PAM-4 Transmission Research Articles

Low-complexity cluster-assisting look-up-table-based Volterra decision-feedback equalizer for IM/DD systems.

To address the challenges posed by chromatic dispersion (CD)-induced power fading and nonlinear signal distortions in double-sideband (DSB) intensity modulation and direct detection (IM/DD) transmission systems, a combination of a Volterra feed-forward equalizer (VFFE) and a Volterra decision-feedback equalizer (VDFE) is widely employed. However, the conventional VFFE-VDFE exhibits significant computational complexity, particularly for longer memory lengths. In this Letter, a low-complexity cluster-assisting look-up table-based VDFE (CLUT-VDFE) is proposed to effectively reduce the computational complexity associated with compensating for CD and nonlinear distortions. By utilizing CLUTs, all multiplication operations required for the implementation of the VDFE are completely eliminated. To validate the effectiveness of the proposed CLUT-VDFE, experiments on a C-band 100-Gb/s PAM-4 transmission system over a 60-km standard single-mode fiber (SSMF) are conducted. The experimental results show that the CLUT-VDFE not only achieves comparable equalization performance to the conventional VDFE but also effectively eliminates multiplication operations and significantly saves 48.8% real-valued additions.

2 × 53 Gbit/s PAM-4 Transmission Using 1.3 μm DML With High Power Budget Enabled by Quantum-Dot SOA

2 × 53 Gbit/s PAM-4 Transmission Using 1.3 μm DML With High Power Budget Enabled by Quantum-Dot SOA

Meter-Scale Long Connectorized Paper-like Polymer Waveguide Film for 100 Gbps Board-Level Optical Interconnects Application.

We design and fabricate meter-scale long connectorized paper-like flexible multimode polymer waveguide film with a large bandwidth-length product (BLP) for board-level optical interconnects application. The measured BLP of the multimode waveguide is greater than 57.3 GHz·m at a wavelength of 850 nm under the strictest overfilled launch condition with a maximum length of 2.1 m and 10-dB insertion loss. The fabricated waveguide films are as flexible as regular printing paper and can be conveniently interfaced with standard mechanically transferable (MT) fiber connectors with low loss. The average insertion loss of the connectorized waveguide is about 0.042 dB/cm with inter-channel crosstalk as low as -46.4 dB, and the bending loss is less than 1 dB at a bending radius of 1 mm under the overfilled launch condition. We also demonstrate a vertical-cavity surface-emitting laser (VCSEL)-based single-lane 100 Gbps PAM4 transmission. Our results show that the meter-scale long paper-like polymer waveguide film has both excellent optical properties and large bandwidth and is ideal for high-speed board-level optical interconnects application with a single-lane data rate of 100 Gbps and beyond, especially those that have a strict requirement on the length of connection and compactness.

Net 30-Tb/s bidirectional IM/DD optical interconnect over 19-core fiber enabled by WDM uplink/downlink twin-SSB PAM-4 transmission.

High-capacity optical interconnects with short reach are hugely demanded driven by the exponential growth of data traffic. In this work, four-channel wavelength division multiplexing (WDM) uplink/downlink twin single-sideband (twin-SSB) signals are implemented by a wavelength selective switch (WSS) at once, which simplifies the structure of multi-channel SSB transmitters and reduces the cost of high-capacity optical interconnect. Compared to a double sideband scheme, it has been experimentally proven that the performance of SSB transmission over standard single-mode fiber (SSMF) at C-band with an ultra-high baud rate has been greatly improved, which has the ability to effectively overcome the power fading induced by chromatic dispersion in an intensity modulation and direct detection (IM/DD) system. Besides, Rayleigh backscattering cross talk introduced by the same wavelengths in single-fiber bidirectional transmission is avoided by the proposed uplink/downlink twin-SSB scheme, which achieves 0.5-dB receiver sensitivity improvement compared to the conventional scheme. Finally, whtat is believed to be a record C-band 36.48-Tb/s PAM-4 4λ WDM IM/DD bidirectional optical interconnect is successfully demonstrated over 1-km 19-core fiber under a 20% soft-decision forward error correction (SD-FEC) threshold of 2.4×10-2 for beyond net 30-Tb/s large-capacity optical interconnection application.

O-band EML and 4-bits DAC enabled 112 GBaud PAM8 transmission over a 10 km SSMF without optical amplifiers.

The increasing traffic of intra-data center networks creates an urgent demand for low-cost and high-speed intensity modulation/direct detection (IM/DD) transmission. In this Letter, we propose a joint scheme combining nonlinear lookup table (NL-LUT) pre-distortion, digital pre-emphasis (DPE), and a digital resolution enhancer (DRE) to achieve high-speed and low-cost IM/DD optical links using a low-resolution digital-to-analog converter (DAC). The NL-LUT and DPE are responsible for the system nonlinear impairments and channel attenuation at high frequency, respectively, while DRE handles the quantization noise (QN) caused by high peak-to-average ratios (PAPRs) and low-resolution DAC. Leveraging the advanced pre-compensation scheme at the transmitter, we experimentally demonstrate the O-band transmission of 124 GBaud and 112 GBaud PAM8 over 2 km and 10 km SSMF, respectively, using only a 4 bits DAC and a simple 5-tap linear equalizer at the receiver-side. To the best of our knowledge, among the reported demonstrations using the low-resolution DAC without an optical amplifier, this work achieves the highest net rates of 310 Gb/s over 2 km and 280 Gb/s over 10 km of SSMF, respectively, which offers a promising solution for low-cost high-speed IM/DD transmission.

Real-time DSP-Free 40 Gbit/s PAM4 transmission over 10 km fiber enabled by optical injection locking of directly modulated laser

We present a comprehensive performance analysis of injection-locked directly modulated laser (DML) for optical communication systems, focusing on both non-return-to-zero (NRZ) and 4-level pulse amplitude modulation (PAM4) signal transmission. We demonstrate real-time PAM4 40 Gbit/s transmission over 10 km of single-mode fiber enabled by optical injection locking without pre-emphasis or post-equalization, achieving a bit error rate (BER) below 10-6, and doubling capacity compared to unlocked transmission with the same laser. Our study investigates the dependence of system performance on the injected power and frequency offset of the master laser. Results indicate that lower injection powers while maintaining a stable locking regime, yield better performance in terms of extinction ratio and BER. Optimized parameters lead to enhanced transmission performance, providing valuable insights into the design and optimization of injection-locked DML systems for optical communication applications employing direct modulation.

Single-mode chirped high-contrast metastructure VCSEL for 106 Gbps PAM4 transmission

Short-reach optical interconnects, employing an 850 nm wavelength multimode vertical cavity surface emitting laser (VCSEL) and multimode fiber (MMF), are confronted with transmission distance limitation as the transmission speed increases. Achieving higher speeds over the same transmission distance necessitates a significant enhancement in the fiber’s effective bandwidth, resulting in substantial costs. Employing single-mode VCSEL with single-mode fiber transmission presents as a more economically viable solution. Here we report, to our knowledge, the first demonstration of an 850 nm single-mode VCSEL using a chirped high-contrast metastructure (HCM) as the top mirror with spatially graded reflectivity to suppress higher-order modes. The chirped HCM top reflector is designed to favor lasing of the fundamental mode. We show stable single-mode lasing with a >40dB side mode suppression ratio (SMSR) up to 15 times threshold current and an open eye through a short single-mode fiber SM800 transmission at 106 Gbps PAM4 modulation. This study highlights the potential for scalable single-mode VCSEL in advanced optical interconnects, providing a cost-effective pathway for high-speed applications.

Simultaneous underwater beam steering and PAM4 transmission enabled by the acousto-optic effect.

In this paper, an underwater wireless optical communication (UWOC) system that can simultaneously achieve beam steering and pulse amplitude modulation (PAM) enabled by the acousto-optic effect is proposed and experimentally demonstrated. An approach to manipulate the driving signal of an acousto-optic modulator (AOM) is utilized to simultaneously achieve precise laser emission angle control and signal modulation, which enables high-speed link-switching for multi-user access. The system is tested in a 7-m water tank with a water attenuation coefficient of 0.87 dB/m, and a bit error rate (BER) of 2.99 × 10-3 is realized at a maximum data rate of 1.6 Mbps. The theoretical deflection angle of a 450-nm laser is 4.6 mrad, while the actual steering range could reach up to 5.2 mrad with a horizontal displacement of 36.4 mm at the receiver plane under experimental conditions. We further measured the relationship between the diffraction efficiency of the 1st-order diffracted light and the amplitude as well as the central frequency of the driving signal, which is essential for the generation of the driving signal. The system provides an effective solution for link-switching in the Internet of Underwater Things (IoUT) and high-throughput underwater wireless optical networks.

Single-Ended PAM-4 Transmitters With Data Bus Inversion and ZQ Calibration for High-Speed Memory Interfaces

Single-Ended PAM-4 Transmitters With Data Bus Inversion and ZQ Calibration for High-Speed Memory Interfaces

Bias-aided DD-LMS equalizer for optical multipath-interference-impaired high-speed PAM4 transmission systems

Multipath interference (MPI) noise induces drastic fluctuations in high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) systems, severely degrading the transmission performance. Here, we propose a bias-aided decision-directed least mean square (DD-LMS) equalizer to eliminate the MPI noise. In the simulation, the proposed bias-aided DD-LMS equalizer could adeptly track and compensate the MPI-impaired PAM4 signal, markedly improving the bit error rate (BER) performance under different MPI levels and laser linewidths. Within a 112 Gbps PAM4 transmission system, the proposed equalizer achieves a MPI tolerance over 4 dB at the KP4-forward error correction (FEC) threshold (2.4 × 10−4), outperforming existing MPI suppression techniques.

Nonlinear equalization based on feature crosses neural networks for High-speed PAM4 transmission

In this paper, a low complexity feature crosses nonlinear equalization(FC-NLE) scheme using an additional feature crosses layer based on deep neural network is proposed. A 120 Gb/s quadratic pulse amplitude modulation (PAM4) transmission system based on intensity modulation and direct detection (IM/DD) is experimentally demonstrated. After a 5 km transmission, the FC-NLE scheme achieves a bit error rate (BER) below the hard-decision forward-error-correction threshold of 7% when the received optical power exceeds 4.5 dBm. This performance surpasses that of the Volterra equalizer(VE) and network-based equalization schemes. k-means clustering and model compression techniques were adopted to effectively diminish the complexity of the FC-NLE. Experimental results show that the performance of the FC-NLE is about 0.5–1.0 dB better than that of the VE, DNN, LSTM equalizers and its complexity is less than 50% of them. Compared with other low complexity network schemes in recent years, FC-NLE still exhibits a performance improvement of more than 1 dB at lower complexity.

Optical Amplifier-Free 100 Gbaud PAM-4 Transmission Over 10 Km SSMF Based on O-Band SiP Transceiver

Optical Amplifier-Free 100 Gbaud PAM-4 Transmission Over 10 Km SSMF Based on O-Band SiP Transceiver

Terminated polymer waveguide with low coupling loss to single-mode fiber for 100 Gbps optical interconnects and beyond.

We designed and fabricated terminated polymer waveguides with low coupling loss to a standard single-mode fiber (SSMF) for 100 Gbps optical interconnects application and beyond. The mode field diameter of the polymer waveguide is designed to perfectly match that of the SSMF with a theoretical coupling loss as small as 0.07 dB. The fabricated polymer waveguides on FR-4 substrates exhibit both a low propagation loss of 0.32 dB/cm and a low coupling loss of less than 0.17 dB to SSMF at a wavelength of 1310 nm. These low loss values indicate good compatibilities with both printed circuit boards and fiber optics. Both single-lane 56 Gbaud (112 Gbps) and 60 Gbaud (120 Gbps) 4-level pulse amplitude modulation (PAM4) optical transmission using terminated polymer waveguides have been successfully demonstrated. The bit error ratio (BER) for PAM4 transmission at both data rates reached a level of 10-4 which is well below the forward error correction (FEC) limitation with a power penalty of less than 1 dB. The bandwidth of the test equipment rather than the polymer waveguide itself limits the high-speed optical interconnect performances in our experiments. The results imply that the polymer waveguide is a promising media for single-lane 100 Gbps optical interconnects application and beyond.

Fractionally spaced nonlinear Tomlinson-Harashima precoding with weight clustering for C-band 100 Gbaud/λ PAM-4 transmission.

A nonlinear Tomlinson-Harashima precoding (NTHP) scheme has been verified for its capability to effectively address both the linear and nonlinear inter-symbol interferences (ISIs) arising in the intensity-modulation direct-detection (IM-DD) fiber optics transmission. Nevertheless, the application of the NTHP scheme may significantly increase the number of levels for the intensity modulated signals, resulting in the reduction of both eye width and receiver sensitivity. Here, we propose a fractionally spaced NTHP with a weight clustering (FS-NTHP-WC) scheme. Consequently, an accurate ISI feedback can be obtained to enlarge the eye width; meanwhile a hardware-efficient implementation without the equalization penalty can be achieved by weight clustering and pruning. When the C-band 100 Gbaud/λ PAM-4 signals are transmitted, our proposed FS-NTHP-WC scheme not only can achieve 0.25 dB and 0.5 dB gains of receiver sensitivity under back-to-back (B2B) and 2-km standard single-mode fiber (SSMF) transmission conditions, respectively, but can also cut down the computational complexity by 90% and 76% in terms of the number of multiplications and additions, respectively, in comparison with the NTHP scheme.

Fabrication Tolerances’ Impact on an ODAC-Based PAM-4 Transmitter

Photonic integrated circuits (PIC) devices are impacted by fabrication tolerances and therefore, prior knowledge of such variations could improve the PIC fabrication process and overall yield. This paper presents a method for predicting the fabrication impacts on a telecommunication optical digital to analog converter (oDAC)-based pulse amplitude modulator level four (PAM-4) transmitter as a case study where the certainty of this passive device is subjected to random variation. Our findings allow us to estimate the production yield in a fabrication scenario using the symbol error rate (SER) benchmark and this contributes to the study of the viability of oDAC PAM-4 transmitters to replace conventional electrical digital to analog converter (eDAC) PAM-4 transmitters.

Low-complexity frequency domain frame synchronization for short-reach IM/DD optical fiber transmission systems

We propose a low-complexity frequency domain frame synchronization method for short-reach intensity modulation and direct detection (IM/DD) systems. A four-level pulse amplitude modulation-training sequence (PAM4-TS) is specially designed for the proposed method, which has an obvious peak in the amplitude spectrum that is higher than the normal signal. The proposed method comprises a coarse synchronization stage and a fine synchronization stage. Firstly, the coarse synchronization stage takes advantage of the feature of PAM4-TS to obtain the approximate position of the frame head by identifying the peak value in amplitude spectrum of the segmented received signal. Then, the fine synchronization stage calculates the correlation between the coarse synchronization result and PAM4-TS by multiplying the two in the frequency domain. Compared with the traditional sliding window correlation method realized in the time domain, both simulation and experimental results of C-band 50 Gbit/s PAM4 transmission demonstrate that the proposed method reduces the multiplication complexity by up to about 96.01% without any additional performance penalty.

Monolithic optical PAM-4 transmitter with autonomous carrier tracking.

We present two single channel optical PAM-4 transmitters, one based on a novel 3-section PN-capacitive micro-ring modulator with on-chip low-power driver and a near-zero power capacitive wavelength locking system and another one based on a 2-section thermally tuned PN micro-ring modulator of the similar size with the same modulator driver. The maximum error-free data-rate of 16 Gb/s and 22 Gb/s at the energy efficiency of 200 fJ/b and 430 fJ/b for the former and the latter transmitters are measured, respectively, and the design trade-offs are discussed. The chips are fabricated in the GlobalFoundries 90 nm CMOS SOI process.

Gradient-descent noise whitening techniques for short-reach IM-DD optical interconnects with severe bandwidth limitation.

Bandwidth limitation in optoelectrical components and the chromatic dispersion-induced power fading phenomenon cause severe inter-symbol interference (ISI) in high-speed intensity modulation and direct detection (IM-DD) optical interconnects. While the equalizer implemented in the receiver's digital signal processing procedure can mitigate ISI, it also inevitably enhances the noise located in the decayed frequency region, known as equalization-enhanced colored noise (EECN). Additionally, the nonlinear impairments of the modulator and photodetector also deteriorate the performance of the IM-DD system, especially for high-order modulation formats. In this work, we propose a gradient-descent noise whitening (GD-NW) algorithm to address EECN and extend it by introducing nonlinear kernels to simultaneously mitigate EECN and nonlinear impairments. The proposed algorithms are compared with conventional counterparts in terms of the achievable baud rate and the receiver optical power sensitivity. As a proof-of-concept experiment, we validate the principles of the proposed algorithms by successfully transmitting 360-GBd on-off-keying (OOK) and 180-GBd 4-level pulse-amplitude-modulation (PAM-4) signals in the back-to-back case under a 62-GHz brick-wall bandwidth limitation. 280-GBd OOK and 150-GBd PAM-4 transmissions are also demonstrated over 1-km standard single-mode fiber with a bit error rate below 7% hard-decision forward error correction aided by the proposed approach.

Simplified Look-Up Table (LUT) Based Digital Pre-Distortion for 100 GBaud/λ PAM-4 Transmission

Simplified Look-Up Table (LUT) Based Digital Pre-Distortion for 100 GBaud/λ PAM-4 Transmission

Tbit/s PAM-4 transmission with a free-running Fabry-Pérot laser as a multi-wavelength source for intra-DCI applications

Tbit/s PAM-4 transmission with a free-running Fabry-Pérot laser as a multi-wavelength source for intra-DCI applications