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

Area of Science:

• Optoelectronics
• Optical Communications
• Materials Science

Background:

• High-speed optical Orthogonal Frequency-Division Multiplexing (OFDM) is crucial for intra-datacenter networks.
• Vertical Cavity Surface Emitting Lasers (VCSELs) are key components in these systems.
• Comparing different VCSEL modes (multi-mode, few-mode, single-mode) is essential for optimizing performance.

Purpose of the Study:

• To comprehensively compare homemade multi-mode (MM), few-mode (FM), and single-mode (SM) VCSEL chips.
• To evaluate their suitability for high-speed optical OFDM transmission applications.
• To identify the optimal VCSEL for intra-datacenter 100-m multi-mode fiber links.

Main Methods:

• Direct encoding of pre-leveled 16-Quadrature Amplitude Modulation (16-QAM) OFDM data using a microwave probe.
• Transmission testing over 100-m OM4 multi-mode fiber (MMF).
• Characterization of VCSELs based on emission aperture, differential quantum efficiency, optical power, and 3-dB encoding bandwidth.

Main Results:

• MM VCSEL (11 μm aperture) offered highest optical power (8.67 mW) but lowest bandwidth (21 GHz).
• SM VCSEL (3 μm aperture) provided highest bandwidth (23 GHz) with minimal heat accumulation.
• FM VCSEL achieved the highest bit rate (96 Gbit/s) in back-to-back testing.
• SM VCSEL demonstrated the best performance for 100-m MMF transmission at 80 Gbit/s with the lowest power penalty (1.77 dB).

Conclusions:

• The SM VCSEL, with its near modal-dispersion-free characteristic, is the most promising candidate for 80-Gbit/s 16-QAM OFDM transmission over 100-m OM4 MMF.
• Trade-offs between signal-to-noise ratio and bandwidth are critical for optimizing VCSEL performance.
• Different oxide-layer confined gain cross-sections influence the performance of VCSELs with identical material structures.