Sunday, October 25, 2020

NTT develops world’s fastest directly modulated laser

 Researchers at NTT have developed the world's fastest directly modulated laser with a 3-dB bandwidth exceeding 100 GHz and capable of transmitting at 256 Gbps over a distance of 2 km.

The research was carried out in collaboration with Professor Fumio Koyama at the Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology.

NTT researchers developed a membrane laser on a silicon (Si) substrate with a thermal oxide film (SiO2). The company says membrane lasers have a large optical confinement factor in the active region and are compact, making it possible to realize directly modulated lasers with low power consumption. On the other hand, since the device is fabricated on a low-thermal-conductivity SiO2 layer, the temperature increase in the active layer due to current injection is large, and even if the current is increased, the relaxation oscillation frequency saturates at about 20 GHz due to saturation of the differential gain.

To suppress the increase in the active region temperature, we fabricated an indium-phosphorus (InP) based membrane laser on a silicon carbide (SiC) substrate (Fig. 3), which has a thermal conductivity approximately 500 times higher than that of SiO2. Since SiC has a lower refractive index than InP, the optical confinement factor is almost the same as that of the device on SiO2. The device was fabricated by direct bonding with ultrathin (40 nanometers) SiO2 between the InP and SiC substrate. 

Assuming a 100-mW heat source, the temperature increase in the active region of a membrane laser with an active layer length of 50 micrometers was significantly reduced from 130.9 to 16.8℃ when the SiO2 thickness was reduced from 2 micrometers to 40 nanometers.