Raman amplifiers are being deployed in almost every new long-haul and ultralong-haul fiber-optic transmission systems, making them one of the first widely commercialized nonlinear optical devices in telecommunications. This paper reviews some of the technical reasons behind the wide-spread acceptance of Raman technology. Distributed Raman amplifiers improve the noise figure and reduce the nonlinear penalty of fiber systems, allowing for longer amplifier spans, higher bit rates, closer channel spacing, and operation near the zero-dispersion wavelength. Lumped or discrete Raman amplifiers are primarily used to increase the capacity of fiber-optic networks, opening up new wavelength windows for wavelength-division multiplexing such as the 1300 nm, 1400 nm, or short-wavelength S-band. As an example, using a cascade of S-band lumped amplifiers, a 20-channel, OC-192 system is shown that propagates over 867 km of standard, single-mode fiber. Raman amplifiers provide a simple single platform for long-haul and ultralong-haul amplifier needs and, therefore, should see a wide range of deployment in the next few years.

Raman amplifiers for telecommunications

This paper reviews recent progress in broad-band Raman amplifiers for wavelength-division-multiplexed (WDM) applications. After the fundamentals of Raman amplifiers are discussed in contrast to erbium-doped fiber amplifiers, a new technique called "WDM pumping" is introduced to obtain ultrabroad and flat gain in Raman amplifiers only using WDM diode pumps. The design issues of this technique are then developed to realize outstanding performances such as 100 nm of flat gain bandwidth, 0.1 dB flatness over 80 nm, and so forth.

Ultrabroad-band Raman amplifiers pumped and gain-equalized by wavelength-division-multiplexed high-power laser diodes

This paper reports comprehensive experimental results on a femtosecond code-division multiple-access (CDMA) communication system test bed operating over optical fiber in the 1.5 /spl mu/m communication band. Our test bed integrates together several novel subsystems, including low-loss fiber-pigtailed pulse shapers for encoding-decoding, use of dispersion equalizing fibers in dispersion compensated links for femtosecond pulse transmission and also in femtosecond chirped pulse amplification (CPA) erbium doped fiber amplifiers (EDFAs), and high-contrast nonlinear fiber-optic thresholders. The individual subsystems are described, and single-user system level experimental results demonstrating the ability to transmit spectrally encoded femtosecond pulses over a 2.5-km dispersion compensated fiber link followed by decoding and high contrast nonlinear thresholding are presented.

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A femtosecond code-division multiple-access communication system test bed

Fiber-optic multi-band transmission (MBT) aims at exploiting the low-loss spectral windows of single-mode fibers (SMFs) for data transport, expanding by $\sim\!11\times$ the available bandwidth of C-band line systems and by $\sim\!5\times$ C+L-band line systems’. MBT offers a high potential for cost-efficient throughput upgrades of optical networks, even in absence of available dark-fibers, as it utilizes more efficiently the existing infrastructures. This represents the main advantage compared to approaches such as multi-mode/-core fibers or spatial division multiplexing. Furthermore, the industrial trend is clear: the first commercial C $+$ L-band systems are entering the market and research has moved toward the neighboring S-band. This article discusses the potential and challenges of MBT covering the ITU-T optical bands O  $\rightarrow$  L. MBT performance is assessed by addressing the generalized SNR (GSNR) including both the linear and non-linear fiber propagation effects. Non-linear fiber propagation is taken into account by computing the generated non-linear interference by using the generalized Gaussian-noise (GGN) model, which takes into account the interaction of non-linear fiber propagation with stimulated Raman scattering (SRS), and in general considers wavelength-dependent fiber parameters. For linear effects, we hypothesize typical components’ figures and discussion on components’ limitations, such as transceivers,’ amplifiers’ and filters’ are not part of this work. We focus on assessing the transmission throughput that is realistic to achieve by using feasible multi-band components without specific optimizations and implementation discussion. So, results are meant to address the potential throughput scaling by turning-on excess fiber transmission bands. As transmission fiber, we focus exclusively on the ITU-T G.652.D, since it is the most widely deployed fiber type worldwide and the mostly suitable to multi-band transmission, thanks to its ultra-wide low-loss single-mode high-dispersion spectral region. Similar analyses could be carried out for other single-mode fiber types. We estimate a total single-fiber throughput of 450 Tb/s over a distance of 50 km and 220 Tb/s over regional distances of 600 km: $\sim\!10\times$ and 8× more than C-band transmission respectively and $\sim\!2.5\times$ more than full C+L.

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Assessment on the Achievable Throughput of Multi-Band ITU-T G.652.D Fiber Transmission Systems

Femtosecond encoder-decoders and ultrafast nonlinear thresholders and their integration in a femtosecond code division multiple access communication system test-bed

The present invention is aimed at providing an optical repeater using Raman amplification that can Raman amplify WDM signal lights propagated through a plurality of transmission systems, with high excitation efficiency and stability. To this end, an optical repeater using Raman amplification of the present invention multiplexes and demultiplexes excitation lights supplied from a plurality of excitation light sources in a star coupler and then supplies the resultant demultiplexed lights to Raman amplification media of a plurality of transmission systems via optical multiplexers, to thereby Raman amplify the WDM signal lights of the respective transmission systems. Moreover, by feedback controlling the power of each excitation light based on the result of monitoring the WDM signal light power after Raman amplification in each transmission system, a stabilized Raman amplification operation with reduced dispersion in the power of each excitation light output from the star coupler is realized.

Optical repeater using raman amplification, wavelength division multiplexed light transmission system, excitation light supply method and excitation light control method for raman amplification

We have proposed a "beyond 100 nm" DRA in which pump-and-signal wavelength-interleaving allocation (PSI) is applied. PSI-DRA has advantages such as low optical noise, low pump power, and a simple configuration without WDM signal multiplexing and de-multiplexing. The bandwidth that we achieved was 136.6 nm, which was 95% of the full bandwidth between 1496 nm and 1640 nm. We confirmed no additional penalty after 120 km transmission in a experiment with our proposed PSI-DRAs.

A broadband distributed Raman amplifier for bandwidths beyond 100 nm

We have demonstrated 240 channel, 10 Gbit/s WDM transmission over 7404 km using all Raman amplifier repeaters with a continuous signal bandwidth of 74 nm and dispersion-managed fiber with our proposed dispersion map for transmission systems based on distributed Raman amplifiers (DRA).

2.4 Tbit/s WDM transmission over 7400 km using all Raman amplifier repeaters with 74 nm continuous single band

The present invention has an object to provide an optical transmission technology using Raman amplification wherein a supervisory signal transferred among a plurality of optical transmission apparatuses is superimposed on a main signal light by using a pumping light for Raman amplification To this end, a Raman amplifier applied with the optical transmission method of the present invention, is provided with a supervisory signal superimposing section for superimposing a supervisory signal transferred among the optical transmission apparatuses on at least one of a plurality of pumping lights of different wavelengths to be supplied to a Raman amplification medium via a multiplexer from a pumping light generating section, thereby transmitting the main signal light propagated through the Raman amplification medium to be Raman amplified, which is superimposed with the supervisory signal

Optical transmission method and optical transmission system utilizing Raman amplification

Erbium-doped fiber amplifiers (EDFAs) with broad signal bandwidth, low NF and high output power, and transmission characteristic improvement using chromatic dispersion management and modulation format are required for long-haul, wavelength-division multiplexing (WDM) transmission systems. We performed a 24 5.3-Gbit/s WDM transmission experiment over 7828 km using gain equalization to compensate for the asymmetry of the EDFA gain characteristics, RZ modulation format, and both pre- and post-compensation of chromatic dispersion.

Naomasa Shimojoh

Papers

Optical repeater using raman amplification, wavelength division multiplexed light transmission system, excitation light supply method and excitation light control method for raman amplification

A broadband distributed Raman amplifier for bandwidths beyond 100 nm

2.4 Tbit/s WDM transmission over 7400 km using all Raman amplifier repeaters with 74 nm continuous single band

Optical transmission method and optical transmission system utilizing Raman amplification

128-Gbit/s WDM transmission of 24 5.3-Gbit/s RZ signals over 7828 km using gain equalization to compensate for asymmetry in EDFA gain characteristics