Raman reaches for ultralong-haul DWDM

Mar 1st, 2002
86947

Mohammed Islam and Mark Nietubyc

Conventional Raman deployment occurs in hybrid configurations with EDFAs, yet wide-band all-Raman amplification is achieved with increased understanding of Raman effects and high-power pump lasers.

Lack of efficient power conversion has limited the role of Raman amplification to enhancing the reach of erbium-doped fiber amplifiers (EDFAs) in long-haul DWDM systems. With the increased understanding of Raman efficiency with respect to the gain media and fiberoptics, and the arrival of more efficient, high-power optical pumping lasers, Raman amplifiers can stand alone without EDFAs as the sole means of amplifying long-haul and ultralong-haul DWDM transport.

In its distributed form, Raman amplification allows for lower signal launch powers to traverse the span above the noise floor while still increasing the optical signal-to-noise ratio (OSNR), effectively enabling increased reach in DWDM systems. To further increase available network capacity, the "wavelength agnostic" mechanics of Raman amplification increase the gain bandwidth of the fiber. Through the careful selection of the number, wavelength, and power of signal pumps, an all-Raman amplified system can operate over a wide band of wavelengths in a simple and cost-effective system.

An all-Raman system consists of a discrete, or lumped, Raman amplification portion amplifying a wide continuous band of wavelengths in a single stage. To extend the reach of such a system, a distributed Raman amplification section is tightly coupled with the discrete portion offering the ability to transmit these signals greater distances. In comparison, a system employing multiple bands of wavelengths requires the segregation of the bands through band couplers, before discrete banded amplification (see Fig. 1). Distributed Raman amplification is then added separately to enhance span OSNR for ultralong link distances.

Raman amplification is governed by the following equation:

where

is the effective fiber length;

is the Raman gain efficiency, with gR the Raman gain coefficient; Aeff is the effective core area of the pump; K, the polarization factor

Pp(z) is the power of the pump; Ps(0)is the power of the signal, and αS is the fiber attenuation coefficient.

Besides the advantage of obtaining gain across a single band without the use of lossy couplers, Raman combines the amplification process with that of chromatic-dispersion compensation. Higher Raman gain efficiency is best attained using a gain fiber with the smallest effective area, Aeff. Of the various fiber types commercially available, dispersion-compensating fiber (DCF), exhibits the smallest Aeff, resulting in the highest Raman gain efficiency. Thus, the discrete Raman amplifier portion of an all-Raman system compensates for chromatic dispersion simultaneously with amplification.

The total system-span budget need not include external per-band-DCF units along with their associated attenuation of the signal wavelengths. While dual-stage discrete amplifiers such as available EDFAs limit the impact of signal attenuation due to high loss of the DCF units, the mid-stage access margin designed into the banded amplifiers may limit the amount of compensation available. With Raman, the DCF provides gain, not loss.

NOISE
There are four primary sources of noise in Raman amplifiers. The first is double Rayleigh scattering (DRS), which is proportional to the length of the fiber and the gain in the fiber. In Raman amplification using several kilometers of fiber, DRS is particularly important. From a practical viewpoint, DRS limits the gain per stage to approximately 10 to 15 dB. Use of isolators between the multiple stages of amplification creates higher-gain amplifiers. For example, a 30-dB discrete Raman amplifier has been demonstrated commercially with two stages of amplification and a noise figure less than 5.5 dB.

The second source of noise arises from the short upper-state lifetime of Raman amplification, as short as 3 to 6 fs. This virtually instantaneous gain can lead to the deleterious coupling of pump fluctuations to the signal. The use of counterpropagating pumps and signals effectively introduces an upper-state lifetime equal to the transit time through the fiber that prevents this coupling. Copropagating pump lasers must have a very low relative-intensity noise, which may require using Fabry-Perot laser diodes instead of grating-stabilized laser diodes.

The third source of noise arises from the phonon-stimulated optical noise created when the amplified wavelength is spectrally close to the pump wavelength.1 At room temperature or above, a population of thermally induced phonons in the glass fiber can spontaneously experience gain from the pumps. This can increase the noise as much as 3 dB for signals close to the pump wavelengths.

Finally, the fourth primary source of noise in Raman amplifiers is amplified spontaneous emission (ASE). As is typical for reasonable signal power levels, signal-ASE beating noise dominates ASE-ASE beating noise. Fortunately, Raman amplifiers have inherently low noise from signal-ASE beating because a Raman system always acts as a fully inverted system.

For example, the signal-ASE noise term is written as

Sase (v) = (G - 1) hv (N2/N2 - N1)

where N2 is the upper-state population and N1 is the lower-state population. For Raman amplifiers the N2/(N2 - N1) term is always equal to one, whereas in EDFAs it is usually greater than one. In EDFAs, the term equals one only for an amplifier fully inverted through the entire length of gain fiber. However, since Raman amplifiers use long fiber lengths, the noise figure includes the small fraction of passive loss from the gain fiber. Nonetheless, discrete Raman amplifiers with noise figures as low as 4.2 dB have been reported.

DESIGN AND OPERATION
In an all-Raman system, greater capacity can be achieved with fewer components (see Fig. 1). The design of multiband systems is complicated by the band-coupling equipment required to segregate the wavelengths, the separate per-band dispersion compensation, and the number of discrete amplifiers, not to mention the numerous fibers and connections required to construct it.

This complexity adds not only cost concerns but also operational concerns. Each of the discrete band amplifiers contributes band-coupler losses approaching 2 to 3 dB. To amplify the different gain bandwidths, optical amplifiers using diverse technologies are required. For example, EDFAs are used for the center wavelengths. The same EDFA technology achieves amplification of the longer wavelengths using higher levels of gain-fiber doping, higher pump powers, and greater gain-fiber lengths. Thulium-doped fiber amplifiers or short-wavelength, tailored, discrete Raman amplifiers achieve short-wavelength amplification.

Management of Raman gain tilt is required in either configuration. While present in any DWDM system, Raman gain tilt is most problematic in systems spanning near or above 100 nm of gain bandwidth, near the peak of the Raman gain spectrum. With an all-Raman system, this tilt management is less complicated because monitoring of the signal wavelengths can occur across the entire band at two positions: immediately before amplification and directly after amplification (see Fig. 2). Straightforward adjustment of the Raman-pump power levels enables dynamic adjustment of any measured tilt or other gain-effecting event, such as channel drops or additions. The very nature of Raman amplification permits this "shaping" of the gain spectrum.

Monitoring and adjustment of the banded system is more intricate, because several monitoring points need different gain adjustments for each discrete amplifier. A further complication is that the greatest amount of Raman gain tilt is most prevalent between the bands (intraband), so careful coordination of all adjustments is required, regardless of band.

Additional reliability without added costs is possible with the all-Raman system. By design, careful selection of wavelengths used in the pumping scheme allows the amplifier to withstand the failure of one or more of the pump sources. Besides transferring power from the pumps to the signal wavelengths, power transfers due to the Raman effect occur among the signal wavelengths (as in the case of Raman gain tilt) and the pump wavelengths. Thus, dynamic power-level manipulation of the remaining pumps can compensate for the failure of a pump source. Other currently available discrete technologies must make use of redundant pumps, thereby increasing costs or risk going opaque when a pump source fails.

To increase capacity, either through the addition of wavelengths or the increase in time-division-multiplexing transmission speeds, total signal power within the transmission fiber will increase. This scenario plays directly into the strengths of Raman. The gain obtained from Raman is greater at the higher input pump powers required by the power levels of these future systems. Even the scheme found in most deployed EDFAs, in which a 980-nm first-stage pump is used in conjunction with a 1480-nm second-stage pump, exhibits the leveling off of their gain profile as they reach saturation and fail to provide the same additional gain for similar increases in pump power.

A comparison of the power-conversion efficiency (PCE) between a 1480-nm pump EDFA and a Raman amplifier shows that at lower power levels, EDFAs exhibit a PCE of more than 30%, whereas a Raman amplifier remains below 20% (see Fig. 3). However, with an increase in required power level, Raman becomes more efficient. Raman amplifier PCE crosses over that of EDFAs at approximately 0.45 W (or about 26 dBm), a level already surpassed in current distributed Raman deployments.

DEPLOYMENT CHALLENGES
Raman amplification is not without some challenges. Aside from the difficult engineering needed to implement the technology, the extended distances of the distributed Raman part of the system will result in a large amount of power in the fiber.

Distributed Raman amplification systems often use more than 1 W (>30 dBm) of power to attain the desired per span amplification. Nonessential office connectors should be removed by splicing to minimize the negative impact of back-reflections and attenuation to the Raman gain mechanisms. Special high-power connectors are necessary. To mitigate exposure risks in the handling of the power levels present in Raman amplifier deployment, automatic laser shutdown (ALS) and personnel training are essential. Even so, the benefits of an all-Raman system outweigh the deployment challenges.

REFERENCE

  1. C. R. S. Fluger et al., Fundamental Noise Limits in Broadband Raman Amplifiers, OFC 2001 (March 2001).

Mohammed Islam is founder and CTO, and Mark Nietubyc is technical marketing manager at Xtera Communications, 500 W. Bethany, Allen, TX 75013. Both can be reached at 972-649-5000.

More in Transport