by Mark Zaacks and David Menashe
Geography often limits the ability to provision repeater (i.e., optical amplification) sites between two remote terminal locations, thus dictating they be connected by a single long fibre span. This can appear in a variety of applications: submarine links (island hopping and oil rigs), links spanning large unpopulated areas (deserts, jungles, and mountain ranges), or disaster-recovery connections for enterprise storage systems, to name a few. Even when repeater sites are technically feasible, a single repeaterless span can reduce operational expenses and increase security.
Long, repeaterless, point-to-point links represent a fast growing segment of the optical communication market, requiring practical and cost-effective approaches that can be rapidly and easily deployed. There are three possible methods to extend the reach of single-span links: unique modulation techniques, better forward-error correction (FEC) algorithms, and advanced optical amplification technology. While the first two methods involve redesign of the system transponder cards, the third can be implemented as an add-on to existing systems. This means the basic system can be cost-effectively designed using standard transponders for most standard links, with advanced amplifier capability added to address only the more challenging long links.
In this article we explore amplification technology for long links, proposing design rules for selecting the most effective approach for each application and considering operational aspects related to the use of these amplifiers.
Optical amplification for long single-span WDM systems involves three principle technologies: high-power telecom-grade single-clad EDFAs, distributed Raman amplification (DRA), and remote optically pumped amplifiers (ROPAs). These technologies involve injecting high output power into the transmission line, requiring careful attention to deployment issues. In particular, the total power propagating at any point in the fibre must be less than 1.2 W as required by IEC 61292-4, and all high-power amplifiers must include automatic power reduction (APR) mechanisms to lower output power in case of a fibre link break.
EDFAs deployed in standard systems typically have output power up to 20 dBm, which can be increased to about 23 dBm using standard designs. Until recently, higher power required double-clad fibre technology pumped by multimode pumps; however, this technology is mainly used in CATV, FTTx, and industrial applications. Developments in high-power 980-nm singlemode pump and erbium-doped fibre (EDF) technology now enable telecom-grade single-clad EDFAs with output power of up to 27 dBm and flat gain over the entire C-band. Such "post-booster" EDFAs can be conveniently deployed following a standard 17-dBm or 20-dBm amplifier, providing extra launch power for specific long spans.
While less common than EDFA technology, Raman technology can contribute significantly to extending span reach. DRA is a process whereby pump energy propagating along the transmission line causes signal amplification through stimulated Raman scattering. Since amplification occurs along the transmission line, as opposed to lumped amplification at the terminals, the signal power is prevented from reaching very low or very high levels, thus improving the noise level or reducing non-linear effects. Typically, multiple Raman pumps are used to achieve a flat gain profile for all WDM channels in the C-band.
DRA comes in two forms: co-propagating and counter-propagating. The former uses pumps placed at the transmission terminal, and the latter uses pumps placed at the receiver terminal. The most commonly used form is counter-propagating DRA, mainly since the signal level close to the receiver terminal is low, avoiding non-linear effects. However, the additional optical signal-to-noise ratio (OSNR) margin provided by co-propagating DRA is required for more demanding applications, as discussed later.
For the most demanding applications ROPA technology is also required. This involves supplying pump power from the receiver terminal to a remote piece of EDF placed up to about 100 km from the terminal. The remotely pumped EDF then functions as an effective repeater site, boosting the signal in mid-span, improving OSNR, and extending overall span reach.
To pump the EDF, 1480-nm pump energy is launched from the receiver terminal together with other Raman pumps at shorter wavelengths. The Raman pumps provide counter-propagating DRA for the signal and amplify the 1480-nm pump energy, enabling it to reach up to 100 km while still maintaining enough strength (>10 mW) to pump the EDF.
These four amplification approaches and their typical placement in a system are shown in Fig. 1.
A basic single span WDM link is shown in Fig. 1 and includes transmitters, multiplexer, booster amplifier, transmission fibre, pre-amplifier, dispersion compensation, demux, and receivers. Also shown are advanced amplification offerings that can be used to extend the reach of the span. The OSNR of this basic link can be calculated as:
OSNR(dB) = 58 + Pout â�� 10log(N) â�� Loss â�� NF
where Pout is the booster output power (in dBm), N is the number of channels, Loss is the loss of the link, and NF is the pre-amplifier noise figure.
For typical links with 20-dBm launch power, 5-dB pre-amp NF, and 40 channels:
OSNR = 57 dB â�� Loss
Thus, assuming OSNR tolerance of 14 dB (ballpark for 10.7-Gbit/sec NRZ and 43-Gbit/sec RZ differential phase-shift keying systems with forward-error correction), the maximum link budget is 43 dB. For longer span reach it is necessary to employ one or more of the four amplification technologies shown as blue boxes in the figure.
To upgrade the basic system beyond 43 dB, it is most economical to add a post-booster following the existing booster to increase the launch power above 20 dBm. According to the IEC 60825 laser safety standard, this requires that the post-booster include APR mechanisms to reduce power in case of a fibre line break. Since a telecom-grade post-booster can provide up to 27-dBm output power (i.e., 7-dB gain assuming a 20-dBm booster, as above), this approach provides an additional 7 dB of span budget, supporting up to 50-dB links for 40 channels. In this case the launch power per channel is 11 dBm, far lower than the non-linear threshold due to stimulated Brillouin scattering (SBS), which is about 18 dBm for transponders equipped with SBS suppression.
For links above 50 dB, counter-propagating DRA with pump power in the range of 0.5-1.2 W can also be used. The blue curve in Fig. 2 shows the OSNR gain (which directly translates into additional link budget) achievable using counter-propagating DRA as a function of total pump power. As shown, using 1.2-W pumps provides an additional 8-dB link budget, bringing the limit for a 40 channel system up to 58 dB.
To increase the link budget even further, it is possible to use a combination of counter-propagating and co-propagating DRAs. For co-propagating DRA, the improvement in OSNR due to increased effective launch power (i.e., actual launch power + net Raman gain), is indicated by the red curve in Fig. 2. Thus, using a combination of co-propagating and counter-propagating DRA, each with 1.2-W pump power, it is possible to support 40-channel links up to 61 dB. Since co-propagating Raman increases the effective non-linear length with respect to SBS, it reduces the non-linear threshold. For 40-channel systems, the launch power per channel is lower than this threshold, but for systems with fewer channels SBS can limit span reach.
In principle, using a higher-power booster can further increase span reach; however, in this case the efficiency of the co-propagating Raman process is decreased due to saturation effects.
To increase the link budget to the range of 70 dB requires ROPAs as well as co- and counter-propagating DRA. For example, if 1480-nm pumps are added to the counter-propagating DRA unit, still restricting the total launched pump power to 1.2 W, then it is possible to pump a piece of EDF placed 85 km from the receiver. The combined effect of the ROPAs and counter-propagating DRA provide 16-dB OSNR gain, bringing the link budget for a 40-channel system to 69 dB.
The table summarises many of the discussed alternatives for a typical 40-channel system, as well as 8- and 80-channel systems. The table shows the maximum link budget in decibels, which can be translated to distance by taking about 0.21 dB/km in typical singlemode fibre attenuation. The distance limitation is extended from about 200 km up to 350 km by using advanced amplification techniques together with industry-standard transponders.
When dealing with advanced high-power amplification, network planners must consider a number of deployment issues. Most important is laser safety as defined by the IEC 60825 standard. To avoid radiation hazards, standard telecom systems must be Class 1M classified with respect to laser safety, typically limiting the optical power in the system to about 20 dBm. To use higher powers and still be Class 1M classified, it is necessary to provide APR mechanisms that reduce output power to a safe level (i.e., below 20 dBm) within hundreds of milliseconds in case of a fibre cut or break. Such mechanisms typically involve simultaneous monitoring of back-reflected power in multiple wavelength bands, and must be able to detect any type of fibre break or open connector (including PC connectors). All APR mechanisms must be implemented in hardware and have full redundancy of critical components to avoid single point of failure.
Another issue is possible damage to equipment and the fibre line itself. While in submarine systems it is common to deploy amplifiers with output powers far exceeding 1 W, in terrestrial systems it is required to limit the power propagating in any section of the fibre to 1.2 W (see IEC 61292-4). This limit is imposed mainly to avoid damage to connectors and the fibre itself (e.g., fibre fuse).
Since the effectiveness of DRA and ROPAs is highly influenced by the fibre line quality, OSNR can quickly deteriorate should the fibre line degrade (e.g., loss due to bent or pressed fibre, or connectors with dirt or air gaps). For this reason, line quality must be constantly monitored to track and detect slow degradation and prevent situations that could eventually lead to transmission failure.
Finally, many telecom systems require the transmission of an optical supervisory channel (OSC) at 1510 nm. Since this channel cannot be amplified using EDFA technology, and Raman gain at this wavelength is less than for C-band signal channels, special methods must be used to allow the OSC to be transmitted across long spans. Such techniques can include amplifying the OSC using semiconductor optical amplifier technology or placing the OSC within the C-band.
Mark Zaacks is director of product line management and David Menashe, PhD, is vice president and chief scientist at RED-C Optical Networks Ltd. (www.red-c.com).