Hsuan-Yun Kao

Blue Laser Diode Based Free-space Optical Data Transmission elevated to 18 Gbps over 16 m

Up to 18-Gbps direct encoding of blue laser diode (BLD) is demonstrated for free-space data transmission. By reshaping the orthogonal frequency multiplexed (16-QAM OFDM) stream with sidelobe filtering, the raw data rate expedites from 17.2 to 18.4 Gbps. Employing an ultrafast p-i-n photodiode with smaller active area diameter and lower noise equivalent power significantly enlarges the data rate by 1.6 Gbps or upgrades the signal-to-noise ratio (SNR) by 0.2 dB. Replacing the 80-mW BLD with the 120-mW one essentially increases the received SNR by 0.4 dB under enhanced modulation throughput. Reinforcing the beam collimation and collection by increasing the numerical aperture with a plano-convex hyper-hemispherical lens further improves the SNR by 0.6 dB. After optimization, the 16-QAM OFDM data with and without sidelobe filtering are respectively delivered at raw data rates of 16.4 and 18 Gbps with spectral-density usage efficiency as high as 4 bit/s/Hz over 16 m in free space, wherein the BLD carried QAM-OFDM data stream remains its capacity after reformation with sidelobe filtering as the superior inter-carrier-interference immunity reinforces.

Comparison of single-/few-/multi-mode 850 nm VCSELs for optical OFDM transmission

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.

Investigation of mirror-resistance reduction in the signal transmission integraty of VCSELs

Through the reduction of mirror-resistance of vertical-cavity surface-emitting lasers, the optical bandwidth of 22.5 GHz is successfully achieved by implementation of zinc-diffusion process. It shows that lower mirror-resistance provides better transmission quality under similar optical bandwidth. With 20 ohm resistance reduction, the bandwidth limitation by electrical parasitics can be effectively improved from 6.4 to 14.71 GHz confirmed by the small-signal extraction results.