Optical Communication Research: Modulating Light at a rate of 1 Tbps

CU  and UCSB in optical communication breakthrough 

Researchers at the Chinese University of Hong Kong (CU) have made a breakthrough in ultrahigh-speed optical communication that could potentially speed up the internet by 10,000 times.

The speed of optical communication is determined by how fast one can turn the light on and off, measured in bits per second (bps). Current commercially available lithium niobate modulators can modulate the brightness of light at a rate of 10 Gbps. The research by Prof Liu Renbao of CU's Department of Physics and his collaborators, Ben Zaks and Mark Sherwin at the University of California Santa Barbara (UCSB) in the US, however, have demonstrated a scheme that is 100 times faster, modulating light at a rate of 1 tbps. This new scheme has the potential to transfer data 10,000 times faster than copper cables, on which the current internet is based.

About five years ago, Prof Liu and Prof Zhu Bang-Fen of Tsinghua University proposed the idea of modulating light in a semiconductor by a strong THz laser. The idea was based on a quantum phenomenon. In a semiconductor, a beam of light can create an electron and a hole left behind that have opposite electric charges and are therefore usually bound to each other. However, if a strong electric field is applied, the electron can suddenly hop away from the hole by tunnelling and then the two particles are accelerated by the electric field.

If the electric field is alternating, the electron and the hole will be driven back to each other once the electric field reverses direction and collide with and annihilate each other into light radiation. Thus, there would be a short pulse of light every time the electric field changes direction. A continuous wave of light passing through the semiconductor would be converted into a periodic sequence of pulses with modulation frequency twice the alternating electric field. Accordingly, the spectrum of the modulated light presents a frequency comb separated by twice the THz frequency.

Such high-speed communication would make it possible to transfer one million ebooks in only one second. "However, the current experiments used free electron lasers produced by large-scale accelerators. That is impracticable for household applications," Prof Liu said. "Hopefully, high-speed transistors may be used in the future to amplify THz signals so as to modulate light in optical fibres. Compact desktop devices are not inconceivable." 

Source:  HKIE Online
Source (see Link):  http://www.hkengineer.org.hk/program/home/articlelist.php?cat=update&volid=143