Quantum Dot DFBs Under Amplitude Modulation

This example demonstrates how a quantum dot gain model generated in Harold can be exported to laser diode and circuit simulator PICWave for 3D dynamic simulations. This short example illustrates one of many benefits offered by quantum dot lasers over quantum wells; a higher achievable modulation speed for signal transmission (AM).

Two distributed feedback (DFB) lasers with HR and AR facets are created with similar epitaxy structures; one using InGaAlAs quantum wells, the other using InGaAs quantum dots. A current above threshold is applied and after both lasers begin to lase, a 20 GHz signal is applied through amplitude modulation (8 mA to 12 mA). While under modulation, the lasing wavelength can be seen to vary (chirp) causing error in the output signal.

The quantum dots DFB exhibit a smaller chirping effect due to the quantum dot’s more symmetric gain spectrum (a consequence of their atomic-like discrete energy levels). While under modulation the standard deviation from the average lasing wavelength was 0.0021. The deviation of wavelengths for the quantum well structure was over 7 times larger at 0.016.

Variation in the lasing wavelength causes an error in the transmitted signal with some ‘on’ an ‘off’ bits being indistinguishable from each other as quantified by the bit error rate (BER) and eye diagrams.

The ‘open’ eye diagram on the left corresponds to the quantum dot DFB with a BER of 1.2 × 10-16. The eye diagram for the quantum well DFB is magnitudes larger at 6.5 × 10-2. These results suggest quantum dot epitaxy structures are a stronger candidate than quantum well structures for high speed amplitude modulation.

Harold is an established heterostructure simulation tool that, with pioneering development with academic leaders, can simulate key properties for quantum dot epitaxy structures. Energy level simulations including 3D stress effects and a distribution of dot sizes, contribute to a full 2D cross section simulation including carrier diffusion and overlaps with optical modes.

PICWave is a time domain simulation tool providing dynamic laser diode simulations. Connecting grating sections, component simulations, and time evolving electrical/ optical sources a range of lasers and PICs can be modelled. Results of Harold’s quantum dot model can be imported to PICWave allowing 3D laser simulations to be run even on a laptop.

For this illustration simulations are performed in 1D+Z though PICWave does allow a 2D waveguide cross section to be set up for 3D dynamic simulations.