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Ultra Low Loss Fiber

This year at ECOC, Sumitomo has a review of optical fiber technology. The data given by the curves of several optical fibers is relatively clear, so make a note.

The figure below is a summary table. After the low-loss optical fiber theory of Academician Kao Kung in 1966, 1970-1980 is the development period of optical fiber manufacturing technology. OVD, MCVD, PCVD, and VAD have been written before. (Collection 2020, page 185-191 Fiber Deposition Process)

From the perspective of application fields, the backbone network began to use fiber and copper to retreat in 1984. In 1995, the Internet promoted the deployment of optical fiber in the metropolitan area network, followed by PON, mobile base stations, and data centers that began to deploy optical fiber on a large scale in recent years.

The main classification is TDM before, TDM+WDM from the 1990s to the current coherent technology.

There are also many types of optical fibers, but there are mainly two types left in the industry. One is ordinary standard single-mode, which is the familiar G.652 series.

The other type is G.654, which continuously reduces loss and nonlinearity. Reduce nonlinear effects.

Several Features of Y8T258 G.654E

Today, I mainly add the technical conditions for reducing losses.

The loss of optical fiber is mainly divided into several categories. One is structural loss, which is the smoothness of the contact surface between the core layer and the cladding layer. This loss is from the early pickling to the later atomic interface deposition. The process is basically finalized and the loss is also very small. . It mainly deals with the loss changes caused by the adoption of new materials or new processes. That is, in order to improve one indicator, another indicator may be changed. Basically controllable.

Absorption loss, for 654 optical fiber, there is no need to consider the two intervals of water absorption peak and infrared absorption, and it is also at the minimum point in the 1550nm DWDM band.

What can really be reduced is Rayleigh scattering.

Rayleigh scattering is related to the density and arrangement structure of components and molecules. When I wrote OTDR before, I said Rayleigh scattering. Particles smaller than 1/10 of the wavelength of light are considered Rayleigh scattering, and particles larger than 1/10 of the wavelength are considered Mie scattering.

The distribution of these atoms in silica is strongly related to the optical loss caused by Rayleigh scattering. There are two things to improve, one is what atoms to choose, and the other is the distribution density of atoms.

Ordinary optical fibers generally choose doping to change the refractive index and design the distribution interval of the refractive index. The radial refractive index distribution of the fiber cross section has different distribution areas in various types.

The first optical fiber is clad with pure silica, with silica doped with TiO2 as the core.

At present, most of the 652 optical fibers are made of pure silica as the cladding, and the core layer is made of silicon oxide doped with GeO2 germanium oxide.

Some manufacturers of 654 ultra-low-loss optical fibers use pure silica as the core layer, and the cladding is doped with fluorine. Chapters 183-185 of the 2020 collection explain why doping the cladding with fluorine can reduce the refractive index, which is related to the size of the atom.

Using pure silicon oxide without doping as the core layer and using fluorine doped as the cladding layer can reduce the loss of the core layer doped with germanium.

The second is to reduce the atomic density of the glass. The method used is to reduce Tf, fictive temperature, which is virtual temperature, what is virtual temperature? It is the equivalent temperature at which the attractive and repulsive forces between atoms are balanced. There is an introduction to glass that the macroscopic is solid, and the microscopic is liquid. Why is it liquid? That is, the atoms still need to move to achieve the balance of force, but the reality does not allow the atoms to move, the internal stress is unbalanced, and there is a “virtual equivalent stress balance point temperature”

Then why not choose stress balance, mainly because the optical fiber is very long, it is impossible to achieve a perfect “atomic distribution” stress balance, and once there is polycrystal, the interface will have stronger light reflection. is a deliberately chosen glassy state.

It is to cool down faster, and don’t let them balance the stress. A simple understanding is a flying magpie. “Flying” is an action. Nature has entered the ice age and froze. The magpie does not fly macroscopically, but the wings fly again when viewed microscopically. of. As long as the sky cools down fast enough, you can see these phenomena.

Glass is such a craft

The distribution rings of silicon and oxygen are different, and the distribution density is also different, which is mainly controlled by the structure of the three-fold ring, four-fold ring and six-fold ring, thereby controlling the density.

The technological method adopted is to control the cooling gradient of optical fiber drawing. Find the most suitable molecular density and find the low loss valley.