# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 1 Introduction  
Near-infrared vertical-cavity surface-emitting lasers (VCSELs) are standard sources for short- to medium-range interconnects, starting from single-mode (SM) board-to-board (0.3-1 m distance) to multi-mode intra-datacenter ones (1-500 m) [1]. For distances above 500 m and a bit error rate (BER) of \(10^{-12}\), the power penalty due to modal and chromatic dispersion and mode partition noise, being typical for multi-mode VCSELs [2], becomes prohibitive [3]. Hyper datacenters are typical examples for using SM fibers for short distances to be supported by Wavelength Division Multiplexing (WDM) [4]. Long-wavelength (LW or short-wavelength infrared, SWIR) VCSELs allow to appreciably increase the distance covered by SM fibers [4] in the next generation of datacenters [5]. C-band VCSELs have the potential for 50 Gbps non-return-to-zero (NRZ) operation and show low latency. The power consumption of both NRZ and four-level Pulse Amplitude Modulation (PAM4) is lower as compared to that of DFB-lasers operating at the same wavelength [5]. C-band VCSELs can support 200 GbE operation [4]. In addition, LW VCSELs are very attractive forSpatial Division Multiplexing (SDM) based on multicore fibers (MCF) or few-mode fibers (FMF) [6; 7; 8]. The development of LW VCSELs is challenging due to critical limitations inherent to InAlGaAsSP/InP and InAlGaAs/GaAs material systems. The main problems of monolithic (grown in a single epitaxial process) GaAs-based VCSELs are the lack of active regions, providing efficient radiative recombination in the targeted spectral range while maintaining a pseudomorphic growth mode, and an abrupt increase in free-carrier absorption as the wavelength increases. At the same time, the small refractive index contrast and low thermal conductivity of ternary and quaternary solid alloys lattice-matched to InP used in the distributed Bragg reflectors (DBRs) as well as the ineffective current confinement (air-gap aperture) are key problems of monolithic InP-based LW VCSELs. This high thermal resistance impairs the heat removal from the active region and leads to a rapid increase in the laser internal temperature. As a result, the poor temperature stability of monolithic 1550 nm InP-based VCSELs limits not only the maximum output power (about 1.6 mW at 20 \({}^{\circ}\)C) but also the high-speed performance (about 4 GHz) as well [9]. Today, the creation of high-speed 1550 nm VCSELs is related to two major technologies. The first one is based on InP-based active regions along with specific hybrid (metal-dielectric) DBRs, e.g., developed by the TUM/Vertilas team [10]. The second one is wafer-fused VCSELs. By this technology, pioneered by the ETH team, AlGaAs/GaAs-based DBRs with InP-based active regions are fused. The second approach allows realizing reliable LW VCSELs, satisfying the GR-468-CORE Telcordia qualification test [11]. In addition, high-contrast gratings (HCG) and micro-electro-mechanical system (MEMS) technologies to create 1550 nm VCSELs were developed [12; 13; 14; 15]. Using the first technology, by replacing only one semiconductor InP-based DBR by a dielectric mirror integrated with a Au-heatsink, a \(-3\) dB modulation bandwidth as large as 12 GHz at 25 \({}^{\circ}\)C and 10 GHz at 85 \({}^{\circ}\)C [16] was achieved. Open eyes of 25 Gbps at 20 \({}^{\circ}\)C and error-free data transmission up to 85 \({}^{\circ}\)C at 12.5 Gbps using NRZ signal modulation were demonstrated [17]. This type of VCSEL [18] presents an interesting alternative for creating large monolithic VCSELs arrays with high power (in the watt regime). In fact, recently, Vertilas announced SWIR VCSEL arrays (800 emitters) that emit 8 W of quasi-continuous power [19; 20] suitable for eye-safe 3D sensing applications, such as face recognition in mobile phones or illumination. Using short-cavity concepts (vertical microcavity design with two dielectric mirrors) made it possible to increase the modulation frequency of a 5 \(\upmu\)m device to 17 GHz with a maximum output power of 3.5 mW [21].