摘要： We propose a polarization-modulation and direct-detection (PolM-DD) bandwidth-enhanced PAM-4 transmission system with a tunable frequency range and demonstrate, for the first time, its performance after transmissions through a standard single mode fiber (SSMF). By adjusting the polarization angle between one principal axis of the PolM and polarizer, the frequency notch, caused by the spectral dispersion inherent in SSMF, can be shifted to higher frequencies. As a consequence, the available 3-dB bandwidth at the baseband is much more enhanced than it would be in the traditional case of an intensity-modulation and direct-detection (IM-DD) system. Since the output of PolM modulation module are intensity modulated signals with a tunable chirp, the system is free of polarization-mode dispersion (PMD). From the analysis in this paper, we conclude that under ideal conditions, the PolM-DD system gains approximately 41.4% in bandwidth over its counterpart IM-DD system. The optimal polarization angle is independent of optical wavelengths and fiber distances, so it can be easily applied in wavelength-division-multiplexing (WDM) systems. In our experiment, 100-Gbps, 84-Gbps, 68-Gbps, 58-Gbps and 52-Gbps PAM-4 signals are successfully transmitted by our PolM-DD system over distances of 5-km, 10-km, 15-km, 21-km and 25-km SSMF, respectively, without dispersion compensation component or single sideband (SSB) modulation. The experimental results are consistent with our analysis and simulations results, and support more than 41% data rate gain compared to the IM-DD system, given comparable component bandwidths.