Varghese A. Thomas

Noise in VCSEL-Based Links: Direct Measurement of VCSEL Transverse Mode Correlations and Implications for MPN and RIN

The correlation properties of vertical-cavity surface-emitting laser (VCSEL) transverse modes determine the noise impairments of VCSEL-based high-speed MMF links. Relative intensity noise and mode partition noise originate from the distinct and aggregate fluctuations of the lasing transverse modes of VCSELs. These noise mechanisms are often the limitations in VCSEL-based optical links particularly 25 Gbaud and higher OOK and PAM-4 links and are an integral part of any link budget including those of the IEEE 802.3 and fiber channel T11.2 standards efforts. Here, we report the direct measurement of the correlation properties of high-speed VCSELs. We show that the corresponding computed mode partition noise (MPN) and relative intensity noise (RIN) powers are inconsistent with the conventional models used by standards groups yet are consistent with link measurements.

Adaptive nonsymmetrical demodulation based on machine learning to mitigate time-varying impairments

We proposed and experimentally demonstrated a machine learning-based nonsymmetrical demodulation technique for a DSP-enabled receiver, with the aim of enabling time-varying nonlinear mitigation. Experimental results showed that nonsymmetrical demodulation can reduce the SER by up to 0.7 decades, when assuming time frames consisting of 10 k symbols and fiber transmission of 250 km. The proposed technique is transparent to the specific source of nonlinearity, which makes it simple yet robust. This machine learning method may also allow simplification of the standard demodulation blocks in particular the equalizer. Employing short time windows for demodulation further enables inline optical monitoring, which is a valuable diagnostic tool for future terabit optical communication systems.