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Bandwidth-limited IMDD systems suffer from the noise boosted by the strong equalization at the receiver. This paper proposes a multiplication-free approach that reduces the impacts of the equalizer-enhanced colored noise through time-interleaving the received symbols and distorting the two-dimensional (2D) constellation such that the noise correlation is subdued. The 2D distortion map is predefined and retrieved from a look-up table. The proposed 2D constellation distortion is evaluated after the linear feed-forward equalizer and Volterra nonlinear equalizer. Experimental results show a BER reduction of 40% when the 2D constellation distortion is employed after the equalizer for the 135 Gbaud PAM4 and 110 Gbaud PAM6 signals. Owing to the proposed approach, we transmit a net data rate of 250 Gbps/λ PAM4 and PAM6 in the O-band over 2 km of SSMF at a BER of 3.8×10 -3 below the 6.7% overhead HD-FEC using a 47 GHz SiP modulator, linear equalization, and the proposed 2D distortion.

High-bandwidth GaN-based mini-LEDs on the c-sapphire substrate are promising candidates for underwater optical wireless communication (UOWC) systems due to their compatibility with the mature LED fabrication process. Here we fabricate and characterize mini-LEDs based on a single-layer InGaN active region with a peak emission wavelength around 484 nm for high-speed UOWC links. Since the LED diameter affects the trade-off between the modulation bandwidth and the optical modulation amplitude, mini-LEDs with varying mesa diameters from 100 µm to 175 µm are fabricated for the measurement. The 150 µm mini-LED with a 3-dB optical bandwidth of 906 MHz performs the best and enables the transmission of a net 4 Gb/s PAM-4 signal over 2 m of underwater distance using only linear equalization. This UOWC system has achieved, to the best of our knowledge, the highest net data rate and the highest data-rate-distance product based on a single-pixel mini-LED.

We introduce the asymmetric-ISI pulse shaping filter with Tomlinson-Harashima precoding to increase the receiver RF swing, and demonstrate 102 Gbaud PAM-4 transmission over 2 km with a BER below 3.8×10-3 using linear equalizer at receiver.

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