Comparison on OM5-MMF and OM4-MMF Data Links With 32-GBaud PAM-4 Modulated Few-Mode VCSEL at 850 nm

Based on the pre-emphasis 4-level pulse amplitude modulation (PAM-4) format at a symbol rate of 32 GBaud, the comparisons among optical PAM-4 data transmissions through back-to-back (BtB), 100-m OM4 multi-mode fiber (MMF), and 100-m OM5-MMF links are discussed in detail by employing the directly modulated few-transverse-mode (FM) vertical cavity surface emitting laser (VCSEL) chip at 850 nm as the carrier. The bare FM-VCSEL chip exhibits an oxide-confined aperture of 5 μm to provide 3-4 transverse modes with low threshold current (Ith) of 0.25 mA and high optical power of 1.2 mW at a bias current of 38 Ith. The impendence of 65 Ω and the 3-dB bandwidth of 24 GHz are obtained by setting the DC point of the FM-VCSEL at 6.25 mA. With the bias current optimization and the pre-emphasis, the receiving bit error ratio (BER) of the 32-GBaud PAM-4 data meets the KP4-forward error correction criterion of 2.4 × 10
^-4
, which enables the FM-VCSEL successfully achieving the 64-Gbit/s data stream transmission through the BtB and 100-m OM4-/OM5-MMF links. As compared to the BtB transmission with a BER of 2.5 × 10
^-8
, replacing OM4 with OM5 for the 64-Gbit/s PAM-4 transmission reduces the receiving power penalty from 3.41 to 0.86 dB and improves the BER from 3.8 × 10
^-5
to 3.8 × 10
^-6
. That is mainly attributed to the higher effective modal bandwidth and compensated modal and chromatic dispersions of the OM5-MMF as compared to the OM4-MMF when delivering the FM-VCSEL carried PAM-4 data stream.