Non-zero dispersion-shifted fiber (NZ-DSF)
Single-mode fiber whose zero-dispersion wavelength is moved close to, but outside, the erbium amplifier band, leaving a small dispersion of a few ps/(nm·km) there. That is enough to suppress four-wave mixing between WDM channels and small enough to need less dispersion compensation than standard fiber. Standardized as ITU-T G.655 and G.656, it was widely installed for long-haul WDM at 10 Gb/s; coherent systems favor standard and large-effective-area fibers.
Non-zero dispersion-shifted fiber (NZ-DSF) is a single-mode fiber designed to have a small but non-zero chromatic dispersion across the C-band, 1530 to 1565 nm, where erbium-doped amplifiers work. Its zero-dispersion wavelength sits just outside the band, so the dispersion there is a few ps/(nm·km) instead of the 17 of standard fiber or the zero of dispersion-shifted fiber. The residual dispersion keeps the channels of a WDM system out of step with each other, which suppresses four-wave mixing between them, while accumulating slowly enough to need about a quarter of the compensation that standard fiber needs. ITU-T G.655 defines it: its G.655.C category requires the magnitude of the dispersion to lie between 1.0 and 10.0 ps/(nm·km) from 1530 to 1565 nm, with either sign allowed.
Why the dispersion is not zero
Dispersion-shifted fiber, with its zero moved to 1550 nm, was designed for single-channel links, where zero dispersion at the operating wavelength removes the pulse spreading. With many channels it fails, because the channels then travel in step and four-wave mixing between them is phase matched: every combination of channels generates new frequencies, and on an equally spaced grid many of them land on other channels as crosstalk that cannot be filtered out. Tkach and colleagues showed in 1995 that a small dispersion removes most of the effect, and NZ-DSF followed.
The phase mismatch between channels apart grows as . At 4 ps/(nm·km) and 100 GHz spacing it is 2.0 km⁻¹, so a mixing product falls out of step with its source within 3.1 km, against 0.73 km in standard fiber. Computed for an 80 km span with a loss of 0.2 dB/km, the efficiency of mixing relative to a fiber with zero dispersion is:
| Dispersion at the channels | Channels 100 GHz apart | Channels 50 GHz apart |
|---|---|---|
| 17 ps/(nm·km), standard fiber | −45.4 dB | −33.1 dB |
| 4 ps/(nm·km), NZ-DSF | −32.5 dB | −20.4 dB |
| 1 ps/(nm·km) | −20.4 dB | −8.9 dB |
NZ-DSF thus gives up about 13 dB of suppression against standard fiber in exchange for a quarter of its dispersion. The four-wave mixing entry gives the formula and the crosstalk this implies at typical channel powers.
Fiber types compared
| Fiber | ITU-T | Zero-dispersion wavelength | Dispersion in the C-band | Use |
|---|---|---|---|---|
| Standard single-mode | G.652 | 1300 – 1324 nm | about 17 ps/(nm·km) at 1550 nm | Most fiber in the ground: access, metro and long haul |
| Dispersion-shifted | G.653 | 1500 – 1600 nm | ps/(nm·km), 1525 – 1575 nm | Single-channel links at 1550 nm; unsuited to dense WDM |
| Non-zero dispersion-shifted | G.655 | Outside the C-band | ps/(nm·km) (G.655.C) | Long-haul WDM built for 10 Gb/s channels |
| Wideband non-zero dispersion | G.656 | Below 1460 nm | Positive and non-zero from 1460 to 1625 nm | WDM across the S, C and L bands |
| Cutoff-shifted, low loss | G.654 | Near 1300 nm | about 20 ps/(nm·km) | Submarine and long-haul coherent links, with large effective areas |
NZ-DSF has a smaller effective area than standard fiber, which raises the intensity for a given power and with it the nonlinear effects the fiber was designed to reduce. Corning's LEAF, a large-effective-area NZ-DSF, raised it to about 70 µm², against about 80 µm² for standard fiber. OFS's TrueWave RS took the other route and reduced the dispersion slope, so that the dispersion varies less across the band and is easier to compensate channel by channel. Both G.655 and G.656 fibers have cable cutoff wavelengths of up to 1450 nm, above the 1260 nm of G.652 fiber, so they are single mode only above that wavelength.
Dispersion management
At 4 ps/(nm·km), a 1000 km link accumulates 4,000 ps/nm, and a signal whose spectrum is 0.16 nm wide, about 20 GHz at 1550 nm, spreads by 640 ps. Links carrying 10 Gb/s intensity-modulated channels therefore still compensate the dispersion, usually with dispersion-compensating fiber at each amplifier site, but about a quarter as much as standard fiber needs over the same distance. Because the dispersion varies across the band, a single compensator matched at one wavelength leaves residual dispersion at the others, which is why the dispersion slope mattered in these fibers.
G.655 allows either sign of dispersion, and the common long-haul fibers have positive dispersion. Fibers with a small negative dispersion in the C-band were also made for metro links with directly modulated lasers, since the transient chirp of such lasers is partly offset by negative dispersion, and a link can combine fibers of both signs so that the accumulated dispersion stays small while the local dispersion stays non-zero.
NZ-DSF and coherent transmission
Coherent receivers remove chromatic dispersion in their digital signal processing, so the accumulated dispersion no longer limits reach, and the filter they need is shorter on NZ-DSF than on standard fiber. The nonlinear interference between channels, which sets the useful launch power and so the reach of coherent links, now decides the choice of fiber. It falls as the local dispersion rises and as the effective area grows, which works against NZ-DSF on both counts, and the Gaussian-noise model of fiber nonlinearity describes this dependence (Poggiolini). Standard fiber and the large-effective-area, low-loss G.654 fibers used in submarine cables and new long-haul routes therefore carry coherent signals further. NZ-DSF remains in service where it was installed and carries coherent traffic; for new coherent routes, the larger dispersion and effective area of standard and G.654 fiber are the reasons to prefer them.
Common questions
What is non-zero dispersion-shifted fiber?
A single-mode fiber whose zero-dispersion wavelength lies just outside the C-band, leaving a small dispersion of a few ps/(nm·km) in the band. The dispersion suppresses four-wave mixing between WDM channels while accumulating more slowly than in standard fiber. It is specified in ITU-T G.655 and G.656.
What is the difference between NZ-DSF and standard single-mode fiber?
Standard fiber (G.652) has its zero-dispersion wavelength near 1310 nm and about 17 ps/(nm·km) of dispersion at 1550 nm. NZ-DSF moves the zero close to the C-band, leaving a few ps/(nm·km) there, so it needs less dispersion compensation, but it has a smaller effective area and is more affected by nonlinear effects.
Why does NZ-DSF not have zero dispersion in the C-band?
Because WDM channels in a fiber with zero dispersion travel in step and generate strong four-wave mixing products on each other's wavelengths. A few ps/(nm·km) of dispersion keeps the mixing out of phase: at 4 ps/(nm·km) the mixing between channels 100 GHz apart is about 32 dB weaker over an 80 km span.
What is ITU-T G.655?
The ITU-T Recommendation for non-zero dispersion-shifted fiber and cable. Its G.655.C category requires the magnitude of the dispersion from 1530 to 1565 nm to lie between 1.0 and 10.0 ps/(nm·km), of either sign; G.656 extends a positive, non-zero dispersion from 1460 to 1625 nm.
Is NZ-DSF still used?
Yes, in the long-haul networks where it was installed for WDM at 10 Gb/s, which now carry coherent signals as well. For new routes carrying coherent channels, standard fiber and large-effective-area G.654 fiber are usually preferred, because their larger dispersion and effective area reduce nonlinear interference.
References: ITU-T Recommendation G.655, Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable. ITU-T Recommendation G.656, Characteristics of a fibre and cable with non-zero dispersion for wideband optical transport. ITU-T Recommendations G.652, G.653 and G.654. R. W. Tkach et al., "Four-photon mixing and high-speed WDM systems," Journal of Lightwave Technology 13, 841 (1995). G. P. Agrawal, Fiber-Optic Communication Systems, 4th ed. (Wiley, 2010). P. Poggiolini et al., "The GN-model of fiber non-linear propagation and its applications," Journal of Lightwave Technology 32, 694 (2014).