Fig. 1. Polarization-diverse full-field receiver structure. PBS, polarization beam splitter; OC, optical coupler; PD, photodiode; Ex, Ey, orthogonal polarizations.

Fig. 2. (a) Experimental setup. ECL, external cavity laser; AWG, arbitrary waveform generator; DP IQM., dual-polarization IQ modulator; VOA, variable optical attenuator; EDFA, erbium-doped fiber amplifier; SMF, single-mode fiber; PC, polarization controller; PBS, polarization beam splitter; OC, 3×3 optical coupler; MZI, Mach–Zehnder interferometer; PD, photodiode; RTO, real-time oscilloscope. (b) DSP stacks. CCDM, constant composition distribution matcher; RRC, root-raised cosine; NGMI, normalized generalized mutual information; MIMO Equ. (w/ w/o Volterra), MIMO equalization (with/without Volterra). (c) Optical spectra of transmitted and received PDM optical signals. (d) Optical spectra of X- and Y-polarization signals with and without polarization fading phenomenon. PF, polarization fading.

Fig. 3. Electrical spectra of the six received photocurrents (a) with polarization fading and (b) without polarization fading. ik(t) (k=1−6) are the six received photocurrents. (c) Electrical spectra of the recovered X- and Y-polarization signals. (d) Frequency response for two of 6×2 MIMO linear equalizers. (e) Measured NGMI and GMI versus CSPR in the 40-km transmission case. (f) Net data rate and GMI versus entropy of signal source.

Fig. 4. (a) Measured NGMI versus OSNR with MIMO linear and nonlinear equalization in the BTB case. (b) Measured NGMI versus OSNR with MIMO linear and nonlinear equalization after 40-km SMF transmission. (c) NGMIs of the four signal bands over 50 measurements with randomly varying polarization state. (d) Constellations of the four signal bands.

Fig. 5. (a) Experimental setup. ECL, external cavity laser; AWG, arbitrary waveform generator; DP IQM., dual-polarization IQ modulator; VOA, variable optical attenuator; EDFA, erbium-doped fiber amplifier; SMF, single-mode fiber; PBS, polarization beam splitter; OC, 3×3 optical coupler; PD, photodiode; EA, electrical amplifier; RTO, real-time oscilloscope. D1, D2, D3, three dispersive elements with a dispersion value of −300  ps/nm. (b) DSP stacks. RC, raised cosine; MIMO Equ. (w/ w/o Volterra), MIMO equalization (with/without Volterra). (c) Optical spectra of PDM signals in the BTB and 80-km SMF case. (d) Optical spectra of X- and Y-polarization signals with and without polarization fading phenomenon.

Fig. 6. (a) Electrical spectra of the six received photocurrents. (b) Measured NGMI versus CSPR in the 80-km transmission case. (c) Measured NGMI versus OSNR with MIMO linear and nonlinear equalization in the BTB case. (d) Measured NGMI versus OSNR with MIMO linear and nonlinear equalization after 80-km SMF transmission. (e) Constellations of the four signal bands.

Fig. 7. Net ESE and net data rate comparison in various direct detection systems beyond 100 Gbit/s. Direct detection systems with modulation dimensions from 1-D to 4-D are compared.

Fig. 8. Polarization-diverse full-field receiver structure based on dispersion elements. PBS, polarization beam splitter; OC, 3×3 optical coupler; DE, dispersive element; PD, photodiode.