In this paper, a hardware-efficient and robust DSP scheme for the coherent digital subcarrier multiplexing (DSCM) system in presence of transmitter (Tx) in-phase/quadrature (IQ) impairments is introduced and demonstrated. An analytic complex model is given to describe the effects of IQ skew, IQ amplitude and phase imbalance (referred to as IQ mixing) on the DSCM signal. Based on the model, residual frequency offset, polarization and carrier phase noise are jointly recovered using frequency pilot tones (FPT) in presence of IQ mixing. This process avoids the coupling effect of phase noise and Tx IQ mixing in conventional schemes, which allows the Tx IQ mixing to be compensated for by the following joint equalization of conjugate symmetric subcarriers. Compared to the conventional scheme based on widely linear equalizer, the polarization and carrier phase tracking capability of the proposed scheme is robust to IQ mixing. Meanwhile, the proposed scheme can tolerate a wide range of Tx IQ mixing in the presence of residual frequency offset and carrier phase noise. Since polarization crosstalk is eliminated, the signals of the two polarizations can be equalized separately in the proposed scheme, thus reducing the computational complexity by half. For a 4-subcarrier DSCM system with baud rate of 32 Gbaud, experimental results show that the Q-factor degradation of the proposed scheme is less than 0.5dB in the presence of Tx IQ time skew within $\pm$ 15 ps, IQ phase imbalance within $\pm$ 20 degrees or IQ amplitude imbalance within $\pm$ 3 dB. With 1dB Q-factor degradation, the proposed scheme can track polarization variation up to 10 Mrad/s, residual frequency offset of 10 MHz or carrier phase noise with 3dB linewidth of 200 kHz in the presence of Tx IQ mixing. Therefore, the proposed scheme provides a hardware-efficient and robust DSP solution for coherent DSCM targeting access network applications.

Intensity-modulated direct-detection (IM/DD) optical systems are most widely employed in short-reach optical interconnects due to their simple structure and cost-effectiveness. However, IM/DD systems face mixed linear and nonlinear channel impairments, mainly induced by the combination of square-law detection and chromatic dispersion, as well as the utilization of low-cost non-ideal transceivers. To solve this issue, recent years have witnessed a growing trend of introducing machine learning technologies such as neural networks (NNs) into IM/DD systems for channel equalization. NNs usually present better system performance than traditional approaches, and various types of NNs have been investigated. Despite the excellent system performance, the associated high computational complexity is a major drawback that hinders the practical application of NN-based equalizers. This paper focuses on the performance and complexity trade-off of NNs employed in IM/DD systems, presenting a systematic review of the current status of NN-based equalizers as well as a number of effective complexity reduction approaches. The future trends of leveraging advanced NN in IM/DD links are also discussed.

In this Letter, we propose and experimentally validate a high-fidelity and adaptive forward-phase-based vibration sensing using a Wiener filter (WF). In commercial coherent digital subcarrier multiplexing (DSCM) systems under external cavity lasers (ECLs), frequency-domain pilot tones (FPTs) in subcarrier intervals are employed for dynamic frequency offset estimation (FOE), carrier phase estimation (CPE), and polarization demultiplexing. The phase estimated by the CPE module is processed with the WF to achieve high-fidelity extraction of the vibration-induced phase. Compared to traditional bandpass filter (BPF)-based phase extraction, the WF method effectively suppresses sensing background noise caused by laser phase noise, improving sensing signal-to-noise ratio (SSNR). Furthermore, without knowing vibration frequency and waveform information, the WF method can adaptively detect various vibration waveforms, including amplitude-modulated continuous wave (AMCW) and frequency-modulated continuous wave (FMCW). In experiments, for 10 kHz sinusoidal vibration detection, the WF method demonstrated an 8 dB SSNR improvement compared to the BPF method. The experiments further verified that both AMCW and FMCW vibrations can be detected adaptively with the WF method using the same filter parameters as for sinusoidal vibrations.

Optical performance monitoring is vital for enabling dynamically reconfigurable optical networks. This paper introduces a monitoring scheme that is robust to transceiver impairments for coherent digital subcarrier multiplexing (DSCM) systems that simultaneously monitors polarization-dependent loss (PDL), differential group delay (DGD), and chromatic dispersion (CD). In the proposed scheme, a pair of frequency domain pilot tones (FPTs) are inserted into the X and Y polarizations of the transmitted dual-polarization signal. Both the signal and the FPTs endure identical channel impairments during fiber transmission. At the receiver side, the transmitted FPTs are extracted to monitor the channel impairments. We begin by establishing a simplified system model using a single frequency tone to analyze the effects of system impairments on the frequency domain pilots. Based on this model, CD, PDL, and DGD are jointly and accurately estimated. In addition, the influences of temporal, amplitude, and phase mismatches between in-phase (I) and quadrature (Q) components of transceivers on channel impairment monitoring are completely eliminated. This achieves a channel impairment monitoring scheme that is robust to transceiver impairments. Experimental verification shows that the proposed scheme can jointly and accurately estimate a wide range of CD, PDL, and DGD. Additionally, the proposed monitoring scheme demonstrates strong robustness against transceiver impairments, rapid polarization fluctuations, and amplified spontaneous emission (ASE) noise.

Coherent detection with baud-rate sampling is considered an effective solution for power-sensitive short-reach optical interconnects. To effectively address transceiver IQ skew under baud-rate sampling, this paper proposes a hardware-efficient and transceiver IQ skew-robust baud-rate DSP scheme in self-coherent transmission systems, where it can further reduce cost and eliminate the impact of frequency offset. The proposed scheme independently performs timing recovery on the in-phase (I) and quadrature (Q) components, naturally compensating for transceiver IQ skew. Additionally, a tap-configurable simplified baud-rate equalizer significantly reduces DSP complexity after precise timing synchronization. Through detailed theoretical and simulation analyses, we reveal that the coupling effects resulting from transmitter IQ skew and path mismatch between the signal and local oscillator (LO) light are critical determinants of DSP performance. When using narrow-linewidth ECL lasers, the proposed scheme shows strong tolerance to this coupling effect. Notably, with wide-linewidth DFB lasers, strict control of transmitter IQ skew or path mismatch is necessary to prevent performance degradation. Experimental results from 56 GBaud polarization-multiplexed 16QAM transmission demonstrate that the proposed scheme shows strong stability in BER performance over a wide range of transceiver IQ skew compared to existing baud-rate DSP schemes, reducing the real-valued multipliers for the equalizer by approximately 71%. Compared to traditional DSP schemes (@2 SPS), the proposed scheme has a minimal receiver optical power (ROP) sensitivity penalty while reducing complexity by about 85%.

A novel, to the best of our knowledge, frequency offset estimation (FOE) scheme is proposed and demonstrated for coherent digital subcarrier multiplexing (DSCM) systems, where frequency offset (FO) leads to severe filtering damage of subcarriers due to bandwidth limitations. The scheme exploits the symmetry of the signal spectrum, which naturally arises from the transmitter’s frequency response and introduces frequency skewness as a cost function to search for FO. To achieve fast FOE, the false position (FP) method is employed to iteratively compute frequency shifts. Experimental results show that within 80% of the cumulative distribution function (CDF), the proposed scheme (with an FFT size of 2048) achieves FOE in just three iterations. In the 4-subcarrier system, the maximum error is less than 250 MHz, while in the 6- and 8-subcarrier systems, the maximum error is less than 80 MHz. In a 6-subcarrier system, the proposed scheme exhibits stable performance under different FO values and received optical power (ROP), even under severe filtering conditions where existing schemes tend to degrade or fail. Therefore, the proposed scheme provides a robust FOE solution for coherent digital subcarrier multiplexing systems with strong filtering impairments.