications in greater detail.
Wavelength converters are used in WDM networks to transform data from one
incoming wavelength to a different outgoing wavelength without any inter-
mediate optical-to-electrical conversion. Chapter 11 on optical amplifiers
describes devices and techniques for doing this.

10.2. MEMS Technology
MEMS is the popular acronym for microelectromechanical systems. These are
miniature devices that combine mechanical, electrical, and optical components
to provide sensing and actuation functions. MEMS devices are fabricated using
integrated-circuit compatible batch-processing techniques and range in size
from micrometers to millimeters. The control or actuation of a MEMS device is
done through electrical, thermal, or magnetic means such as microgears or
movable levers, shutters, or mirrors. The devices are used widely for automo-
bile air bag deployment systems, in ink-jet printer heads, for monitoring
mechanical shock and vibration during transportation of sensitive goods, for
monitoring the condition of moving machinery for preventive maintenance, and
in biomedical applications, for patient activity monitoring and pacemakers.
Figure 10.1 shows a simple example of a MEMS actuation method. At the top of
the device there is a thin suspended polysilicon beam that has typical length,
width, and thickness dimensions of 80, 10, and 0.5 µm, respectively. At the bottom
there is a silicon ground plane which is covered by an insulator material. There is
a gap of nominally 0.6 µm between the beam and the insulator. When a voltage is
applied between the silicon ground plane and the polysilicon beam, the electric
force pulls the beam down so that it makes contact with the lower structure.
MEMS technologies also are finding applications in lightwave systems for vari-
able optical attenuators, tunable optical filters, tunable lasers, optical add/drop
multiplexers, optical performance monitors, dynamic gain equalizers, optical
switches, and other optical components and modules. For example, Fig. 10.2
illustrates the use of MEMS technology to make a tunable VCSEL. This is

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