July 28, 2015

NATURE: Compact Optical Data Transmission

(July 28, 2015)  Component for Energy-efficient Optical Communication between Silicon Chips Presented in “Nature Photonics.“ / Micrometer-size Component fast Converts Electrical Signals into Optical Signals

Compact optical transmission possibilities are of great interest in faster and more energy-efficient data exchange between electronic chips. One component serving this application is the Mach-Zehnder modulator (MZM) which is able to convert electronic signals into optical signals. Scientists of the KIT and the ETH in Zurich developed a plasmonic MZM of only 12.5 micrometers length which converts digital electrical signals into optical signals at a rate of up to 108 gigabit per second, and presented this device in the “Nature Photonics” scientific journal. (DOI 10.1038/nphoton.2015.127).

“Optical technologies offer an enormous potential especially in transmitting data between computer chips,” explains Manfred Kohl of the KIT. The EU project he directs, NAVOLCHI, Nano Scale Disruptive Silicon-Plasmonic Platform for Chip-to-Chip Interconnection, developed the plasmonic modulator (an electric-to-optical converter) which is the basis of the current MZM. “Compact optical transmitter and receiver units could exceed the speed limits of present-day electronic systems and help get rid of the bottlenecks in data centers.”

The current publication presents an MZM only 12.5 micrometers long, which is roughly one tenth the thickness of a hair. It consists of two arms, each of which contains one electro-optical modulator. Each modulator is made up of a metal-insulator-metal waveguide with a gap approximately 80 nanometers wide and filled with an electro-optical polymer, and sidewalls made of gold which, at the same time, act as electrodes. The electrodes carry a voltage which is modulated in line with the digital data. The electro-optical polymer changes its index of refraction as a function of the voltage. The waveguide and the coupler made of silicon route the two parts of a split light beam to the gaps or from the gaps.

In the respective gap, the light beams of the waveguides initiate electromagnetic surface waves, the so-called surface plasmons. The voltage applied to the polymer modulates the surface waves. Modulation is different in both gaps but coherent, as the same voltage is applied with different polarities. After passing through the gaps, the surface waves initially enter the output optical waveguides as modulated light beams and are then superimposed. The result is a light beam in whose intensity (amplitude), the digital information was encoded.

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