Page 175 - Optical Communications Essentials

Active Optical Components

Active Optical Components 165

an elastomer (from the words elastic and polymer). Elastomeric material can be
stretched as much as 300 percent, as opposed to less than 1 percent for silicon.
As a result, CMEMS devices require much lower voltages to achieve a given
mechanical deflection, and for equivalent voltages their mechanical range of
motion is much larger than that with silicon MEMS.
Depending on the technology used, MEMS devices may have some reliability
issues if the manufacturer does not follow strict fabrication procedures. One
major failure problem is stiction, which is the tendency of two silicon surfaces
to stick to each other permanently. A second concern is how to keep contami-
nants such as dust and saw-generated particles away from MEMS structures
during singulation, which is the process of cutting up a large fabricated wafer
into individual MEMS devices. A third issue involves packaging the devices so
that no liquid, vapor, particles, or other contaminants are present to cause mal-
functions in the moving parts of the MEMS device.

10.3. Variable Optical Attenuators
Precise active signal-level control is essential for proper operation of DWDM
networks. For example, all wavelength channels exiting an optical amplifier need
to have the same gain level, certain channels may need to be blanked out to per-
form network monitoring, span balancing may be needed to ensure that all sig-
nal strengths at a user location are the same, and signal attenuation may be
needed at the receiver to prevent photodetector saturation. A variable optical
attenuator (VOA) offers such dynamic signal-level control. This device attenuates
optical power by various means to control signal levels precisely without disturb-
ing other properties of a light signal. That means a VOA should be polarization-
independent, attenuate light independently of the wavelength, and have a low
insertion loss. In addition, it should have a dynamic range of 15 to 30dB (a con-
trol factor ranging from 30 to 1000).
The control methods include mechanical, thermooptic, MEMS, or electrooptic
techniques. The mechanical control methods are reliable but have a low dynamic
range and a slow response time. Thermooptic methods have a high dynamic
range, but are slow and require the use of a thermoelectric cooler (TEC), which
may not be desirable. The two most popular control methods are MEMS-based
and electrooptic-based techniques. For MEMS techniques an electrostatic actu-
ation method is the most common and well developed, since integrated-circuit
processes offer a wide selection of conductive and insulating materials. In this
method a voltage change across a pair of electrodes provides an electrostatic
actuation force. This requires lower power levels than other methods and is the
fastest.
When wavelengths are added, dropped, or routed in a WDM system, a VOA
can manage the optical power fluctuations of this and other simultaneously
propagating wavelength signals. Figure 10.3 shows a typical VOA package. Large
power handling capabilities are possible with bigger package sizes. Table 10.1
shows some representative operational parameter values for a VOA.

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