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Effects of External Optical Feedback on PAM4 Signal in VCSEL-Based SMF Link
IEEE Photonics Technology Letters ( IF 2.6 ) Pub Date : 2020-07-15 , DOI: 10.1109/lpt.2020.3002240
M. S. Kim , B. G. Kim , S. H. Bae , Y. C. Chung

We evaluate the effects of the external optical feedback on the on-off keying (OOK) and four-level pulse amplitude modulation (PAM4) signals in the single-mode fiber (SMF) link implemented by using a commercial 10G-class 1.55- $\mu \text{m}$ vertical-cavity surface-emitting laser (VCSEL). The results show that the PAM4 signal is much more sensitive to the external optical feedback than the OOK signal. For example, in the back-to-back condition, an error floor is observed at the bit-error rate (BER) of $\sim 8\times 10 ^{\mathbf {-3}}$ for the 10-Gbaud PAM4 signal when we set the optical feedback ratio to be −14 dB. In comparison, the 10-Gbaud OOK signal has a power penalty of only 0.4 dB (@BER $= 5\times 10 ^{-5}$ ) under the same conditions. The results also show that the PAM4 signal becomes more sensitive to the optical feedback after the SMF transmission due to the shallowed BER curve by the fiber dispersion. In addition, we confirm that the effects of the optical feedback are independent of the baud rate. From these results, we conclude that the error-free transmission of the high-speed PAM4 signal cannot be achieved if there is even one bad fiber connector having >2.5% reflection in the VCSEL-based SMF link.

中文翻译:

外部光反馈对基于 VCSEL 的 SMF 链路中 PAM4 信号的影响

我们评估了外部光反馈对单模光纤 (SMF) 链路中开关键控 (OOK) 和四级脉冲幅度调制 (PAM4) 信号的影响,该链路使用商用 10G 级 1.55- $\mu \text{m}$ 垂直腔面发射激光器 (VCSEL)。结果表明,PAM4 信号对外部光反馈比 OOK 信号敏感得多。例如,在背靠背条件下,误码率 (BER) 为 $\sim 8\times 10 ^{\mathbf {-3}}$ 当我们将光反馈比设置为 -14 dB 时,对于 10-Gbaud PAM4 信号。相比之下,10-Gbaud OOK 信号的功率损失仅为 0.4 dB (@BER $= 5\times 10 ^{-5}$ ) 相同条件下。结果还表明,由于光纤色散使 BER 曲线变浅,因此 PAM4 信号在 SMF 传输后对光反馈变得更加敏感。此外,我们确认光反馈的影响与波特率无关。根据这些结果,我们得出结论,如果在基于 VCSEL 的 SMF 链路中,即使有一个坏光纤连接器的反射率大于 2.5%,也无法实现高速 PAM4 信号的无差错传输。
更新日期:2020-07-15
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