Nonlinearity mitigation in phase-sensitively amplified optical transmission links

The fundamental limitations in fiber-optic communication are caused by optical amplifier noise and the nonlinear response of the optical fibers. The quantum-limited noise figure of erbium-doped fiber amplifier (EDFA) or any phase-insensitive amplifier is 3 dB. However, the noise added bythe amplification can be reduced using phase-sensitive amplifier (PSA) whose quantum-limited noise figure is 0 dB. PSAs can also compensatefor the nonlinear distortions from the optical fiber with copier-PSA implementation. At the transmitter, a copier which is nothing but aphase-insensitive amplifier is used to create a conjugated copy of the signal. The signal and idler are co-propagated in the span, experiencingcorrelated nonlinear distortions. The nonlinear distortions are reduced by the all-optical coherent superposition of the signal and idler in thePSA.In this work, an analytical investigation is performed for the nonlinearity mitigation using the PSAs, by calculating the residual nonlineardistortion after the coherent superposition in PSAs. The optical bandwidth and the dispersion map dependence on the nonlinearity mitigationin the PSAs are analytically and experimentally studied. A modified Volterra nonlinear equalizer (VNLE) is used to reduce the residual nonlineardistortions after PSAs. Experiments were performed to show that PSAs can mitigate cross-phase modulation (XPM), which was evidentby observing the constellation diagrams. The maximum allowed launch power increase was also measured to quantify the XPM mitigation. Tothe best of our knowledge, this is the first experiment that showed the mitigation of XPM in a phase-sensitively amplified transmission link.Also, the effectiveness in mitigating self-phase modulation (SPM) and XPM using a PSA is studied

Phase-sensitive amplifiers for nonlinearity impairment mitigation in optical fiber transmission links

The fundamental limitations in fiber-optic communication are caused by optical amplifier noise and the nonlinear response of the optical fibers. The quantum-limited noise figure of erbium-doped fiber amplifiers (EDFAs) or any phase-insensitive amplifier is 3 dB. However, the noise added by the amplification can be reduced using phase-sensitive amplifiers (PSAs), whose quantum-limited noise figure is 0 dB. PSAs can also compensate for the nonlinear distortions from the optical transmission fiber in the copier-PSA implementation. At the transmitter, a copier which is nothing but a phase-insensitive amplifier, is used to create a conjugated copy of the signal. The signal and idler are then copropagated in the fiber link, experiencing correlated nonlinear distortions. The nonlinear distortions are reduced by the all-optical coherent superposition of the signal and idler in the PSA.In this work, an investigation is made for the nonlinearity mitigation using the PSAs, by calculating the residual nonlinear distortion after the coherent superposition in a copier-PSA link. The nonlinearity mitigation efficiency in PSA links is studied with respect to modulation formats, symbol rates and number of wavelength channels. The effectiveness of nonlinearity mitigation is found to increase with higher-order modulation formats. However, the efficiency of nonlinearity mitigation decreases with increasing number of wavelength channels and increasing symbol rate resulting in larger residual nonlinear distortions. A modified Volterra nonlinear equalizer (VNLE) is implemented to reduce the residual nonlinear distortions after PSAs in single- and multi-channel PSA links. Cross-phase modulation mitigation using PSAs is also demonstrated

Modulation format dependence on transmission reach in phase-sensitively amplified fiber links

Optical Society of America under the terms of the OSA Open Access Publishing Agreement We quantify the maximum transmission reach for phase-insensitive amplifier (PIA) and phase-sensitive amplifier (PSA) links with different modulation formats and show that the maximum transmission reach increase (MTRI) when using PSAs compared to PIAs is enhanced for higher-order modulation formats. The higher-order modulation formats are more susceptible to smaller phase rotations from nonlinearities, and PSAs are efficient in mitigating these smaller phase distortions. Numerical simulations were performed for single- and multi-span PIA and PSA links with single and multiple wavelength channels. We obtain a significant enhancement in the MTRI with PSAs compared to PIAs when using higher-order modulation formats for both the single- and multi-channel systems in single- and multi-span links. We verify the enhancement with a single-span, single-channel system experiment. We also demonstrate, for the first time, a 64-QAM modulation format fiber transmission in phase-sensitively amplified link, with a 13.3-dB maximum allowable span loss increase compared to a phase-insensitively amplified link

Phase-sensitively amplified wavelength-division multiplexed optical transmission systems

The throughput and reach in fiber-optic communication links are limited by in-line optical amplifier noise and the Kerr nonlinearity in the optical transmission fiber. Phase-sensitive amplifiers (PSAs) are capable of amplifying signals without adding excess noise and mitigating the impairments caused by the Kerr nonlinearity. However, the effectiveness of Kerr nonlinearity mitigation depends on the dispersion pre-compensation in each span. This paper investigates dense wavelength-division multiplexed PSA-amplified links using joint processing with a less complex digital domain Volterra nonlinear equalizer at the receiver. Both numerically and with experiments, it is shown that this significantly reduces the impact of the dispersion pre-compensation in each span. Also, with simulations, a substantial improvement in transmission reach is demonstrated for PSA links

Comparison of Physical Realizations of Multidimensional Voronoi Constellations in Single Mode Fibers

We investigate experimentally and numerically the impact of using different fiber dimensions to spread out the 32-dimensional Voronoi constellations. We find similar performance in experiments and less than 5.4% reach improvements in long-haul transmission simulations by spreading the constellation dimensions over time slots compared to wavelength

Waveguide tapering for improved parametric amplification in integrated nonlinear Si3N4 waveguides

In this paper, we propose and numerically investigate waveguide tapering to improve optical parametric amplification in integrated nonlinear Si3N4 circuits. The phase matching condition of parametric amplification changes along the length of uniform Si3N4 waveguides, due to the non-negligible propagation loss, potentially causing peak-gain wavelength shifts of more than 20 nm. By tapering the waveguide width along propagation, we can achieve a 2.5 dB higher maximum parametric gain thanks to the improved phase matching, which can also broaden the amplification bandwidth. Therefore, the length of an optimally tapered Si3N4 waveguide can be 23% shorter than a uniform one in the case of a 3.0 dB/m propagation loss and a single continuous-wavelength pump. Quasi-continuous tapers are efficient to approximate continuous ones and might simplify the fabrication of long tapered nonlinear Si3N4 waveguides, which are promising for optical signal processing and optical communications

Frequency-comb-calibrated swept-wavelength interferometry

Lasers are often used to characterize samples in a non-destructive manner and retrieve sensing information transduced in changes in amplitude and phase. In swept wavelength interferometry, a wavelength-tunable laser is used to measure the complex response (i.e. in amplitude and phase) of an optical sample. This technique leverages continuous advances in rapidly tunable lasers and is widely used for sensing, bioimaging and testing of photonic integrated components. However, the tunable laser requires an additional calibration step because, in practice, it does not tune at a constant rate. In this work, we use a self-referenced frequency comb as an optical ruler to calibrate the laser used in swept-wavelength interferometry and optical frequency domain reflectometry. This allows for realizing high-resolution complex spectroscopy over a bandwidth exceeding 10 THz. We apply the technique to the characterization of low-loss integrated photonic devices and demonstrate that the phase information can disentangle intrinsic from coupling losses in the characterization of high-Q microresonators. We also demonstrate the technique in reflection mode, where it can resolve attenuation and dispersion characteristics in integrated long spiral waveguides

Physical Realizations of Multidimensional Voronoi Constellations in Optical Communication Systems

Abstract:Multidimensional geometric shaping has been shown to outperform uniform quadrature amplitude modulation (QAM) in optical communication systems but the complexity of symbol decision and bit mapping can often be significant as dimensionality increases. In this paper, a low-complexity geometric shaping method based on multidimensional lattices is investigated both in experiments and simulations. The modulation formats designed based on this method are called Voronoi constellations (VCs) and we study them in 8, 16, and 32 dimensions. We obtain transmission reach improvements of up to 22 and 70% for VCs compared to 4QAM and 16QAM, respectively, in nonlinear long-haul fiber transmission. Moreover, we compare different physical realizations of multidimensional VCs over wavelengths, polarizations, and time slots in both the Gaussian and nonlinear fiber channels. We demonstrate that different physical realizations perform similarly in the fiber-optic back-to-back channel. However, in long-haul transmission systems, spreading the dimensions over time slots can increase the transmission reach up to 4% compared to wavelengths and polarizations. Furthermore, the mutual information and generalized mutual information are estimated and compared to QAM formats at the same spectral efficiencies

Experimental Demonstration of 8-Dimensional Voronoi Constellations with 65,536 and 16,777,216 Symbols

We experimentally demonstrate high-cardinality, low-complexity Voronoi constellations based on the E8 lattice over multiple time slots with OSNR and launch power gains of up to 1.7 and 2.4 dB for back-to-back and 80 km fiber transmission, respectively, compared to QAM formats