For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 μm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 μm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

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A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100 m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5 μm, demonstrates high external quantum efficiency, provides an optical power of 2.2 mW at 38 times threshold condition, and exhibits 3 dB direct-modulation bandwidth beyond 22 GHz at a cost of slight heat accumulation. At a DC bias point of 5 mA (22.6I th) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100 Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100 m OM4 MMF with a receiving power penalty of 4 dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92 Gb/s.