Observation of guided acoustic-wave Brillouin scattering noise and its compensation in digital coherent optical fiber transmission.

We describe the first observation of guided acoustic-wave Brillouin scattering (GAWBS) phase noise in a digital coherent optical fiber transmission. GAWBS noise, which is a forward lightwave generated by thermally excited vibration modes in a cylindrical fiber structure, occurs coherently not only in a signal at a single carrier frequency, but also in modulated wide-band optical signals. Since the signal-to-GAWBS-noise ratio is independent of signal power, it has caused problems in various fields including quantum optics. We point out that GAWBS noise exists even in a digital coherent transmission system such as quadrature amplitude modulation (QAM) and degrades the transmission performance since the phase noise is inevitably included within the bandwidth of the transmitted data. We propose two analogue and one digital method to compensate for the GAWBS noise and demonstrate improved performance in a QAM digital coherent transmission.

Experimental and numerical comparison of probabilistically shaped 4096 QAM and a uniformly shaped 1024 QAM in all-Raman amplified 160 km transmission.

We describe an experimental and numerical comparison of a probabilistically shaped (PS) 4096 QAM signal and a uniformly shaped 1024 QAM signal. Both modulation formats have the same transmission rate and a spectral efficiency of 15.3 bit/s/Hz. In the computational simulation, we compared the generalized mutual information (GMI) of both modulation formats with bit-wise soft decision decoding and bit-wise hard decision decoding. For bit-wise hard decision decoding with an overhead of 7%, a shaping gain of 1.8 dB was attained. Then we successfully transmitted a single-channel PS-4096 QAM signal for the first time in an all-Raman amplified 160-km link, in which the transmission performance was improved compared with a uniformly shaped 1024 QAM with the same transmission rate. Transmissions with a high QAM multiplicity were achieved by using an optical phase-locked loop (OPLL) and a frequency stabilized fiber laser locked to an acetylene absorption line. Thanks to a shaping gain based on a bit-wise hard decision decoder, the 1.9-dB power margin, which agreed with the simulation result to within 0.1 dB, was increased after transmission.