# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 12 Applications of HCFs in Networks and Systems
### DWDM Networks  
For high-performance DWDM networks, the most important advantages of NANF/DNANF fibers over conventional SMF or PSCF fibers are:  
1. Very low optical nonlinearity, \(\geq\)1000\(\times\) weaker than in SMF;
2. Ability to carry high optical power;
3. Lower attenuation and/or new low-loss bands;
4. Low chromatic dispersion (Figure 16).  
In networks spanning long distances, the low latency of NANF/DNANF fibers is beneficial too. HC-PBGFs are not suitable due to high attenuation, typically 3-5 dB/km, narrow passbands (Figure 4), and high dispersion (Figure 8)--see data in Table 2.  
A gas-filled NANF has a very low nonlinear refractive index (n\({}_{2}\)), Brillouin and Raman effects. When its attenuation is low enough (\(\leq\)0.20 dB/km) and the \"water peak\" (Figure 14) is suppressed, its use in high-capacity DWDM systems shall bring several improvements over systems with G.652.D [29] or G.654.E [78] solid-core single-mode fibers:  
* Elimination of nonlinear optical effects (SPM, XPM, FWM, SRS, SBS);
* Wider bandwidth with suitable optical amplifiers;
* Increase in transmit power per channel and repeater spacing;
* Lower demand for dispersion compensation and FEC;
* Fewer amplifiers in metro and long-distance networks [130,131]. Disadvantages of NANFs in this application include:
* Impossibility of Raman amplification in the transmission fiber;
* Fire and eye safety issues due to high power carried by the fiber.  
The authors of [132] analyzed the benefits of increasing the power launched into an NANF or similar fiber even to kW levels, but there is a limit imposed by the heating of splices. As presented in Section 10, it is difficult to reduce the loss of NANF-NANF fusion splice below 0.1-0.2 dB. For a 0.1 dB loss, 2.4% of the power arriving via the fiber will escape from the splice, mostly to be absorbed by the plastic parts of the splice protector and the cassette holding it; with 1 kW in the fiber, there is 24 W to dissipate, which is too much. The safe limit is around 50 W in the fiber.  
Low-loss longwave bands located between 1625 nm and \(\approx\)2100 nm, depending on fiber design, will only be useful with suitable amplification technology. In the absence of Raman amplification in HCF, this leaves Thulium-Doped Fiber Amplifier (TDFA). The bandwidth added this way can be, by using a set of dedicated amplifiers serving several sub-bands, extended to some 1625-2050 nm (146.3-184.6 THz) [133], adding to the bandwidth of C and L bands, \(\approx\)1528-1610 nm (186.3-196.3 THz).  
The total bandwidth usable in DWDM networks with NANFs and sets of EDFA and TDFA amplifiers can be estimated as 10 + 38 = 48 THz, approx. five times the C + L bandwidth available today. Further capacity gains, by a factor of 4 or more, will result from the lack of signal deterioration and noise produced by nonlinear effects in the fiber, like cross-phase mixing (XPM). A higher signal-to-noise ratio enables the use of larger modulation constellations, and increasing the spectral efficiency up to \(\approx\)14 bit/s\(\cdot\)Hz will be possible.  
Restrictions imposed by noise accumulation in amplifier chains will not change, because the noise figure of optimized TDFA is similar to one of EFDA: 5-6 dB [134]. A lower attenuation of NANF like 0.05-0.10 dB/km would help, thus reducing the number of amplifiers.  
The main barrier to the introduction of HCFs to metro and core DWDM networks will be cost of the fiber. However, while distances of hundreds or thousands of kilometers are typical in this segment, a single pair of new fibers shall suffice at the beginning.