# Long-Wavelength VCSELs: Status and Prospects
## 1 Introduction  
According to Yole Intelligence's annual 'VCSEL--Market and Technology Trends 2022 report [1], the datacom sector of the vertical-cavity surface-emitting lasers (VCSELs) market is growing at 22.2%/year, regaining the lead from the mobile and consumer sector, which are growing at 15.7%/year.  
The use of long-wavelength (1300-1550 nm) single-mode (SM) VCSELs makes it possible to reduce the modal and chromatic dispersion in an optical link and, as a result, to extend its reach. Moreover, spatial division multiplexing (SDM) transmission by multi-core fibers using long-wavelength (LW or short-wavelength infrared, SWIR) VCSELs are enabling many larger-scale data center networks [2] than presently possible. A review of data communication and telecommunication by LW VCSELs can be found in ref. [3]. 1550 nm SM VCSELs are promising not only for next-generation short-reach optical interconnects but also for SDM transceivers. In fact, the large output power and dynamic characteristics of 1550 nm VCSELs [4] demonstrate their potential for large distance transmission (e.g., 1 km), where high bit rate SM emission is a prerequisite for operating narrow wavelength-division multiplexing systems (e.g., for 5 km distance). Moreover, 1550 nm widely tunable micro-electro-mechanical system (MEMS) VCSELs are used for wavelength DM passive optical network (WDP-PON) systems [5] with VCSEL-based SFP+ modules and THz photomixing [6].  
Due to the absence of mode-hopping effects in short cavities, where longitudinal-mode spacing is larger than the mirror stopband, 1950 nm range MEMS VCSELs can be applied for tunable diode laser absorption spectroscopy (TDLAS) [7]. LW VCSELs (1300-2300 nm) can also be used for gas sensing, including HF, H\({}_{2}\)O, NH\({}_{3}\), CH\({}_{4}\), HCl, H\({}_{2}\)S, CO\({}_{2}\), CO, N\({}_{2}\)O, ethylene oxide, and combustibles [8; 9].  
1300 nm VCSELs are of particularly strong importance for hybrid integration with silicon photonics, providing integrated modulators and InP- and GaAs-based integrated photonic circuits [10; 11; 12; 13]. Other emerging markets for 1300 nm VCSELs include eye-safe-based Laser Imaging Detection and Ranging (LiDARs) [14] and light sources for 5G (fifth generation) mobile and optical wireless applications [13]. High-power SM VCSELs operating uncooled up to large on-chip temperatures are demanded. Modulation bandwidths >5 GHz are preferable for time-of-flight applications to realize short raise and fall times <1 ns. Novel proximity sensors may be based on 1380 nm VCSELs, enabling integration with organic light-emitting diodes (OLEDs). A shift of the VCSEL emission wavelength to 1380 nm will enable using these VCSELs behind OLED displays, replacing the laser diodes that are being currently used, e.g., in the proximity sensor under the display in the iPhone 14 Pro\(\copyright\)[1]. Therefore, the first large-scale application of LW VCSELs might be proximity sensors for smartphones. TRUMPF Photonic Components GmbH aims to bring the first SWIR VCSEL products to the high-volume market in 2025 [15; 16].  
In this paper, we present a review of the current status of LW VCSEL development and the relevant technologies. In Section 2, the various distributed Bragg reflectors (DBRs) that are used for the fabrication of 1300-1550 nm spectral range VCSELs are compared. In Section 3, we focus on the approaches to realize strong current confinement. The present status of monolithic LW VCSELs grown on GaAs and InP substrates is presented in Sections 4 and 5, respectively. Hybrid VCSELs with active regions grown on InP wafers will be discussed in Section 6. Approaches for the fabrication of LW VCSEL by wafer fusion techniques are described in Section 7.