Optical Solitons and Frequency Comb Generation - Abstract
Passively mode-locked semiconductor lasers provide an efficient and cost effective opportunity to generate short optical pulses. Additional to the gain section they inhibit a saturable absorber section, which leads to pulsed regime that are energetically favourable compared to continuous wave operation. Under certain conditions, e.g. a large cavity round-trip time $T$ relative to the carrier recovery in the gain $gamma_g^-1$ ,a multi-stability can be found between the stable off-solution and the periodic fundamental and harmonic mode locking solutions , which is an indication for localized structures in the time domain. As the gain recovery time is the slowest time scale of the system, pulses propagating along the cavity only interact via the charge carriers in the gain medium. Therefore, additional pulses can be individually addressed inside the cavity by inducing a modulation to the gain. These localized structures have been investigated experimentally and theoretically by using a generic model based on delay differential equations. In order to describe the effects of the cavity configuration on the emergence of localized structures, we utilize a DDE model also considering the characteristics of a V-shaped external cavity geometry . These types of lasers qualify for possible applications of localized pulses such as optical buffers or on demand pulse systems. We perform a bifurcation analysis as well as direct numerical integration, in order to unravel the influence of the cavity geometry on the existence of localized pulses in these types of lasers. We find a multi-stability of the fundamental and harmonic mode-locking solutions below the lasing threshold. From this starting point, we show that introducing asymmetries to the cavity, i.e. shifting the gain-ship to either the absorber or the out-coupling lens, can be beneficial for the formation of localized structures.  M. Marconi, J. Javaloyes, S. Balle, M. Giudici, How Lasing Localized Structures Evolve out of Passive Mode Locking, Phys Rev Lett. 112, 223901 (2014).  J. Hausen, S. Meinecke, B. Lingnau, and K. Lüdge, Pulse Cluster Dynamics in Passively Mode-Locked Semiconductor Vertical-External-Cavity Surface-Emitting Lasers, Phys. Rev. Appl. 11, 044055 (2019).