Tunable spectral asymmetry at the facets of a chirped tapered quantum‑dot superluminescent diode

A wide spectral asymmetry between the wide and narrow facets of a two-section tapered quantum dot (QD) superluminescent diode (SLD) emitting around 1240 nm was observed and investigated. This asymmetry, as characterized by the mismatch in the center wavelengths of the wide and narrow facet spectra, was found to be tunable and had some dependence on the magnitude of the difference in current densities applied to each section of the device. A maximum spectral mismatch of 14 nm was observed when the current density difference between the two SLD sections was 1.5 kAcm2.This spectral asymmetry presents an unexplored degree of freedom which could be exploited via multiplexing from a single device to optimize spectral bandwidth. Furthermore, potentially useful output powers of up to 50 mW were observed from the narrow facet of the SLD, which could again be exploited via single device multiplexing to increase output power, with little to no cost to spectral bandwidth. The experimental findings were analyzed using a rate-equation based QD model considering the QD ensemble inhomogeneous broadening, the multilayer chirped active material, the spatial distribution of the QD carriers and the spectral and spatial distribution of the photons in the SLDs. The numerical simulations were able to predict the asymmetric output powers extracted from the SLD facets, mainly to due to different equivalent material losses experienced by the forward and backward fields in the weakly gain guided tapered device. Simulations were also able to predict the spectral distribution of the optical fields at the output facets.