Silicon photonics ushers in 100G networks

Modulators and Detectors
Modulators: 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 and higher.

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.


Detectors: 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.

Andy 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.