Abstract

Superluminescent light emitting diodes (SLD) offer the prospect of high power and wide bandwidth sources for various sensing and spectroscopy application and in particular for optical coherent tomography, where it is required broad band and very bright light sources with a short temporal coherence. In the last years the Quantum Dot (QD) semiconductor materials have been promising used for the realization of QD-SLD [1], thanks to the possibility of tailoring the optical gain bandwidth with a proper engineering of the QD size and composition. Despite the growing importance of this kind of light sources, up to now only few and simple models [1] have been presented in the literature for the theoretical study of these devices. The purpose of this work is to present a detailed numerical modelling for the analysis and design of QD-SLD. The model we propose includes the peculiar characteristics of the QD material system such as for example the non-homogeneous distribution of the QD size (inhomogeneous gain broadening) and the presence of other states (wetting layer and SCH states) besides the states confined in the QDs. Since the output power of the SLD is given by the spontaneous emission amplified by the propagation in the device, the numerical model is based on a travelling power approach: the SLD is divided in several section whose photon density are obtained from the propagation of the forward and backward photons of the two nearest sections. Furthermore, given the importance of the analysis of the SLD emission spectra for any practical applications, the model is also based on a spectral slicing approach. In each SLD longitudinal section we calculate the true spontaneous emission (TSE) and the gain spectra solving a Multi Population Rate Equation system[2]. In this way the non-uniform QD ensemble is represented by several sub-groups, coupled through the wetting layer, each having a different energy for the states confined in the dots (ground states, GS, and excited state, ES). The TSE obtained by the carrier occupation of the various states is then divided in several wavelength intervals, each giving the contribution of that section to the forward and backward photons. These photon densities are then propagated with the material gain of that particular section. We therefore account for the spatial hole burning effect in the longitudinal direction of the SLD and we can calculate how much the carrier population in the GS and ES are depleted by the propagating photons. The inclusion of this process is fundamental for a correct evaluation of the output power and output spectrum of the SLD. As an example we show in Fig. 1 the calculated power vs current characteristics of a 3 mm InAs/GaAs QD-SLD with both anti-reflection coated facets. The inset reports the output spectra at 1=70 mA and at 1=450 mA. For the particular QD material we analyze, when the output power from the ES equals the power from the GS (maximum bandwidth, 1=70 mA), the output power is quite low (3 mW). Whereas at the maximum of the output power (1=450 mA) the GS is suppressed of 9.3 dB (Fig. 2, dashed line), with a consequent reduction of the emission bandwidth. This is due to the higher ES gain and due to the suppression of the GS gain caused by spatial hole burning. The present model has therefore been used to propose solutions to improve the bandwidth. For example the SLD has been divided in two sections injected with different currents: the first section (1.5 mm with 20 mA current injection) provides the GS power; the second section (3 mm with 450 mA injection) amplifies the GS emission provided by the first section and also generates the ES power. In this case the GS suppression is reduced to 3.7 dB (Fig.2, solid line). Optimizing the injected current and/or the length of the sections the GS and ES power can be equalized keeping at the same time very high output power. In conclusion we present a MPRE model for the analysis and design of QD-SLD. The model is used to calculate the power vs current characteristic and the output spectra of the SLD and propose design solution to improve the emission bandwidth of the device.

© 2007 IEEE