The coherent digital subcarrier multiplexing (DSCM) has recently been proposed as a cost saving solution to realize flexible and scalable point-to-multipoint (PTMP) networks. In this work, a hardware-efficient, ultra-fast and joint polarization and carrier phase tracking scheme for the DSCM systems based on frequency domain pilot tones (named as PTJ) is proposed and demonstrated via both simulations and experiments. By extracting the amplitude and phase of the pilot tones with a low-pass filter (LPF), the transmission matrix of random birefringence and carrier phase can be obtained prior to subcarrier demultiplexing and channel equalization. For the hardware-efficient implementation, a simplified LPF based on sliding window averaging (SWA) is proposed to filter out the pilot tones. Compared to previous scheme using pilot tones to track polarization and carrier phase in a cascaded manner (named as PTC), our proposed scheme shows better BER performance over wide polarization rotation rates and reduces more than 90% of computation resources by the SWA and joint tracking of polarization and carrier phase. The experimental results show that the proposed scheme can track polarization rotation rate up to 20 Mrad/s with the OSNR penalty of 0.2 dB at 7% FEC threshold. This indicates that an OSNR sensitivity gain of 2.8 dB is achieved compared with the PTC scheme. The proposed scheme provides a hardware-efficient and fast-tracking solution to recovery the polarization and carrier phase for digital subcarrier multiplexing (DSCM) and polarization division multiplexing (PDM) based optical fiber communication system.

We propose and experimentally demonstrate a dual-polarization (DP) transceiver imbalance monitoring scheme, which is applicable for the coherent system using square QAM signals. The scheme can separate/monitor various kinds of transceiver imbalances and has technical advantages of multi-dimension, wide monitoring range, and modulation-format-transparency. In the proposed scheme, a polarization scrambler is configured between transmitter (Tx) and receiver (Rx). After coherent detection, the proposed digital signal processing (DSP) modules are implemented to monitor transceiver imbalance based on the received signals. To simplify the DSP modules, the implementation sequence of DSP modules is in reverse order compared to the order in which the sequence of impairments is introduced. Firstly, Godard timing error detection (TED) and Gram-Schmidt orthogonalization procedure (GSOP) is used to monitor and compensate Rx imbalances. After compensating the Rx imbalances, Tx imbalance monitoring is implemented. Complex/real maximum likelihood independent component analysis (ML-ICA) and Godard-TED are used to monitor Tx imbalance. Finally, the amplitude ratios of the final output signal and Tx in-phase/quadrature (IQ) amplitude imbalance are fed back to mitigate the interaction of Tx imbalances on the signal power and improve monitoring accuracy. The transceiver IQ and x-polarization/y-polarization (XY) imbalances are separated for the first time by using the frequency offset (FO) naturally existing in the system and the continuous polarization rotation induced by a polarization scrambler. The separation method combined with the reverse algorithm design and amplitude ratio feedback mechanism makes the proposed scheme achieve the most multi-dimensional imbalance monitoring by far. The simulation and experimental results verify that the proposed scheme is modulation-format-transparent and can separate/monitor multi-dimensional transceiver imbalances within a wide range.

This article presents a novel scheme based on underwater optical wireless communication and positioning (UOWC-UOWP) for an underwater wireless sensor network (UWSN). A dual-hop system is adopted to extend the limited coverage area of the UWSN with mobile nodes, consisting of a grade-index plastic optical fiber (GI-POF) link and a wireless link. Specifically, the mini-LED offers high-speed and flexible uplink transmission and the high-power LED enables both downlink transmission and positioning. The time frame design and code division multiplexing access (CDMA) support multi-user signal transmission. Proof-of-concept experiments with GI-POF and wireless link demonstrate the feasibility of the proposed scheme, which offers full-duplex communication, high-precise positioning and extended distance. To the best of our knowledge, the UOWC-UOWP convergent system is proposed for the first time. It has great potential to support high-capacity transmission between mobile nodes for future underwater transmission scenarios.

In this paper, integrated scheme of enhanced vibration-sensing and digital subcarrier multiplexing (DSCM) communication based on frequency-domain pilot tones (FPTs) is proposed and demonstrated experimentally. In the scheme, FPTs are employed for multiple receiver digital signal processing (DSP) modules including frequency offset estimation (FOE), carrier phase estimation (CPE) and polarization demultiplexing. Meanwhile, with the recovered phase after CPE, the vibration-induced phase is extracted for sensing with band-pass filter within a frequency range of 7 kHz to 50 kHz. Thanks to two FPTs, dynamic FOE performance is enhanced under the frequency drift of 100 kHz ECLs compared to the conventional FOE methods, such as Max-Fast Fourier Transform (Max-FFT) and Viterbi-Viterbi (VV) algorithm. This enhancement can effectively suppress the background noise of the recovered phase after CPE, which inherently can subsequently improve the vibration sensing ability at a low frequency. In the experiment, sensing signal-to-noise ratio (SSNR) at 10 kHz is improved by 10 dB with the aid of FPTs. As a result, the vibration of frequency down to 10 kHz is successfully detected based on forward phase-based sensing in integrated DSCM communication system using commercial 100 kHz ECLs. The proposed integrated solution is compatible with the existing optical networks employing commercial ECLs to support intelligent operation and maintenance.

We have proposed and demonstrated a novel switchable single-longitudinal-mode (SLM), dual-wavelength erbium-doped fiber laser (DWEDFL) assisted by Rayleigh backscattering (RBS) in a tapered fiber in a ring laser configuration. The RBS feedback in a tapered fiber is a key mechanism as linewidth narrowing for laser output. A compound laser cavity ensured that the EDFL operated in the SLM state and a saturable absorber (SA) is employed to form a gain grating for both filtering and improving wavelength stability. The fiber laser can output dual wavelengths simultaneously or operate at single wavelength in a switchable manner. Experiment results show that with the proper SA, the peak power drift was improved from 1–2 dB to 0.31 dB and the optical signal to noise ratio was higher than 60 dB. Under the assistance of RBS feedback, the laser linewidths are compressed by around three times and the Lorentzian 3 dB linewidths of 445 Hz and 425 Hz are obtained at 1550 nm and 1554 nm, respectively.

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.