Polarization division multiplexing for optical data communications

Multiple parallel channels are ubiquitous in optical communications, with spatial division multiplexing (separate physical paths) and wavelength division multiplexing (separate optical wavelengths) being the most common forms. In this research work, we investigate the viability of polarization division multiplexing, the separation of distinct parallel optical communication channels through the polarization properties of light. We investigate polarization division multiplexing based optical communication systems in five distinct parts. In the first part of the work, we define a simulation model of two or more linearly polarized optical signals (at different polarization angles) that are transmitted through a common medium (e.g., air), filtered using aluminum nanowire optical filters fabricated on-chip, and received using individual silicon photodetectors (one per channel). The filter model is based upon an input optical signal formed as the sum of the Stokes vectors for each individual channel, transformed by the Mueller matrix that models the filter proper, resulting in an output optical signal that impinges on each photodiode. The simulation results show that two and three channel systems can operate with a fixed-threshold comparator in the receiver circuit, but four channel systems (and larger) will require channel coding of some form. The entire simulation model is designed in Cadence tools and the receiver (including optics) is compatible with standard CMOS fabrication processes. In the second part of the work, we design and manufacture a two channel chip that is used as the light receiver to confirm the simulation results from the first part of the research. Since logistics for the receiver’s chip testing were not favorable we constrained our testing to single channel operation, which we demonstrated functionality using both electrical and optical inputs. In addition, we used data from a pair of optical imagers (one linear and the second with a logarithmic response) to investigate the noise properties of both the optical and electrical signals within the system. In the third part of the work, we provide examples of channel coding that enable the four channel system to operate with positive noise margins. In the fourth part of the work, we define an end-to-end simulation model of two, three or four channel systems that utilize air, fiber, and a pair of mirrors in the optical path from transmitter to receiver. Each of these systems is shown to have positive noise margins (albeit using channel coding on the four channel editions); however, there are many circumstances where the noise margins are quite small. In the final part of the work, we examine the trade-offs between number of channels, signal power, and noise margins, including the use of pulse amplitude modulation within the two channel system.