Integrating a Modified Pulse Amplitude Modulation (PAM4) Scheme with Two Spatially Multiplexed Channels in Optical Fiber Communication Systems

Abstract

A multi-input multi-output (MIMO) based optical fiber multiplexing architecture known as Spatial Domain Multiplexing (SDM) adds a new dimension to optical fiber communication systems by spatially reusing the optical frequencies. SDM allocates different spatial location for different input signals based on the input launch angles. These input signals propagate helically inside the carrier fiber. They abide total internal reflection and exit as concentric donut shaped rings while travelling the fiber. These SDM channels do not exhibit any crosstalk or intersymbol interference. SDM systems can also complement different modulation techniques such as Return to Zero (RZ), Non-Return to Zero (NRZ), and Pulse Amplitude Modulation (PAM) to transmit either single-bit/symbol or multi-bits/symbol in the input data stream. Employing multi-level modulation scheme such as PAM can multiply the bandwidth by multiple folds. For example, a PAM4 signal utilizes two bits/symbol and effectively multiplies the data rate by two. In this endeavor, a modified PAM4 signal generation scheme by employing incoherent addition of three lasers’ intensities is presented, which could reduce the cost & power consumption rate of optical transceivers and simplify the system architecture. This simplified PAM4 scheme when combined with a two channel SDM system, increases the bandwidth by a factor of two, thereby giving an efficiency equivalent to four bits/symbol or the PAM16 scheme. The proposed scheme works at different communication windows, thus laying the pathway for future architectures embedded with SDM and WDM signals.