Different values for the linewidth enhancement factor of a Quantum-Dots laser obtained using optical and electrical modulation

Abstract

Summary form only given. The linewidth enhancement factor (α-factor), defined as the ratio of carrier-induced variation of real and imaginary parts of the material's susceptibility, has a great importance for semiconductor lasers, as it influences linewidth, chirp, mode stability, laser dynamics, laser behavior in presence of optical feedback. Given that a low value for the α-factor is considered good, Quantum-Dot lasers (QDLs) are particularly interesting as they should exhibit a near-zero value for the α-factor, thanks to their atom-like transition [1]. However, measured values for QDLs range from almost zero to more than 10 [1,2], i.e., a range that extends well beyond the natural discrepancies that are expected when different measuring methods are applied.To further investigate this issue, we applied for the first time the most reliable measuring technique for the a- factor, i.e. the Fiber Transfer Function (FTF) method, to a Fabry-Perot QD laser emitting around 1300nm, fabricated using high performance InAs-In-GaAs stacked QD layers grown by MBE [3]. As shown in Fig. 1, the modulated light emitted from the QDL is passed through a Fabry-Perot air filter to select a single longitudinal mode, it is amplified by two SOAs to compensate for the losses of the highly dispersive fiber, and it is finally detected by a high-speed photodiode connected to a network analyzer. The chirp is assessed by measuring the transfer function of the fiber span in the electrical frequency domain. We introduced the novelty of comparing two methods for the modulation of the laser. Firstly, we used conventional current modulation, and measured an a-factor value around 3 (see Fig. 2, black trace). Secondly, we applied external optical modulation (at a wavelength that is offset with respect to the QDL lasing modes, to avoid locking) by injecting into the QDL the light from an ECL that is amplitude-modulated by a Mach-Zehnder LiNbO3 modulator, and obtained an a-factor value very close to zero (see Fig. 2, red trace).Our measurements prove that QD laser indeed exhibit a low intrinsic a-factor when only the QD carrier population is involved, but this positive feature is jeopardized upon current modulation.