Pulse Amplitude Modulation Signal Research Articles

Fiber-optic parametric amplifier-based all-optical format conversion enabling optical networks interconnection with different modulation formats

A fiber-optic parametric amplifier (FOPA)-based all-optical format conversion (AOFC) scheme is proposed and numerically simulated to achieve optical networks interconnection with different modulation formats. In this scheme, the power transfer function (PTF) and the relative phase shift (RPS) functions between the input and output optical signals for the single-pump FOPA are derived and numerically analyzed. For the input 20 Gbps bipolar 4-ary pulse amplitude modulation (PAM4) signal, it can be converted into a BPSK signal (BPSK1) based on the power saturation characteristic of the PTF. By adjusting the input power of the input PAM4 signal, it can also be converted into a level-swapping PAM4 signal based on the single-pump FOPA. When the level-swapping PAM4 signal is sent into the single-pump FOPA with suitable power again, another BPSK signal (BPSK2) could also be generated. When the PAM4-to-BPSK format converter based on the single-pump FOPA is combined with the dual-pump FOPA (namely phase-sensitive amplifier, PSA), the 16-ary quadrature amplitude modulation (16QAM) signal can also be de-aggregated into four BPSK signals or two quadrature phase shift keying (QPSK) signals. The constellation diagrams, the power waveforms, the error vector magnitude (EVM) and the bit error rate (BER) of the relevant optical signals are measured to evaluate the scheme performance. The scheme can be deployed at the optical gateway to realize optical networks interconnection using different modulation formats.

Hybrid lithium tantalite-silicon integrated photonics platform for electro-optic modulation.

Integrated electro-optic modulators are key components in photonic integrated circuits. Silicon photonic technology is considered to be promising for large-scale and low-cost integration. However, silicon does not exhibit any Pockels effect, and the electro-optic modulator based on free-carrier dispersion suffers from challenges such as high-power consumption, limited bandwidth, and large optical propagation loss. Here, a new, to the best of our knowledge, hybrid lithium tantalite-silicon platform is proposed for electro-optic modulators based on the Pockels effect. Benefiting from the strong Pockels coefficients of a thin-film lithium tantalite, a hybrid microring-based modulator is demonstrated. The quality factor and the extinction ratio of the hybrid microring are 1.7 × 104 and 10 dB, respectively. The linear bidirectional wavelength tuning efficiency is measured as 12.8 pm/V. The measured 3-dB bandwidth is > 20 GHz. High-quality eye diagrams of 20 Gbps non-return-to-zero signal and 20 Gbps four-level pulse amplitude modulation signals are generated experimentally. The proposed platform extends the toolbox of silicon photonics technology, which paves the way for high-speed modulators and phase shifters in optical communication and optical phased array.

Performance vs. complexity in NN pre-distortion for a nonlinear channel.

Optical communications at high bandwidth and high spectral efficiency rely on the use of a digital-to-analog converter (DAC). We propose the use of a neural network (NN) for digital pre-distortion (DPD) to mitigate the quantization and bandlimited impairments from a DAC in such systems. We experimentally validate our approach with a 64 Gbaud 8-level pulse amplitude modulation (PAM-8) signal. We examine the NN-DPD training with both direct and indirect learning methods. We compare the performance with typical Volterra, look-up table (LUT) and linear DPD solutions. We sweep regimes where nonlinear quantization becomes more prominent to highlight the advantages of NN-DPD. The proposed NN-DPD trained via direct learning outperforms the Volterra, LUT and linear DPDs by almost 0.9 dB, 1.9 dB and 2.9 dB, respectively. We find that an indirect learning recurrent NN offers better performance at the same complexity as Volterra, while a direct learning recursive NN pushes performance to a higher level than a Volterra can achieve.

Adaptive decision threshold for an optical multipath-interference-impaired short-reach 50-Gbps PAM4 transmission.

The short-reach optical transmission systems based on intensity modulation and direct detection (IM/DD) are gradually evolving into the networks with complex link topologies and connections, especially inside the data center. Multipath interference (MPI) introduces irregular fluctuations in the 4-level pulse amplitude modulation (PAM4) signals and therefore affects the transmission performance. This Letter proposes an adaptive decision threshold (ADT) scheme to dynamically update the decision threshold, which can track the signal fluctuations in real time and mitigate the impact of MPI noise on the transmitted PAM4 signals. Numerical simulation results present that the proposed ADT scheme can significantly improve the bit error rate (BER) performance of the MPI-impaired PAM4 transmission system considering different MPI levels and laser linewidths. In a 50-Gbps PAM4 transmission system, the ADT can improve the MPI tolerance by more than beyond 6 dB when the BER reaches the KP4-forward error correction (FEC) criterion (2.4 × 10-4), presenting a better denoising performance than the existing MPI-mitigation algorithms A1 and A2. Moreover, the ADT scheme offers a lower computation complexity compared with A1 and A2, making it more practical for implementation.

Simultaneous dual-channel data transmission through a multimode fiber via wavefront shaping

The increasing demand for transmission capacity in fiber-optic communications makes multimode fibers (MMFs) attractive by enabling simultaneous multi-channel data transmission. However, inherent mode crosstalk among transmission channels limits its applicability. In this Letter, we propose to overcome this problem via wavefront shaping. By actively modulating the incident light with a special wavefront, not only the transmitted power for a specific channel can be enhanced but also crosstalk among different transmission channels can be mitigated. As a proof of concept, we experimentally demonstrated wavefront shaping assisted dual-channel optical communications through a single MMF. 10 Gbaud four-level pulse amplitude modulation signals were sent through both channels simultaneously, enabling a total bit rate of 40 Gbps. Both channels were analyzed with bit error rates below the hard-decision forward error correction limit, confirming the validity of the proposed scheme.

Simplified THP and M-Log-MAP Decoder Based Faster Than Nyquist Signaling for Intra-Datacenter Interconnect

In this paper, a joint application of simplified Tomlinson-Harashima precoding (THP) and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -log-maximum a posteriori probability ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -log-MAP) decoder at transceivers is proposed for low complexity faster than Nyquist (FTN) 4-ary pulse amplitude modulation (PAM4) signaling. A THP with only two taps is applied at the transmitter for FTN-induced inter-symbol interference elimination and equalization-induced noise amplification issue mitigation, which effectively relieves the burden of the receiver. At the receiver, a low complexity M-log-MAP decoder is equipped for residual impairments cancellation, thus a feedback channel is not required to accurately obtain the channel state information for the transmitter. Taking advantage of this collaborative approach, the proposed scheme performs much better than applying the two schemes separately. We experimentally demonstrate the proposed scheme in 70, 75, and 80GBaud PAM4 systems with a 32-GHz brick-wall filter, corresponding to 0.914, 0.853, and 0.8 FTN compression factor, respectively. The results show that the 80GBaud PAM4 signal is successfully transmitted over 2-km single mode fiber with the bit-error-rate (BER) under the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . It is also observed that the proposed scheme shows about 0.7dB receiver sensitivity improvement compared to the error propagation-free decision feedback equalizer, namely the performance upper bound of the THP, at BER of 7% HD-FEC threshold in a 75GBaud FTN PAM4 system. According to the experimental results, the proposed scheme is a promising solution for bandwidth-starved low-cost intra-datacenter interconnect.

Reconfigurable Generation of PAM-4 Signal Based on Fano Effect for Optical Interconnect Systems

We present a novel low-power architecture for the generation of multilevel pulse amplitude modulation (PAM-4) signal generation. A new structure of a micro-ring resonator based on a 4´4 MMI (Multimode interference) coupler is proposed to control the coupling coefficient and Fano shapes. Based on this structure, a high linearity of the transmission is created, compared with the Mach Zehnder Interferometer (MZI) conventional structure. Here, instead of using directional couplers and MZI as shown in the previous reports, in our method only two 4´4 multimode interference structures on silicon on insulator (SOI) waveguides has been used. The new design is compatible and suitable for the current CMOS fabrication technology. In this work, the special design is as follows: one of two MMI couplers is used to be a micro-ring resonator and two segmented phase shifters are used in the micro-ring resonator to generate 4 levels of the PAM-4 signal. The micro-ring resonator is controlled to work at the over-coupled region, so the Fano effect can be generated. This proposed PAM-4 architecture uses the generated Fano effect, therefore an extreme reduction in power consumption can be achieved. A large fabrication tolerance of ±500 nm and a compact footprint of 10´500 µm2 can be carried-out. The device is simulated and optimally designed using the FDTD (finite difference time domain) and EME (Eigenmode Expansion). This architecture can be useful for optical interconnects and data center network applications.&#x0D;

100Gb/s coherent optical secure communication over 1000 km based on analog-digital hybrid chaos.

In recent years, the transmission capacity of chaotic secure communications has been greatly expanded by combining coherent detection and multi-dimensional multiplexing. However, demonstrations over 1000 km fiber are yet to be further explored. In this paper, we propose a coherent optical secure transmission system based on analog-digital hybrid chaos. By introducing an analog-digital converter (ADC) and a bit extraction into the feedback loop of entropy source, the broadband analog chaos is converted into a binary digital signal. This binary digital signal is then mapped to a 65536-level pulse amplitude modulation (PAM) signal and injected into the semiconductor laser (SL) to regenerate the analog chaos, forming a closed loop. The binary digital signal from the chaos source and the encrypted signal are transmitted via wavelength division multiplexing (WDM). By using conventional digital signal processing (DSP) algorithms and neural networks for post-compensation, long-haul high-quality chaotic synchronization and high-performance secure communication are achieved. In addition, the probability density distribution of the analog chaotic signal is effectively improved by adopting the additional higher-order mapping operation in the digital part of the chaos source. The proof-of-concept experimental results show that our proposed scheme can support the secure transmission of 100 Gb/s quadrature phase shift keying (QPSK) signals over 1000 km of standard single-mode fiber (SSMF). The decrypted bit error rate (BER) reaches 9.88 × 10-4, which is well below the 7% forward error correction (FEC) threshold (BER = 3.8 × 10-3). This research provides a potential solution for high-capacity long-haul chaotic optical communications and fills the gap in secure communications based on analog-digital hybrid chaos.

All-optical wavelength and format conversion of DQPSK-to-PAM4 using DLI and pump assisted NOLM

An all-optical wavelength and format conversion scheme of 20 Gbps differential quadrature phase shift keying (DQPSK)-to-4-ary pulse amplitude modulation (PAM4) signals based on the delay line interferometer (DLI) and the pump assisted nonlinear optical loop mirror (NOLM) is proposed and numerically simulated. The input DQPSK signal is converted into two star-16-ary quadrature amplitude modulation (star-16QAM) signals (namely, star-16QAM-I and star-16QAM-Q) using DLI with the time delay of 1-symbol and different linear phase shifts of π/4 and −π/4, respectively. The star-16QAM signals are converted into the on-off keying (OOK) signals (namely, OOK-I and OOK-Q) using the pump assisted NOLM based on the self-phase modulation (SPM) and the cross-phase modulation (XPM) in the highly nonlinear fiber (HNLF). The two OOK signals can be aggregated into different types of PAM4 signals by adjusting their power ratio (PR) and relative phase shift (RPS) flexibly. Error vector magnitude (EVM) and bit error rate (BER) for the input and output optical signals are calculated to verify the scheme performance. Compared to the DQPSK signal with an input optical signal-to-noise ratio (OSNR) of 30 dB, there are about 2.5 dB penalty of receiver OSNR for the converted OOK-I and OOK-Q signals. For the corresponding aggregated equidistant PAM4 and equiamplitude PAM4 signals, they have about 6.3 dB and 12.1 dB penalty of receiver OSNR. The proposed scheme of DQPSK-to-PAM4 can be deployed at the all-optical gateway to bridge the long-hual and short-reach optical networks.

High-speed electro-optic modulation in topological interface states of a one-dimensional lattice

Electro-optic modulators are key components in data communication, microwave photonics, and quantum photonics. Modulation bandwidth, energy efficiency, and device dimension are crucial metrics of modulators. Here, we provide an important direction for the miniaturization of electro-optic modulators by reporting on ultracompact topological modulators. A topological interface state in a one-dimensional lattice is implemented on a thin-film lithium-niobate integrated platform. Due to the strong optical confinement of the interface state and the peaking enhancement of the electro-optic response, a topological cavity with a size of 1.6 × 140 μm2 enables a large modulation bandwidth of 104 GHz. The first topological modulator exhibits the most compact device size compared to reported LN modulators with bandwidths above 28 GHz, to the best of our knowledge. 100 Gb/s non-return-to-zero and 100 Gb/s four-level pulse amplitude modulation signals are generated. The switching energy is 5.4 fJ/bit, owing to the small electro-optic mode volume and low capacitance. The topological modulator accelerates the response time of topological photonic devices from the microsecond order to the picosecond order and provides an essential foundation for the implementation of large-scale lithium-niobate photonic integrated circuits.

Level Allocation of Four-Level Pulse-Amplitude Modulation Signal in Optically Pre-Amplified Receiver Systems

Level Allocation of Four-Level Pulse-Amplitude Modulation Signal in Optically Pre-Amplified Receiver Systems

A Decision Feedback Equalization Algorithm Based on Simplified Volterra Structure for PAM4 IM-DD Optical Communication Systems

A novel simplifying Volterra structure algorithm is proposed for an intensity modulation direct detection (IM-DD) optical fiber short distance communication system using the decision feedback equalization algorithm (DFE). Based on this algorithm, the signal damage for the four-level pulse amplitude modulation signal (PAM-4) is compensated, which is caused by device bandwidth limitation and dispersion during transmission. Experiments have been carried out using a 25 GHz Electro-absorption Modulated Laser (EML), showing that PAM-4 signals can transmit over 10 km in standard single-mode fiber (SSMF). The 112 Gbps and 128 Gbps signals can reach the error rate threshold of KP4-FEC (BER = 2 × 10−4) and HD-FEC (BER = 3.8 × 10−3), respectively. The simplified principle and process of the proposed Volterra-based equalization algorithm are presented. Experimental results show that the algorithm complexity is greatly reduced by 75%, which provides effective theoretical support for the commercial application of this algorithm.

Adaptive parallel decision deep neural network for high-speed equalization.

The equalization plays a pivotal role in modern high-speed optical wire-line transmission. Taking advantage of the digital signal processing architecture, the deep neural network (DNN) is introduced to realize the feedback-free signaling, which has no processing speed ceiling due to the timing constraint on the feedback path. To save the hardware resource of a DNN equalizer, a parallel decision DNN is proposed in this paper. By replacing the soft-max decision layer with hard decision layer, multi-symbol can be processed within one neural network. The neuron increment during parallelization is only linear with the layer count, rather than the neuron count in the case of duplication. The simulation results show that the optimized new architecture has competitive performance with the traditional 2-tap decision feedback equalizer architecture with 15-tap feed forward equalizer at a 28GBd, or even 56GBd, four-level pulse amplitude modulation signal with 30dB loss. And the training convergency of the proposed equalizer is much faster than its traditional counterpart. An adaptive mechanism of the network parameter based on forward error correction is also studied.

Dynamic analysis of PAM-4 IM/DD OAM-based MGDM transmission enabled by mode-group filter approach.

Mode-group-division multiplexing (MGDM)-based intensity modulation direct detection (IM/DD) transmission is an attractive approach to increase the capacity for short-reach optical communication. In this Letter, a simple but versatile scheme of mode group (MG) filtering for MGDM IM/DD transmission is proposed. The scheme is applicable to any mode basis in the fiber, and it satisfies the needs of low complexity, low power consumption, and high system performance. By employing the proposed MG filter scheme, a total raw bit rate of a 152-Gb/s multiple-input-multiple-output (MIMO)-free IM/DD co-channel simultaneous transmit and receive system based on two orbital angular momentum (OAM) MGs, each carrying a 38-GBaud four-level pulse amplitude modulation (PAM-4) signal, is experimentally demonstrated over a 5-km few-mode fiber (FMF). The bit error ratios (BERs) of the two MGs are below the 7% hard-decision forward error correction (HD-FEC) BER threshold at 3.8×10-3, using simple feedforward equalization (FFE). Furthermore, the dependability and robustness of such MGDM links are of great significance. Thus, the dynamic evaluation of BER and signal-to-noise ratio (SNR) for each MG is tested over 210 minutes under different conditions. In the dynamic cases, all the BER results using the proposed scheme can be below 1×10-3, which further confirms the stability and feasibility of our proposed MGDM transmission scheme.

Generation of high-speed PAM-4 signal with 3-bit DAC enabled by CRD-NS in optical interconnect.

In this paper, we experimentally demonstrated a 2-km high-speed optical interconnection with pulse-shaped pre-equalized four-level pulse amplitude modulation (PAM-4) signal generated by a 3-bit digital-to-analog converter (DAC) with the aid of in-band quantization noise suppression techniques under different oversampling ratios (OSRs) to reduce the influence of quantization noise. The simulation results show that the quantization noise suppression capability of high computational complexity digital resolution enhancer (DRE) is sensitive to taps number of the estimated channel and match filter (MF) response when OSR is sufficient, which will lead to further significant computational complexity increase. To better accommodate this issue, channel response-dependent noise shaping (CRD-NS) which also takes channel response into consideration when optimizing quantization noise distribution is proposed to suppress the in-band quantization noise instead of DRE. Experimental results show that about 2 dB receiver sensitivity improvement can be achieved at the hard-decision forward error correction (HD-FEC) threshold for 110 Gb/s pre-equalized PAM-4 signal generated by 3-bit DAC when the traditional NS technique is replaced by the CRD-NS technique. Compared to the high computational complexity DRE technique, in which channel response is also considered, negligible receiver sensitivity penalty is observed for 110 Gb/s PAM-4 signal, when the CRD-NS technique is utilized. Considering both the system cost and bit error ratio (BER) performance, the generation of high-speed PAM signal with 3-bit DAC enabled by the CRD-NS technique is regarded as a promising scheme for optical interconnection.

Support 16384QAM mobile fronthaul based on 3 amplitude quantization Delta-Sigma Modulation

We experimentally demonstrated 16384-ary quadrature amplitude modulation (16384QAM) mobile fronthaul based on 3-amplitude quantization high-pass Delta-sigma Modulation (DSM) in the O band intensity-modulated direct detection (IM/DD) system. By using 3-amplitude quantized high-pass DSM, we successfully quantize the analog waveform of intermediate frequency (IF) Orthogonal Frequency Division Multiplexing (OFDM) signal into 3-Level Pulse Amplitude Modulation (PAM3) signal, and 1 GHz bandwidth 16384-QAM and 1024QAM are successful transmission in 20Gb/s IM/DD link. 0.63% Error Vector Magnitude (EVM) of 16384QAM and 0.7% EVM of 2048QAM are achieved after 25km SSMF and 50km SSMF transmission, respectively. The signal-to-noise ratio (SNR) of the retrieved OFDM signal is 43 dB, which shows that the proposed scheme can support 16384QAM mobile fronthaul network.

Silica micro-rod resonator-based Kerr frequency comb for high-speed short-reach optical interconnects.

Conventional data center interconnects rely on power-hungry arrays of discrete wavelength laser sources. However, growing bandwidth demand severely challenges ensuring the power and spectral efficiency toward which data center interconnects tend to strive. Kerr frequency combs based on silica microresonators can replace multiple laser arrays, easing the pressure on data center interconnect infrastructure. Therefore, we experimentally demonstrate a bit rate of up to 100 Gbps/λ employing 4-level pulse amplitude modulated signal transmission over a 2 km long short-reach optical interconnect that can be considered a record using any Kerr frequency comb light source, specifically based on a silica micro-rod. In addition, data transmission using the non-return to zero on-off keying modulation format is demonstrated to achieve 60 Gbps/λ. The silica micro-rod resonator-based Kerr frequency comb light source generates an optical frequency comb in the optical C-band with 90 GHz spacing between optical carriers. Data transmission is supported by frequency domain pre-equalization techniques to compensate amplitude-frequency distortions and limited bandwidths of electrical system components. Additionally, achievable results are enhanced with offline digital signal processing, implementing post-equalization using feed-forward and feedback taps.

C-band 100-GBaud PS-PAM-4 transmission over 50-km SSMF enabled by FIR-filter-based pre-electronic dispersion compensation.

Chromatic dispersion (CD) is always an obstacle to C-band high-speed intensity modulation and direct detection (IM/DD) transmissions, especially with a fiber reach of > 20 km. To reach beyond net-100-Gb/s IM/DD transmission over 50-km standard single mode fiber (SSMF), we for the first time present a CD-aware probabilistically shaped four-ary pulse amplitude modulation (PS-PAM-4) signal transmission scheme with a FIR-filter-based pre-electronic dispersion compensation (FIR-EDC) for C-band IM/DD transmission system. With the help of the FIR-EDC at the transmitter, 100-GBaud PS-PAM-4 signal transmission at 150-Gb/s line rate and 115.2-Gb/s net rate over 50-km SSMF is realized with only feed-forward equalization (FFE) at the receiver side. The superiority of the CD-aware PS-PAM-4 signal transmission scheme over other benchmark schemes has been successfully verified by experiments. Experimental results show that 24.5% improvement of system capacity is obtained by the FIR-EDC-based PS-PAM-4 signal transmission scheme in comparison to the FIR-EDC-based on-off keying (OOK) signal transmission scheme. Compared with the FIR-EDC-based uniform PAM-4 signal transmission scheme or the PS-PAM-4 signal transmission scheme without EDC, the capacity improvement obtained by the FIR-EDC-based PS-PAM-4 signal transmission scheme becomes more profound. The results show the potential and feasibility of such CD-aware PS-PAM-4 signal transmission scheme applied in CD-constrained IM/DD datacenter interconnects.

Ultrahigh-Speed Optical Interconnects With Thin Film Lithium Niobate Modulator

The exponential growth in data-driven applications like artificial intelligence and metaverse poses an increasing demand for integrated optical interconnects with efficient and cost-effective high-capacity solutions. For short-reach optical interconnects dominated by intensity-modulated direct detection (IM-DD), high-speed and large-capacity transmission enabled by high-bandwidth electro-optic devices is always highly desired. In this article, based on a thin film lithium niobate (TFLN) modulator with a 3-dB bandwidth larger than 110 GHz fabricated by ourselves, we experimentally demonstrate ultrahigh-speed optical interconnects with probabilistic shaping pulse amplitude modulation (PS-PAM) signals. With linear pre-equalization and Volterra nonlinear equalizer (VNLE), a 120 Gbaud PS-PAM16 signal with a bit rate up to 408 Gb/s is generated. Compared to the regular-PAM8 signal with the same bit rate of 360Gb/s, the PS-PAM signal demonstrates a receiver sensitivity gain of 2.2 dB. With the help of VNLE, 384.5 Gb/s PS-PAM16 signal can be transmitted over 500 m standard single-mode fiber (SSMF) at the C band. The ultra-broadband TFLN enabled transmission provides a promising solution for future 400G per wavelength data-center interconnects.

Application Research of CRZ-4PAM Optical Access Signals

In this paper, the generation and transmission of chirped return-to-zero 4-ary pulse amplitude modulation (CRZ-4PAM) signals in passive optical networks (PONs) is proposed and investigated. The optical spectra, time-domain phase and chirp change diagrams of CRZ-4PAM signals are measured, and their eye-diagrams before and after transmission are also analyzed. We demonstrate that CRZ-4PAM signals after transmission over 20km single mode fiber (SMF) at 10Gb/s can be directly detected by using one photo detector, and the original data can also be restored by one M-ary threshold detector and one PAM sequence decoder. The results demonstrate that CRZ-4PAM downlink signals can be received effectively at the BER value of 10-6, and exhibits 2.4 dB receiver power penalty compared with the back to back case.