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.

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

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.

Analysis of nonlinearity mitigation using phase-sensitive optical parametric amplifiers.

Phase-sensitive optical parametric amplifiers (PSAs) can provide low-noise optical amplification while simultaneously mitigating nonlinear distortions caused by the Kerr effect. However, nonlinearity mitigation using PSAs is affected by link parameters, and imperfect link design results in residual nonlinear distortions. In this paper, we use first-order perturbation theory to describe these residual nonlinear distortions, and develop a way to mitigate them using a modified third-order Volterra nonlinear equalizer (VNLE) in the receiver. Using numerical simulations, we show that our proposed VNLE reduces the residual nonlinear distortions in links using in-line PSAs for several combinations of symbol rates and modulation formats, and can increase the maximum transmission distance by up to 80%. We also perform a proof-of-concept experiment and confirm that our modified VNLE can mitigate the residual nonlinear distortions on a 10-Gbaud 16QAM signal after transmission through a 10×80-km link with in-line PSAs.

Single-photodiode per polarization receiver with signal-signal beat interference suppression through heterodyne detection.

We show that a simplified, single-photodiode per polarization heterodyne receiver is able to directly suppress signal-signal beat interference (SSBI), without the need for cancellation in the digital domain. We characterize performance degradation due to SSBI, and show that a strong LO in the receiver can mitigate SSBI. Transmission of 400 Gb/s-class signals is shown over single fiber spans of up to 160 km, and over field-deployed metropolitan area fiber. These results indicate that a single photodiode can be used to receive complex optical signals in high speed fiber systems without the need for SSBI cancellation in the digital domain.

Phase-sensitive amplifier link with distributed Raman amplification.

We demonstrate long-haul transmission using a hybrid amplifier approach combining distributed Raman amplification and lumped phase-sensitive amplification. Aside from the well-known resulting SNR improvement, distributed Raman amplification is included in an effort to improve the nonlinearity mitigation capability of the phase-sensitive amplifiers. When changing from phase-insensitive operation to phase-sensitive operation in a link employing distributed Raman amplification, the transmission reach at BER = 10-3 is increased from 15 to 44 spans of length 81 km while simultaneously increasing the optimal launch power by 2 dB.

Cycle-slip-less low-complexity phase recovery algorithm for coherent optical receivers.

We propose and experimentally validate a blind phase recovery algorithm based on tracking low-frequency components of the phase noise, which we call "filtered carrier-phase estimation (F-CPE)." Tracking only the low-frequency components allows F-CPE to reduce the computational complexity by using a frequency-domain equalizer and to simplify the partitioning of a 16 quadrature amplitude modulation (16QAM) constellation. Further, this approach eliminates cycle slips by suppressing the impact of amplified spontaneous emission on phase noise estimation. The experimental results demonstrate cycle-slip-free operation for 15 and 32 GBd 16QAM signals. Additionally, the proposed method showed similar or better sensitivity compared with the blind-phase-search algorithm, near standard forward error correction thresholds of modern wavelength division multiplexing systems.

All-optical digital-to-analog converter based on cross-phase modulation with temporal integration.

We propose and experimentally demonstrate an all-optical digital-to-analog converter based on cross-phase modulation with temporal integration. The scheme is robust for driving signal noise due to the low-pass filtering feature of the temporal integrator. The proof-of-concept experiment demonstrates the generation of pulse-amplitude modulation (PAM) sequences up to eight levels. The performance of random PAM 2 and PAM 4 signals with different optical signal-to-noise ratios of the binary driving signal is also investigated. The scheme is scalable for high-speed operation with an appropriate dispersion profile of the nonlinear medium.

Optoelectronic method for inline compensation of XPM in long-haul optical links.

We propose an optoelectronic subsystem for inline compensation of XPM-induced phase noise in long-haul optical links. Based on intensity-dependent phase rotation, these compensators are used to suppress XPM on a span-by-span basis. Using a 7-channel hybrid WDM system as a test case, our simulations show that this technique is effective at suppressing XPM in dispersion managed and unmanaged links for various transmission distances.

Experimental demonstrations of dual polarization CO-OFDM using mid-span spectral inversion for nonlinearity compensation.

We experimentally demonstrate fiber nonlinearity compensation in dual polarization coherent optical OFDM (DP CO-OFDM) systems using mid-span spectral inversion (MSSI). We use third-order nonlinearity between a pump and the signal in a highly nonlinear fiber (HNLF) for MSSI. Maximum launch powers at FEC threshold for two 10 × 80-km 16-QAM OFDM systems were increased by 6.4 dB at a 121-Gb/s data rate and 2.8 dB at 1.2 Tb/s. The experimental results are the first demonstration of using MSSI for nonlinearity compensation in any dual polarization coherent system. Simulations show that these increases could support a 22% increase in total transmission distance at 1.2-Tb/s system without increasing the number of inline amplifiers, by extending the fiber spans from 90 to 110 km. When spans of 80 km are used, simulations reveal that MSSI system performance shows less degradation with increasing transmission distance, and an overall transmission distance increase of more than 70% is expected using MSSI.

Phase-coherent lightwave communications with frequency combs.

Fiber-optical networks are a crucial telecommunication infrastructure in society. Wavelength division multiplexing allows for transmitting parallel data streams over the fiber bandwidth, and coherent detection enables the use of sophisticated modulation formats and electronic compensation of signal impairments. Optical frequency combs can replace the multiple lasers used for the different wavelength channels. Beyond multiplexing, it has been suggested that the broadband phase coherence of frequency combs could simplify the receiver scheme by performing joint reception and processing of several wavelength channels, but an experimental validation in a fiber transmission experiment remains elusive. Here we demonstrate and quantify joint reception and processing of several wavelength channels in a full transmission system. We demonstrate two joint processing schemes; one that reduces the phase-tracking complexity and one that increases the transmission performance.

Antibody Treatment against Angiopoietin-Like 4 Reduces Pulmonary Edema and Injury in Secondary Pneumococcal Pneumonia.

Secondary bacterial lung infection by Streptococcus pneumoniae (S. pneumoniae) poses a serious health concern, especially in developing countries. We posit that the emergence of multiantibiotic-resistant strains will jeopardize current treatments in these regions. Deaths arising from secondary infections are more often associated with acute lung injury, a common consequence of hypercytokinemia, than with the infection per se Given that secondary bacterial pneumonia often has a poor prognosis, newer approaches to improve treatment outcomes are urgently needed to reduce the high levels of morbidity and mortality. Using a sequential dual-infection mouse model of secondary bacterial lung infection, we show that host-directed therapy via immunoneutralization of the angiopoietin-like 4 c-isoform (cANGPTL4) reduced pulmonary edema and damage in infected mice. RNA sequencing analysis revealed that anti-cANGPTL4 treatment improved immune and coagulation functions and reduced internal bleeding and edema. Importantly, anti-cANGPTL4 antibody, when used concurrently with either conventional antibiotics or antipneumolysin antibody, prolonged the median survival of mice compared to monotherapy. Anti-cANGPTL4 treatment enhanced immune cell phagocytosis of bacteria while restricting excessive inflammation. This modification of immune responses improved the disease outcomes of secondary pneumococcal pneumonia. Taken together, our study emphasizes that host-directed therapeutic strategies are viable adjuncts to standard antimicrobial treatments.IMPORTANCE Despite extensive global efforts, secondary bacterial pneumonia still represents a major cause of death in developing countries and is an important cause of long-term functional disability arising from lung tissue damage. Newer approaches to improving treatment outcomes are needed to reduce the significant morbidity and mortality caused by infectious diseases. Our study, using an experimental mouse model of secondary S. pneumoniae infection, shows that a multimodal treatment that concurrently targets host and pathogen factors improved lung tissue integrity and extended the median survival time of infected mice. The immunoneutralization of host protein cANGPTL4 reduced the severity of pulmonary edema and damage. We show that host-directed therapeutic strategies as well as neutralizing antibodies against pathogen virulence factors are viable adjuncts to standard antimicrobial treatments such as antibiotics. In view of their different modes of action compared to antibiotics, concurrent immunotherapies using antibodies are potentially efficacious against secondary pneumococcal pneumonia caused by antibiotic-resistant pathogens.