RF Analysis of a Sub-GHz InP-Based 1550 nm Monolithic Mode-Locked Laser Chip

Abstract

We report a monolithic sub-GHz repetition rate mode-locked laser with record low pulse-to-pulse RMS timing jitter of 3.65 ps in the passive mode locking regime. We analyse the optical pulse generation in passive and hybrid mode-locking operating regimes, finding narrower RF tone linewidth in the passive regime, attributed to the improved contact structure of the gain sections. The noise performance is also characterized in passive and hybrid regimes, showing RMS integrated timing jitter of approximately 600 fs. For hybrid modelocking, the repetition rate can be varied over a large range from 880 to 990 MHz. We observe broad pulse widths of few hundred picoseconds attributed to the (long folded) waveguide architecture and on-chip multimode interference mirrors. This device subjects a stand-alone, ultra-compact, mode-locking based clock source to realize frequency synthesizers operating over a frequency range from sub-GHz up to approximately 15 GHz.

Extending hybrid integrated diode lasers to multi-frequency oscillation

“…Reducing the repetition rate of a mode-locked diode laser may seem straightforward to achieve, simply by choosing a longer cavity length, because this increases the roundtrip time. However, for monolithic devices, the laser cavity length is typically limited to at most a few centimeters [65,[169][170][171], due to intrinsically high linear and nonlinear loss of the semiconductor material. This can mean that for longer cavities, in the first place the laser threshold may not be reached.…”

“…Table 1 provides an overview of the lowest repetition rates to date for integrated mode-locked diode lasers based on standard (AM) mode-locking techniques [56,63,65,140,[169][170][171][172][173][174][175][176], adapted from the overviews provided in Ref. [63] and [177].…”

“…For monolithic mode-locked lasers, the longest cavity lengths were realized by employing reduced-loss passive waveguides for part of the cavity, resulting in cavity lengths of about 41 mm [65,170,171]. There, the loss was reduced by having part of the resonator made with a different epitaxial structure, where the multi-quantum well active layer is etched away before the other layers are grown [178].…”