Mobile fronthaul requires an efficient transfer of radio signals between cell-site remote radio heads (RRHs) and a centralized baseband unit (BBU) to satisfy diverse requirements on system capacity, latency and cost. Both analogue radio-over-fiber (A-RoF) and digital radio-over-fiber (D-RoF) enable the realization of the mobile fronthaul transmission link. A-RoF can achieve high spectral efficiency while it is susceptible to the noise and inherent nonlinearities during the optical transmission. Nevertheless, D-RoF can achieve high signal fidelity using high quantization bits while it is commonly considered as low spectral efficiency. Therefore, the spectral efficiency comparison between D-RoF and A-RoF is generally not a fair one under the same constraint of the optical transmission link. In this paper, we theoretically analyze the recovered signal fidelity for D-RoF and A-RoF under the constraint of spectral efficiency and channel condition. We reveal that under the same channel condition, D-RoF can achieve the same spectral efficiency as A-RoF with the same recovered signal-to-noise ratio (SNR). Provided that D-RoF consumes more bandwidth, it offers the key advantage of D-RoF where the achievable SNR exhibits exponential improvement as a trade-off of the link bandwidth. If keeping the same quantization bits for radio signals, the required SNR threshold for D-RoF decreases exponentially with respect to the link bandwidth especially for using high quantization bits. Extensive experiments are carried out to confirm the theoretical analysis over the same constraint of the optical transmission link.

The recent decade has witnessed the evolution and enormous demand of the high-capacity data center interconnect (DCI), in such application scenarios spectral efficiency, optical field recovery and cost-effectiveness are highly desirable. Compared to coherent detection, direct detection does not require local oscillators, resulting in a simpler transceiver structure and thus a lower-cost solution for DCIs. To overcome chromatic dispersion induced power fading, field recovery enabled by direct detection has drawn extensive research interests. To date, various proposed direct detection schemes concentrate mainly on single sideband (SSB) modulation. Compared to double sideband (DSB) modulation, however, one sideband of SSB is unfilled and hence SSB poses higher requirement on the receiver bandwidth. Besides, the complexity and cost of implementing precisely aligned optical filtering poses burden for adopting SSB. Accordingly, we recently proposed a novel direct detection scheme called carrier assisted direct detection (CADD), which can realize the field recovery of complex-valued DSB signals with the aid of an optical carrier. In this letter, we carry out the first-time experimental demonstration of the CADD receiver. 54-Gb/s OFDM signals are transmitted over 160-km standard single-mode fiber with digital chromatic dispersion compensation at the receiver, indicating the ability to reconstruct optical field of DSB signals. For signal-signal beat interference (SSBI) which extensively exists in direct detection, experimental results show that merely two iterations of cancellation are sufficient to mitigate SSBI. More importantly, compared to various SSB based receiver schemes, the requirement of receiver bandwidth for DSB based CADD scheme is relaxed by 41%.

There has been renewed interest in coherent detection to provide high spectral efficiency transmission for short-reach optical interconnects. However, the expensive high-stable laser sources may preclude its potential application. To balance the high performance and low cost for short-reach optical networks, the self-coherent homodyne receiver has been investigated, where the dual-polarization (DP) signal and the remote local oscillator (LO) originating from the same laser source co-propagate over a duplex fiber, enabling a remarkable tolerance of laser linewidth. However, the fast evolution of the state of polarization (SOP) of remotely delivered LO becomes problematic for self-coherent homodyne detection systems. To combat the polarization wandering of the remote LO, the complicated adaptive/automatic polarization controller (APC) has been deployed with only up to a few hundred rad/s polarization tracking speed. In this paper, we propose a complementary polarization-diversity coherent receiver (C-PDCR) for self-coherent homodyne detection using remote LO. The proposed C-PDCR features rapid polarization tracking for remote LO utilizing electronic digital signal processing (DSP). The robustness of the proposed C-PDCR is verified and demonstrated with a 1.08-Tb/s line rate (net rate 769.4 Gb/s over 45-GHz electrical bandwidth) using DP PCS-256QAM signal under rapid polarization rotation rate up to 314 krad/s in the experiment, which is more than 3-orders improvement of magnitude compared to the prior reports.

This paper reviews recent progress on different high-speed optical short- and medium-reach transmission systems. Furthermore, a comprehensive tutorial on high-performance, low-cost, and advanced optical transceiver (TRx) paradigms is presented. In this context, recent advances in high-performance digital signal processing algorithms and innovative optoelectronic components are extensively discussed. Moreover, based on the growing increase in the dynamic environment and the heterogeneous nature of different applications and services to be supported by the systems, we discuss the reconfigurable and sliceable TRxs that can be employed. The associated technical challenges of various system algorithms are reviewed, and we proffer viable solutions to address them.

Direct detection attracts much attention for its simplicity compared with coherent detection. In this Letter, we propose for the first time, to the best of our knowledge, a high-dimensional Stokes vector direct detection (HD-SVDD) receiver for mode-division multiplexing transmission in few-mode fibers where the coupled modes can be recovered without resorting to coherent detection. To the best of our knowledge, the first high-dimensional Stokes vector reception based on the proposed HD-SVDD receiver has been successfully demonstrated with a dual-spatial and dual-polarization mode at 60 Gb/s over a 200 m two-mode fiber.

Coherent detection is advantageous in achieving high electrical spectral efficiency (ESE) relative to direct detection (DD), due to its capability of field recovery. However, the stringent requirement on the laser sources prevents it from being widely utilized in cost-sensitive short-reach applications. Carrier-assisted differential detection (CADD) was proposed to retrieve the optical field without needing for a narrow-linewidth local oscillator (LO) in a receiver. To realize a compact CADD receiver, silicon photonic (SiP) integration is therefore of particular interest since it leverages the standard complementary metal-oxide semiconductor (CMOS) compatible fabrication, offering high yield, low cost, and compact footprint. The fabrication process does not introduce III-V materials as the CADD receiver is LO-free. In this article, we fabricated a SiP integrated CADD receiver and demonstrated the transmission and reception of a 240-Gb/s orthogonal frequency division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal transmitted over an 80-km single-mode fiber (SMF) link, with a bit error ratio (BER) below the 25% overhead (OH) soft-decision forward error correction (SD-FEC) threshold. To the best of our knowledge, we achieve a record 5.2-b/s/Hz ESE per polarization for an integrated DD receiver.