Four Wave Mixing in Silicon Photonics - Optical Signal Processing by Silicon Photonics - page 104

keying (OOK) format, for example better receiver sensitivity associated with bal-
anced detection, larger dispersion tolerance and better resilience to fiber nonlinear
effects. With the recent interest in phase-modulated signals for optical communi-
cation systems, it is interesting to look at wavelength conversion methods suitable
for such signals.
Parametric amplification is promising because of their large and flexible gain
bandwidth. Operation of parametric amplification with CW pumps would be desir-
able for many practical applications. This would provide gain or idler conversion
efficiency independent of time, which is convenient for amplifying or convert-
ing input signals with arbitrary modulation formats. However, recently available
highly nonlinear silicon waveguides have a zero-dispersion wavelength, which can
vary by several nanometers over a few centimeters. This in turn prevents the phase
matching condition from being maintained at its optimum value along the wave-
guide. As a result, to date, CW parametric amplifiers have not exhibited gain spec-
tra as wide as that by theory. In contrast, by using a pulsed pump, which consists
of high-peak power pulses, and relatively short waveguide length, one can obtain
very wide (>200 nm) gain spectra, with a shape very close to that predicted by
theory.
8.5.2 Wavelength Conversion by FWM in SOI Waveguides
The flexibility of very high capacity optical fiber networks solely depends upon
all optical wavelength conversion. Among many wavelength conversion schemes,
exploiting FWM [ 5 ] in silicon waveguides has gained considerable attraction due
to its transparency to modulation formats and bitrates, along with its high inte-
gration and compatibility with electronic devices. The silicon waveguides are the
good candidate for FWM applications due to unbelievable reduction in cost and
size of silicon photonic devices.
The parametric process of FWM in silicon waveguides [ 6 ] is the result of non-
linear response of bound electrons to an applied field. The inherent properties of
very high nonlinearity and tight light confinement of silicon waveguides enable it
to provide silicon compatible chip scale wavelength converters, ready to replace
the long highly nonlinear-silica fibers (HNLSF).
Utilization of nondegenerate FWM in semiconductor optical amplifiers (SOA)
is a common approach to realize frequency shifting [ 7 - 9 ] or dispersion shifted
silica fibers may be used. Recently, wavelength conversion for 10, 20, 40, and
160 Gb/s has been demonstrated in SOI waveguides [ 10 - 12 ]. In addition, other sig-
nal processing devices such as optical modulation [ 13 ], optical switching [ 14 , 15 ],
optical amplification [ 16 ], and lasing [ 17 ] have also been presented, benefitting
with the recent advancements in CMOS-compatible silicon photonics.
In this chapter, it has been investigated that the conversion efficiency in
accordance with the pump/signal powers and wavelengths for 1 cm long silicon
waveguide while taking into account the noise and losses as noise figure (NF).