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This article will discuss the collaboratively formed cross-industry open cables concept for characterizing optical performance of undersea cables with the intent of assessing and understanding their capacity potential. The article proposes definitions of two critical nonlinear and linear performance metrics for open cables: GSNR (Gaussian or generalized signal to noise ratio) and SNRASE (Signal to noise ratio amplified spontaneous emission), including effects such as GAWBS (guided acoustic wave Brillouin scattering) and signal droop. Measurement methodologies for these metrics are proposed, with considerations for limitations and impact of the test conditions and characteristics of the transponders used. Expanded definitions are offered to enable variable symbol rate transponders to be used for measurement, with considerations for scaling of SNR values. Considerations for using these metrics for capacity assessment and applying these techniques to concatenated multi-segment systems are introduced. Recommendations on key parameters for system specification, system characterization, and proposals for SNR-based performance budgeting tables are also discussed as foundational elements to enabling accurate estimation of the capacity potential of a subsea open cable.

We consider both physical and economic aspects of submarine transmission to identify optimal designs of submarine space division multiplexed (SDM) systems. We focus on single mode-based SDM systems as the first practical step for implementation of high capacity, low cost-per-unit-capacity systems based on SDM principles. We review power efficiency models in EDFA-based long-haul systems and show existence of optimum SNR. We demonstrate relationship between the power efficiency and cost-per-unit-capacity optimizations for submarine links. We also describe a detailed model that combines physical and economic aspects of the submarine links. The model uses “least amount of technological change” approach based on the existing components and simple scaling rules for repeater and cable designs. The model results suggest that 32-fiber SDM systems are a practical spot for near-future systems close to cost-per-unit-capacity limit of the wet-plant. Further increase of the number of fibers in a cable has diminishing returns for current technologies and cost structure. We also discuss requirements for transponders in terms of spectral efficiency and nonlinearity compensation.

We discuss space division multiplexing as a means of increasing link capacity for power limited transmission in undersea communication. Theoretical study of erbium-doped fiber amplifiers-based space division multiplexing (SDM) transmission shows the existence of optimal values of spectral efficiency, signal-to-noise ratio, and span length that maximize power efficiency. We experimentally confirm the existence of an optimal spectral efficiency and demonstrate power efficient SDM transmission using 12-core multicore fiber. Modulation format, amplifier, and link design are discussed in connection to SDM power efficient transmission.

We demonstrate 70.46 Tb/s transmission over 7,600 km and 71.65 Tb/s transmission over 6,970 km with C+L band EDFAs using a multidimensional coded modulation format with hybrid probabilistic and geometric constellation shaping. The proposed format (4D-PS-9/12-56APSK) is designed to approach Shannon limit and to improve the performance of nonlinearity compensation over conventional and probabilistically shaped 2-D formats. The receiver DSP uses multistage nonlinearity compensation that includes fast least-mean-square equalizer and generalized filter in addition to digital back propagation. The adaptive linear filters are aided by coded modulation decisions. We study the contribution of each algorithm and show an average total nonlinearity compensation benefit of 1.4 dBQ at the designed amplifier power.

We use multicore fiber to demonstrate power efficient, high capacity ultralong-haul spatial division multiplexing (SDM) transmission. Feasibility of 105.1 Tb/s transmission over 14,350 km in a 12-core fiber is shown using a power efficient modulation format and an optimized optical system design. Pump power equivalent to a single 800 mW laser is used to provide amplification for all 12 SDM paths.

The capacity of optical communication systems can be continuously scaled by utilizing spatial division multiplexing (SDM) technology. However, this scaling has significant impact on the required optical and electrical power as the number of EDFAs increase. This constraint is crucial in long-haul undersea optical systems where electrical power for the entire system is fed from the landing points. Successful implementation of SDM in undersea systems will require both signal modulation and amplification technologies with high power efficiency. In this work, we use 8-D coded modulation and simple EDFA design to demonstrate 100 Gb/s transmission over 9750 km with record high power efficiency as defined by the ratio of capacity-distance product to the total amplifier output power.

We demonstrate the existence of an optimal spectral efficiency that maximizes the power efficiency and/or capacity of a power limited erbium-doped fiber amplifier (EDFA)-based space division multiplexing transmission system. Optimal spectral efficiency observed experimentally is in good agreement with a theoretical estimate.

We demonstrate that channel spacing can be reduced to values smaller than the Nyquist channel spacing over transoceanic distance. Modulation memory induced by constrained transmitter bandwidth together with multisymbol detection can reduce intersymbol interference for systems with sub-Nyquist channel spacing. We transmit 198 × 100 G bandwidth constrained polarization-division-multiplexed return-to-zero quaternary phase shift keying channels with 400% spectral efficiency over 6860 km using 52 km spans of 150 μ m2 fiber and simple single-stage erbium-doped fiber amplifiers without any Raman amplification. We also show that 100 G coherent nonlinear performance scales differently with distance on uncompensated dispersion maps compared with direct detection transmission.

This paper investigated the impact of receiver dispersion slope compensation for 40-Gb/s transoceanic transmission over conventional nonzero dispersion shifted fibers. Various differential phase-shift keying (DPSK) modulation formats were experimentally compared at 42.8 Gb/s [to account for forwarded error correction (FEC) overhead] with dispersion slope compensators at the receiver. These transmission measurements were performed in a circulating loop over a transatlantic distance of 6250 km using a variety of channel spacings, relative polarizations, and synchronous modulation techniques. All formats benefited from receiver dispersion slope compensation. For orthogonally polarized channels on 133-GHz spacing, the return-to-zero DPSK (RZ-DPSK) format performed the best; all channels (18 /spl times/ 40 Gb/s) propagated with > 13.5-dB Q-factor and with > 4-dB FEC margin. Whereas for copolarized channels on 100-GHz spacing, carrier-suppressed return-to-zero (CSRZ)-DPSK performed the best; all channels (25 /spl times/ 40 Gb/s) propagated with > 3-dB FEC margin. Moreover, it was shown that parallel launch only suffered a penalty of /spl sim/ 0.2 and /spl sim/ 0.5 dB relative to the orthogonal launch for 133and 100-GHz channel spacing, respectively. Finally, it was demonstrated that copolarized 40 Gb/s RZ-DPSK worked as well as 10 Gb/s RZ-ON-OFF keying (RZ-OOK) for the same spectral efficiency (30%) over the 6250 km of conventional nonzero dispersion shifted fibers (NZ-DSF) originally designed for 10 Gb/s transmission.

This work presents the first field trial using the return-to-zero differential-phase-shift-keying (RZ-DPSK) modulation format. A 96/spl times/10-Gb/s RZ-DPSK field trial was conducted over a 13 100-km optical undersea path by double passing the installed 6550-km underwater link which was deployed with non-slope-matched submarine fibers. All channels performed with more than a 3-dB forward-error correction margin, including channels that accumulated over /spl plusmn/13 000ps/nm of dispersion. It is also shown that the RZ-DPSK format has similar residual dispersion and channel power tolerance for both slope-matched and non-slope-matched fibers. Furthermore, it is demonstrated that the chirped RZ-DPSK format could further improve system performance by 1-2 dB.

Product accumulate (PA) codes were proposed as a class of high-rate low-complexity, capacity-approaching codes on additive white Gaussian noise (AWGN) channels. In this paper, we investigate the performance of the PA codes on intensity modulated optical fiber channels where the amplified spontaneous emission (ASE) noise dominates all other noise sources. We consider binary on-off keying (OOK) modulation and iterative soft-decision message-passing decoding for the PA codes. Three channel models for the ASE noise dominated channel are investigated: asymmetric chi-square, asymmetric Gaussian, and symmetric Gaussian channels (i.e., AWGN). At low signal-to-noise ratios (SNRs), due to the lack of tight bounds, code performance is evaluated using simulations of typical PA coding schemes. For high SNRs that are beyond simulation capabilities, we derive the pairwise error probability of the three channels and explore an average upper bound on the bit-error rate over the ensemble of PA codes. We show that AWGN channels, although fundamentally different from chi-square channels, can serve as a reference to approximate the performance of high-rate PA codes.

We present an experimental investigation of 40 Gb/s transmission technology aimed at an aggregate capacity exceeding 1 Tb/s over multithousand kilometer transmission distances. These studies have been performed using a variety of dispersion management and modulation techniques. The transmission distances investigated range from a minimum of 2000 km, which is considered a regional undersea cable distance, up to a transatlantic distance of 6200 km. Our regional distance experiments were performed using both slope-matched and conventional dispersion maps where the accumulated dispersion becomes large for the edge channels. Dispersion map studies showed that slope-matched fiber performs better than nonslope-matched fiber (NZDSF) beyond 2000 km due to its larger effective area and lower accumulated dispersion slope. The demonstration of 38/spl times/40 Gb/s over 6200 km was the first transoceanic length experiment using 40 Gb/s DWDM channels. This was achieved with a relatively simple amplifier chain that uses only C-band EDFAs. Modulation format studies showed that RZ performs better over transoceanic distance, carrier-suppressed RZ performs better with nonslope-matched fiber at a distance of 2055 km, and prefiltered carrier-suppressed RZ is more suited for higher spectral efficiency. Experimental techniques for high bit-rate experiments are presented.

Nonlinear interaction between signal channels may result in significant performance degradation in transoceanic dense wavelength-division-multiplexing (WDM) transmission systems operated at high bit rates. In this paper, we report that the nonlinear crosstalk for systems with high spectral efficiency can be suppressed if WDM channels are launched with orthogonal relative states of polarization. It is shown that for adjacent channels (that experience the strongest nonlinear interaction) the launched relative state of polarization is maintained over the large distance of low polarization-mode dispersion fiber typically used in undersea systems. Numerical studies, as well as experimental results, are presented for standard and dispersion slope-matched dispersion maps.

The rapid growth of long-haul wavelength-division multiplexing (WDM) fiber-optic telecommunications challenges lightwave system designers to increase the transmission capacity per fiber pair, while reducing the time to market. This makes it essential to have fast and accurate computer modeling tools to aid the systems design. There is a natural engineering tradeoff between simulation speed and accuracy; fast approximations tend to be inaccurate, while the exact treatment of the physical processes affecting transmission are nearly impossible to capture in a time-efficient algorithm. The proof of a successful tool development lies in the comparison of simulation results to transmission system measurements. In this paper, we discuss a new simulation technique based on careful evaluation of the key physical effects that produce system impairments. We show that this new approach can produce fast and accurate simulations of long-haul WDM transmission systems.

We introduce a signal processing approach to compensate for the timing jitter produced by the acoustic effect in soliton communications. The other main sources of timing jitter, the Gordon-Haus effect and the polarization effect, are inherently stochastic. By contrast, the acoustic effect is deterministic and becomes the dominant source of bit error rates in standard soliton systems when the bit rates are more than 10 Gb/s and the transmission distance is more than several thousand kilometers. We exploit the deterministic nature of the acoustic effect to introduce a scheme that predicts the amount of timing jitter as a function of the previous transmitted bits and uses the information to adjust the sampling period of the received soliton pulses. We demonstrate successful application of the scheme by simulations and discuss implementation issues.

We numerically analyze the effectiveness of a nonlinear optical loop mirror (NOLM) based on standard communication fiber with randomly varying birefringence for demultiplexing streams of picosecond pulses at 100 GHz. A broad switching window of about three pulse full-width at half maximum (FWHM) can be obtained. The device performance is defined by the pulse duration, the dispersion of the fiber, and the fiber length. We show that imperfect averaging of the randomly varying birefringence causes amplitude fluctuations on the NOLM transmission curve. We also show that the Raman self-frequency shift does not affect the NOLM switching characteristics at picosecond pulse durations.

One of the main mechanisms that limits the bit rate and transmission distance in soliton communication lines is the so called “long-range” interaction of solitons [1]. The physical reason of this interaction is the electrostrictional excitation of the acoustic waves by light pulses and back-influence of the acoustic wave on solitons [2]. In soliton communication lines the interaction of solitons with the acoustic wave results in the additional to the Gordon-Haus effect [3] timing jitter of solitons [4,5]. Although the timing jitter of solitons can be significantly reduced by guiding filters [6], the detailed investigation of the acoustic effect is still important because the guiding filters cannot totally suppress the timing jitter.

The effect of Raman gain parametric suppression is investigated both theoretically and experimentally. Good agreement between theory and experiment is obtained. The analysis of the effect of Raman nonlinearity on the process of modulation instability in the region of negative group velocity dispersion of fibers is presented. New dependencies for the modulational instability frequency and increment are calculated. >

Dispersion compensation placed at the end of a nonlinear system affects both signal variance and pulse distortion. This letter shows experimentally and theoretically that optimum performance for a wavelength-division-multiplexed test bed of 4870 km is achieved by selecting dispersion compensation to minimize pulse distortion rather than signal variance.

A dispersion management scheme that is capable of supporting both dispersion-managed solitons and nonreturn-to-zero (NRZ) data formats as well as intermediate formats is demonstrated for error-free long-haul transmission at 10 Gb/s in excess of 9500 km. This experimental tool ran be used to directly compare the different data formats. Our results suggest that a format between standard solitons and standard NRZ, such as dispersion-managed solitons or phase-modulated NRZ, may have significant advantages.

We calculate the distribution function of the degree of polarization of an initially polarization scrambled signal propagating long distances in an optical fiber. We show that a significant repolarization can occur for modern-day parameters. We estimate how far the polarization dependent loss must be reduced before repolarization becomes negligible.

We report on stable soliton propagation experiments in a fiber transmission system consisting of more than 90% normal dispersion fiber. The transmission system has a dispersion map much like that used in nonreturn-to-zero (NRZ) pulse transmission but with a path-average anomalous dispersion of +0.1 ps/nm-km. A stable soliton pulse train at 8 GHz has been observed after 28 Mm of propagation in a 108-km recirculating fiber-optic loop. A significant enhancement in the average soliton power required for stable transmission in a dispersion map with alternating signs of dispersion is experimentally demonstrated for the first time. Theoretical modeling of our experiment is in good agreement with our findings.