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(a) The block diagram of the proposed J-CPR algorithm. (b) Illustration of a CS event and the detection of the CS. The number of subcarriers is 4, and only the phase estimations in X polarization are plotted.
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In this work we numerically and experimentally investigate the impairments related to laser phase noise in coherent digital subcarrier multiplexing (SCM) systems. Based on the evaluation in both back-to-back and transmission scenarios and the performance comparison between single carrier (SC) and SCM systems, we show that even though SCM systems su...
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High bit rates optical communication systems pose the challenge of their tolerance to linear and nonlinear fiber impairments recently,the use of electronic processing for the mitigation of signal distortion in optical communication has attracted increasing interest due to its low cost and size relative to alternative optical techniques and the pote...
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... In this scenario, a digital subcarrier multiplexing (DSCM) system is studied by researchers extensively. By dividing a high symbol rate single carrier (SC) into multiple low symbol rate subcarriers and multiplexing these subcarriers in the frequency domain by DSP, the DSCM systems have high tolerances for CD [1,2], EEPN, and fiber nonlinearity [3][4][5][6][7]. These features are favored by large capacity and long-distance transmission. ...
... Additionally, CDCs on subcarriers are also beneficial to reducing EEPN because EEPN originates from local laser phase noise passing through the CDC in receiver DSP. The more dispersion the CDC compensates, the more severe EEPN would be [3]. In the meantime, FOE in a DSCM system should be implemented before subcarrier demultiplexing, as the subcarrier demultiplexing algorithm would fail with a large frequency offset (FO) presented. ...
A digital subcarrier multiplexing (DSCM) system has been proposed as a possible solution for large capacity and long-distance coherent optical transmissions due to its high tolerances for chromatic dispersion (CD), equalization-enhanced phase noise (EEPN) and fiber nonlinearity. In a DSCM receiver, for subcarrier-demultiplex to occur properly, frequency offset estimation (FOE) must be implemented before demultiplexing. It is beneficial to decrease complexity and EEPN by compensating CD on each subcarrier. Therefore, a high CD tolerance is indispensable for the FOE algorithm in a DSCM receiver. However, the mainstream blind FOE algorithms for single-carrier systems, such as the 4th power fast Fourier transform algorithm, could not work for DSCM systems. To deal with this challenge, a pilot tone-based FOE algorithm with high CD tolerance is proposed and verified using simulations and offline experiments. The final estimation accuracy of about 10 MHz of the proposed two-stage FOE is achieved at low computational complexity. Simulations and offline experiments show that DSCM systems with the proposed algorithm have a 0.5~1 dB Q-factor improvement over Nyquist single-carrier systems.
... Then power normalization is used to control the power of the optical signal, and for simplicity, the power values of the two channels are set to be the same. The laser phase noise, which is modeled by a wiener process [28], is loaded before the two optical carriers being multiplexed into the optical fiber channel that is composed of standard single-mode fiber (SSMF) and EDFA. The main fiber channel parameters are shown in Table I. ...
Recently, data-driven fiber channel modeling methods based on deep learning have been proposed in optical communication system simulations. We investigate a new data-driven method based on the deep neural network (DNN) to model the nonlinear fiber channel with the characteristics of attenuation, chromatic dispersion, amplified spontaneous emission noise, self-phase modulation (SPM), and cross-phase modulation (XPM). Demonstration in multiple dimensions, including constellations, optical waveforms, spectra, and the normalized mean square error, shows that DNN can approach the transfer function of the fiber channel accurately. Additionally, the DNN shows good generalization for modulation formats and wavelength schemes. Besides, the time complexity of DNN-based method for modeling nonlinear fiber channel is reduced significantly (96.5%) compared to the conventional model-driven method, which is based on the split-step Fourier method. This work demonstrates that the DNN can model accurately the nonlinear fiber channel that takes account of both SPM and XPM. Therefore, it can contribute to the application of data-driven methods in modern optical communication system simulations and designs.
... IQ skew, IQ imbalance, phase error, and IQ crosstalk were used to examine typical transceiver impairments. We also evaluated the effectiveness of the proposed method by testing 29-GBaud dual-SC PM-64QAM transmission, because the penalty from laser phase noise becomes more severe as the symbol rate decreases [14], [15]. On the other hand, an SC signal shows a relatively low impact from equalizationenhanced phase noise (EEPN) after transmission, but EEPN is outside the scope of this paper; instead, the details are described in ref. [15]. ...
... We also evaluated the effectiveness of the proposed method by testing 29-GBaud dual-SC PM-64QAM transmission, because the penalty from laser phase noise becomes more severe as the symbol rate decreases [14], [15]. On the other hand, an SC signal shows a relatively low impact from equalizationenhanced phase noise (EEPN) after transmission, but EEPN is outside the scope of this paper; instead, the details are described in ref. [15]. The 29-GBaud signals were separated by 1.8-GHz guard bands. ...
High-order modulation formats with high symbol rates are promising candidates for meeting the growing traffic requirements of digital coherent transmission. In using these modulation formats, the impact of transceiver impairment on the signal quality becomes significant. In this paper, we propose an 8×2 widely linear (WL) multiple-input multiple-output (MIMO) equalizer with independent complex filtering on IQ signals to mitigate linear transceiver impairments. Furthermore, we extend the equalizer for digital subcarrier (SC) multiplexing. Accordingly, a 16×4 WL MIMO equalizer is also proposed to mitigate linear transceiver impairments, through application to a pair of SC inputs. Experimental results show that, for transmission of a 58-GBaud polarization-multiplexing 64 quadrature amplitude modulation (PM-64QAM) single-carrier signal, the proposed method effectively compensates for various receiver impairments and also mitigates transmitter (Tx) impairments. Q-penalties of 0.1 dB up to a 10-ps skew, 2.5-dB gain imbalance, 7.2-degree phase error, and –15-dB crosstalk in Tx were achieved for 100-km standard single-mode fiber transmission. Moreover, for a 29-GBaud PM-64QAM dual-SC signal, the proposed 16 × 4 WL MIMO equalizer shows transceiver impairment mitigation performance similar to that in the single-carrier case, even though dual-SC transmission is less tolerant of laser phase noise.
... A single-line, highly tunable, low linewidth, and high power source is highly desirable for applications such as spectroscopy, interferometry, and metrology [1][2][3][4], as well in communications [5][6][7][8][9]. They may also find uses as optical tweezers [10], for the detection of gravitational waves [11], and more. ...
We propose and implement a tunable, high power and narrow linewidth laser source based on a series of highly coherent tones from an electro-optic frequency comb and a set of 3 DFB slave lasers. We experimentally demonstrate approximately 1.25 THz (10 nm) of tuning within the C-Band centered at 192.9 THz (1555 nm). The output power is approximately 100 mW (20 dBm), with a side band suppression ratio greater than 55 dB and a linewidth below 400 Hz across the full range of tunability. This approach is scalable and may be extended to cover a significantly broader optical spectral range.
... Therefore, the correlation between the GAWBS phase noise of the signal and the PT decreases as the group delay increases due to the chromatic dispersion, resulting in imperfect compensation of the GAWBS phase noise. It has been found that the influence of chromatic dispersion (CD) on the laser phase noise can be mitigated with a dispersion management scheme [24]. However, the influence of CD on GAWBS phase noise is still inevitable even with dispersion management, since the GAWBS noise is randomly generated in space and time in the transmission fiber. ...
We describe experimental and numerical results regarding the influence of chromatic dispersion in optical fibers on guided acoustic-wave Brillouin scattering (GAWBS) phase noise compensation with a pilot tone (PT). We compared the compensation performance for GAWBS phase noise generated in an ultra-large-area fiber (ULAF) where DULAF = 21 ps/nm/km with that in a dispersion-shifted fiber (DSF) where DDSF = -1.3 ps/nm/km and found that the performance depends strongly on chromatic dispersion. The numerical analysis shows that the group delay between the signal and PT caused by chromatic dispersion reduces the GAWBS noise correlation between them, which degrades the compensation performance for GAWBS phase noise. It is clarified that a condition for effective GAWBS compensation is that the group delay between the signal and PT should be less than half the period of the GAWBS phase noise component.
... A single-line, highly tunable, low linewidth, and high power source is highly desireable for applications such as spectroscopy, interferometry, and metrology [1][2][3][4], as well in communications [5][6][7][8][9]. They may also find uses as optical tweezers [10], for the detection of gravitational waves [11], and more. ...
We propose a simple approach to implement a tunable, high power and narrow linewidth laser source based on a series of highly coherent tones from an electro-optic frequency comb and a set of 3 DFB slave lasers. We experimentally demonstrate approximately 1.25 THz (10 nm) of tuning within the C-Band centered at 192.9 THz (1555 nm). The output power is approximately 100 mW (20 dBm), with a side band suppression ratio greater than 55 dB, and a linewidth below 400 Hz across the full range of tunability. This approach is scalable and may be extended to cover a significantly broader optical spectral range.
... This causes a finite linewidth, so that their relative phase drifts with time results in a phenomenon commonly referred to as phase noise [31]. The phase fluctuation t ( ) of the lasers, which is often quantified by the laser linewidth , is generally be modeled as a Wiener process [32,33]: ...
... = H t t T ( ) ( ) n is the impulse response of the fiber channel with only CD effects where the relative time delay T n between the phase-locked subchannels caused by the CD-induced walk-off effect can be expressed as [33,34]: ...
... In order to distinguish this unique phase-drift phenomenon from the conventional CPN in phase-locked superchannel systems, we rename it here as "dispersion-induced phase offset (DIPO)". Note that this DIPO effect is similar to the dispersionenhanced phase noise (DEPN) phenomenon previously discussed in direct [35,36] and reduced-guard-interval coherent [34,[37][38][39][40] optical detection orthogonal frequency-division multiplexing (DDO-OFDM & RGI-CO-OFDM) and digital subcarrier multiplexing (SCM) [33,41] systems. ...
Phase-locked WDM superchannel transmission systems have been demonstrated to effectively provide reduced DSP complexity. Nonetheless, the subchannels may be affected by laser and nonlinear phase noises, which can be effectively compensated using joint carrier phase recovery (JCPR) techniques. In this work, for the first time, we demonstrated via extensive numerical simulations that the subchannels experience intra-channel dispersion-induced phase-offset (DIPO), which significantly degrade the phase coherence and hence performance of JCPR schemes. In addition, we described in detail and identified the origin of DIPO as the interaction between the LO laser linewidth and the dispersive walk-off effect among the subchannels after long-haul dispersion-unmanaged fiber transmissions. Comprehensive simulation results revealed that the channel spacing, fiber length and the number of subchannels within the superchannel enhances the DIPO. Consequently, the effect of DIPO on JCPR performance is severe for all outer subchannels than the inner subchannels. Finally, we proposed and demonstrated a modified JCPR where simulation results showed that the BER limit for soft-decision FEC could still be achieved in a 50 GHz-spaced 5-channel 45 Gbaud 16-QAM phase-locked WDM superchannel transmission system over a 3000 km fiber link with phase noise effects from both laser linewidth and fiber nonlinearities.
... For a 200-kHz linewidth the penalties are considerably higher. Joint-sub-band phase recovery can alleviate the impact of phase noise in MSB systems [41], [42], [43], but this solution may impair the parallel nature of this type of implementation. Finally, it should be noted that, although the phase noise penalties in systems with a high number of sub-bands can be non-negligible, they can be further suppressed with lower linewidth lasers, or higher symbol rates, which is a trend in high-bit-rate systems. ...
Mode-multiplexed optical transmission is subject to mode coupling and potentially large differential mode delays. In most recent implementations, these effects are compensated for at the receiver by complex adaptive multiple-input multiple-output (MIMO) equalizers. Although frequency-domain MIMO equalization requires a moderate complexity compared to time-domain equalization, the long required fast Fourier transforms (FFTs) may face implementation issues. In this paper, we evaluate an alternative transceiver architecture based on sub-band partitioning, implemented by filter banks, which enables concurrent time-domain equalization. Single-carrier (SC) and multi-sub-band (MSB) mode division multiplexing (MDM) transmission are simulated using frequency-domain equalization (FDE) and time-domain equalization (TDE), respectively. Their performance is compared in terms of static transmission performance, channel tracking capability, phase noise tolerance, and computational complexity. The results indicate that compared with an equivalent SC solution, the MSB architecture provides a high degree of parallelism at the cost of a penalty of 0.7-to-1.3 dB for a laser linewidth of 25-to-100 kHz and a moderate increase in complexity.
... Therefore, the possibility of phase slips in the unwrapped phase estimates renders simple phase averaging ineffective. In [17], the authors propose a method for compensating these phase slips by comparing the phase estimates of a particular subcarrier with the average of all other subcarriers. The performance enhancements are promising for reasonably small linewidth symbol time products (1 × 10 −4 ). ...
We experimentally demonstrate pilot aided compensation of relative phase noise (RPN) in a distributed Raman amplified coherent fiber optic system with forward pumping. This technique effectively eliminates the error floor that arises when conventional phase noise compensation algorithms track RPN, especially in the context of higher order modulation formats and low baud rate signals. Extrapolating the experimental results, we also demonstrate through measurement and simulation, the effectiveness of pilot aided RPN compensation in a 25 Gbaud, 16-QAM modulated digital-subcarrier system with 2.5 Gbaud and 5 Gbaud subcarriers.
... superchannel, the variance of EEPN which is proportional to the baud rate is expected to decrease compared to single carrier superchannels [26]. ...
We propose a simplified digital phase noise compensation technique for a Nyquist pulse shaped digital subcarrier multiplexed (DSCM) coherent optical transmission system, employing an optical frequency comb based on Quantum dot passive mode locked laser (QD-PMLL). Our results show that the impact of dominant common mode phase noise can be efficiently compensated at the receiver by digitally mixing the data sideband with the complex conjugate of the residual carrier component. This digital mixing technique resulted in better BER performance compared to the conventional m th power Viterbi-Viterbi algorithm for QPSK and blind phase noise compensation for 16-QAM modulation formats especially in the presence of large phase noise. To this end, exploiting the mutual coherence between the mode locked comb lines of QD-PMLL, we numerically demonstrate its potential applicability as a transmission source for coherent optical superchannel transmission.
