We report the experimental demonstration of single wavelength terabit free-space intensity modulation direct detection (IM-DD) system employing both orbital angular momentum (OAM) multiplexing and polarization division multiplexing (PDM). In our experiment, 12 OAM modes with two orthogonal polarization states are used to generate 24 channels for transmission. Each channel carries 30 Gbaud Nyquist PAM-4 signal. Therefore an aggregate gross capacity record of 1.44 Tb/s (12 × 2 × 30 × 2 Gb/s) is acheived with a modulation efficiency of 48 bits/symbol. After 0.8m free-space transmission, the bit error rates (BERs) of all the channels are below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10(-2). After applying the decision directed recursive least square (DD-RLS) based filter and post filter, the BERs of two polarizations can be reduced from 5.3 × 10(-3) and 7.3 × 10(-3) to 2.2 × 10(-3) and 3.4 × 10(-3), respectively.
Nyquist pulse shaping is a promising technique for high-speed optical fiber transmission. We experimentally demonstrate the generation and transmission of a 1.76Tb/s, polarization-division-multiplexing (PDM) 16 quadrature amplitude modulation (QAM) Nyquist pulse shaping super-channel over 714km standard single-mode fiber (SSMF) with Erbium-doped fiber amplifier (EDFA) only amplification. The superchannel consists of 40 subcarriers tightly spaced at 6.25GHz with a spectral efficiency of 7.06b/s/Hz. The experiment is successfully enabled with the modified single carrier frequency domain estimation and equalization (SCFDE) scheme by performing training sequence based channel estimation in frequency domain and subsequent channel equalization in time domain. After 714km transmission, the bit-error-rate (BER) of all subcarriers are lower than the forward error correction limit of 3.8 × 10(-3).
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