A bandwidth-efficient and low-cost spectral-null-filling scheme simultaneously delivering a 56-Gbps 4-level pulse amplitude modulation (PAM-4) digital signal and a 10×400-MHz analog radio frequency (RF) signal on a single wavelength is demonstrated for high-capacity wired and wireless optical access networks. The data rate of the PAM-4 signal is consistent with the next-generation passive optical network (PON), and the PAM-4 signal inherently provides a spectral null at 28 GHz, which can be filled with the RF signal exactly at the 5G-specified millimeter wave (mmW) band. Since pulse shaping is not essential in this hybrid transmission system, a low-cost 2-bit digital-to-analog converter (DAC) is adopted for the PAM-4 signal generation. Volterra nonlinear equalizer is adopted to effectively eliminate the analog-to-digital crosstalk without reserving a frequency gap in between. The digital-analog hybrid signal is simultaneously transmitted over a 25-km standard single-mode fiber in telecom C-band and the impact of chromatic dispersion on the number of supported mmW bands is analyzed. In addition, three types of hybrid transmitters are experimentally demonstrated, among which the first-time proposed IQ-modulator-based hybrid transmitter shows the best performance.

We propose a novel physical layer security enhanced digital filter multiple access based passive optical network (DFMA-PON), for the first time to our best knowledge, by using phase masking and hybrid time-frequency domain chaotic scrambling. In the proposed secure DFMA-PON, all digital orthogonal filters have different random phase and the random phase is controlled by a hyper-chaotic Chen system. The hyper-chaotic Chen system is also adopted for the generation of scrambling matrices. In our demonstration, an encrypted signal has been experimentally transmitted over a 25-km standard single mode fiber (SSMF) in an intensity modulation/direct-detection (IM/DD) DFMA-PON system. The experimental results verify that various attacks can be effectively prevented and indicate the proposed scheme as a promising solution for physical layer secure DFMA-PON systems.

Filter bank multi-carrier (FBMC) using offset quadrature amplitude modulation (OQAM) has recently attracted increasing interests for the next-generation high-speed optical transmission systems, due to the salient FBMC modulation advantages in terms of high spectrum efficiency by completely removing the cyclic prefix (CP). However, high peak-to-average power ratio (PAPR) still plays a key role of constraining the performance of FBMC systems. In this paper, we investigate a single sideband (SSB) pruned discrete Fourier transform (DFT)-spread FBMC for direct-detection (DD) passive optical network systems (PONs). It is demonstrated that the PAPR of the SSB pruned DFT-spread FBMC signal with the overlapping factor of 1.56 outperforms the conventional FBMC signal and the simple DFT-spread FBMC signal by ~2 dB and 1 dB at the complementary cumulative distribution function (CCDF) of 1×10 $^-3$ , respectively. Finally, by using the pruned DFT-spread FBMC with low PAPR, the aggregate data rate of 50-Gbit/s can be achieved after transmission over 50-km single mode fiber (SMF) below the 7%hard decision forward-error correction (HD-FEC) threshold in the DD PONs.

As the demand for high data volumes keeps increasing in optical access networks, transmission capacities and distance are becoming bottlenecks for passive optical networks (PONs). To solve this problem, a novel scheme based on multi-twin single sideband (SSB) modulation with direct detection is proposed and investigated in this paper. At the central office, two SSB signals are generated simultaneously with the same digital-to-analog converters (DACs). The twin-SSB signal is not only robust against frequency selected power fading introduced by chromatic dispersion (CD), but also improves the spectral efficiency (SE). By combining a twin-SSB technique with multi-band carrier-less amplitude/phase modulation (multi-CAP), different optical network units (ONUs) can be supported by flexible multi-band allocation based on software-reconfigurable optical transceivers. The Kramers–Kronig (KK) scheme is adopted on the ONU side to effectively mitigate the signal–signal beat interference (SSBI) induced by the square-law detection. The proposed system is extensively studied and validated with four sub-bands using 50 Gbps 16 quadrature amplitude modulation (QAM) modulation for each sub-band using numerical simulations. Digital pre-equalization is introduced at the transmitter-side to balance the performance of different ONUs. After system optimization, a bit error rate (BER) threshold for hard decision forward error correction (HD-FEC) code with 7% redundancy ratio (BER = 3.8 × 10⁻³) can be reached for all ONUs over 50-km standard single-mode fiber.

The dissipative Kerr soliton (DKS) microcomb has been regarded as a highly promising multi-wavelength laser source for optical fiber communication, due to its excellent frequency and phase stability. However, in some specific application scenarios, such as direct modulation and direct detection (DM/DD), the relative intensity noise (RIN) performance of Kerr optical combs still fails to meet the requirements. Here, we systematically investigate the key factors that contribute to the power fluctuations in DKS combs. By exploiting the gain saturation effect of the semiconductor optical amplifier (SOA), the RIN of an on-chip DKS microcomb is effectively suppressed, achieving a maximum reduction of about 30 dB (@600 kHz offset frequency) for all comb lines. Moreover, such DKS comb RIN suppression technology based on an SOA chip can eliminate the need for additional complex feedback control circuits, showcasing the potential for further chip integration of the ultra-low-RIN DKS microcomb system.