Locking of laser cavity solitons trapped by defects in VCSELs

Summary form only given. Laser cavity solitons (LCS) are transverse, nonlinear, self-localized and dissipative states that possess both translational and phase invariance. LCS interaction and synchronisation properties in broad-area semiconductor lasers have the potential for massive parallelism and the formation of complex arrays. Phase-locked bound states of solitons have been predicted in mode-locked lasers for the temporal case [1] and in lasers with saturable absorbers for the spatial case [2]. Corresponding phase-quadrature states have been observed in fibre lasers [1].Here we present a different kind of soliton locking. We demonstrate experimentally and theoretically Adlertype locking and synchronization of spatial LCS in a vertical-cavity surface-emitting laser (VCSEL) with an external volume Bragg grating (VBG) that provides frequency-selective feedback (see left panel of Fig.1) [3]. In particular we explain the role played by defects resulting from fluctuations during the epitaxial growth process. The experiment has been performed with a temperature tuned 981 nm VCSEL of 200 μm circular aperture and a VBG with a single reflection peak at 981.1 nm, and a peak reflectivity of 99% [3]. The external cavity for the frequency-selective feedback is arranged in a self-imaging configuration that maintains the high Fresnel number of the VCSEL cavity (see the left panel of Fig. 1). This potentially enables large parallelism of solitons and hence the prospect of synchronisation in large disordered networks.A piezo-electric transducer was used to minutely tilt the external cavity´s end reflector (VBG) with respect to the optical axis. The differential change of the external cavity length for the two LCS leads to a differential change in feedback phase. In this way the detuning between two LCS can be tuned. During the scan, a region of frequency and phase locking appears, identified in the right panel of Fig. 1 by the region of high fringe visibility in the fa- field. This illustrates that locking - once achieved - is a robust phenomenon. We have theoretically and numerically investigated the interaction of pinned LCS upon the change of the difference in the depth of the trapping defects for both class-B semiconductor [4] and Ginzburg-Landau [5] models. In the locking region we recover an Adler scenario, where the fringe phase varies smoothly and quasilinearly with the detuning of the external cavity (see the red curve in the right panel of Fig. 1). The transitions to and from frequency and phase-locking are rather abrupt. Finally we discuss theoretical and experimental spectra in the locked and unlocked regions and the effect of the external cavity modes. Independent switching and erasure of locked and unlocked LCS is demonstrated.