Integrated CMOS-Compatible Mode-Locked Lasers and Their Optoelectronic Applications

Mode-locked lasers producing femtosecond pulses show ultra-low timing jitter. Upon photo detection the harmonics of the photo current at multiples of the pulse repetition rate, show ideally ultra-low phase noise. Here, we review the scaling behind this ultra-low jitter sources and their potential impact in optoelectronic systems. Therefore, integrated CMOS-compatible mode-locked lasers producing femtosecond pulses are expected to deliver trains of ultrashort pulses with unprecedented low timing jitter, and enable microwave signals at every harmonic of the fundamental repetition rate with ultra-low phase noise after detection in a very compact format. In combination with tunable lasers and frequency comb technology also high precision optical signals can be synthesized for miniaturized optical clocks and optical spectroscopy systems. We review recent progress towards chip-scale mode-locked lasers in the femtosecond regime using rare-earth doped gain media. Current limitations, can be overcome with improved low-loss integrated waveguides and novel gain deposition technology. In addition, a suite of integrated optical components in silicon photonics technology is discussed that enables the implementation of integrated frequency combs, and, therefore optical synthesizers or ultra-low noise microwave sources based on direct optical frequency division.