Role of dynamical injection locking and characteristic pulse events for low frequency fluctuations in semiconductor lasers
We investigate the dynamics of semiconductor lasers subject to time-delayed optical feedback from the perspective of dynamical self-injection locking. Based on the Lang-Kobayashi model, we perform an analysis of the well-known Low Frequency Fluctuations (LFFs) in the frequency-intensity plane. Moreover, we investigate a recently found dynamical regime of fragmented LFFs by means of a locking-range analysis, spectral comparison and precursor pulse identification. We show that LFF dynamics can be explained by dynamical optical injection locking due to the delayed optical feedback. Moreover, the fragmented LFFs occur due to a re-injection locking induced by a particular optical pulse structure in the chaotic feedback dynamics. This is corroborated by experiments with a semiconductor laser experiencing delayed feedback from an optical fiber loop. The dynamical nature of the feedback injection results in an eventual loss, but also possible regaining, of the locking, explaining the recently observed phenomenon of fragmented LFFs. The complex dynamics of semiconductor lasers subject to time-delayed optical feedback originally represented a nuisance and impacted the performance of these lasers negatively. The dynamical phenomenon of Low Frequency Fluctuations (LFFs) is a prominent example1–11 that has widely been studied due to its ubiquity and the variety of time scales involved in the dynamics.12 LFFs are characterized by fast intensity fluctuations occurring during a gradual power-buildup phase and erratic subsequent sudden power dropouts. These power dropouts recur on a much slower time scale than the fast intensity pulsations and even slower than the time scale introduced by the delay. In this paper, we apply a complementary perspective to the usual approach to interpret the emergence of the recurring LFF pattern. We conceptually treat the feedback as an externally injected drive signal to the otherwise stable free-running laser and view the emission properties as the result of dynamical injection-locking between the laser and its own delayed output. Extending the earlier work which introduced a similar perspective,2,13 we show that LFF dynamics is associated with dynamical injection locking and its eventual loss in the high gain region (HGR), and, moreover, can also exhibit the possibility of sudden regaining of the locking to the high gain region.