As the internet grows ever larger, there is an increasing need for innovation in the core technologies which drive the backbone of the internet.
Today a majority of the data transmission is carried out using optical telecommunication, i.e. by encoding information in light and sending it through optical fibres. As the amount of data generated per second increases, so does the bandwidth needs, and thusly the power consumption of the transmitters, links, switches, and receivers involved in the process.
One widely used technique for increasing data transmission bandwidth in a single fibre is by using several colours simultaneously to transmit light. Each colour (wavelength) channel can carry independent data and travel alongside all the other colours without mixing. This technique is called wavelength division multiplexing (WDM) and ordinarily requires one laser for each wavelength channel.
Spectral buffer regions are placed around each laser to avoid any overlap in the channels, at the cost of spectral efficiency. However, a frequency comb, is a laser-like source which contains many well-defined and equally spaced wavelengths. It can be used as a WDM light source without the need for spectral buffer regions as it inherently locks the spacing between the wavelength peaks and thus completely prevents any channel overlap.
Because the frequency comb is driven by a single laser and can potentially replace hundreds of individual lasers the technique is expected to reduce the power consumption of high bandwidth transmission.
The researchers at the Technical University of Denmark and Chalmers Technical University have for the first time tried to find the limit of just how much data a single optical frequency comb can support. They have managed to fabricate and test a microchip capable of producing a frequency comb powerful enough to support a record high data transmission rate of 1.84 Petabit-per-second from a single light source, which is twice the global current internet bandwidth usage[1].
The chip is a photonic chip meaning it uses light instead of electricity. It is made of silicon nitride in a ring structure and is capable of converting light from a single laser into a frequency comb using third order nonlinear effects. Data is encoded on the generated frequency comb by splitting up the light in different wavelength channels and encoding each channel separately. The generated frequency comb was so powerful that it could be further split into 37 copies which could each carry different data. Researchers from Fujikura Ltd. provided a 7.9 km long 37-core fibre which allowed the Scandinavian researchers to transmit all 37 copies simultaneously in different fibre cores inside one fibre.
The system was tested by the researchers transmitting 37 independent copies of 224 individual wavelength channels leading to a total of 8.228 data channels. In total this represents a data rate of 1.84 Petabit-per-second which corresponds to uploading 75 million Instagram photos every second, or streaming 122 million Netflix movies in 4k resolution.
The experimental results also allowed the researchers to benchmark their theoretical understanding and even improve their modelling. Theory indicates that the current limitation is the available equipment and not the micro chip itself. While a lot of work still has to be done on the practical implementations, the theory suggests that these systems could be scaled up from the demonstrated 37 channels to hundreds or even thousands of fibre channels, which if proven true, could potentially pave the way for more energy efficient optical communication systems in the future.
While the experiment did use an external pump laser and external data modulators, there is a lot of research at the moment on integrating such elements on chips. The hope is that we in the future will be able to package everything on chip and drastically decrease both size and energy consumption of these frequency comb based transmission systems.
Hear professor Leif Katsuo Oxenløwe explain the results on BBC below.
[1] https://www.itu.int/itu-d/reports/statistics/2021/11/15/international-bandwidth-usage/
For excerpts of the global media coverage of the result, please visit:
Nature photonics altmetric
Interview by Washington Post
Interview by New Scientist