Photonic Crystal Lasers for Chip-to-Chip and On-Chip Optical Interconnects

Abstract

A directly modulated photonic crystal (PhC) laser with a wavelength-scale cavity is a key component for chip-to-chip and on-chip optical interconnections because it enables us to achieve ultralow operating energy. A buried heterostructure, where the wavelength-scale active region is buried with an InP layer, is essential for achieving ultralow operating energy because it improves both carrier and photon confinement in the active region and the InP layer improves the thermal conductivity of the device. We call this a lambda-scale embedded active-region PhC, or LEAP, laser. We have already developed an electrically driven LEAP laser with a threshold current of 4.8 μA and operating energy of 4.4 fJ/bit. However, to employ the LEAP laser in chip-to-chip optical interconnects, the output power should be increased. To increase it, we increase the number of multiple quantum wells and reduce the device series resistance. The device exhibits 39.3-μW output power with threshold current of 36 μA. As a result, we measure the bit error rate with 10-Gb/s signal without using an optical amplifier. For an on-chip optical interconnect, the monolithic integration of a laser, waveguide, and photodetector is the main issue because on-chip optical interconnect requires extremely small energy consumption to overcome its electrical counterpart; therefore, unwanted optical loss such as coupling loss should be minimized. We have fabricated a monolithic optical link, in which a LEAP laser exhibits a threshold current of 22 μA. The integrated optical link operates at an ultralow energy of 28.5 fJ/bit. These results indicate that the LEAP laser is highly suitable for use as a transmitter in computercom applications.