Carl R. Davidson

Long-period fiber-grating-based gain equalizers

Long-period fiber gratings are used to f latten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3.

Wavelength division multiplexing in long-haul transmission systems

The Erbium-Doped Tiber Amplifier (EDFA) has had a profound impact on the design, operation and performance of transoceanic cable transmission, and is central to the expected proliferation of cable systems. Laboratory experiments have demonstrated 100 Gb/s over transoceanic distances using Wavelength Division Multiplexing (WDM) techniques. These large transmission capacity experiments have resulted from an increased understanding of the effects that can limit performance of WDM systems. Important strides have been made in areas of dispersion management, gain equalization, and modulation formats which have made possible the demonstration of large data transmission capacity. This paper reviews experimental techniques developed to improve the performance of long-haul WDM transmission systems based on the Non-Return-to-Zero (NRZ) format, and other non-soliton methods.

Circulating loop transmission experiments for the study of long-haul transmission systems using erbium-doped fiber amplifiers

A circulating loop transmission experiment is a useful tool for the research and development of long-haul transmission systems that use erbium-doped fiber-amplifier repeaters. Circulating loop techniques, applied to an amplifier chain of modest length, can provide an experimental platform to study a broad range of transmission phenomena for EDFA-based transmission systems. A loop experiment attempts to simulate the transmission performance of a multi-thousand kilometer long system by reusing or recirculating an optical signal through a modest length amplifier chain of tens to hundreds of kilometers. This paper reviews original loop techniques developed to study various parameters of long-haul transmission using erbium-doped fiber-amplifiers repeaters for undersea systems. Transmission experiments were performed at transoceanic distances with 33 and 46 km amplifier spacing, operating at 5 and 10 Gb/s. Experimental data is provided here for various parameters, including bit error ratio, Q-factor, and spectral broadening. >

20 Tbit/s Transmission Over 6860 km With Sub-Nyquist Channel Spacing

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.

Modeling of transoceanic fiber-optic WDM communication systems

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.

Transmission of 96

We demonstrated that simple pre-filtering at the transmitter to constrain the channel bandwidth together with a maximum a posteriori probability (MAP) detection algorithm can significantly improve spectral efficiency (SE). 96 × 100-Gb/s bandwidth-constrained polarization-division-multiplexing return- to-zero-QPSK channels were transmitted with 300% SE over 10 610 km using 52-km spans of 150-μ m 2 fiber and simple single-stage erbium-doped fiber amplifiers without any Raman amplification. We also achieved 400% SE over 4370 km using similar techniques.

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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.

100-Gb/s Bandwidth-Constrained PDM-RZ-QPSK Channels With 300% Spectral Efficiency Over 10610 km and 400% Spectral Efficiency Over 4370 km

A repeater spacing of 240 km was experimentally demonstrated over a distance of 5280 km in an 8/spl times/2.5 Gb/s wavelength-division-multiplexing (WDM) transmission experiment. This long-span length was achieved by a combination of Raman amplification, a locally pumped erbium-doped fiber (EDF), and two remotely pumped EDFs. The total gain obtained from those four gain elements was about 50 dB. The performance demonstrated in this experiment was comparable to that of a system of similar length and capacity using conventional erbium-doped fiber amplifier (EDFA) technology at a repeater spacing of 80 km.

Long-haul 40 Gb/s DWDM transmission with aggregate capacities exceeding 1 Tb/s

We present the first multi-terabit/s transoceanic length transmission demonstration using the RZ-DPSK format at a 10 Gb/s channel data rate. Error-free performance for 373-10 Gb/s RZ-DPSK channels after 11,000 km was demonstrated.

240-km repeater spacing in a 5280-km WDM system experiment using 8 x 2.5 Gb/s NRZ 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.