Page 262 - Introduction to Information Optics

4.5. Summary 24 7

The commercial 32 x 32 port switch by Agilent Technologies has a specifica-
tion of — 50 dB cross talk. The switching operation is digital, and switching
time for the device is ~ 10ms. Typical operating voltages are 15 and 5 V DC,
Optical alignment can be easily performed, and, once manufactured, no
alignment change is necessary. The switch has no moving mechanical parts,
which may translate to longer lifetimes and better reliability. The switch
architecture is flexible and allows signal ports to be added and dropped for free.
By contrast, in a beam-steering MEMS switch additional mirror pairs must be
added to accomplish these functions, resulting in a larger switch.

4.5. SUMMARY

This chapter discussed three very important types of optical switching
devices: ultrafast all-optical switches based on nonlinear optical effects, fast
electro-optic modulators, and massive parallel switching devices using micro-
electromechanical systems.
In ultrafast optical switches, three different schemes were discussed. They are
nonlinear Fabry-Perot etalons, nonlinear directional couplers, and nonlinear
optical loop mirrors. Nonlinear Fabry-Perot etalons are the first all-optical
switches experimentally demonstrated. They consist of a Fabry-Perot cavity
within a nonlinear optical medium. Switches arrays can be formed for parallel
signal processing. However, current etalon switching devices require a high
holding power, which prevents them from practical applications. NLDC has
high contrast and can be implemented using integrated waveguides as well as
optical fibers. However, it requires a 2n nonlinear phase shift to perform the
switching, and it is difficult to form large arrays. NOLM is an interferometric
switching device. It requires a n nonlinear phase shift to perform switching. The
contrast is not as high as that in NLDC. It can be implemented using optical
fiber or a semiconductor optical amplifier as a nonlinear element. Asymmetric
arrangement of SOA in a NOLM has proven to be a very effective scheme to
perform ultrafast switching for digital optical signal processing.
Fast electro-optic modulators are essential devices to convert electrical data
into optical ones in optical communication systems. These devices are made by
using an electrical pulse to induce a dielectric change in the medium through
which the (carrier) optical signal is to pass. Three schemes were discussed in
this chapter. Direct modulation of diode lasers is a simple and direct approach
to the generation of coded optical pulses. External electrical signal modulates
the gain in a laser diode. The main advantage of this scheme is that it can
generate modulated optical signals without the use of a separate modulator;
therefore the unit is very compact. Modulation bandwidths up to several tens
of GHz have been achieved. However, direct current modulation produces