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.

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.

We introduce and experimentally demonstrate a modified Kramers-Kronig (KK) receiver with direct detection (DD) to realize an accurate optical field reconstruction at low carrier-to-signal power ratio (CSPR) condition. In our scheme, an exponential operation is applied to a single-sideband (SSB) signal, rather than a continuous-wave tone adding method in the conventional KK scheme. By this means, the minimum phase condition (MPC) is inherently satisfied, thus the optical field can be reconstructed accurately through KK relation at a low CSPR, leading to a high receiver sensitivity in a DD system. A proof-of-concept experiment is conducted, in which an SSB signal is generated based on an 8-GBaud 16-quadrature amplitude modulation (QAM) data stream and then transmitted over an 80-km single mode fiber (SMF). By using the modified KK technique, 1.1- and 0.9-dB sensitivity improvements are achieved in the optical back-to-back case and after the fiber transmission, respectively. The proposed and the conventional KK schemes show similar BER performances at the optimum CSPR value with a high received optical power. In addition, we perform numerical simulations to study the impacts of bandwidth limitation and chromatic dispersion on the proposed minimum phase signal.

Standard coherent communication technology is being introduced to short-reach optical communications to further scale the link throughput by leveraging more degrees of freedom of an optical channel. However, the cost, footprint, and power consumption of the coherent transceivers may impede its application to short-reach optical interconnects. Silicon photonics technology has realized the monolithic integration of various optoelectronic components for coherent transceivers on a single chip to lower the cost but except for a silicon laser. Heterogeneous integration has been developed to realize distributed feedback (DFB) laser sources, which are not well suited for standard coherent communication. Therefore, there is a trade-off between the receiver complexity and the cost of laser sources. Self-coherent homodyne detection as a compromising solution of coherent detection for short-reach applications has been studied to utilize the un-cooled DFB lasers. In this paper, as one realization of self-coherent homodyne detection without optical polarization control, the first integrated complementary polarization-diversity coherent receiver (C-PDCR) chip for addressing the complexity issue has been successfully demonstrated with rapid polarization tracking of remotely delivered local oscillator (LO). To the best of our knowledge, the fastest reported polarization tracking speed is achieved and reaches 9 Mrad/s with negligible performance degradation based on electronic MIMO DSP given the system symbol rate and MIMO coefficients updating conditions/algorithms for a 70-Gbaud dual-polarization (DP) PCS-64QAM signal with 630-Gb/s line rate (net rate 485 Gb/s). The maximal transmission capacity of this on-chip C-PDCR can reach 714 Gb/s (net rate 540 Gb/s over 35 GHz electrical bandwidth).

Data center interconnects require cost-effective photonic integrated optical transceivers to meet the ever-increasing capacity demands. Compared with a coherent transmission system, a complex-valued double-sideband (CV-DSB) direct detection (DD) system can minimize the cost of the photonic circuit, since it replaces two stable narrow-linewidth lasers with only a low-cost un-cooled laser in the transmitter while maintaining a similar spectral efficiency. In the carrier-assisted DD system, the carrier power accounts for a large proportion of the total optical signal power. Reducing the carrier to signal power ratio (CSPR) can improve the information-bearing signal power and thus the achievable system performance. To date, the minimum required CSPR is ∼7 dB for all the reported CV-DSB DD systems having electrical bandwidths of approximately half of baud rates. In this paper, we propose a deep-learning-enabled DD (DLEDD) scheme to recover the full optical field of the transmitted signal at a low CSPR of 2 dB in experiment. Our proposal is based on a dispersion-diversity receiver with an electrical bandwidth of ∼61.0% baud rate and a high tolerance to laser wavelength drift. A deep convolutional neural network enables accurate signal recovery in the presence of a strong signal-signal beat interference. Compared with the conventional method, the proposed DLEDD scheme can reduce the optimum CSPR by ∼8 dB, leading to a significant signal-to-noise ratio improvement of ∼5.8 dB according to simulation results. We experimentally demonstrate the optical field reconstruction for a 28-GBaud 16-ary quadrature amplitude modulation signal after 80-km single-mode fiber transmission based on the proposed DLEDD scheme with a 2-dB optimum CSPR. The results show that the proposed DLEDD scheme could offer a high-performance solution for cost-sensitive applications such as data center interconnects, metro networks, and mobile fronthaul systems.