# Long-Wavelength VCSELs: Status and Prospects
## 8 Conclusions  
Over the past 20 years, a variety of technologies have been developed for LW VCSELs. Presently, two of them have emerged, leading to excellent results. The first one is based on  
Figure 5: 100 GBASE–LR4 standard mask test for 1550 nm MBE-grown WF VCSEL for a bit rate of 25.78 Gbit/s with BER 9E–9 and the corresponding: (**a**) BER Contour; (**b**) BER H Bathtub; (**c**) BER Eye; and (**d**) SP In PDF Eye. Reprinted/adapted with permission from Ref. [169]. 2017, IEEE. Figure 6: Measured (points) small signal modulation response \(S_{21}\) and the corresponding curve-fits (solid lines) for MBE-grown 1550 nm VCSEL for different drive currents (**left panel**); Eye diagram and bit error ratio bath tub curve for a data rate of 37 Gbit/s across a 3 m SMF (**right panel**). Reprinted/adapted with permission from Ref. [4]. 2023, IEEE. hybrid short-cavity VCSELs, and the second one uses wafer-fused VCSELs. Both of them are based on the BTJ concept for optical and current confinement. The excellent dynamic performance of hybrid short-cavity VCSELs (about 17 GHz and 22 GHz modulation bandwidths at 1300 nm and 1550 nm, respectively) is related to the combination of an ultra-short semiconductor cavity (-1.5-\(\upmu\)m) and two dielectric DBRs. This allows for minimizing the mode volume and increasing the D-factor (the proportionality factor of the relaxation resonance frequency versus the square root of the drive and threshold currents difference). Using a flip-chip process, double-mesa structures are used to reduce the mesa capacitance (to increase the parasitic frequency) and improve heat spreading [175]. This fabrication technology requires dielectric DBRs based on toxic materials (AlF\({}_{3}\)/ZnS) and complex processing steps [177], which might create difficulties for the mass production of such VCSELs. Based on wafer fusion technologies, VCSELs reach record 6 mW single-mode output power at both 1300 and 1550 nm, due to the better thermal conductivity of AlGaAs/GaAs DBRs as compared to high-contrast dielectric mirrors. The thermal conductivity of WF VCSELs [166] is about five times greater than that of hybrid short-cavity VCSELs [28] for the same size as BTJ. The creation of large internal temperatures at large currents combined with low heat dissipation limits the maximum current and bandwidth. The data rate of WF-VCSELs is in addition limited by the parasitic cut-off (due to high parasitic capacity) at low frequencies and currently reaches 13 GHz modulation bandwidth, yielding a data rate of 37 Gbit/s for NRZ modulation at 1550 nm VCSELs [4]. WF VCSELs also fulfilled the reliability conditions according to the GR-468-CORE Telcoria qualification test [147]), making them attractive for industrial applications. Further increases in data rates of WF VCSELs can be achieved by reducing the parasitic capacitance, adjusting photon lifetime, the gain-to-cavity detuning, and increasing the heat dissipation. Conceptualization, A.E. and D.B. ; writing--original draft preparation, A.B., E.K., L.K. and S.B. ; writing--review and editing, L.N., D.B. and S.-C.T. ; supervision, A.E. and D.B. ; project administration, L.K. ; funding acquisition, A.E. and D.B. All authors have read and agreed to the published version of the manuscript. The authors from CAS acknowledge support from the National Key R&D Program of China (2021 YFB2801000) for the large signal modulation measurements and analysis of 1550 nm WF VCSELs. The authors from ITMO University acknowledge support in part by project no. 2019-1442 for the static characteristics analysis of 1550 nm WF VCSELs and in part by the Priority 2030 program for the small signal modulation characteristics analysis of 1550 nm WF VCSELs. [PERSON] and [PERSON] acknowledge the support of the CAS PIFI program for the analysis of the large signal modulation characteristics and small signal modulation characteristics of 1300 nm WF VCSELs, respectively.