Insufficient system bandwidth and chromatic dispersion (CD) of fiber are major limitations for high-speed four-level pulse amplitude modulation (PAM4) direct detection systems. In this paper, we investigate C-band 112-Gb/s PAM4 optical transport systems over 10-km and 20-km standard single-mode fiber (SSMF) transmission, which suffers from both bandwidth limitation and CD-induced power fading. Advanced digital signal processing techniques are proposed to handle those channel distortions. At the transmitter, a low-complexity 1 + αZ $^-1$ finite impulse response (FIR) filter coarsely pre-compensates system bandwidth without a preliminary channel estimation. At the receiver, a modified feed-forward equalizer and decision feedback equalizer (FFE-DFE) is employed to compensate residual insufficient bandwidth and CD-induced power fading. Moreover, a novel multi-symbols joint decision is proposed for receiver-side FFE-DFE to mitigate symbol error propagation and correct possible previous error symbols. Experimental results show that the proposed pre-equalization technique exhibits a better transmission performance, which reduces bit error ratio (BER) from 3.3e-3 to 5.5e-4 for optical back to back transmission. As for 10-km transmission case, FFE-DFE with 5-symbols joint decision achieves a BER of 7.3e-5, which is better than the BER of 1.6e-4 with ideal error propagation-free FFE-DFE (EPF-FFE-DFE). Furthermore, for 20-km transmission scenario, FFE-DFE with 8-symbols joint decision achieves a BER of 6e-4, which is also a superior result than the BER of 9.1e-4 with EPF-FFE-DFE.

In this paper, a memory polynomial equalizer combined with decision feedback equalizer (MPE-DFE) is proposed to eliminate channel distortions for intensity modulation and direct detection (IM/DD) systems. Compared with traditional feedforward equalizer and decision feedback equalizer (FFE-DFE), the proposed MPE-DFE introduces extra square terms and cubic terms to jointly equalize chromatic dispersion and nonlinear distortions. We demonstrated a C-band 56-Gb/s four-level pulse-amplitude modulation (PAM4) system over 80-km standard single mode fiber (SSMF) transmission. Experimental results show that the proposed MPE-DFE achieved up to 6.2 dB higher SNR than traditional FFE-DFE. Moreover, the achieved bit error ratio (BER) with MPE-DFE reaches 3.1 × 10⁻³, which is below 7% feedforward error correction (FEC) threshold of 3.8 × 10⁻³. To the best of our knowledge, we achieved a record transmission distance for C-band 56-Gb/s PAM4 signal with only electrical equalization at the receiver.

In this letter, we propose layered/enhanced asymmetrically clipped optical single-carrier frequency-division multiplexing (L/E-ACO-SCFDM) for optical-wireless communications. L/E-ACO-SCFDM has a lower computational complexity and peak-to-average power ratio (PAPR) than L/E-ACO orthogonal frequency-division multiplexing (L/E-ACO-OFDM). The computational complexity of the simplified transmitter in L/E-ACO-SCFDM with R layers is (2−2/2R)O(N), which is lower than the computational complexity of (2−2/2R)O(Nlog2N) in L/E-ACO-OFDM. At a complementary cumulative distribution function of 10−3, the PAPR of L/E-ACO-SCFDM is approximately 4.2, 3.4, and 2.7 dB lower than that of L/E-ACO-OFDM for 2, 3, and 4 layers, respectively. The simulation results indicate that L/E-ACO-SCFDM has better performance than L/E-ACO-OFDM under the transmitter nonlinearity and multipath fading.

In this investigation, a convex-shaped fiber ring (CSFR)-based single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) is proposed and experimentally verified. The CSFR, composed of only two optical couplers and forming a double-ring architecture, is employed to serve as a high quality mode filter to eliminate the dense longitudinal modes of the EDFL. The unpumped erbium-doped fiber in the Sagnac loop is utilized to produce the ultra-narrowband auto-tracking filter effect as a saturable absorber to guarantee the SLM out. The experimental results show that the linewidth of the proposed EDFL is 840 Hz with a high optical signal to noise ratio of 63 dB. In addition, the stability of the EDFL with the maximum output power fluctuation of 0.01 dB and the maximum central wavelength variation of 0.01 nm, in both the short-term and the long-term observations. Hence, we obtained a SLM EDFL with extremely high stability, ultra-narrow linewidth and high optical signal to noise ratio (OSNR).

Faster-than-Nyquist (FTN) signal achieves higher spectral efficiency and capacity compared to Nyquist signal due to its smaller pulse interval or narrower subcarrier spacing. Shannon limit typically defines the upper-limit capacity of Nyquist signal. To the best of our knowledge, the mathematical expression for the capacity limit of FTN non-orthogonal frequency-division multiplexing (NOFDM) signal is first demonstrated in this paper. The mathematical expression shows that FTN NOFDM signal has the potential to achieve a higher capacity limit compared to Nyquist signal. In this paper, we demonstrate the principle of FTN NOFDM by taking fractional cosine transform-based NOFDM (FrCT-NOFDM) for instance. FrCT-NOFDM is first proposed and implemented by both simulation and experiment. When the bandwidth compression factor α is set to 0.8 in FrCT-NOFDM, the subcarrier spacing is equal to 40% of the symbol rate per subcarrier, thus the transmission rate is about 25% faster than Nyquist rate. FTN NOFDM with higher capacity would be promising in the future communication systems, especially in the bandwidth-limited applications.