Contents I. Introduction
As the rapid development of cloud services, real-time video, and online social networks, there are urgent needs of the growth of the transmission capacity in optical fiber communications systems. Various transmission formats using multilevel modulation based on intensity modulation and direct detection (IM-DD) have been proposed to realize low-cost, high-capacity in short-reach transmission system, such as orthogonal frequency division multiplexing (OFDM) [1], discrete multi-tone (DMT) [2], carrier-less amplitude and phase (CAP) [3] and pulse amplitude modulation (PAM) [4]–[8]. Among them, PAM is a promising modulation scheme in short reach transmission system due to its low complexity, flexible implementation and simple structure. However, compared to traditional on-off keying (OOK) signal, PAM4 signal is more sensitive to the linearity of components at both transmitter and receiver sides. Although employing equalization schemes at either transmitter or receiver sides could mitigate the impact of nonlinearity [9], it would increase the complexity and cost. Recently, the singlemode fiber (SMF) capacity limit has reached 100 Tb/s [10], and space-division multiplexing (SDM) technique based on multi-core fiber (MCF) in optical fiber communications has attracted a lot of research interest since it offers hundred times the capacity of SMF -based transport systems. Besides, in MCF, signals can be transmitted independently in each core with low crosstalk (typically less than −45 dB) [11]–[13]. Therefore, MCF is attractive for capacity enlargement. Directly modulated lasers (DMLs) are a low cost transmitter for short-reach systems due to their small footprint, low power dissipation and high output optical power. However, the ability to generate bit rates beyond 10-Gb/s with advanced modulation formats has been limited by the strict requirements on the laser drive current [14]. In this paper, we have experimentally demonstrated 10- and 20-Gb/s PAM4 generation using two DMLs and one photodiode (PD) over 10-km MCF. Compared to conventional schemes, the proposed scheme not only can provide similar performance, but also is insensitive to the offset of the DML's bias current.
