To facilitate the next generation of ultrafast optical transmission

According to data released by the Ministry of Internal Affairs and Communications, Internet traffic in Japan in May 2015 was estimated at about 4.4 Tbps. This was an increase of about 1.5 times the total measured results for the previous year. The rate of increase in the numbers also exceeded that of the year before, showing that broadband use in Japan is growing at an accelerating pace. The probable causes of this are the popularization of mobile devices such as smartphones and tablets and the spread of video distribution services, and this upward trend is likely to continue in future.

To address this problem, there is a need for technology that will facilitate even larger capacity communications. Attracting attention in this respect is the optical digital coherent system, which curbs the effects of polarization mode dispersion, wave dispersion, etc. that deteriorate transmission quality in conventional optical fiber cables. This is a system of detecting light phase information from transmission data using digital signal processing, and has the characteristic of enabling the long distance transmission of a lot of information over a small bandwidth.

Furukawa Electric has developed various products as optical components for optical digital coherent systems. We have contributed, through optical digital coherent systems, to the construction of a 100 Gbps large-capacity transmission system.

In an optical digital coherent system, the signal is demodulated by the intervention of a signal light and local light. Both light sources therefore require a narrow oscillation spectrum width that can provide stable intervention. The “narrow-linewidth full-band tunable laser” is a optical source that achieves a narrow linewidth of 500 kHz or less and a high output power of 40 mW or more. Being able to freely select the wavelength opens the way for flexible transmission path design, thus helping to keep costs down.

An optical digital coherent system also requires the propagation of a phase modulation signal with two polarized waves bisecting each other at right angles, and the signal receiver that has a role in this needs a polarization multiplexer and a coherent mixer. The “coherent mixer with built-in polarization multiplexer” is, as its name suggests, a product with a polarization multiplexer and a coherent mixer on one chip. Not only have we achieved miniaturization, we have also removed the need for joining the two components together.

This is what Toshikazu Mukaihara of Furukawa Electric’s Telecommunications & Energy Laboratory had to say about the future development of optical digital coherent systems.
“When trouble occurred at the verification stage of the large-capacity transmission system, we spent several days on site at our customer’s location to resolve the problems together, before coming up with the product. We have utilized this experience again in our new system construction. To cope with ever-increasing traffic in future, there will be a need for optical components that have high functionality, small size, and low power consumption. We hope to respond to this market and customer demand through technical innovation using our base technologies in materials and optics.”