We demonstrate 20-Gb/s upstream transmission in the wavelength-division-multiplexed passive optical network based on reflective semiconductor optical amplifiers (RSOAs) and direct-detection receivers by using the polar return-to-zero (RZ) four-level pulse amplitude modulation (4-PAM) format. In order to compensate for the limited modulation bandwidth of the RSOA, an optical delay interferometer is used together with an electrical equalizer at the receiver. We successfully transmit the 20-Gb/s polar RZ 4-PAM signal over the 20-km long loopback link without using optical dispersion compensator. We also show that the proposed polar RZ 4-PAM format provides superior performances to the conventional RZ 4-PAM format due to the up-down symmetry of its waveform.
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Shim et al. (2015) studied this question.
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