All-Optical Frequency Shifting - Optical Signal Processing by Silicon Photonics - page 82

Wavelength conversion over the entire ITU S, C, and L bands at a data rate
of 40 Gb/s is demonstrated using silicon photonic waveguides, a continuous-wave
pump, and a non-return-to-zero (NRZ) input signal of wavelength 1,513.7 nm
is up-converted across nearly 50 nm, resulting in a minimum power penalty of
2.9 dB on the converted signal at a BER of 10 − 9 [ 11 ].
160-Gb/s pulsed return-to-zero (RZ) wavelength conversion within a CMOS-
compatible silicon chip device comprising 1.1 cm long and a 290-nm × 660-nm
cross section has been published recently which is the highest data rate achieved
till date for a single-channel conversion in silicon, with a 21-nm conversion range
and − 15.5 dB conversion efficiency using continuous-wave (CW) pump produc-
ing moderate pulse broadening, Fig. 5.2 [ 12 ].
The new schemes were developed during the 1990s for making wavelength
converters; in the present technology the wavelength converters change the
input wavelength to a new wavelength without modifying the data content of the
signal.
A simple scheme uses an optoelectronic regenerator; this scheme is relatively
easy to implement as it uses standard components. In this scheme an optical
receiver first converts the incident signal at the input wavelength λ 1 into an electri-
cal bit pattern, which is then used by a simple transmitter to generate the optical
signal at the desired wavelength λ 2. Advantages include insensitivity to the input
polarization while the drawback includes limited transparency to bit rate and data
format and speed limited by electronics [ 13 ].
7.3 Optical Modulation
Nowadays, silicon-photonic chip-scale active and passive devices are a hot and
overwhelming topic for researchers in this field. The devices being proposed and
demonstrated as a result of this titanic research and evolution of nanofabrication
techniques incorporate silicon lasers, amplifiers, modulators, photo-detectors,
wavelength converters, optical logic gates, optical buffers, biosensors, etc. The
list of silicon photonic devices is increasing and applications of these nanoscale
devices will continue at a great pace [ 14 - 16 ]. The efficient fiber to silicon-wave-
guide coupling (from cross-sectional dimensions of several micrometers to a few
hundreds of nanometers in centimeter lengths) enables the better use of silicon
waveguides.
Waveguides and devices with enhanced third-order nonlinearities in polymer
silicon which is essential for nanometer scale photonic signal processing devices
have been patented by researchers at the University of Washington [ 17 ]. They used
these slot waveguides having closed as well as linear or circuitous formations in
devices, i.e., variable delay lines, optical logic gates, optical multiplexers, optical
self-oscillators, and optical clock generators. Passive silicon devices such as split-
ters, bends, couplers, and filters have been designed but the signal cannot be mod-
ulated while propagating through them [ 18 , 19 ]. Optical switches and modulators