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In this letter, transmission of directly-modulated PAM-4 in C-band is experimentally demonstrated. Instead of introducing any optical devices for dispersion management, we employ receiver-side equalization consisting of Volterra-based FFE and Volterra-based DFE, to mitigate the signal distortions induced by the frequency chirp effect and chromatic dispersion. Results indicate that, PAM-4 transmissions of 56 Gbps over 60-km and 60 Gbps over 80-km are achieved respectively under the HD-FEC limit ( $3.8\times 10^{-3}$ ) and SD-FEC limit ( $2.2\times 10^{-2}$ ).

Due to the explosive growth of traffic demands, large-scale optical cross-connects (OXCs) are highly desired in next-generation optical networks. However, the scalability of classical OXCs is not good in terms of the size of optical switching modules and cabling complexity to meet application requirement. To address this issue, this letter proposes a two-phase approach to construct modular large-scale OXCs, using a set of small-size OXC modules. We first decompose each optical space switch (OS) in a classical OXC to a Clos network and then merge the corresponding links and decomposed optical switching modules of different Clos networks to form small-size OXC modules, which finally yields a modular OXC. We show that the proposed modular OXC has good scalability and acceptable physical-layer performance.

Four-level pulse amplitude modulation (PAM4) has been considered as a low-cost modulation format for capacity upgrade in digital mobile fronthaul; however, the bit error ratio increases due to the reduced Euclidean distance. In this letter, a novel scheme is proposed to suppress bit-error-induced error vector magnitude (EVM) distortion upon radio signal in PAM4-based fronthaul. The scheme is realized by pairing one high-order bit and one low-order bit onto one PAM4 symbol and setting a portion of low-order bits to “0.” As a result, sample bits with high priority are delivered by PAM2 symbols with small error probability, and sample bits with low priority are transmitted by PAM4 signal with high spectral efficiency. Without any modification of the hardware, the proportion between PAM2 and PAM4 can be flexibly adjusted according to the quality of optical link. Experiments validate that, under respective optimal PAM2/4 proportion, EVM of long term evolution-advanced like radio signal decreases by at least 20 dB, and up to 1024-quadratic-amplitude modulation can be supported.

The power depletion and crosstalk induced by stimulated Raman scattering in wavelength division multiplexing-based mobile fronthaul are investigated by simulation and experiment for the first time to the best of our knowledge. The simulation results show that the maximum power depletion is 1.7 dB in the case of 40-km fiber transmission with four fronthaul wavelength pairs. However, for the future hybrid fiber and radio access network, where the whole $\text{C}\,+\,\text{L}$ wavebands may be employed, the power depletion will be increased to 11 dB. Meanwhile, in order to verify the simulation, we conducted an experiment with four fronthaul wavelength pairs. The measured power depletion is consistent with the simulation results. As for the Raman crosstalk, the experimental results indicate that it varies from the fiber lengths and has the low-pass characteristic. These results provide a valuable reference for the wavelength allocation in the future hybrid fiber and radio access network.

We propose and demonstrate a physical layer security scheme via chaotic partial transmit sequence (PTS) technique in orthogonal frequency-division multiplexing passive optical network (OFDM-PON). A 4-D hyper chaos is employed to generate the chaotic partition information, the chaotic phase weighing factors in PTS as well as the chaotic training sequence for OFDM symbol synchronization, which provides a huge key space to enhance the physical layer confidentiality. Transmission of 8.9-Gb/s 16 quadrature amplitude modulation encrypted OFDM signals is demonstrated over 20-km standard single-mode fiber, which shows reliable robustness against exhaustive attacks. Moreover, the proposed scheme improves the OFDM transmission performance due to the effective reduction of the peak-to-average power ratio of the OFDM signals by chaotic random PTS.

A generation approach of photonic logic minterms and maxterms, which is fully reconfigured minterms and maxterms for four-input 40-Gb/s differential phase-shift keying (DPSK) signals, has been demonstrated based on delay interferometers and semiconductor optical amplifiers. The maxterms are directly derived by inverting the corresponding minterms, according to their complementary characteristics. Reconfigurations of the scheme, enabled by tuning either the phase shift of delay interferometer or input wavelength, are shown among 16 minterms, as well as corresponding maxterms. A full set of minterms and maxterms are captured with clear open eye diagrams and extinction ratios of over 10 and 8 dB, respectively, which indicate the potential of further realizing arbitrary complex logic operations.

An extinction ratio tunable optical fiber polarization interferometer (OFPI) is proposed and implemented to increase the optical modulation depth in radio-over-fiber (RoF) systems by suppressing the optical carrier power. The extinction ratio of the OFPI can be simply tuned by adjusting the polarization state of the launched RoF signal. Experimental results illustrate that the receiver sensitivity and dynamic range of RoF systems are greatly improved using the proposed scheme.

The Clos network is used for optical cross-connect and optical packet switch node, and has a large number of cross-points if the number of wavelengths per fiber is more than that of fibers per node. In this letter, we propose a cost-effective permuted Clos network to reduce the number of crosspoints. We further investigate the placement of fixed wavelength converter groups (WCGs) shared inside the permuted Clos network. On the contrary, a normal Clos network has to be equipped with expensive tunable WCGs. The simulation result implies that equipment of about 50% WCGs achieves similar performance as the full wavelength interchanging node does.

We experimentally demonstrate subnanosecond tunability of a heterodyne receiver employing a fast-switching local laser oscillator. A

We propose and demonstrate a novel in-band optical signal-to-noise-ratio (OSNR) and spectrum monitoring technique based on swept coherent detection. This technique measures the signal and the in-band amplified spontaneous emission spectra by properly adjusting the polarization states of the signal and the local oscillator. A high resolution of 0.002 nm is achieved. By integrating the detected spectra of the signal and the noise, the in-band OSNR can be obtained in a very precise manner. We monitor the OSNR values with high accuracy in the range of 10-30 dB.