Ultrafast harmonic modelocking of monolithic compound-cavity laser diodes incorporating photonic-bandgap reflectors

Abstract—We present the first demonstration of reproducible harmonic mode-locked operation from a novel design of mono-lithic semiconductor laser comprising a compound cavity formed by a 1-D photonic-bandgap (PBG) mirror. Mode-locking (ML) is achieved at a harmonic of the fundamental round-trip frequency with pulse repetition rates from 131 GHz up to a record high fre-quency of 2.1 THz. The devices are fabricated from GaAs–Al-GaAs material emitting at a wavelength of 860 nm and incorpo-rate two gain sections with an etched PBG reflector between them, and a saturable absorber section. Autocorrelation studies are re-ported which allow the device behavior for different ML frequen-cies, compound cavity ratios, and type and number of intra-cavity reflectors to be analyzed. The highly reflective PBG microstruc-tures are shown to be essential for subharmonic-free ML opera-tion of the high-frequency devices. We have also demonstrated that the single PBG reflector can be replaced by two separate features with lower optical loss. These lasers may find applications in ter-ahertz imaging, medicine, ultrafast optical links, and atmospheric sensing. Index Terms—Microwave generation, mode-locked lasers, optical modulation, optoelectronic devices, quantum-well lasers, semiconductor lasers, submillimeter wave generation, submil-limeter wave modulation. I