Microwave Photonics—Design of a Fiber Optic Recirculating Loop

Abstract

This article outlines recent Johns Hopkins University Applied Physics Laboratory (APL) work on a fiber optic recirculating loop (RCL) system and describes some of the important design decisions. Optical RCLs were originally designed as a means to study long-haul data transmission systems in a compact, less-expensive manner. For this project, however, the RCL is used to transmit repeated radio frequency (RF) signals within a larger optical system. At its core, an RCL consists of a length of fiber and an amplifier. An optical switch is used to let an encoded RF signal enter the loop, while an optical coupler is used to let the encoded RF signal exit the loop. We made multiple design decisions while making this system. Most important, chromatic dispersion of the optical fiber disrupts transmission of the recirculated RF signal. To account for this, we evaluated multiple optical fiber types, encoded the RF signal using a single-sideband technique, and incorporated a programmable optical filter with dispersion compensation capabilities. Moreover, polarization-dependent loss (PDL) and polarization-mode dispersion (PMD) within optical components are compounded as light recirculates. To accommodate for this, we incorporated a polarization scrambler in the design. In this article, we walk through the RCL design process and mention the contributions of each optical component to the final design.