Narrow-bandwidth silicon photonic CROW filter for carrier-extracted self-coherent (CESC) detection.

In this Letter, we report a second-order silicon photonic (SiP) coupled resonator optical waveguide (CROW) filter with an ultra-narrow 10-dB bandwidth of 1.75 GHz and a high extinction ratio (ER) of ∼50 dB. By utilizing this CROW filter, we demonstrated an innovative self-coherent detection, called carrier-extracted self-coherent (CESC) detection.

Weight-adaptive joint mixed-precision quantization and pruning for neural network-based equalization in short-reach direct detection links.

Neural network (NN)-based equalizers have been widely applied for dealing with nonlinear impairments in intensity-modulated direct detection (IM/DD) systems due to their excellent performance. However, the computational complexity (CC) is a major concern that limits the real-time application of NN-based receivers. In this Letter, we propose, to our knowledge, a novel weight-adaptive joint mixed-precision quantization and pruning approach to reduce the CC of NN-based equalizers, where only integer arithmetic is taken into account instead of floating-point operations.

Modified SiP single-polarization CADD receiver.

In cost-sensitive application scenarios, increasing the data rate per channel under a limited receiver bandwidth is critical, and thus, the transceivers with low costs and high electrical spectral efficiencies (ESEs) are highly desirable. In this Letter, we demonstrate a modified silicon photonic (SiP) carrier-assisted differential detection (CADD) receiver with a record ESE for single polarization. The ESE of the conventional CADD is mainly limited by the transfer function that originated from the optical delay and hybrid.

Impact of non-Gaussian noise on GMI and LDPC performance in neural network equalized systems.

In this Letter, the impact of non-Gaussian noise caused by a nonlinear equalizer on low-density parity-check code (LDPC) performance is investigated in a 25-km 50-Gb/s pulse amplitude modulation4 (PAM4) direct detection system. The lookup table (LUT)-based log-likelihood ratio (LLR) calculation method is proposed to enhance the LDPC performance for the non-Gaussian noise case. Compared to the conventional LLR calculation method based on Gaussian distribution, the proposed method can improve 0.

Phase noise-induced interference for coherently detected OTDR systems.

The phase noise-induced interference (PNII) in coherently detected OTDR systems is investigated. A close-form relationship between signal to (interference) noise ratio (SNR) and laser linewidth is derived for the first time, to the best of our knowledge, and numerical simulations are conducted to verify the theoretical results. Additionally, the proportion of noise composition of PNII is studied.

Deep-learning-enabled high-performance full-field direct detection with dispersion diversity.

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.

Feature issue introduction: ultra-wideband optical communications.

This Feature Issue covers the important aspects to develop ultra-wideband optical communication systems including optoelectronics, impairment modeling and compensation, optical amplification, superchannel and multi-band transmission and control, and so forth. This Introduction provides a summary of the articles on these topics in this Feature Issue.

Carrier-assisted differential detection with reduced guard band and high electrical spectral efficiency.

For high-capacity and short-reach applications, carrier-assisted differential detection (CADD) has been proposed, in which the optical field of a complex-valued double sideband (DSB) signal is reconstructed without using a sharp-edge optical bandpass filter or local oscillator laser. The CADD receiver features a transfer function with periodical nulls in the frequency domain, while the signal-signal beat interference (SSBI) is severely amplified around the frequency nulls of the transfer function. Since the null magnitude at the zero frequency is inevitable, a guard band is required between the carrier and the signal, leading to a higher receiver bandwidth and implementation cost.

Carrier assisted differential detection with a generalized transfer function.

Direct detection capable of optical field recovery not only enables high-order modulation for high spectral efficiency (SE) but also extends the transmission reach by digital compensation of linear channel impairments such as chromatic dispersion. Recently, to bridge the gap between direct detection and coherent detection, carrier assisted differential detection (CADD) has been proposed for the reception of complex-valued double-sideband signals. In this paper, we extend the concept CADD to a general selection of the transfer functions, beyond the originally-proposed delay interferometer.

Polarization-diversity receiver using remotely delivered local oscillator without optical polarization control.

Silicon photonics coherent transceivers have integrated all the necessary optics except the lasers. The laser source has become a major obstacle to further reduce the cost, footprint, power consumption of the coherent transceivers for short-reach optical interconnects. One solution is to utilize remotely delivered local oscillator (LO) from the transmitter, which has the benefits of relaxing the requirements of wavelength stability and laser linewidth and simplifying the digital signal processing (DSP) of carrier/phase recovery.

Cascade recurrent neural network-assisted nonlinear equalization for a 100 Gb/s PAM4 short-reach direct detection system.

We propose a novel, to the best of our knowledge, cascade recurrent neural network (RNN)-based nonlinear equalizer for a pulse amplitude modulation (PAM)4 short-reach direct detection system. A 100 Gb/s PAM4 link is experimentally demonstrated over 15 km standard single-mode fiber (SSMF), using a 16 GHz directly modulated laser (DML) in C-band. The link suffers from strong nonlinear impairments which is mainly induced by the mixture of linear channel effects with square-law detection, the DML frequency chirp, and the device nonlinearity.

Carrier-assisted differential detection.

To overcome power fading induced by chromatic dispersion in optical fiber communications, optical field recovery is a promising solution for direct detection short-reach applications, such as fast-evolving data center interconnects (DCIs). To date, various direct detection schemes capable of optical field recovery have been proposed, including Kramers-Kronig (KK) and signal-signal beat interference (SSBI) iterative cancellation (IC) receivers. However, they are all restricted to the single sideband (SSB) modulation format, thus conspicuously losing half of the electrical spectral efficiency (SE) compared with double sideband (DSB) modulation.

Computational complexity comparison of feedforward/radial basis function/recurrent neural network-based equalizer for a 50-Gb/s PAM4 direct-detection optical link.

The computational complexity and system bit-error-rate (BER) performance of four types of neural-network-based nonlinear equalizers are analyzed for a 50-Gb/s pulse amplitude modulation (PAM)-4 direct-detection (DD) optical link. The four types are feedforward neural networks (F-NN), radial basis function neural networks (RBF-NN), auto-regressive recurrent neural networks (AR-RNN) and layer-recurrent neural networks (L-RNN). Numerical results show that, for a fixed BER threshold, the AR-RNN-based equalizers have the lowest computational complexity.

Squeezing out the last few bits from band-limited channels with entropy loading.

The past decade witnessed the stirring development of advanced optical modulations and digital signal processing, which have been pushing optical transmission systems towards the capacity limit. Recent research has sought to squeeze out the last few bits from bandwidth-limited optical channels. One straightforward path is to expand the signal spectrum beyond the bandwidth limit while keeping the single-carrier modulation, which inevitably induces huge inter-symbol interference.

High-dimensional Stokes vector direct detection over few-mode fibers.

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.

Investigation of single- and multi-carrier modulation formats for Kramers-Kronig and SSBI iterative cancellation receivers.

The Kramers-Kronig (KK) receiver has recently attracted significant attention due to its capability of field recovery with direct detection. Under minimum phase condition, the KK receiver may use either single- or multi-carrier modulation formats. In this Letter, we investigate the appropriate modulation formats for both KK and signal-signal beat interference (SSBI) iterative cancellation (IC) receivers.

.Maximizing the spectral efficiency of Stokes vector receiver with optical field recovery.

Stokes vector receivers (SVR) bridge the 4-D (i.e. dual-polarization complex signals) coherent detection and the conventional intensity-only 1-D direct detection (DD).

A Photonic Switch Based on a Hybrid Combination of Metallic Nanoholes and Phase-change Vanadium Dioxide.

A photonic switch is an integral part of optical telecommunication systems. A plasmonic bandpass filter integrated with materials exhibiting phase transition can be used as a thermally reconfigurable optical switch. This paper presents the design and demonstration of a broadband photonic switch based on an aluminium nanohole array on quartz utilising the semiconductor-to-metal phase transition of vanadium dioxide.

Direct detection of the optical field beyond single polarization mode.

Direct detection is traditionally regarded as a detection method that recovers only the optical intensity. Compared with coherent detection, it owns a natural advantage-the simplicity-but lacks a crucial capability of field recovery that enables not only the multi-dimensional modulation, but also the digital compensation of the fiber impairments linear with the optical field. Full-field detection is crucial to increase the capacity-distance product of optical transmission systems.

Complex imaging via coherent detection.

Complex imaging via coherent detection is proposed for acquiring two-dimensional (2-D) nearfield optical image that recovers amplitude and phase simultaneously. Based on the proposed complex imaging, we experimentally demonstrate the technique by detecting few-mode-fiber (FMF) modes with high extinction-ratio, and perform mode decomposition and differential mode delay (DMD) measurement.

Single-shot distributed Brillouin optical time domain analyzer.

We demonstrate a novel single-shot distributed Brillouin optical time domain analyzer (SS-BOTDA). In our method, dual-polarization probe with orthogonal frequency-division multiplexing (OFDM) modulation is used to acquire the distributed Brillouin gain spectra, and coherent detection is used to enhance the signal-to-noise ratio (SNR) drastically. Distributed temperature sensing is demonstrated over a 1.

Maximum likelihood sequence estimation for optical complex direct modulation.

Semiconductor lasers are versatile optical transmitters in nature. Through the direct modulation (DM), the intensity modulation is realized by the linear mapping between the injection current and the light power, while various angle modulations are enabled by the frequency chirp. Limited by the direct detection, DM lasers used to be exploited only as 1-D (intensity or angle) transmitters by suppressing or simply ignoring the other modulation.

Duobinary pulse shaping for frequency chirp enabled complex modulation.

The frequency chirp of optical direct modulation (DM) used to be a performance barrier of optical transmission system, because it broadens the signal optical spectrum, which becomes more susceptible to chromatic dispersion induced inter-symbol interference (ISI). However, by considering the chirp as frequency modulation, the single DM simultaneously generates a 2-D signal containing the intensity and phase (namely, the time integral of frequency). This complex modulation concept significantly increases the optical signal to noise ratio (OSNR) sensitivity of DM systems.

High-fidelity angle-modulated analog optical link.

There has long existed a debate over whether analog or digital optical link is more suitable for wireless convergence applications. Digital link achieves the highest fidelity, with the sacrifice of huge bandwidth due to the high resolution of digitization, and large power consumption due to the exhaustive digital data recovery. Analog link avoids these drawbacks, but it inevitably suffers from the SNR degradation.