Page 251 - Introduction to Information Optics

236 4. Switching with Optics

refractive index is around 3.5. Therefore, the reflection off a single air-to-
semiconductor or semiconductor-to-air interface at normal incidence is about
30%. This loss can be eliminated by integrating the modulator with the laser
source. Other approaches, including antireflection coatings, can be used to
reduce this loss to values to around 0.1 dB.
Main advantages of MQW devices in comparison with LiNbO 3 modulators
include (I) the feasibility of monolithically integrated structures combining
several functions onto the same device, (2) polarization-insensitive operation,
and (3) low drive voltage. Integrated semiconductor laser-modulator struc-
tures have been reported with high on-off ratios, low drive voltage, and high
bandwidth [34, 36].

4.4. OPTICAL SWITCHING BASED ON MEMS

In addition to ultrafast serial processing speed, optical switches can also be
used for massive parallel connections. If we build a 1000 x 1000 array with
each switch operating on a moderate 1 /zs cycle time, terahertz processing speed
can be realized. Switches using waveguides (including optical fibers) utilize long
interaction lengths and confined high-optical intensity to build up phase
changes for switching. However, a high degree of parallelism is unlikely using
waveguides, since large arrays of switching fabric are difficult to implement.
Therefore, these switches are mainly used for ultra-high-speed serial process-
ing and small arrays of crossbar routing networks. Optical bistable etalons
have the potential for massive parallel switching networks. In fact, switching
arrays have been fabricated and tested using multiple quantum wells and
self-electro-optic devices (SEED) for signal processing applications. However,
current bistable devices require a holding power of at least 10 mW. For a
1000 x 1000 array, the required power would be 10 kW, and most of the power
would be absorbed by the device and converted into heat. This would prevent
the use of current bistable devices for practical applications.
An attractive scheme is switching devices using microelectromechanical
systems (MEMS). MEMS are integrated microdevices or systems combining
electrical and mechanical components fabricated using integrated circuit (1C)
compatible batch-processing techniques. MEMS are fabricated using microen-
gineering and have a size ranging from micrometers to millimeters. These
systems can sense, control, and actuate on the micro scale and function
individually or in arrays to generate effects on the macro scale. MEMS can be
used to provide robust and inexpensive miniaturization and integration of
simple elements into more complex systems. Current MEMS applications
include accelerometers; pressure, chemical, and flow sensors; micro-optics;
optical scanners; and fluid pumps [37, 38].