Single Mode Fiber: The center core is very thin (the core diameter is generally 9 or 10 μm), and the single-clad outer diameter is 125 μm, which is expressed as 9/125
μm. Only one mode can be transmitted (degenerate two polarization states). Therefore, the inter-mode dispersion is small, suitable for remote communication, but there are
also material dispersion and waveguide dispersion, so that single-mode fiber has high requirements on the spectral width and stability of the light source, that is, the spectral
width is narrow and stable. It’s better.
It was later found that at a wavelength of 1310 nm, the total dispersion of the single mode fiber is zero. From the loss characteristics of the fiber, 1310nm is just a low loss
window of the fiber. Thus, the 1310 nm wavelength region has become an ideal working window for fiber-optic communication and is also the main working band of practical
fiber-optic communication systems. The main parameters of the 1310nm conventional single-mode fiber are determined by the International Telecommunication Union ITU-
T in the G652 recommendation, so this fiber is also called G652 fiber. The vast majority of fiber optic cables that have been laid in China are such fibers. With the successful
advancement of the fiber optic cable industry and semiconductor laser technology, the operating wavelength of the fiber line can be transferred to a lower loss (0.22 dB/km)
1550 nm fiber window.
G.653 single mode fiber
A single-mode fiber that meets ITU-TG653 requirements, often referred to as Dispersion Shifted Fiber, has a zero-dispersion wavelength shifted to a very low loss of 1550
nm. However, the existence of DSF is seriously insufficient. There are harmful nonlinear effects such as four-wave mixing in the low dispersion region around 1550 nm, which
hinders the application of the fiber amplifier in the 1550 nm window.
Four-Wave Mixing (FWM), also known as four-phonon mixing, is an inter-wave coupling effect produced by the real part of the third-order polarization of a fiber medium. It
is due to the interaction of two or three light waves of different wavelengths. This results in the generation of so-called mixing products at other wavelengths or new light
waves in the sidebands. This interaction may occur between signals in a multi-channel system and can produce multiple parametric effects such as triple frequency, sum
frequency, and difference frequency.
The reason why the four-wave mixing occurs is that the light at one of the incident lights changes the refractive index of the optical fiber, and the phase of the optical wave
changes at different frequencies, thereby generating a new wavelength of light.
In a DWDM system, four-wave mixing becomes a major factor in nonlinear crosstalk when the channel spacing and fiber dispersion are sufficiently small and phase matching
is satisfied. When the channel spacing reaches below 10 GHz, the impact of the FWM on the system will be most severe.
Addax security engineers believe that the impact of four-wave mixing on the DWDM system is mainly manifested in (1) generating new wavelengths, causing loss of optical
energy of the original signal, affecting the signal-to-noise ratio of the system; (2) If the new wavelength produced is the same as or overlaps with the original wavelength,
serious crosstalk is generated. The generation of four-wave mixing requires the phase matching of each signal light. When each signal light is transmitted near the zero
dispersion of the optical fiber, the influence of material dispersion on the phase mismatch is small, so it is easy to satisfy the phase matching condition and easily generate
four waves. Mixing effect.
The channel spacing of current DWDM systems is generally 100 GHz, and zero-dispersion causes four-wave mixing to become the main reason. Therefore, when G.653 fiber
transmission DWDM system is used, it is easy to generate a four-wave mixing effect, and G.652 or G. At 655 fiber, it is not easy to produce a four-wave mixing
effect. However, G.652 fiber has a certain dispersion in the 1550nm window storage. When transmitting the 10G signal, the dispersion compensation should be added. The
dispersion of G.655 fiber in the 1550nm window is small, which is suitable for transmission of a 10G DWDM system…
G.655 single mode fiber
Single mode fiber that meets ITU-TG 655 requirements, often referred to as a non-zero dispersion shifted fiber or NZDSF (=NonZero Dispersion Shifted Fiber). It belongs to
the dispersion-shifted fiber, but the dispersion is not zero at 1550 nm (the dispersion value corresponding to the range of 1530-1565 nm according to ITU-TG655 is 0.1-6.0
ps/nm.km), which is used to balance the four-wave mixing. Linear effect.
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