Contents 1. Introduction
With the ever growing demand for network capacity, Ethernet data rate has evolved from 10G to 100G. Since the early 100G optical transmission experiment by electrical time-division multiplexing (ETDM) [1], a variety of approaches have been demonstrated to achieve 100G [2]–[5]. Among them, coherent detection has become prevalent thanks to powerful electronic digital signal processing (DSP) [2]–[5]. Nevertheless, coherent system is rather sophisticated which involves polarization multiplexer/demultiplexer, dual IQ modulators, local oscillator and dual pairs of balanced receivers. This results in relatively high cost that restricts its application primarily in long-haul communication. In contrast, direct detection (DD) [6]–[10] has simpler transmitter/receiver implementation and is thus more cost-effective. Early DD schemes apply single-sideband (SSB) modulation at transmitter. At receiver, a crucial problem for these schemes is how to eliminate the signal-to-signal beat noise (SSBN). The most straightforward method is to leave gaps in signal spectrum at frequency where SSBN exists [6], which effectively reduces the electrical SE by half. To avoid the frequency gaps, Baseband SSB is proposed [7] which uses high carrier to signal ratio to restrain the SSBN, but its receiver sensitivity is greatly reduced. Virtual SSB DD [8] implements iterative FFT/IFFT operation at receiver to digitally subtract SSBN from signal at the price of high computational complexity. 100G single channel direct detection is achieved in [9] by using polarization multiplexing (Pol-Mux) at transmitter, but it has complexity close to coherent detection. To further enhance the spectral efficiency (SE), Blockwise Phase Switching (BPS) [10] is introduced which adopts double sideband (DSB) modulation while only one photo-detector is needed at receiver. Nevertheless, repetitive signals in time domain shrink its SE by 1/2 and sophisticated DSP is required to eliminate SSBN. In this paper, we propose and demonstrate a novel DD scheme called signal carrier interleaved direct detection (SCI-DD), whose concept is similar to digital self-coherent detection (DSCD) for differential DPSK signal [11]. By using DSB modulation at transmitter, SCI-DD maintains a high electrical SE; and by using a pair of balanced photo-detectors at receiver, SSBN is intrinsically eliminated without resorting to any sophisticated DSP. To the authors' best knowledge, we demonstrate the first 100-Gb/s single-wavelength single-polarization DD reception after transmission of 80-km standard single mode fiber (SSMF) without optical compensation or pre-equalization.
