RF line width and integrated RIN study of a single-section quantum dot comb laser

Optical frequency combs generated by self mode-locking of single-section quantum dot based semiconductor lasers are ideal sources for applications in high capacity optical interconnects or high precision dual comb spectroscopy. We investigate a 1mm long InAs/InGaAs quantum dot semiconductor laser both experimentally and by simulations using a time-domain traveling-wave model. We observe that by increasing the injection current, the laser output exhibits an unlocked multi-mode behavior above the lasing threshold up to a certain current were the modes lock due to an internal non-linear effect in the active laser medium. This phase locking is experimentally and numerically observed by RF beat note line width analysis as well as by integrated relative intensity noise analysis. Both of these properties are significantly reduced above this locking threshold. The lowest experimentally measured RF line width amounts to 20 kHz, while for lower currents prior to the threshold the line width can be as high as hundreds of MHz. Our simulations confirm this threshold behavior and the simulated spectra are in good qualitative and quantitative agreement.