Long-Haul OFDM transmission using a recirculating Fiber loop

Abstract

Optical recirculating loops have been employed systematically over the years in the study of long-haul transmission systems. The loop technique attempts to simulate the performance of these systems by recirculating optical data through fiber spans and amplifier chains. The technique became widely accepted due to the reduced number of components used to achieve simulations of long-distance communications. Orthogonal frequency division multiplexing (OFDM) is an important method due to its resilience towards polarization mode (PMD) and fiber chromatic dispersions (CD). However, a high peak-to-average power ratio (PAPR) is usually common in these signals, presenting peaks that are saturated by amplitude-limited equipment, consequently distorting the signals and reducing system performance. To overcome this limitation a phase modulation of the conventional OFDM is employed, generating a Constant Envelope OFDM (CEOFDM), reducing the PAPR significantly. Both methods may be realized either with direct detection (DDO) or with coherent optical detection (CO). For long-haul systems, CO has shown superior performance. On the other hand, DDO has a simpler layout, making it more cost-effective. By designing and testing a recirculating fiber loop, the parameters of long distance communications can be tested efficiently, acquiring an important tool for performance comparisons over different optical communications systems. Through this technique a DDOOFDM and a DDO-CE-OFDM systems are tested for long distance optical communications with regards to numerous different parameters. The modulator bias, phase modulation index, fiber input power, transmission distances and modulation formats are thoroughly discussed for both systems and performance comparisons are made. Distances as long as 1.300 km are reached at rates of 1.41 Gb/s and 700 km for 2.82 Gb/s