Contents I. Introduction
Coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission technique [1] is promising in enabling next-generation flexible optical transport networks via dynamically allocating optical carrier frequency and transmission data rate. First, the coherent detection technique enables optical carrier frequency to be gridless. This is because by properly tuning the local oscillator at the receiver, we can almost continuously demodulate an optical carrier at any frequency. Second, by changing the number of loaded OFDM subcarriers and their individual modulation formats, the optical OFDM technique is flexible in allocating signal spectrum and data rate. There have been extensive experiments on CO-OFDM transmission systems in the literature [1] [2]. These experiments are however mostly based on point-to-point configuration, not presented in the context of a network. Meanwhile, from the theoretical and simulation perspective, many studies were performed for the OFDM-based optical transport networks [3] [4], which show that the OFDM-based optical networks can achieve much better spectrum efficiency than the conventional optical networks that follow the strict ITU-T frequency grids in G.694.1 (although ITU - T is now developing flexible grids in a revision of G.694.1). However, there are very few actual experimental demonstrations to realize such flexible OFDM-based optical networks except one experiment that demonstrates a non-coherent detection-based OFDM optical network [5]. The coherent-detection-based transmission technique is expected to achieve much better transmission performance than the non-coherent transmission technique, which is important for long-haul optical transport networks. In this study, we experimentally demonstrate the concept of coherent-detection-based OFDM (CO-OFDM) optical transport network. As an important advantage (also different from the work in [5]), we employ the wavelength filtering capability of coherent detection when building reconfigurable optical add-drop multiplexers (ROADM) as well as operating the CO-OFDM network. The application of this optical filtering capability to ROADM based on optical network has received interest from researchers only in recent years. We verify several important subsystems and conceptual operations. The subsystems include (i) two different types of ROADMs that take advantage of coherent detection capability for wavelength selection and (ii) advanced coherent detection-based optical OFDM transmission systems whose bit-rates are flexible to be adjusted from 35.9 Gb/s to 897 Gb/s. Rather than taking standard bit-rates such as 40 Gb/s and 1 Tb/s in many studies, here we intentionally adopt some arbitrary bit - rates aiming to emphasize the flexibility of the CO-OFDM network in adapting optical channel bit-rates. The conceptual operations include (i) CO-OFDM optical channel switching, (ii) CO-OFDM optical channel bandwidth elasticity, and (iii) CO-OFDM optical channel wavelength tunability.
