All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 84

An optical modulator makes use of the LiNbO 3 material and a Mech-Zehnder
(MZ) interferometer for intensity modulation. The refractive index of electro-
optic materials such as LiNbO 3 can be changed by applying external voltage.
LiNbO 3 modulators with a bandwidth of 10 GHz were available commercially by
1998, and the bandwidth increased to 40 GHz by 2000. The second category of
optical modulator is electro absorption modulators, that they made by using the
same semiconductor material that is used for the laser; thus the two can be easily
integrated on the same chip. The electro absorption modulator makes use of the
Franz-Keldysh effect in which the band gap of a semiconductor decreases when an
electric field is applied across it [ 26 ].
Other materials can also be used to make the external modulators, for example,
the modulators have been fabricated using electro-optic polymers; such modula-
tors may find applications in light wave systems (Figs. 7.3 ).
Hewlett Packard Company also patented their design of optical modulator in
2009 including electrically controlled ring resonator, comprising:
1. a waveguide for guiding an optical signal,
2. a ring resonator disposed in evanescent communication with the waveguide for
at least one predetermined wavelength of the optical signal, and
3. a semiconductor PN junction which receives an electrical control signal.
A resonance state of the ring resonator at the predetermined wavelength is thus
controlled by the electrical signal, which controls a free carrier population in the
resonant light path, and the optical signal is thereby modulated according to the
electrical control signal [ 28 ]. A similar high speed ring optical modulator is pat-
ented by the Electronics and Telecommunications Research Institute, Daejeon [ 29 ].
7.4 All-Optical Switching
All-optical switching and interconnection based on SOI are fitting the need of
the hour due to their promise of fulfilling the ultrahigh bit rates of hundreds of
Tbits/s inside and between computer chips. These computer chips will contain
about 1,000 cores and more than 10 tera flip-flops in 2020 [ 30 ]. Many schemes
have been developed for performing the switching operations, mechanical switch-
ing is the simplest to understand. A micro-electro-mechanical system (MEMS) is
also used for switching. Semiconductor waveguides can also be used for making
the optical switches in the form of direction couplers, MZ interferometers, or Y
junctions [ 31 ].
The optical cross-connect (OXC) performs the same function as provided by elec-
tronic digital switches in telephone networks. The design and fabrication of OXCs
has remained a major topic of research since the arrival of WDM systems [ 32 ].
The future computer architecture is looking curiously toward silicon photonics
for mature solution but the on-chip integration of all-optical silicon-based switch-
ing is challenging due to its weak nonlinear optical properties. The switching in