Invited Article: Mitigation of dynamical instabilities in laser arrays via non-Hermitian coupling

Abstract

Arrays of coupled semiconductor lasers are systems possessing complex dynamical behavior that are of major interest in photonics and laser science. Dynamical instabilities, arising from supermode competition and slow carrier dynamics, are known to prevent stable phase locking in a wide range of parameter space, requiring special methods to realize stable laser operation. Inspired by recent concepts of parity-time (PT ) and non-Hermitian photonics, in this work we consider non-Hermitian coupling engineering in laser arrays in a ring geometry and show, both analytically and numerically, that non-Hermitian coupling can help to mitigate the onset of dynamical laser instabilities. In particular, we consider in details two kinds of nearest-neighbor non-Hermitian couplings: symmetric but complex mode coupling (type-I non-Hermitian coupling) and asymmetric mode coupling (type-II non-Hermitian coupling). Suppression of dynamical instabilities can be realized in both coupling schemes, resulting in stable phase-locking laser emission with the lasers emitting in phase (for type-I coupling) or with π/2 phase gradient (for type-II coupling), resulting in a vortex far-field beam. In type-II non-Hermitian coupling, chirality induced by asymmetric mode coupling enables laser phase locking even in presence of moderate disorder in the resonance frequencies of the lasers. arXiv:1802.05439v2 [physics.optics]

Nonlinear mode competition and symmetry-protected power oscillations in topological lasers

“…Looking beyond this symmetry class, it will be worthwhile to explore the role of nonlinear distributed gain and loss in topological-insulator lasers [30,31], where topological edge states align continuously along an edge band. This is a scenario which has been predicted to be more fragile against the carrier dynamics in the medium [53], but is generally expected to benefit from non-hermitian effects, as has already been demonstrated for complex and directed coupling [54]. It would therefore be desirable to classify in general which nonlinearly extended dynamical symmetries can exist in these and other universality classes of topological systems, and whether this leads to novel operation regimes as described here for the case of non-hermitian chargeconjugation symmetry.…”

Supersymmetric Microring Laser Arrays

“…While the single longitudinal mode lasing was addressed by parity-time symmetry [21], novel strategy and symmetry consideration are required to support single transverse supermode lasing. Sophisticated laser-array designs based on gain tailoring, special diffractive or non-Hermitian coupling are proposed to mitigate the complexity of transverse supermode lasing [22][23][24].…”

Parity anomaly laser

“…A limitation of the strained lattice and topological midgap mode proposals is that they are based on large lattices described by band structures. For integrated laser applications, small arrays of several to dozens of individual elements are preferable because slow dynamical instabilities typically inhibit synchronization of larger arrays [16,17]. Furthermore, lasing based on localized topological zero modes is not compatible with large mode volumes, suffering from mode competition when realistic (saturable) gain is taken into account [18].…”

“…In contrast to the 1D topological laser arrays, here the zero mode uniformly excites all the pumped sites, so it can saturate the gain and maintain single mode operation over a wider range of parameters. To account for gain saturation we consider a class B laser model describing coupled resonators with embedded InGaAsP quan- tum wells [13,16,17],…”

“…Ref. [17]). The zero mode is linearly stable if none of its perturbation eigenvalues have a positive real part.…”