Abstract:A theoretical investigation of the equalization-enhanced phase noise (EEPN) and its mitigation is presented. We show with a frequency domain analysis that the EEPN results from the non-linear inter-mixing between the sidebands of the dispersed signal and the noise sidebands of the local oscillator. It is further shown and validated with system simulations that the transmission penalty is mainly due to the slow optical frequency fluctuations of the local oscillator. Hence, elimination of the frequency noise bel… Show more
“…In these regimes, the impact falls off inversely proportional to the weighting factor in equations (7–8) which falls off inversely proportional to frequency f as shown in ref. 17. Thus, an upper cut-off frequency f
cutoff can be considered beyond which the impact of frequency noise can be neglected similar to the one derived for white frequency noise in ref.…”
Section: Resultsmentioning
confidence: 99%
“…The received signal output as per eqs (7) and (8) in case of is given by the integral over all delayed versions, each corresponding to a LO sideband, of the demodulated signal. The residual Fourier spectrum of the LO,, can hence be thought of as a weighting factor in eqs (7) and (8) which falls off with the increase in the modulation frequency for the same frequency deviation 17 . Thus the MME also falls off with the increase in modulation frequency.Figure 5( a ) BER vs.…”
Section: Resultsmentioning
confidence: 99%
“…We recently revisited the phenomenon for the white FN case. We showed that EEPN induced penalty is mainly due to the low frequency fluctuations of the laser 17 . We also provided and experimentally validated closed form expressions for parameters essential for system design in the presence of EEPN 18, 19 .…”
Coherent communication networks are based on the ability to use multiple dimensions of the lightwave together with electrical domain compensation of transmission impairments. Electrical-domain dispersion compensation (EDC) provides many advantages such as network flexibility and enhanced fiber nonlinearity tolerance, but makes the system more susceptible to laser frequency noise (FN), e.g. to the local oscillator FN in systems with post-reception EDC. Although this problem has been extensively studied, statistically, for links assuming lasers with white-FN, many questions remain unanswered. Particularly, the influence of a realistic non-white FN-spectrum due to e.g., the presence of 1/f-flicker and carrier induced noise remains elusive and a statistical analysis becomes insufficient. Here we provide an experimentally validated theory for coherent optical links with lasers having general non-white FN-spectrum and EDC. The fundamental reason of the increased susceptibility is shown to be FN-induced symbol displacement that causes timing jitter and/or inter/intra symbol interference. We establish that different regimes of the laser FN-spectrum cause a different set of impairments. The influence of the impairments due to some regimes can be reduced by optimizing the corresponding mitigation algorithms, while other regimes cause irretrievable impairments. Theoretical boundaries of these regimes and corresponding criteria applicable to system/laser design are provided.
“…In these regimes, the impact falls off inversely proportional to the weighting factor in equations (7–8) which falls off inversely proportional to frequency f as shown in ref. 17. Thus, an upper cut-off frequency f
cutoff can be considered beyond which the impact of frequency noise can be neglected similar to the one derived for white frequency noise in ref.…”
Section: Resultsmentioning
confidence: 99%
“…The received signal output as per eqs (7) and (8) in case of is given by the integral over all delayed versions, each corresponding to a LO sideband, of the demodulated signal. The residual Fourier spectrum of the LO,, can hence be thought of as a weighting factor in eqs (7) and (8) which falls off with the increase in the modulation frequency for the same frequency deviation 17 . Thus the MME also falls off with the increase in modulation frequency.Figure 5( a ) BER vs.…”
Section: Resultsmentioning
confidence: 99%
“…We recently revisited the phenomenon for the white FN case. We showed that EEPN induced penalty is mainly due to the low frequency fluctuations of the laser 17 . We also provided and experimentally validated closed form expressions for parameters essential for system design in the presence of EEPN 18, 19 .…”
Coherent communication networks are based on the ability to use multiple dimensions of the lightwave together with electrical domain compensation of transmission impairments. Electrical-domain dispersion compensation (EDC) provides many advantages such as network flexibility and enhanced fiber nonlinearity tolerance, but makes the system more susceptible to laser frequency noise (FN), e.g. to the local oscillator FN in systems with post-reception EDC. Although this problem has been extensively studied, statistically, for links assuming lasers with white-FN, many questions remain unanswered. Particularly, the influence of a realistic non-white FN-spectrum due to e.g., the presence of 1/f-flicker and carrier induced noise remains elusive and a statistical analysis becomes insufficient. Here we provide an experimentally validated theory for coherent optical links with lasers having general non-white FN-spectrum and EDC. The fundamental reason of the increased susceptibility is shown to be FN-induced symbol displacement that causes timing jitter and/or inter/intra symbol interference. We establish that different regimes of the laser FN-spectrum cause a different set of impairments. The influence of the impairments due to some regimes can be reduced by optimizing the corresponding mitigation algorithms, while other regimes cause irretrievable impairments. Theoretical boundaries of these regimes and corresponding criteria applicable to system/laser design are provided.
“…In our previous paper, we presented a theoretical investigation of the EEPN, and its mitigation [12]. We showed with a frequency domain analysis that the EEPN results from the nonlinear inter-mixing between the sidebands of the dispersed signal and the noise sidebands of the local oscillator.…”
Section: Introductionmentioning
confidence: 99%
“…In this letter, a realistic implementation of the approach proposed in [12], to achieve EEPN mitigation, is presented. This is accomplished by utilizing low bandwidth implementation of DCE interferometric hardware and simple DSP processing comprising of an interpolator and an integrator.…”
A method for mitigating local oscillator (LO) phase noise-induced impairment, also known as equalization-enhanced phase noise, in coherent optical systems is discussed. The method is suitable for real-time implementation and requires hardware with a bandwidth much lower than the signal baud rate, even for a system utilizing conventional semiconductor laser as LO. We evaluate the required parameters like interpolation technique, electrical signal-to-noise ratio at digital coherence enhancement (DCE) front end, for long haul transmission links having quadrature phase shift keying and 16-quadrature amplitude modulation formats. We show that the method can be implemented using a low-speed DCE front end and a simple digital linear interpolator with small (<1 dB) implementation penalty even in cases that would otherwise result in error floor.Index Terms-Coherent receivers, quadrature amplitude modulation (QAM), equalization-enhanced phase noise, laser linewidth, optical communication.
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