High-speed transmission of adaptively modulated optical OFDM signals over multimode fibers using directly Modulated DFBs

A novel optical signal modulation concept of adaptively modulated optical orthogonal frequency division multiplexing (AMOOFDM) is proposed, and a comprehensive theoretical model of AMOOFDM modems is developed. Numerical simulations of the transmission performance of the AMOOFDM signals are undertaken in unamplified multimode fiber (MMF)-based links using directly modulated distributed feedback (DFB) lasers (DMLs). It is shown that 28 Gb/s over 300 m and 10 Gb/s over 900 m transmission of intensity modulation and direct detection (IMDD) AMOOFDM signals at 1550 nm is feasible in DML-based links using MMFs with 3-dB effective bandwidths of 200 MHz/spl middot/km. Apart from a higher signal capacity, AMOOFDM also has a greater spectral efficiency and is less susceptible to different launching conditions, modal dispersion, and fiber types, compared with all existing schemes. In addition, a large noise margin of about 15 dB is also observed. The bits of resolution of analog-to-digital converters (ADCs) and the cyclic prefix of AMOOFDM symbols are the main factors limiting the maximum achievable performance, on which the influence of DMLs is, however, negligible under the optimum operating condition.

I. Introduction

Most installed Ethernet backbones based on multimode fibers (MMFs) operate at bit rates of about 1 Gb/s, which is inadequate for current and emerging demand. Great effort has been expended on exploring cost-effective solutions for upgrading 1 Gb/s Ethernet backbones to 10 Gb/s and above [1], [2]. Enterprise customers prefer to use their already-installed MMFs rather than pulling new fibers to upgrade their backbones as significant cost savings can be realized. Installed MMF links, however, exhibit considerable bandwidth variation. Field measurements showed that [3] the 3-dB bandwidth under radially overfilled launching (ROFL) at 1.3 ranges from 200 to 2600 with 17% being even less than the overfilled launching (OFL) bandwidth–length product specification of 500 at this wavelength, and that the abovementioned 3-dB bandwidth deviation is almost doubled for central launching condition at 1.3 . However, there are still about 3% MMFs having the 3-dB effective bandwidth less than the OFL bandwidth–length product specification of 500 . In addition, the 3-dB effective bandwidth is also very sensitive to different restricted launching conditions including central, small offset, and conventional offset launching [4].