FBGs vs. DCFs
As mentioned earlier, insertion loss is one of the largest drawbacks when it comes to utilizing DCFs for dispersion compensation. For example, commercial DCFs for 100- to 120-km standard single-mode fiber compensation have about 10 dB of insertion loss, whereas a continuous FBG-DCM compensating the same span length would only have between 3 to 4 dB (and below 3 dB for a channelized FBG solution).
Furthermore, the DCF has a loss that’s approximately linear with the compensated length, whereas loss is more or less constant in FBGs (Fig. 3).
Insertion loss is a major cost driver in optical networks, because it directly affects the amount of amplification needed. Keeping down the numbers of amplifiers isn’t just a key issue in terms of cost, though. It’s also a fact that erbium-doped fiber amplifiers (EDFAs) actually add strongly wavelength-dependent dispersion—negatively affecting system performance as their numbers increase.
Another benefit of the FBG-DCM is its resilience to withstand high optical power. In contrast to DCFs, which display severe nonlinearity issues at quite moderate optical powers, the FBG-DCM can tolerate the highest optical power commonly found in any optical network without inducing any such effects.
Accurate dispersion compensation becomes more stringent when increasing the bit rate. Slightly depending on modulation format, the dispersion tolerance is proportional to the square of the bit rate. Typically, the chromatic dispersion tolerance for a 10G transmission line is above 1000 picoseconds per nanometer (ps/nm). But when considering optical transport at 40G, this tolerance typically falls well below 100 ps/nm.
DCF-based compensation often displays a high degree of wavelength-dependent residual dispersion due to manufacturing and design issues, which leads to inadequate slope matching. This behavior is especially noticeable for DCFs targeting non-zero dispersion-shifted fiber (NZ-DSF; e.g., LEAF) compensation, but also exists somewhat for standard single-mode-fiber (SMF) optimized DCFs.
Low residual chromatic dispersion is an important requirement, particularly in high-bit-rate applications and where full-wavelength-band dispersion compensation is desirable. Thus, the ability of FBG technology to tailor the FBG’s compensation behavior to fit virtually any dispersion and dispersion slope characteristic becomes a key advantage.
Figure 4 illustrates a comparison between a typical DCF and FBG compensation of NZ-DSF. It can clearly be seen that a significant wavelength-dependent dispersion variation exists for the DCF. In practice, this means that the different channels being transmitted throughout the C-band would experience different compensations, and worst-case, some channels may not work properly.
To overcome the severe dispersion requirements imposed by high-bit-rate transport, a number of strategies were developed. One way to increase the dispersion tolerance is to move away from simple digital encoding formats, e.g. on-off keying (OOK), and start employing more dispersion-tolerant formats such as duobinary and differential quadrature phase-shift keying (DQPSK).
Utilizing new modulation schemes will certainly increase the tolerance to chromatic dispersion. Consequently, many system vendors and operators are turning to tunable dispersion compensators (T-DCMs) for future systems.
T-DCMs allow the system vendor to basically use 10G design rules for 40G networks, since it has the potential to increase the dispersion tolerance tenfold. As such, the original 10G link can remain largely intact. In addition, the T-DCM will also handle time-varying dispersion changes induced by normal temperature variations along the fiber.
FBG-based technology has proven very suitable for T-DCM. FBG-based adaptive dispersion compensation is commercially available today, and tunable FBGs are being considered as the technology of choice in numerous 40G and 100G optical systems being developed.
Low-cost architectural strategies
The specific cost savings achievable by introducing FBG-based dispersion compensation is closely related to the specific topology of an optical transport link. However, some general and straightforward examples immediately stand out.
By making good use of the low insertion loss, the equivalent of hundreds of kilometers of SMF dispersion compensation can be concentrated in single nodes. This is especially interesting to achieve cost-effective point-to-point networks that don’t require distributed dispersion compensation.
The low loss and high-power tolerance further provide network designers with the possibility of placing the compensation either directly after the multiplexer on the transmitter side or after the booster (placing the DCM will be governed by optical signal-to-noise ratio (OSNR) requirements and/or terminal equipment layout). In the case of DCF-DCM, issues normally arise either from high loss limiting the amount of dispersion compensation close to the transmitter, or the introduction of high nonlinearity penalties if placed directly after the booster.
Networks requiring distributed dispersion compensation, often an architecture used when requirement on signal fidelity at each node is vital, typically rely on the use of mid-stage access amplifiers to accommodate this aspect.
In some cases, drawing on the low insertion loss of the FBG-DCM, which in turn enables a simple in-line approach, can actually eliminate the need for mid-stage access amplifiers in these networks. If such a strategy is fully implemented in a network, the amplifier-related cost saving per span may reach as high as 40% (Fig. 5).
Even in networks that normally don’t use mid-stage access amplifiers, insertion-loss-related cost saving could still be significant. By simply utilizing amplifiers with less available output power, the savings on amplifications can be in the area of 20% for a standard 80-km span.
In green-field projects or in networks where hut skipping is of interest, the low loss of the FBG-DCM can directly be translated to a reach advantage. A FBG-DCM would support full dispersion compensation of a 25% longer span than an equivalent DCF-based solution (Fig. 6), leading to significant savings when it comes to both CAPEX and OPEX.
FBG-based chromatic dispersion management provides the telecom industry with unparalleled possibilities when in comes to cost and performance network optimization. The increased focus on cost, especially considering the future of 40 and 100G networks, is effectively addressed by this unique and, in many aspects, disruptive technology.
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