Virtual Carrier-Assisted Direct-Detection MB-OFDM Next-Generation Ultra-Dense Metro Networks Limited by Laser Phase Noise

The impact of the phase-to-intensity conversion of the laser phase noise due to the optical band selector (BS) used for band extraction on the performance of direct-detection (DD) optical multiband orthogonal frequency-division multiplexing (MB-OFDM) networks employing single-sideband (SSB) modulation is assessed. Compared to conventional double-sideband DD systems, in which the amplitude shape of the optical filter is quasi-symmetric relative to the centre of the signal spectrum, ultra-dense optical SSB-MB-OFDM networks employing virtual carrier-assisted DD are differently affected by the phase-to-intensity conversion. This is due to the detuned BS required by these networks, which modify the converted phase noise (CPN) features. A reduction of the power spectral density of the CPN achieving 30 dB can occur along the frequency range in which the BS bandwidth and detuning lead to a filtering attenuation that is similar in both sides of the virtual carrier. For single band, when the ratio between the BS detuning and bandwidth increases from 0% to 28%, an error vector magnitude improvement of 7 dB due to the CPN reduction is observed. The optical signal-to-noise ratio (OSNR) penalty due to the laser phase noise in 112-Gb/s SSB-MB-OFDM networks comprising 12 OFDM bands and employing an ultra-dense band spacing of 3.125 GHz is lower than 1 dB for external cavity lasers and is 1.6 dB for distributed feedback lasers. These OSNR penalties are obtained for an optimized ratio between the BS detuning and bandwidth of 14%.