Effects of External Optical Feedback on PAM4 Signal in VCSEL-Based SMF Link

M S Kim,S H Bae,B G Kim,Y C Chung

doi:10.1109/lpt.2020.3002240

Abstract

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-μm 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 ~8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> 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 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> ) 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.

Highlights

V ERTICAL-CAVITY surface-emitting laser (VCSEL) is widely used in various short-haul applications due to its low cost, low power consumption, and small size
When we set the optical feedback ratio to be -14 dB, an error floor is observed at the bit-error rate (BER) of ∼8.0 × 10−3 in the back-to-back condition for the 10-Gbaud PAM4 signal, while the on-off keying (OOK) signal operating at the same baud rate has a power penalty of only 0.4 dB (@BER = 5 × 10−5) under the same condition
We evaluated the effects of the external optical feedback on the OOK and PAM4 signals operating at various baud rates in a short-haul single-mode fiber (SMF) link implemented by using a commercial 10Gclass 1.55-μm vertical-cavity surface-emitting laser (VCSEL)

Summary

Introduction

V ERTICAL-CAVITY surface-emitting laser (VCSEL) is widely used in various short-haul applications due to its low cost, low power consumption, and small size. When we set the optical feedback ratio to be -14 dB (which can be caused by a bad fiber connector), an error floor is observed at the BER of ∼8.0 × 10−3 in the back-to-back condition for the 10-Gbaud PAM4 signal, while the OOK signal operating at the same baud rate has a power penalty of only 0.4 dB (@BER = 5 × 10−5) under the same condition.

Results

Conclusion