In this paper, we propose a receiver architecture capable of compensating for a time-varying signal distortion induced by the transmitter analog components. In the proposed architecture, the novel ...transmitter distortion compensating (TDC) block is implemented outside of the feedback control loop of the adaptive equalizer to suppress the feedback delay by optimizing the parameter used in the least mean square (LMS) algorithm. The proposed TDC block is designed for compensating for the DC offset, the IQ amplitude imbalance, the IQ orthogonality error, and the IQ timing skew. Our simulation results show that the proposed scheme significantly reduces the required signal-to-noise ratio penalty from the theoretical limit, which is imposed due to the transmitter components distortion. Furthermore, our theoretical analysis confirms that the delay induced in the feedback loop of the adaptive equalizer determines the upper bound of the LMS step size under the stable condition, hence allowing us to maximize the tracking speed of our receiver.
This paper presents analytical results on longitudinal power profile estimation (PPE) methods, which visualize signal power evolution in optical fibers at a coherent receiver. The PPE can be ...formulated as an inverse problem of the nonlinear Schrödinger equation, where the nonlinear coefficient (and thus signal power) is reconstructed from boundary conditions, i.e., transmitted and received signals. Two types of PPE methods are reviewed and analyzed, including correlation-based methods (CMs) and minimum-mean-square-error-based methods (MMSEs). The analytical expressions for their output power profiles and spatial resolution are provided, and thus the theoretical performance limits of the two PPE methods and their differences are clarified. The derived equations indicate that the estimated power profiles of CMs can be understood as the convolution of a true power profile and a smoothing function. Consequently, the spatial resolution and measurement accuracy of CMs are limited, even under noiseless and distortionless conditions. Closed-form formulas for the spatial resolution of CMs are shown to be inversely proportional to the product of a chromatic dispersion coefficient and the square of signal bandwidth. With MMSEs, such a convolution effect is canceled out and the estimated power profiles approach a true power profile under a fine spatial step size.
A scheme that compensates the waveform distortion induced by nonlinear interchannel crosstalk such as four-wave mixing (FWM) and cross-phase modulation as well as self-phase modulation in ...phase-locked wavelength-division-multiplexing transmission systems is proposed. Reduction of FWM-induced waveform distortion by controlling the phase relationship between neighbouring channels and its cancellation by precompensation is successfully demonstrated
Ultra-wideband (UWB) wavelength division multiplexed (WDM) transmission using high-order modulation formats is one of the key techniques to expand the transmission capacity per optical fiber. For UWB ...systems, the nonlinear interaction caused by inter-band stimulated Raman scattering (SRS) must be considered. Therefore, we have proposed and demonstrated a scheme to optimize the fiber input powers for UWB transmission systems considering the signal power transition caused by the inter-band SRS. We demonstrated a single-mode capacity of 150.3 Tb/s using the proposed power optimization scheme with 13.6-THz UWB in the S -, C -, and L -bands over 40-km transmission. Spectral efficiency of 11.05 b/s/Hz was achieved with 272-channel 50-GHz spaced WDM signals of 45-GBaud polarization division multiplexed 128 quadrature amplitude modulation.
This paper presents a linear least squares method for fiber-longitudinal power profile estimation (PPE), which estimates the optical signal power distribution throughout a fiber-optic link at a ...coherent receiver. The method finds the global optimum in the least squares estimation of the longitudinal power profiles; thus, its results closely match the true optical power profiles and locate loss anomalies in a link with high spatial resolution. Experimental results show that the method achieves accurate PPE with an RMS error of 0.18 dB from OTDR. Consequently, it successfully identifies a loss anomaly as small as 0.77 dB, demonstrating the potential of a coherent receiver in locating even splice and connector losses. The method is also evaluated under WDM conditions with optimal system fiber launch power, highlighting its feasibility for use in practical operations. Furthermore, the fundamental limit for stable estimation and the spatial resolution of least-squares-based PPE are quantitatively discussed in relation to the ill-posedness of the PPE by evaluating the condition number of the nonlinear perturbation matrix.
In polarization-multiplexed digital coherent systems, polarization dependent loss (PDL) is a dominant factor limiting the capacity and distance of fiber-optic transmission systems. In this paper, we ...propose polarization-wise power profile estimation (PPE) based on a linear least squares algorithm to estimate the value and location of excessive PDLs. The method visualizes polarization-wise signal power profiles at a coherent receiver with high sensitivity. We also experimentally demonstrate that PPE can detect PDL in an optical multi-span transmission link using the method. The experiment was performed using dual-polarization transmission over a 3-span × 50-km link, with a PDL resolution of 1-3 dB and a spatial step size of 1 km. In high power conditions, all estimated PDL values agree with actual values with estimation errors of well below 0.5 dB.
Optical transmission links are generally composed of optical fibers, optical amplifiers, and optical filters. In this paper, we present a channel reconstruction method (CRM) that extracts physical ...characteristics of multiple link components such as longitudinal fiber losses, chromatic dispersion (CD), multiple amplifiers' gain spectra, and multiple filters' responses, only from receiver-side (Rx) digital signal processing (DSP) of data-carrying signals. The concept is to reconstruct a virtual copy of an actual transmission channel in the digital domain, where optical fibers and amplifiers are modeled as the split-step Fourier method for the Manakov equation while optical filters are emulated as complex-valued finite impulse response filters. We estimate the model parameters such as losses, CD, gains, and filter responses from boundary conditions, i.e., transmitted and received signals. Experimental results show that, unlike traditional analog testing devices such as optical time-domain reflectometers and optical spectrum analyzers, CRM visualizes multi-span characteristics of fibers, amplifiers, and filters in Rx DSP, and thus localizes anomaly components among multiple ones without direct measurement.
In the optical communication, ultra-wide band (UWB) wavelength division multiplexing (WDM) transmission systems have been gathering considerable attention because it is becoming rapidly difficult to ...increase capacity per optical fiber within conventional limited bandwidths. Our aim is to extend the wavelength for UWB WDM systems from the widely used C or L bands to additional bands such as the S, E, and O bands. For these wavelengths to be used in a practical manner, it is necessary to study how repeater configurations such as multiplexer and optical amplifiers and inter-channel interference caused by stimulated Raman scattering (SRS) will impact optical transmission systems. We study these problems through two UWB WDM experiments. In the first, we demonstrate the first ever 5-band WDM transmission and devise a format for allocating wavelength adaptive modulation to exhaustively use wavelength dependent optical signal-to-noise ratio (OSNR). The experiment setup includes 5 channel signals each allocated in each band and 5-band WDM repeater constructed of WDM coupler, 3-dB couplers, and four type of optical amplifiers. In the second, we investigate the inter-channel interference effect from SRS between 35-channel S- and 40-channel L-band 16QAM signal transmission over 210 km. The experimental results show that there is only optical power transition between S and L bands and that no nonlinear crosstalk interference is observed.
We present a simple nonlinear digital pre-distortion (DPD) of optical transmitter components, which consists of concatenated blocks of a finite impulse response (FIR) filter, a memoryless nonlinear ...function and another FIR filter. The model is a Wiener-Hammerstein (WH) model and has essentially the same structure as neural networks or multilayer perceptrons. This awareness enables one to achieve complexity-efficient DPD owing to the model-aware structure and exploit the well-developed optimization scheme in the machine learning field. The effectiveness of the method is assessed by electrical and optical back-to-back (B2B) experiments, and the results show that the WH DPD offers a 0.52-dB gain in signal-to-noise ratio (SNR) and 6.0-dB gain in optical modulator output power at a fixed SNR over linear-only DPD.
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