Unrepeatered Submarine Links Benefit From All-Raman Technology By Daryl Chaires A worldwide increase of broadband penetration has created a renewed interest in fiber optic network builds and upgrades of existing links. These builds and upgrades include both terrestrial and submarine networks that support a worldwide exchange of information across public and private networks. New builds and upgrades are required to support efficient communications between detached regions and provide previously underserved areas with access to vast amounts of content located anywhere in the world. Prior consideration of builds and upgrades in many of these areas could not be financially justified. Today’s economic justification has been made easier by the recent introduction of technologies that enable more bandwidth to be transmitted even greater distances on all fiber types, giving service providers better options for new builds and upgrades.
achieved by implementing distributed Raman amplification (DRA) to improve the noise performance of their systems. In an unrepeatered application, DRA is placed in-line with receive erbium doped fiber amplifiers to effectively reduce the amplified spontaneous emission (ASE) noise contribution of the amplifiers. This improves the system noise performance while also increasing the receive channel input power level, leading to an increase of transmission distance. The result is an incremental improvement of distance. However, it falls short of maximizing the transmission distance. While distance improvements are important for new systems, operators of unrepeatered submarine links also require increased capacities for existing links. The addition of Raman amplification to existing systems cannot overcome the limited 33 nm spectral window of erbium doped fiber amplifiers.
For transport, all-Raman technology provides unprecedented reach for unrepeatered submarine DWDM systems. Commercially available systems, now allow unrepeatered transmission of 450 km with near term enhancements supporting 500 Km. Just as importantly, the technology helps service providers to extend the life of their current investment by significantly increasing the capacity of existing routes. All-Raman technology provides amplification over a much broader range of wavelengths compared to the Erbium-doped fiber amplifiers (EDFAs), used on many of the existing routes. Amplification over a wide spectrum provides flexibility for wavelength allocations resulting in increased capacity over a larger array of fiber types. In fact, an all-Raman DWDM system is uniquely capable of breathing new life into old fiber routes. It is the best option for maximizing capacity on DWDM challenged DSF and LS fibers. Without the performance enhancements of an all-Raman system, service providers would be forced to prematurely abandon low capacity systems in favor of more efficient new builds. It would simply be too costly to operate these links while also installing new links to support higher capacity demands.
Maximum benefit for high-capacity DWDM transmission on terrestrial and unrepeatered submarine routes is only achieved through the implementation of all-Raman technology. An all-Raman system utilizes properties of distributed Raman amplification as well as lumped Raman amplification to maximize both distance and capacity. Lumped Raman amplification (LRA) also referred to as discrete Raman amplification, can be used to enable access to previously unused spectral windows. This is achieved by tailoring pump sources to manipulate specific spectral windows thus perform signal amplification over a much wider spectrum than that supported by EDFAs. All-Raman technology has been implemented to manipulate wavelengths over a 100 nm spectral window.
The benefits of Raman amplification are well known and a number of DWDM manufacturers are exploiting the performance benefits of Raman amplification to extend the transmission distance of their unrepeatered submarine and terrestrial products. This is generally
One of the benefits of all-Raman amplification for unrepeatered submarine networks was published at ECOC in September 2005. The paper, All-Raman Unrepeatered transmission over 440Km of Standard PSCF, published the results of a lab test confirming the feasibility of transmitting high-capacity signals (8 X 10 Gb/s) over a distance of 440 Km without the use of any passive or active in-line components. The demonstration spot lighted the characteristics of an all-Raman system which positions it to maximize the distance and capacity of unrepeatered submarine links. The demonstration used silica fiber that is typical for unrepeatered submarine systems. The minimal loss window of this type of fiber is centered approximately
at 1580 nm, which is outside the optimal operating range of Erbiumdoped amplifiers. The DWDM equipment used tightly integrated distributed Raman amplification (DRA) and lumped Raman amplification (LRA) to over come the loss (75.3 dB @ 1550 nm) and simplify the overall dispersion compensation. It did not use any special modulation techniques, but did implement enhanced forward error correction that yielded a measured gross coding gain of 8.5 dB. The ability to tailor the pumps to optimize the distributed gain of both the post amplifier and the pre-amplifier resulted in an improvement of transmission over both an EDFA only system and an EDFA plus Raman as shown in figure 1. The demonstration not only confirms the feasibility of transmitting high-capacity signals over a distance of 450 Km without the use of in-line equipment or special modulation formats, it lays the foundation to achieve transmission distances beyond 500 Km. The wide-spectrum enabled by an all-Raman system using tightly integrated DRA and LRA, provides access beyond the C-band to include areas of the S-band and L-band. This additional spectrum enables flexible wavelength allocations that can be used to increase the number of transmitted channels for various fiber types. Dispersion shifted fiber, for example, is generally limited to less than 8 X 10 Gb/s channels over the typical 250 Km to 300 Km unrepeatered link. However, by selecting wavelengths outside the C-band, it is possible to achieve 30 X 10 Gb/s over that same distance. Another practical benefit of this wide spectrum was recently demonstrated to provide a cost-effective option for increasing the capacity of unrepeatered submarine cables equipped with remote optically pumped amplifiers (ROPA). Systems using ROPAs which were installed a few years ago, generally supported a maximum of 16 2.5 Gb/s channels over an unrepeatered distance of 250 to 300 Km. Influenced by demand for more bandwidth and 10 Gb/s wavelengths, service providers are seeking simple cost effective ways to upgrade these systems to N X 10 Gb/s. The goal of the demonstration was to show that this type of upgrade is possible without the extra cost associated with retracting the cable and removing the ROPA. The demonstration confirmed that the ROPA, when left in place, does not interfere with the transmission
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