Researchers achieve data speed record on optical fibre

As data traffic increases, there is a need for miniaturised optical transmitters and receivers that operate with high-order multi-level modulation formats and faster data transmission rates. Researchers have developed a new compact indium phosphide (InP)-based coherent driver modulator (CDM) and achieved a high baud rate and transmission capacity per wavelength.

CDMs are optical transmitters used in optical communication systems that can put information on light by modulating the amplitude and phase before it is transmitted through optical fibre. Josuke Ozaki from the NTT Innovative Devices Corporation in Japan said that services that need data capacity, such as video distribution and web conferencing services, are widespread, with more services to be introduced in the future. To realise the new services, the total data rate of optical transmission systems needs to be increased.

“If the optical transmission capacity is insufficient, it will be difficult to realise new convenient services and data society. In addition, the development of an optical transmitter that covers the C+L band in a single module enables flexible network operation and reduces equipment costs,” Ozaki said.

The baud rate measures the speed of data transmission and indicates the number of signal changes that occur every second in a communication channel. With higher baud rates, the bandwidth of the modulation signal required for each channel increases and fewer channels can be transmitted in the conventional C-band. This makes it more important to extend the wavelength bandwidth from the C-band to the L-band. Together, they are referred to as the C+L band.

While modulators made from the semiconductor InP have excellent optical and radio frequency characteristics, they also exhibit strong wavelength dependence that makes it difficult to extend their wavelength range. To overcome this, the researchers developed a novel InP modulator chip with an optimised semiconductor layer and waveguide structure that can operate over a wide wavelength range. Using the new modulator chip enabled the researchers to achieve a CDM with an InP modulator chip that can transmit in C+L band and has a package body measuring 11.9 x 29.8 x 4.35 mm³.

In the C+L band, the new CDM exhibited an electro-optic 3 dB bandwidth of more than 90 GHz, an insertion loss at maximum transmission of less than 8 dB, and an extinction ratio of 28 dB or more. The researchers also applied the new CDM in experiments using 180 Gbaud probabilistically constellation-shaped 144-level quadrature amplitude modulation (PCS-144QAM) signals, demonstrating a net bit rate of 1.8 Tbps over 80 km standard single mode fibre in the C+L band.

According to the researchers, this is the first time an InP-based CDM has been shown to operate in the C+L bands and the world record’s transmission capacity per wavelength has been reported for a CDM. “The next step is to further increase the baud rate for a higher transmission speed. In doing so, it is important to find new modulator’s structure and assembly configuration, including a driver die and a package that can achieve higher EO bandwidth with both lower power consumption and smaller form factor,” Ozaki said.

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