At 1550 nm the loss of any optical fiber is minimum. Its 0.2dB/km. Low loss means the distance between 3R repeater and Optical Amplifier is large.

Thank you.

the answer is simple: Its due to the absorption characteristics of the glass material used in fibers. For short wavelengths Rayleigh scattering of inhomogeneities becomes important and towards UV wavelengths electronic absorption starts to kick in. At around 1400nm there is a strong absorption from OH^- groups. This leaves two windows for telecommunication with similarly low absorption: one around 1300nm and another one around 1500nm. For larger wavelengths infrared absorption starts to increase. You can find hundreds of glass attenuation spectra on the internet.

Also Erbium Doped Fiber Amplifier (EDFA) work exactly at this wavelength.

I think mainly this is due to the materials absorption. This is called the operation window.

Because the attenuation at this wavelength is minimun compared with 1300nm (second window) or 850nm (first window). In 1550nm we obtain a relative minimun in the total atenuation.

I worked in the telecom industry at the time of telecom boom. The magic word was "1.5 um" indeed. We even joked that those who can spell it would be hired on the spot:) But the source of the buzz was the optical fiber (or fibre). Just take a look at on-line catalogs from Corning, Thorlabs or Newport and check the specs with plots of typical SMF fibers used for telecom purposes at 1.5 um. So, I totally agree with Bert that is it's material properties of silica and core-cladding devices (the fiber) including absorption, scattering, dispersion, non-linearity, cost (!) etc. At that time fiber was the magic bullet that was supposed to answer long haul, metro- and the "last mile" (i.e., fiber-to-the home) demands. All the rest like advances in optoelectronics, solid state technology and so called photonic integrated circuits came second because they were driven by the fiber. Basically, the motto was "What can you do at 1.5 um and how can we integrate it in our networks? ".

At around 1.3 - 1.5 micron, both Rayleigh scattering and IR absorption (shadow overtones) makes a valley region (minimum absorption), which is idea for long distance data transmission using silica fibers. Since optical communication reflects silica, this is much more established as the low loss window. This can be shifted to other wavelengths by material modification. However, silica is one of those magic materials which can not be replaced easily. There are certain fibers based on aluminates, fluorides, chgs, etc. exhibiting different low loss windows; but they are for special uses such as long wavelength lasers and short distance applications. The other factors as stipulated in above discussion makes them less competative over silica for wide scale usage.

The window at 1.3 um actually has a zero group velocity dispersion, though with the advent of external modulators and linear amplifiers that the group velocity dispersion at the lowest loss window at 1.5 um can be tolerated.

Because the loss attains its absolute minimum value of about 0.2 dB/km around 1550 nm. The 1550 nm window has become extremely important in view of the availability of optical amplifiers (erbium-doped fiber amplifiers) and wavelength division multiplexing (WDM) transmission system.

Also, 1.5 um is a eye safe wavelength. This is not a show stopper, but a nice feature.

S

Since the loss is minimal.

"The window at 1.3 um actually has a zero group velocity dispersion, though with the advent of external modulators and linear amplifiers that the group velocity dispersion at the lowest loss window at 1.5 um can be tolerated."

Chromatic Dispersion used to be considered a disadvantage of the 1.5 micron window, but now it is now realised that dispersion is vital in suppressing nonlinear cross-talk in fibres carrying multiple wavelengths (DWDM). Fibre optical nonlinearity limits the power one can launch down a fibre, so limits the range of the system.

Adnvances in coherent detection and signal processing have now enabled the dispersion to be compensated electronically. Coherent detection preserves all the optical phase and amplitude information, converting it into the electrical domain. The chromatic dispersion is deterministic, so it becomes possible to perfectly compensate the chromatic dispersion at the receiver.

Chromatic dispoersion then becomes a benefit rather than a problem.

Put simply, the different propagation velocity of different wavelength channels, ensures that they slip past each other as they propagate along the fibre, so the effect of each bit in one channel does not cumulatively interfere with a single bit in another, as they both propagate along the length. The walk-off desynchronises them.

Dispersion is also a benefit within a single high capacity channel for similar reasons.

For many years fibre (and optical component) manufacturers devised complex means to optically compensate the chromatic dispersion. It is now best to use high dispersion fibre in situations where one can afford the complexity to compensate it electronically. In long high capacity systems, where the complex terminals are a minor part of the total system cost.

Paradoxically, this has made much of the earlyer installed simple single-mode fibre design, capable of carrying much higher capacity than the later more expensive disperion shifted fibres (which also had higher loss).

This philosophy does not apply to shorter distance systems where minimum fibre loss is less important, and minimum terminal cost is essential. Simple intensity modulated systems operating at 1.3 microns are then much more attractive.

See: Analytical model for the design of multispan DWDM transmission systems

H. Louchet, A. Hodzic, K. Petermann, A. Robinson, R. Epworth. IEEE Photonics Technology Letters 02/2005

Bert's comment is a concise and sufficient answer to what I think you're looking for; see my attachment for a helpful visualization of that.  Image is from Marek Wartak's Computational Photonics book.

Central wavelength of 1550 nm around C-band (1525-1565 nm) is popularly known as zero loss window, means silica fiber gives least attenuation at this wavelength, moreover, EDFA provides second largest peak gain (first is at 1530 nm) at 1550 nm, this makes overall double benefit to use 1550 nm as information signal wavelength.

because the loss is minimal and eye safe!

We live in an information age. We live in the era of globalization. To distribute data in global scale represents an industry and requires ultralow loss medium material. Silica can be purified to a very high level with a cheap price. So, silica fiber is an industry standard, and the 1.5um wavelength region is selected out for applications. A lot of optical products and instruments are designed for this SPECIAL wavelength region.

Because it is much easier to amplify over long distances

There are several reasons, the first is that between 1500-1650nm (third window) the optical power loss in the glass is smaller than at 850nm and 1300nm.

Second, erbium-doped fiber amplifiers (EDFAs) easily provide amplification in the 1530-1560nm band (now reaching 1610nm). EDFAs are the most widely used amplifiers in optical communications.

Third, the standard fiber chromatic dispersion (NDSF) is high enough around 1550nm to prevent DWDM channels from interfering with each other. If the chromatic dispersion were too low (which happens around 1300nm on standard fibers), the channels start to model each other and add noise to the system effectively.

The above provided answers cover everything. Some people still argue that there is no need for research on 1550nm (due to CD effects) if transmission loss is not an issue and EDFA is not required. Even if the 1300nm window is selected for the downlink transmission in a WDM system, we still need a different optical window 1550nm for the uplink in a duplex. Using two different windows will always be desirable, hence the research on both of them will be required by the researchers.

low attenuation 0.2dB/km at that wavelength