Optical feedback stabilization of a frequency comb generated by a self-mode-locked quantum dot laser emitting at 1255 nm
In this contribution we investigate experimentally and by modeling the mode spacing tuning range and stability of optical frequency comb lines generated by a 1 mm long self mode-locked single section quantum dot semiconductor laser subject to dual-cavity all optical self-feedback. The external optical feedback cavities have an optical length of 9.4 m and 16.5 m and are implemented by optical fibers. The optical feedback strengths are below 0.02% back reflected onto the as-cleaved laser facet. The line-width of the laser mode-beating frequency equals optical frequency mode spacing of 40.67 GHz amounts to 1.4 MHz in the free-running case. By fine-delay tuning of both optical feedback lengths, we find a comb-line spacing tuning range of 70 MHz. The radio-frequency linewidth decreases by a factor of 700 down to 2 kHz for particular adjusted optical feedback cavity lengths, thus improve the mode spacing stability. We validate the experimental findings by a simple and universal stochastic time-domain model.