Multimode VCSEL Enables 42-GBaud PAM-4 and 35-GBaud 16-QAM OFDM for 100-m OM5 MMF Data Link

By enhancing the differential gain and reducing the capacitance of the 850-nm multi-mode vertical cavity surface emitting laser (VCSEL) with an analog bandwidth beyond 25 GHz via the use of InGaAs/AlGaAs quantum wells and multiple oxide confinement layers, the transmission of directly encoded 4-level pulse amplitude modulation (PAM-4) data at 42 GBaud and 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM OFDM) data at 35 GBaud are demonstrated. After passing through 100-m OM5 multimode fiber (MMF), the detailed comparison between the VCSELs designed with different aperture sizes of 5.5/7.5 μm is performed. The 7.5-μm-aperture VCSEL provides the higher power with larger quantum efficiency but exhibits the narrower 3-dB bandwidth and higher noise level than those of the 5.5-μm-aperture VCSEL. Shrinking the oxide-confined aperture to 5.5 μm assists the VCSEL to expand its 3-dB bandwidth to 25.2 GHz and suppresses its relative intensity noise to -135 dBc/Hz, which contributes to support the highest data rate up to 84 and 140 Gbit/s, respectively, for PAM-4 and 16-QAM OFDM data under forward error correction criterion at optical back-to-back condition. Even after transmitting through 100-m-long OM5-MMF, the allowable data transmission rate still remains at 80 Gbit/s for PAM-4 and 120 Gbit/s for 16-QAM OFDM with their receiving power penalty of 3.24 and 3.1 dB, respectively, when the data is carried by the 5.5-μm-aperture VCSEL. Such a newly designed VCSEL structure promotes its allowable bandwidth to manifest the feasibility toward 50-GBaud per channel capacity for future data center applications.