Laser Frequency Noise in Coherent Optical Systems: Spectral Regimes and Impairments

Abstract

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.

Digital nonlinearity compensation in high-capacity optical communication systems considering signal spectral broadening effect

“…It is worth mentioning that the phase noise contributions of both transmitter and LO lasers were neglected in the above studies. In practical optical transmission systems, the laser phase noise will interact with electronic dispersion compensation modules in both linear and nonlinear compensation schemes, and this will lead to an effect of equalisation enhanced phase noise (EEPN) 39 – 41 . The performance of high-capacity optical communication system will be further degraded by the EEPN, and the distortions will be more serious for outer channels than the central channel 42 .…”

Phase Noise Effects on Phase-Sensitive OTDR Sensors Using Optical Pulse Compression

“…where E(t) is a periodic complex function representing the PPA or the LFM modulations, ω 0 is the center radial frequency and φ(t) represents the laser's phase noise. The stochastic properties of φ(t) and their characterization, in narrow-linewidth laser sources, have been studied extensively in recent years [17], [18], [19], [20], [21], [22]. While these properties differ significantly from one type of laser to another, it is generally accepted that φ(t) can be modeled as a sum of terms, each with a different Power Spectral Density (PSD).…”

Refined method to extract frequency-noise components of lasers by delayed self-heterodyne

“…the field of coherent optical communication [23][24][25] as lasers with lower linewidths allow for advanced modulation formats capable of increased transmission rates. As an example, transmitting 64-QAM signals has been reported to require effective linewidths of 1.2 kHz at 40 Gbit/s [26].…”