A digital signal processing (DSP) scheme based on Volterra equalizer (VE) combined with adaptive noise-whitening post-filter and maximum likelihood sequence detection (MLSD) is proposed to mitigate nonlinear impairments in vertical-cavity surface-emitting lasers (VCSEL) multimode fiber (MMF) system. Successfully transmission of 108 Gb/s, 100 Gb/s and 60 Gb/s 4-ary pulse amplitude modulation (PAM4) signal over 5 m, 160 m and 460 m OM3-MMF is demonstrated below the 7% overhead hard-decision forward error correction (HD-FEC) bit error rate (BER) threshold by using a 20-GHz class VCSEL at 850 nm. Linear pre-equalization is applied to mitigate severe bandwidth limitation of the system. Our experimental results show that the scheme can well mitigate modulation nonlinearity induced by VCSEL and fiber nonlinearity induced by MMF. The BER decreases about two order of magnitude compared to linear equalizer after 100 m OM3-MMF transmission for 100 Gb/s PAM4 signal.
We have experimentally demonstrated a direct-detection (DD) 112-Gbit/s 16 quadrature amplitude modulation (QAM) transmission over single-span 140-km standard single mode fiber (SSMF) with Kramers-Kronig receiver and a sparse I/Q Volterra filter (VF). The sparse I/Q VF was proposed in our previous work and it is based on dual-input real-valued Volterra series and ℓ1-regularization method. In this paper it is used for compensating the nonlinear distortion in a short-reach DD optical 16-QAM signal transmission system. In back to back case, sparse I/Q VF represents the great compensation ability to the saturation effect of the electrical amplifiers and the nonlinear sinusoidal transfer function of I/Q modulator. It provides around 1-order magnitude improvement of BER when reducing 84% complexity from full I/Q VF. For fiber transmission case, sparse I/Q VF can mitigate the fiber nonlinearity effectively and it achieves single-span 140-km transmission at hard-decision forward error correction (HD-FEC) threshold of 3.8 ×10-3 with less than half complexity of full I/Q VF. Besides, optical signal noise ratio (OSNR) performance at 120 km is measured and sparse I/Q VF reduces the required OSNR at HD-FEC threshold by 1.3 dB. In a word, we investigate the performance of sparse I/Q VF in short-reach optical 16-QAM transmission system and sparse I/Q VF reveals its potential in the growing short-reach applications, such as data center inter-connection and metropolitan area network.
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