Integrated laser-modulator with a universal InGaAs/InAs tunnel coupled quantum-well on quantum dot medium

Monolithic integration of a laser diode with a modulator usually requires complicated and potentially defect-generating processing steps such as epitaxial regrowth or bonding. Alternatively, the same medium grown in a single epitaxial run can be used in both gain and electro-absorption section. We demonstrate a universal active medium with tunnel coupled InGaAs short period superlattice (SPSL) quantum well grown on InAs quantum dots (QW-on-QDs) in an AlGaAs waveguide. In this configuration, the QW serves as a tunnel injector of electrons into the QDs in the forward-biased gain section, while in the reverse-biased modulator section the InGaAs QW acts as a wavelength matched quantum Stark effect electro-absorber. The optimum QW and QD ground state separation is close to 30meV for efficient tunnel injection into QDs to increase the gain. The 3x(QW-on-QDs) double heterostructure with separate 0.3μm waveguide was grown by molecular beam epitaxy on n-type GaAs substrate. The device was processed into two-section dies with an intermediate distributed Bragg reflector (DBR) between the sections. Focused ion beam air-gap DBR fabrication with low electrical leakage and required (~30%) optical mode reflection was employed. The laser-modulator performance with slope efficiency over 50%, 6dB power modulation, linear dynamic range>24dB at 2GHz was shown with a 200μm long modulator section and low optical feedback from the modulator to laser.