Communication and Device Technology

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Photonics-electronics convergence hardware technology, aimed at revolutionizing the network

Photonics-electronics convergence hardware technology, supporting large-capacity optical transmission technology and revolutionizing networks

Computing infrastructure has developed in accordance with Moore's Law, which states that the number of transistors on microchips will double every 18 months. However, due to restrictions on microfabrication and transistor density, processing speed and power consumption of electronic circuits is nearing its limits.

For this reason, photonics-electronics convergence hardware technology has become the focus of research as one innovative technology for optical communication devices. Photonics-electronics convergence hardware technology functionally combines optical devices and electronic devices used for communication devices in order to achieve higher performance, higher functionality, and more compact size that could not be achieved with a single device.

While there is already some hardware in use that combines optical devices and electronic devices, these was made by merely adding individual functions. Photonics-electronics convergence hardware combines optical devices and electronic devices to form a single device, achieving performance that could not be achieved by merely adding functions. Photonics-electronics convergence hardware can be realized with an integrated circuit because multiple optical circuits and electronic circuits are concentrated on one platform and operated by coordinated control of functions. This achieves high performance that cannot be achieved using only optical devices or electronic devices.

“Digital coherent optical transmission technology” is becoming popular as a technology to realize optical transmission of over 100 Gbps. This requires miniaturization of digital coherent optical transmission devices such as “digital coherent transceivers”.

Silicon photonics technology, the source of the paradigm shift of communication hardware?

Silicon photonics technology has made incredible progress towards solving the issue of miniaturization. Silicon photonics technology is aimed at photonics-electronics convergence by integrating silicon optical circuits and silicon electronic circuits by converting optical elements to silicon and making ultra-miniaturized integrated circuits.

With silicon, it is possible to utilize sophisticated mass production equipment that can produce electronic circuits at low cost, and it is expected that optical devices that support networks will have both higher functionality and lower cost. At the same time, it is expected that the power consumption of network systems can be reduced by integrating optical circuits and the electronic circuits that control the optical circuits, and photonics-electronics integration through the application of optical communication technology to signal transmission in LSI.

One of the greatest strengths of silicon photonics technology is that it can realize “ultra-miniaturization of optical circuits”. Compared to glass, silicon has a higher refractive index, so it can be used to integrate a large number of optical circuits, giving it the advantage of making it possible to economically make integrated circuits with mature semiconductor manufacturing technology. The processing dimensions and bending radius of the waveguide are orders of magnitude smaller than that of the glass (SiO2) material generally used in optical circuits, and optical circuits which used to be several centimeters in size, can be kept to millimeters or smaller. Furthermore, it is possible to integrate them with a silicon CMOS integrated circuits, which are well suited to high-performance signal processing, and a system-on-chip (SoC) that combines light and electrons can be realized at low cost and with compact size.

Optical modulators using silicon photonics do not require temperature modulation, unlike conventional optical modulators which use compound semiconductors, so its thickness can be reduced by eliminating the thermoelectric cooling element (TEC). Furthermore, it does not require a hermetic package, unlike optical modulators that use compound semiconductors, whose performance deteriorates due to moisture. In contrast to a hermetic package that requires light to enter and exit through the lens, a non-hermetic package allows an “edge connector” that bonds the optical fiber to the edge of the package, so its thickness can be further reduced by omitting the lens. At present, optical modulators and coherent receivers can be integrated on a chip that is just a few millimeters square, fitting on the tip of a finger.

This combination of photonic and electronic technologies is thought to hold the key to further network innovation. Using these technologies, NTT aims to build a photonics network, a system that combines light and electrons. A photonics network is composed of optical transceivers that transmit and receive electrical information as optical signals from electronic devices including servers and routers, and node devices that switch the destination of information (switching function by electricity or light), which are realized by combining electronic devices and photonic devices.

Recently, NTT has realized a nano-optical modulator that operates with the lowest energy consumption in the world and an “optical transistor” that converts optical input signals into other light then outputs it. With this technology, it is possible to introduce advanced signal processing technology using light into processor chips through nanoscale photonics-electronics integration. In the future, we aim to use this technology to realize unprecedented ultra-low energy consumption, high-speed computing platforms.

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