Novel Dispersion-Division-Multiplexing Technique for Enabling Double the Spectral Efficiency in Transmission and Routing of Photonic and Microwave Networks

University Of Southern California, Los Angeles CA

Investigators

Abstract

It has long been a comfortable fallacy that the optical fiber has infinite bandwidth, and, therefore, we have bandwidth to burn. With the explosion of multiple-channel communications through the use wavelength-division-multiplexing (WDM), available bandwidth now takes center stage. When comparing optical communications with other types of communications, we lag far behind in terms of spectral efficiency (i.e., bits/Hz) and we are not even close to the Shannon capacity limit. In fact, higher spectral efficiency is probably the area of least creative activity within the field, and yet it holds the promise for revolutionary increases in future systems capacity. One of the technical areas within optical communications that has shown the most bandwidth waste and has many future potential applications is subcarrier multiplexing (SCM) of many lower-speed channels on a single wavelength carrier. In SCM, half the data bandwidth is wasted since either: (i) transmitting double-sideband signals will, in essence, duplicate the data being sent given that each sideband carries the full information needed for reception, or (ii) transmitting single-sideband signals requires that the mirror frequencies on the other side of the carrier wave (i.e., where the second sideband would have been located) remain unoccupied by any other data signals. Subcarrier multiplexing has many advantages for future optical communications systems, including: (a) the transmission of many lower-speed digital data signals on a single high-capacity wavelength, (b) the transmission of many analog-modulated signals, (c) the transmission medium for networks that accommodate wireless microwave-based optical signals, and (d) the transmission of control labels for efficient routing in packet-switched networks. We propose implementing a new multiplexing scheme that enables double the spectral efficiency for optical systems that use subcarrier multiplexing. The scheme is called dispersion-division-multiplexing (DDM) and relies on placing two data channels simultaneously in the same frequency space away from an optical carrrier. For example, one double-sideband data channel is transmitted conventionally, and a second double-sideband channel is transmitted in the shadow of the first channel. Shadowing occurs due to the chromatic dispersion of the optical fiber. Due to dispersion, each of the two double sidebands of a single channel will travel at slightly different speeds. These sidebands will periodically be out-of-phase will each other relative to the optical carrier wave, thereby canceling each other and inducing RF power fading (i.e., a nearly complete disappearing of the given channel's power at a receiver). One channel would be faded when the other channel would be in-phase and have full power. By using a tunable dispersion-inducing element, a receiver can change the relative phase between each cannel's two sidebands and thus recover either of the two frequency-co-located channels. Our research program will investigate unique functionalities, opportunities, and limitations when using spectrally-efficient dispersion-division-multiplexing that could bear significant fruit in the 5-10 year time frame. The key fundamental features of our systems and networking research program are as follows. We will: (a) demonstrate highly-spectrally-efficient DDM for many WDM channels, (b) demonstrate the utility of DDM for interconnecting nodes in a wireless-based photonic network, (c) demonstrate SCM label swapping and routing in a WDM photonic network, and (d) investigate the fundamental limitations of generating, transmitting, and receiving DDM channels when considering SNR, extinction ratio, and data bandwidth. TABLE OF CONTENTS