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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 discuss technologies that enable high-capacity and power-efficient undersea transmission. We review recent long-haul transmission experiments using conventional single mode fibers and multicore fibers for space division multiplexing.

We review our recent work on ultra-long-haul wavelength division multiplexed (WDM) transmission with high spectral efficiency (SE) employing tight pre-filtering and multi-symbol detection. We start the discussion with a theoretical evaluation of the SE limit of pre-filtered modulation in optical fiber communication systems. We show that pre-filtering induced symbol correlation generates a modulation with memory and thus, a higher SE limit than that of the original memory-less modulation. We also investigate the merits of utilizing the pre-filtering induced symbol correlation with multi-symbol detection to achieve high SE transmission. We demonstrate transoceanic WDM transmission of a pre-filtered polarization division multiplexed return-to-zero quaternary phased shift keying (PDM-RZ-QPSK) modulation format with multi-symbol detection, achieving 419% SE which is higher than the SE limit of the original memory-less PDM-RZ-QPSK format.

Today’s submarine transmission systems carry tens of Tb/s of transmission capacity. The talk will examine specifics of undersea transmission, modern enabling technologies and their limitations. The fundamental issues challenging further long-term transmission capacity growth will be reviewed.

Using single-stage C-band EDFAs equalized to 41 nm, we transmit 152 x 200 Gb/s PDM 16QAM channels with 6.0 b/s/Hz spectral efficiency over 9,748 km enabled by Nyquist spectral shaping and digital back propagation. 76 x 400 Gb/s channels are also transmitted over 8,665 km detecting two 200 Gb/s channels simultaneously using a single wideband receiver. Digital back propagation benefit versus channel pre-emphasis, transmission distance and wavelength are experimentally investigated.

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.

We transmit 160 x 100 G PDM RZ 16 QAM channels with 5.2 bits/s/Hz spectral efficiency over 6,860 km. There are more than 3 billion 16 QAM symbols, i.e., 12 billion bits, processed in total. Using coded modulation and iterative decoding between a MAP decoder and an LDPC based FEC all channels are decoded with no remaining errors.

We investigate the spectral efficiency (SE) limit of pre-filtered modulation in optical fiber communication systems. We show that pre-filtering induced symbol correlation generates a modulation with memory and, thus, a higher SE limit than that of the original memoryless modulation. The SE limits of a series of modulation formats with varying number L of correlated symbols producing 50% of the original bandwidth are evaluated, which approach the SE limit of a modulation format with 50% pre-filtering as L → ∞. We show that the SE limit of a modulation format with L correlated symbols approximates a lower bound for the SE limit of the corresponding pre-filtered format.

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. >

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.

A major source of errors in soliton WDM transmission is collision-induced timing jitter. Because standard transmission lines have lumped amplification, the four-wave mixing fields from soliton collisions grow uncontrollably even when the spacing between amplifiers is much smaller than the soliton collision length, adding amplitude and timing jitter to the jitter due to ideal soliton collisions. Previously, it was proposed to use dispersion-decreasing fiber to reduce the four-wave mixing fields. In this paper, we show that dispersion management is a viable alternative to dispersion-decreasing fiber. Our computational results show that solitons in dispersion-managed fibers can actually suffer less impairment than solitons in ideal dispersion-decreasing fibers with the same average dispersion when the channel spacing is

We transmit 250x100G PDM RZ-16QAM channels with 5.2 b/s/Hz spectral efficiency over 5,530 km using single-stage C-band EDFAs equalized to 40 nm. We use single parity check coded modulation and all channels are decoded with no errors after iterative decoding between a MAP decoder and an LDPC based FEC algorithm. We also observe that the optimum power spectral density is nearly independent of SE, signal baud rate or modulation format in a dispersion uncompensated system.

The first transoceanic transmission experiment using optical differential quadrature phase-shift keying (DQPSK) is presented. The long-haul transmission capabilities of RZ-DQPSK in a 64-channel WDM system have been demonstrated by transmitting 12.5 Gbit/s RZ-DQPSK.