The performance of high-speed intensity modulation direct detection (IM-DD) transmissions is severely degraded by the linear inter-symbol interference (ISI), due to both bandwidth-constrained optoelectrical components and the chromatic dispersion (CD). Meanwhile, nonlinear ISI due to intensity modulation and square-law detection can further deteriorate the IM-DD system performance significantly. Here, we first propose three metrics to quantify those impairments in IM-DD transmissions. Then, we compare both the BER performance and computational complexity of different DSP schemes to enable short-reach high-speed IM-DD transmissions with severe bandwidth constraint, using a C-band PAM-4 experimental test-bed with an end-to-end 10-dB bandwidth of 17 GHz. Those DSP schemes include receiver-side feedforward equalization (FFE), receiver-side Volterra filter equalization (VFE), transmitter-side Tomlinson-Harashima precoding (THP) together with receiver-side FFE, transmitter-side THP together with receiver-side VFE, channel-shortening filter (CSF) together with maximum likelihood sequence estimation implemented with linear branch metrics (MLSE-LI-BM), and CSF together with MLSE implemented with nonlinear branch metrics (MLSE-NL-BM). Our experimental results indicate that, only the combination of CSF and MLSE-NL-BM enables a 112 Gbit/s transmission over 5 km fiber at the KP4-FEC threshold, but with an increased computational complexity.

We investigatethe potential of space division multiplexing (SDM) for the power-over-fiber (PWoF) technique, in terms of weakly-coupled multicore fiber (WC-MCF) design and experimental verification. Our simulation results indicate that, the SDM-enabled PWoF scheme is superior to the wavelength division multiplexing (WDM) enabled counterpart, due to the scalability of the energy light delivery and the save of wavelength division multiplexers. Then, we experimentally demonstrate the co-transmission of optically carried fifth generation new radio (5G NR) signals at 1550-nm and the 60-W energy light at 1064-nm over the 1-km weakly-coupled seven-core fiber (WC-7CF) fronthaul link. Since the optical power delivery characteristic per the WC-MCF core is similar to that of the standard single-mode fiber (SSMF), the SDM-enabled PWoF scheme can evenly distribute the high-power energy light into multiple spatial channels, to maximize the efficiency of the optical power transition efficiency (OPTE). Consequently, the total collected optical power of the energy light over the 1-km WC-7CF is 4.6 times higher than that of the 1-km SSMF, mainly due to the insertion loss of the fan-in and fan-out (FIFO) devices. Meanwhile, the error-vector magnitude (EVM) of 1.5-Gbit/s 5G NR signal is 0.28%, when the received electrical signal power is −25 dBm. The EVM value of the co-transmitted 5G NR signal with the 60-W energy light only has a fluctuation of 0.01% over six hours. The SDM-enabled PWoF is useful for the optically powered 5G pico-cells, with the capability of high-speed access and centralized management.

The number of base stations (BSs) for the fifth generation (5G) wireless network is substantially increased, as each coverage is greatly reduced. Therefore, both the miniaturization and the densification of BSs suffer from the challenges of electrical power supply and deployment cost. Here, we present an optically powered 5G fronthaul network, in support of the co-propagation of spatial-division-multiplexing (SDM) energy light and wavelength-division-multiplexing (WDM) 5G new radio (NR) signals over the weakly-coupled multicore fiber (WC-MCF). When the 60-W energy light at 1064.8-nm is equally distributed among the outer six cores, and the 9-Gbit/s 5G NR WDM signals are transmitted over the central core of 1-km WC seven-core fiber (WC-7CF), we can collect total 11.9-W electrical power at the remote node, for the purpose of optically powered small cells. Meanwhile, the error-vector magnitude (EVM) values of 1.5-Gbit/s 5G NR 64-level quadrature amplitude modulation orthogonal frequency division multiplexing (64QAM-OFDM) signals at the central frequency of 3.5 GHz fluctuate within a range of 0.3%∼0.39%, under a received electrical power of −25 dBm, for all six-wavelength channels. Six optically powered small cells are equipped with the characteristics of centralized management and flexible access-rate.

For the digital subcarriermultiplexing (DSM) transmission, it is compulsory to realize the frequency offset estimation (FOE) and compensation (FOC) before the subcarrier de-multiplexing and the chromatic dispersion (CD) compensation (CDC). Here, we demonstrate a blind FOE method for the spectral-efficient DSM signals with a sharper root raised cosine (RRC) spectral shaping, with the capabilities of the transparent modulation format operation, a wide FOE range, and a high FOE accuracy. By reusing the spectral information for the CDC, the proposed FOE method is hardware efficient. Moreover, the proposed FOE method is numerically identified robust to transmission impairments, including the amplified spontaneous emission (ASE) noise, the optical filtering effect, and the accumulated CD, with an estimation error of less than 35 MHz under a FFT size of 2048. The correct function of the proposed FOE method is finally experimentally verified for 25 Gbaud 16QAM DSM signals under scenarios of both back-to-back (B2B) and standard single-mode fiber (SSMF) transmissions.

A broadband digitizer is proposed and experimentally demonstrated based on photonic time stretch (PTS). In this scheme, a complementary single-sideband modulation architecture in combination with successive digital difference operation and phase compensation are employed to simultaneously eliminate dispersion-induced power fading problem, remove dynamic pulse envelope, suppress modulation nonlinearity impairment and maintain broadband waveform fidelity. In the proof-of-concept experiment, a PTS preprocessor with a stretch factor of 8.77 and a time window of 1.8 ns is realized, where the maximum power degradation in the frequency range up to 25 GHz is 5 dB. By using this preprocessor, single-tone microwave signals in the frequency range of 2 to 25 GHz are digitized with effective number of bits larger than 3.4 bits. For single-tone input microwave signals at 12 and 15 GHz, the 2 ^nd -order harmonic suppression ratios are enhanced by 11.51 and 16.86 dB, respectively. For a two-tone microwave signal at 8 and 10 GHz, the synthetic waveform fidelity after time stretch is maintained by carrying out phase compensation.

An approach for generating phase-coded coherent microwave pulse trains at high frequencies is proposed and demonstrated based on an actively mode-locked optoelectronic parametric oscillator (AML-OEPO), where an electrical mixer is inserted into the cavity of an optoelectronic oscillator (OEO) to achieve both mode locking and parameter oscillation. The driving signal applied to the mixer is a low-frequency sinusoidal signal with voltage polarity coding, where the frequency is the same as the free spectral range (FSR) of the OEO cavity, and the duration of each voltage polarity coding bit is identical to the loop delay. As a result, phase-coded coherent microwave pulse trains can be generated, where the pulse interval is equal to the loop delay due to the active mode locking effect, and the phase coding period is equal to a multiple integer of the loop delay due to parameter oscillation. The enhancement of the signal period and the highly coherent characteristic are beneficial for breaking the contradiction between unambiguous detection range and ranging resolution in pulse radars. In the experiment, phase-coded microwave pulse trains with either 13-bit barker codes or 7-bit M codes are generated at 15.026 GHz. The autocorrelation calculation result of the phase-coded microwave pulse train with 13-bit barker codes shows a high peak-to-sidelobe ratio, verifying high coherence.