This work investigates the effects of the confinement factor on the linewidth enhancement factor in hybrid silicon quantum dot comb lasers, which is a key parameter involved in frequency comb generation. Experiments are performed on two laser devices sharing the same gain material with slightly different cavity designs resulting in different confinement factors. The results highlight that a lower confinement factor leads to a smaller carrier-induced refractive index variation and a larger differential gain, together resulting in a smaller linewidth enhancement factor, which in turn translates into different sets of performance regarding the feedback applications. This paper brings novel insights on the fundamental aspects of quantum dot comb lasers and provides new guidelines of future on-chip light sources for integrated wavelength-division multiplexing applications.
On-chip integration of semiconductor lasers have shown a growing interest in recent years, especially for the development of photonic integrated circuits (PICs) which are of paramount importance for high-speed communication within and between data centers, and fast on-board data exchanges. For all these applications, a key challenge remains the stability of the laser sources integrated on a PIC in presence of external optical feedback with the view to avoid integrated bulky and costly optical isolation. In this study, the effects of external optical feedback are investigated in hybrid InAs/InP quantum dot comb lasers on silicon. The design of the cavity includes a semiconductor optical amplifier section, a saturable absorber and an on-chip external cavity incorporating a vertical coupler. We measured the resulting feedback properties with respect to the operation conditions (bias current and voltage) and to the length of the saturable absorber. We show that under most operating conditions, the laser remains stable against optical feedback, only few regimes of operation occur, which either improve or degrade the frequency comb and/or the radio-frequency beatnote power of the laser.
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