# Impact of Device Topology on the Performance of High-Speed 1550 nm Wafer-Fused VCSELs
## 4 Conclusions  
A detailed experimental analysis of the impact of device topology on the performance of 1550 nm VCSELs with an active region based on thin InGaAs/InAlGaAs quantum wells and a composite InAlGaAs BTJ was performed. The VCSELs structures were fabricated by wafer fusion of InP-based optical cavity structures with two AlGaAs/GaAs DBRs grown by MBE. Optimization of the composite InAlGaAs BTJ allowed us to find a balance between the mode volume and the single-mode output power. Moreover, doping adjustments of the overgrown n-InP layers help to find a compromise between low electrical resistance and high parasitic cutoff frequency. The high-speed performance of the lasers of \(L\)-type device topology (with the largest mesa sizes) is mainly limited by electrical parasitics and shows a noticeable damping of the relaxation oscillations. An analysis of \(S_{11}(f)\) in the \(S\)-type device topology (with the smallest mesa sizes) leads to the conclusion that the key mechanism limiting our high-speed performance is the damping of the relaxation oscillations. The record values of the modulation bandwidth (13 GHz) and data transmission (37 Gbps in BTB configuration for NRZ direct modulation) are achieved at 20 \({}^{\circ}\)C for our 1550 nm WF VCSELs. The high output power and dynamic performance demonstrate the potential of our 1550 nm InGaAs QW-based WF VCSELs for large distance transmission systems (i.e., beyond \(\sim\)1 km), where a high bit rate SM emission is required. Such lasers provide the basis for narrow WDM systems (with 5 nm channel spacing). Our monolithic VCSEL allows to realize high-capacity and compact short-reach optical interconnects for datacenters and arrays for 3D sensing and illumination applications. Another possible market of single-mode LWIR VCSELs is gas sensing tunable diode laser spectroscopy (TDLS).  
Figure 7: **(a)** Eye diagram (left) and bit error ratio (right) bathtub curve for a data rate of 25 Gbps for a 1550 nm InGaAs QW-based WF VCSEL with 6 \(\upmu\)m BTJ diameter and \(L\)-type device topology across a 3 m SMF; **(b)** the eye diagram (left) and bit error ratio (right) bathtub curve for a data rate of 37 Gbps for a 1550 nm InGaAs QW-based WF VCSEL with 6 \(\upmu\)m BTJ diameter and \(S\)-type device topology across a 3 m SMF. The temperature of measurements was 20 \({}^{\circ}\)C. Figure 7b is reprinted, with permission, from Ref. [39]. 2023, IEEE.  
**Author Contributions:** Conceptualization, S.B.; data curation, A.B. ([PERSON]), A.K., A.G., A.V., K.V., M.B., S.H. and Y.K.; investigation, V.N., S.B. and S.-C.T.; writing--original draft, A.B. ([PERSON]), E.K., L.K., N.M. and S.B.; writing--review and editing, I.N., V.U. and D.B.; 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.  
**Funding:** The authors from CAS acknowledge support by the National Key R&D Program of China (2021 YFB2801000) for the large-signal modulation measurements of 1550 nm WF VCSELs. The authors from ITMO University acknowledge support in part by the Ministry of Science and Higher Education of the Russian Federation, project no. 2019-1442 (project reference number FSER-2020-0013) for the static characteristics measurements of 1550 nm WF VCSELs and in part by the Advanced Engineering Schools Federal Project for the small-signal modulation characteristics measurements of 1550 nm WF VCSELs. [PERSON] and [PERSON] acknowledge the support of the CAS President's international fellowship initiative grants No. 2023 YTA0007 and No. 2023 YTB0002 for the analysis of the static characteristics and small-signal modulation characteristics of 1550 nm WF VCSELs, respectively.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.