Contents 1. Introduction
In recent years, there has been an explosion of interest in coherent-optical orthogonal-frequency-division-multiplexing (CO-OFDM) techniques for optical transmission and network [1]‑[3]. Compared with conventional single-carrier WDM scheme in which a large guard band is required to avoid interference between two adjacent channels, the OFDM can have small or even no guard bands enabled by orthogonal band multiplexed (OBM) [4]. A high-speed CO-OFDM superchannel can consist of some parallel individually-modulated subbands, which greatly reduce the bandwidth requirement for optical and electrical components. 400-Gb/s and 1-Tb/s experiments have been demonstrated with high spectral efficiency [5]‑[7]. The significant change in transmission technique has much influence on current optical network. Recently, there have been some discussions for network architecture based on OFDM, such as spectrum-sliced ‘elastic optical path’ (SLICE) with flexible allocation and efficient utilization of the spectral resource [8] and variable-bandwidth wavelength crossconnect [9]. The sub-wavelength routing overcome the bottleneck of rigid large granularity stranded bandwidth and can achieve efficient traffic allocation.
