. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Serrano, A.R.C.; de Dios Fernandez, C.; Cano, E.P.; Ortsiefer, M.; Meissner, P.; Acedo, P., "VCSEL-Based Optical Frequency Combs: Toward Efficient Single-Device Comb Generation", IEEe Photonics Technology Letters, (2013Letters, ( ), 25(20), 1981Letters, ( ,1984 such as Mode-Locking Laser Diodes (MLLD) [7] or microresonators [8]. These devices can be divided according to their repetition frequency, being MLLDs in the order of several GHz, and microresonators in the order of few hundreds of GHz. Microresonators are recent and very promising devices able to generate extremely wide OFCGs, but their high repetition frequencies make them unsuitable for a significant number of applications [8]. On the other hand, MLLDs have been widely used during the last decades, but they still present important drawbacks, like the absence of continuous tunability of the repetition frequency and the need of specially designed structures that nowadays are still far from offering reliability and repeatability in the manufacturing processes for commercial purposes.In this sense, some recent works have recovered a wellknown technique for inducing pulsed operation in a semiconductor laser, Gain-Switching (GS), in order to implement multi-GHz OFCGs that are to overcome some of these drawbacks associated with MLLDs [4], [9]. Although they offer much less optical span than MLLDs, GS-based OFCGs offer wide tunability range, high correlation between optical modes, and low-cost; as they can be implemented using commercial semiconductor lasers. Nevertheless, if standard edge-emitting lasers are used, the amount of direct modulation power needed for an OFCG featuring 8-10 lines is about 0.5-1 W, which makes necessary the use of Radiofrequency (RF) power amplifiers [4], [10]. Moreover, the generated optical spectra are not flat, and additional stages based on nonlinear techniques and comprising several external components are usually needed to obtain flat-topped pulses, such as cascaded Intensity or Phase Electro-Optical Modulators (EOMs) [11][12][13] and Four Wave Mixing (FWM) [14].For this reason, different types of semiconductor lasers have been used under GS regime in the search of compact, commercial devices based pulsed sources, and, Vertical Cavity Surface Emitting Lasers (VCSELs) are promising candidates [15][16][17][18][19]. VCSELs need very few current to operate (under 10 mA), and their integration capabilities as well as the capacity for on-chip testing allow for low cost optical subsystems with an excellent energy efficiency. Moreover, they have demonstrated wide wavelength tuning capabilities (in excess of 100 nm) with direct control of the cavity length using membranes [20]. In this sense, VCSELs would lead to ...

Abstract-In this letter, we explore the performance of an optical frequency comb generator (OFCG) sources based on vertical-cavity surface-emitting laser diodes. The direct gain switching (GS) technique, the indirect electro-optical (EO) technique, and a combination of both have been experimentally evaluated. We have observed that this later combination gives birth to an enhanced OFCG that offers a tunable improved comb in terms of frequency span, flatness, and coherence with respect to the sole EO approach, as it partly inherits the high-efficiency qualities of the GS-OFCG while offering a wider span.