Directly modulated DFB laser on SiO₂/Si substrate for datacenter networks

Reducing the operating energy of a distributed feedback (DFB) laser is a critical issue if we are to use the device as a directly modulated light source employing wavelength division multiplexing technologies in short-distance datacom networks. A membrane buried heterostructure (BH) DFB laser on a SiO₂ layer is one candidate for reducing the operating energy because it provides a strong carrier and optical confinement in the active region. For low-cost fabrication, we have proposed and developed a fabrication procedure that employs the buried growth of an InP layer by using a directly bonded InP-based active layer on a SiO₂/Si substrate, which enables us to use a large-scale Si wafer. To overcome the problem of the difference between the thermal expansion coefficients of Si, SiO ₂, and InP, we have used a thin active layer (∼250 nm) on a SiO₂/Si substrate as a template for the epitaxial growth of a III-V compound semiconductor. A lateral current injection structure is essential for fabricating a device with a 250-nm-thick template. Our fabricated DFB laser with a 73-μm cavity length exhibits a threshold current of 0.9 mA for continuous operation at room temperature and achieves lasing at up to 100 °C. We have also demonstrated 171-fJ/bit operation with a 25.8-Gb/s NRZ signal. These results indicate that the BH DFB laser on a SiO₂/Si substrate is highly suitable for use as a transmitter for datacom applications.