Recirculating Loop Experiments Research Articles

(INVITED)Reconfigurable topology testbeds: A new approach to optical system experiments

Optical transmission systems provide high capacity, low latency and jitter, and high reliability for city-scale networks. Recirculating loop experiments have facilitated the study of signal propagation in long-haul optical transmission systems. However, they are unsuited for developing control and management software for city-scale optical networks with dozens or hundreds of reconfigurable optical add drop multiplexer (ROADM) units, diverse interconnection topologies, and dynamic traffic patterns. Large-scale testbeds can help, but may be inflexible and time- or cost-prohibitive. Reconfigurable testbeds such as COSMOS enable piece-wise emulation of a city-scale network by applying space and wavelength switching, dual-use software-defined networking (SDN) controllers, and comb sources, while digital twin models enable software emulation. Results from the development of a digital twin for COSMOS are presented for optical amplifiers and stimulated Raman scattering (SRS) including both analytical and machine learning (ML) models.

Performance Monitoring for Live Systems with Soft FEC and Multilevel Modulation

Performance monitoring is an essential function for margin measurements in live systems. Historically, system budgets have been described by the Q-factor converted from the bit error rate (BER) under binary modulation and direct detection. The introduction of hard-decision forward error correction (FEC) did not change this. In recent years technologies have changed significantly to comprise coherent detection, multilevel modulation and soft FEC. In such advanced systems, different metrics such as (nomalized) generalized mutual information (GMI/NGMI) and asymmetric information (ASI) are regarded as being more reliable. On the other hand, Q budgets are still useful because pre-FEC BER monitoring is established in industry for live system monitoring. The pre-FEC BER is easily estimated from available information of the number of flipped bits in the FEC decoding, which does not require knowledge of the transmitted bits that are unknown in live systems. Therefore, the use of metrics like GMI/NGMI/ASI for performance monitoring has not been possible in live systems. However, in this work we propose a blind soft-performance estimation method. Based on a histogram of log-likelihood-values without the knowledge of the transmitted bits, we show how the ASI can be estimated. We examined the proposed method experimentally for 16 and 64-ary quadrature amplitude modulation (QAM) and probabilistically shaped 16, 64, and 256-QAM in recirculating loop experiments. We see a relative error of 3.6%, which corresponds to around 0.5 dB signal-to-noise ratio difference for binary modulation, in the regime where the ASI is larger than the assumed FEC threshold. For this proposed method, the digital signal processing circuitry requires only a minimal additional function of storing the L-value histograms before the soft-decision FEC decoder.

Impact of Random Group Delay Ripple on the Cascadability of Wavelength Select Switches for Nyquist-Pulse-Shaped Signals

We report on the impact that the random group delay ripple of wavelength select switches has on Nyquist-pulse-shaped signals and on the cascadability of wavelength select switches (WSSs) for use as wavelength-based routers within optical communication networks. We show how to simulate the appropriate WSS filter response that accounts for the random group delay ripple and use this within a numerical simulator to predict the performance of communication networks based on WSSs. We find that the random group delay ripple could lead to large bit error rates in recirculating loop experiments, whereby the signal sees the same group delay upon each pass through the loop; though, as in a realistic network, if independent WSSs with uncorrelated group delay ripples are concatenated together then we predict that the system performance would be limited by the accumulated amplified spontaneous emission noise in the network. These results indicate the suitability of WSSs to construct large scale networks with wavelength-based routing functionality extending over 30 switching nodes.

Experimental Investigation of Crosstalk Penalties in Multicore Fiber Transmission Systems

We experimentally study the impact of crosstalk in multicore fibers, using polarization-multiplexed quadrature phase-shift keying signals. Using a 7-core fiber, we perform single-span transmission experiments, where the level of crosstalk to the core under test can be varied. We find the penalty in the required optical signal-to-noise ratio (OSNR), compared to a system with no crosstalk, for different signal-to-crosstalk ratios and at different pre-forward-error-correction target bit-error rates (BERs). We show that, for a 1-dB penalty, a 15.7-dB signal-to-crosstalk ratio can be tolerated at BER = 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . We also perform recirculating loop experiments with varying amount of crosstalk per span to find the impact on the achievable transmission distance.

Cascadability and reshaping properties of a saturable absorber inserted inside a RZ transmission line for future 160-Gbit/s all-optical 2R-regenerators

In this prospective work, we analyze the behavior of a quantum-well microcavity saturable absorber component cascaded into a 100-km SMF RZ transmission line in order to annihilate the ghost-pulse phenomenon in the following simplified “…010101…” 160-Gbit/s 2-bit pattern at 1555nm. Recirculating-loop experiments show a maximal ghost-pulse extinction up to 11.6dB as well as an intensity extinction ratio enhancement higher than 6dB over at least 800km of propagation.

Impact of the in-line dispersion-compensation map on four-wave mixing (FWM)-impaired optical networks

The impact of the dispersion-compensation map on four-wave mixing-impaired nonreturn-to-zero modulated wavelength-division-multiplexing systems is investigated. Dedicated worst-case scenarios are found analytically and confirmed by recirculating loop experiments.

Filter concatenation penalties for 10-Gb/s chirped transmitters suitable for WDM metropolitan area networks

The impact of filter concatenation effects on the performance of wavelength-division-multiplexing (WDM) metropolitan area transparent optical networks utilizing cost-effective 10-Gb/s transmitters is investigated with recirculating loop experiments. We considered directly modulated lasers and electroabsorption modulator integrated distributed feedback WPM lasers (electroabsorption-distributed feedback) as cost-effective 10-Gb/s transmitters. The filter concatenation performance for thin-film multilayer interference filters designed for dense WDM networks with both 200- and 100-GHz channel spacing is studied.

Study of system performance in a 107-km dispersion-managed recirculating loop due to polarization effects

We investigate the polarization evolution for both signal and noise in two 107-km recirculating loops with polarization-dependent loss per round-trip of 0.35 dB and less than 0.1 dB, respectively. When the system is optimized, in the first case, both signal and noise are polarized, while in the second case, the signal tends to depolarize due to the noise. We experimentally measured and theoretically simulated the Q factor distribution, which is far from what is expected for straight-line systems, after 5000 km in the second case. We also suggest a simple method for obtaining the same Q distribution in recirculating loop experiments as expected in straight-line experiments.

Polarisation-independent InP push-pullMach-Zehnder modulator for20 Gbit/s soliton regeneration

A new polarisation-independent InP Mach-Zehnder interferometer provides adjustable phase/intensity optical modulation under independent 20 GHz electrical control of the electrodes. Validation of the component as a regenerator for soliton systems is made through recirculating loop experiments. Error free transmission at 20 Gbit/s over more than 40000 km is obtained in push-pull mode for comparison with 8000 km under single-electrode control.

System design of filter-guided soliton transmission considering amplitude noise and timing jitter

The system design strategy of basing soliton transmission on the signal frequency sliding technique is discussed while considering amplitude noise and timing jitter. The dependence of system performance on various parameters associated with signal frequency sliding are investigated at given bit rates and transmission distances; the signal-to-noise ratio (SNR) due to amplitude noise is theoretically estimated. These estimated are well supported by the results of recirculating loop experiments at 10 Gb/s. The total SNR is then maximized by optimizing the fiber dispersion, through the tradeoff between the effect of amplitude noise and that of timing jitter.

Dispersion-managed solitons in a fiber loop with in-line filtering

We investigate both numerically and experimentally soliton propagation in a fiber loop with dispersion management, in-line filters, and frequency shifting. More than 90% of the fiber in the loop is in the normal-dispersion regime, but the net dispersion is anomalous. Stable pulses in the loop have an enhanced power relative to solitons in a fiber with uniform dispersion equal to the loop's path-averaged dispersion. Because the loop's path-averaged dispersion is small, the in-line filtering and the frequency shifting play an important role in pulse shaping. Recirculating loop experiments that demonstrate stable pulse propagation over 28,000 km are consistent with results from computer modeling.

Timing jitter due to carrier linewidth of laser-diode pulse sources in ultra-high speed soliton transmission

We theoretically analyze the timing jitter due to both the carrier phase noise of laser-diode (LD) pulse sources and the Gordon-Haus effect in soliton transmission. A formula is derived for the timing jitter in terms of the carrier linewidth, one of the measurable parameters common to all types of LD pulse sources. The transmission distance restricted by the timing jitter is analyzed, and the carrier linewidths required for ultra-long distance and ultra-high speed soliton transmission are estimated as well. Recirculating loop experiments at 10 Gb/s are demonstrated using two pulse sources; a gain-switched DFB-LD and a sinusoidally driven monolithically integrated MQW-DFB-LD/MQW-EA modulator, which have different carrier linewidths due to their different pulse formation processes. The difference in the carrier linewidths of the two pulse sources is measured by the proposed technique which is based on the optical heterodyne method. The observed difference between the two pulse sources in terms of timing jitter accumulation and timing jitter reduction with optical bandpass filters for the two pulse sources well support the theoretical predictions. >

Eight-wavelength, densely-spaced coherent WDM recirculating-loopexperiments at 2.5 Gbit/s over 600 km

Eight-wavelength, densely-spaced coherent CPFSK recirculating-loop experiments at 2.5 Gbit/s have been demonstrated over 600 km through 30 cascaded EDFAs. The WDM signals were densely packed in a bandwidth of only 1.4 nm, indicating the effective use of optical frequency resources by coherent schemes.

2.5 Gbit/s, 4 channel, 1200 km densely spaced coherentWDM recirculating-loop experiments

2.5 Gbit/s, 4 channel, densely spaced coherent WDM recirculating-loop transmission experiments were performed over 1200 km using unequal channel spacings to demonstrate the alleviation of the effect of accumulated gain ‘unflatness’ of EDFAs.

Bit-error-rate characterization of IM-DD ultralong-distance optical communication systems with Er-doped fiber amplifiers using a recirculating loop

The authors have evaluated the bit error-rate characteristics of the IM-DD ultralong-distance optical communication systems employing Er doped fiber amplifiers. A 2.5- and 5-Gb/s IM-DD nonreturn-to-zero signal was analyzed by recirculating-loop experiments. They used a fiber loop of four Er-doped fiber amplifiers and three spans of dispersion-shifted single-mode fibers for the experiments. A 16,000-km transmission at 2.5 Gb/s and an 11,000-km transmission at 5 Gb/s were successfully achieved. The allowable wavelength-tunable range of the optical transmitter for the 9000-km transmission was also measured at the bit rate of 5 Gb/s. The experimental results show the possibilities and limitations of transoceanic optical communication systems using Er-doped fiber amplifier repeaters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Signal linewidth broadening due to nonlinear Kerr effect in long-haul coherent systems using cascaded optical amplifiers

Signal spectral linewidth broadening due to the interaction between nonlinear Kerr effect and spontaneous emission from optical amplifiers in long-haul coherent optical fiber communication systems using optical amplifiers is examined theoretically and experimentally. By theoretical consideration and recirculating-loop experiments, it is shown that this effect becomes an essential limitation on maximum transmission length in transoceanic coherent optical fiber submarine cable systems. >

Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal spectral linewidth broadening due to the Kerr effect in long-haul coherent optical fibre communication systems is examined theoretically and experimentally. By recirculating loop experiments, it is shown that linewidth broadening becomes serious in a system of more than a few thousand kilometres.

Helium gas flow monitor

In this prospective work, we analyze the behavior of a quantum-well microcavity saturable absorber component cascaded into a 100-km SMF RZ transmission line in order to annihilate the ghost-pulse phenomenon in the following simplified “…010101…” 160-Gbit/s 2-bit pattern at 1555 nm. Recirculating-loop experiments show a maximal ghost-pulse extinction up to 11.6 dB as well as an intensity extinction ratio enhancement higher than 6 dB over at least 800 km of propagation.

The Transport of Radioactive Corrosion Products in High-Temperature Water—I. Recirculating loop Experiments

A stainless-steel loop containing recirculating, pressurized water at 300°C and neutral pH has been used to investigate the processes by which surfaces in nuclear reactor coolant circuits can become contaminated by radioactive corrosion products. By measuring deposition rates of activity onto metals (usually stainless steel) under different conditions and by examining exposed surfaces, it was deduced that the corrosion of the test surface controls the activation. Hence, the traditional phenomenological method of describing activation by first-order deposition and release coefficients is an oversimplification.