Modulators and Detectors
: The lowest-cost, most efficient scheme is to directly
convert the electrical lanes to optical. This means that the modulators
must work at the highest electrical data rate so the conversion can
happen. 100G nets are physically four 25G electrical lanes (which are
treated by all of the network elements as a 100G pipe) so the modulator
must be 25G or faster. There are other constraints: the drive voltage
must be CMOS compatible; the modulator must exhibit great extinction
ratio at 25G; it must be low power; it must work over a broad spectrum
of light; and it has to be small.
Kotura has developed an electro-absorption (EA) modulator
that is 25 times smaller than a traditional Mach-Zehnder Interferometer (MZI)
style modulator (see below for additional details). The length of the
EA modulator is only 55 um; a competing MZI version would be measured
in millimeters. This small size reduces the drive capacitance (less than
25 fF) and power consumption. Additionally, the drivers can be
implemented in pure CMOS – and the modulator works at speeds of 40GHz
WDM multiplexers: After the four modulators convert
the electrical signals to optical, they must be combined into a single
waveguide. Kotura uses an echelle grating to accomplish this at low loss
and small size. Although only four channels are needed today, the same
WDM echelle grating can easily combine 40 or more channels in the
future. On the receiver side, a demultiplexer works in reverse to
separate one stream of light into four independent waveguides.
1: An abstract view of a WDM demultiplexer. One input channel with
many parallel channels on different wavelengths of light is separated by
an Echelle grating before conversion to electrical.
Four integrated germanium detectors convert the optical signals to
electrical signals. Like the modulator, the detectors must be small,
high speed and highly efficient. Kotura’s germanium detectors are fully
integrated and far exceed the 25 Ghz speeds required for 100G.
is right; silicon photonics is hot. Never before have all of these
functions been integrated into a single chip. Silicon integration
eliminates the need for expensive assembly of hundreds of piece parts.
Kotura’s Optical Engine takes advantage of low-cost lasers, electronics
packaging and WDM to provide an innovative solution for multiple data
center network applications. In the future, silicon photonics will not
only enable widespread adoption of 100G networks; it will also scale
interconnects to 400G and one terabits in the future.