Parameters Of Optical Fiber - Technopediasite

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Friday, January 15, 2021

Parameters Of Optical Fiber

Parameters Of Optical Fiber : This question is often asked by friends related to telecom, what are the parameters of optical fiber? In this article, the parameters of optical fiber will be explained, one who reads the entire article can get complete information. Optical parameters will help to know which are the most important features and limitations of these cables and their strength.

There are two groups of optical fiber parameters that incorporate each of them, and they are structural parameters and transmission parameters. They all establish the conditions under which you can perform data transmission. Now we will talk about structural parameters and transmission parameters in detail.
Optical Fiber


Optical Fiber Structural Parameters

Optical fiber structural parameters are those that relate to the geometry and characteristic structure of each fiber. These parameters determine and characterize the types of optical fibers in the market, so that the classification of fibers is directly dependent on them.  fiber but the most important are mentioned below-

Refractive index profile, which allows fibers with different dispersion.

Size of core and cladding of optical fiber, which determine the type of propagation: That is single-mode or multimode optical fiber.

The mode field diameter of optical fiber cable, which indicates how the geometric distribution of light occurs in diffusion mode.

Numerical aperture of optical fiber, which indicates the number of rays that can enter the core of an optical fiber transmission.

The cutoff wavelength of the optical fiber, which determines that the optical fiber is transmitted over only one mode of propagation.

Optical Fiber Transmission Parameters

Signal attenuation of optical fiber is one of the most important properties, as it largely determines the maximum separation between a transmitter and a receiver. Since the repeater is expensive to install and maintain, the degree of attenuation in the optical fiber has a large impact on the cost of the system. 

When signals travel by optical fiber, the signal gets broadened, which is what we call the distortion mechanisms. If the applied pulses travel sufficiently far, they will eventually overlap with neighboring pulses, causing errors in the receiver output. Thus we can say that signal distortion mechanisms thus limit the information carrying capacity of a fiber.

Optical Fiber Attenuation

Light that travels through an optical fiber loses power as it moves along it, and therefore, with distance. Standoff losses limit the transmission distance and depend on the wavelength of the light and material through which it propagates.

Signal attenuation is defined as the ratio of optical output power P(out) to optical input power P(in) to optical output power. Attenuation express by the symbol α is commonly used to express decibels per kilometer. α=10/L Log(P(in)/P(out).

It is not convenient to use fibers below than 800 nm due to high attenuation by Rayleigh scattering. Above 1600 nm, attenuation problems are presented with the effect of infrared radiation. The lowest losses are at wavelengths of 1550 nm, a value heavily used for long-distance transmission, while the highest values   are at shorter wavelengths.

Therefore, optical fiber communications usually work in the same wavelength region as one of the following "telecom windows"-

➤Earlier window of 800–900 nm was originally used. Therefore, the first telecommunications window is only suitable for short-distance transmissions. 

➤The second telecommunications window uses wavelengths around 1.3 μm, where the loss of silica fiber is very low and the chromatic dispersion of the fiber is very weak, so that dispersion widening is minimized. Normally this window was used for long-haul transmission.

➤The third telecom window, now very widely used, uses wavelengths around 1.5 μm. Silica fiber losses are the lowest in this region, and erbium-doped fiber amplifiers are available that provide very high performance. Fiber dispersion is usually asymmetrical but can be tailored with great flexibility.

Signal attenuation in fiber does not depend on bandwidth and modulation, because the carrier frequency exceeds several orders of magnitude frequency modulation that does not occur in other conventional waveguides. The attenuation of light in an optical fiber occurs as a result of various effects and the losses can be classified into external and internal losses:

Intrinsic Losses

The nature of such damage is due to intrinsic factors of the fiber, and is therefore characteristic of the construction and operational work of the same. As is also the case with external losses, the origin of these losses is a decrease in the transmitted signal strength, reducing the amplitude of the signal.

Absorption due to impurities is a major cause of signal loss in optical fibers. The most common impurity is OH-molecule, which, despite being hardened, remains in the fiber as a residue in manufacturing techniques. Here we can say that the fundamental absorption at 2700 nm and its harmonics produce three absorption peaks at 1383 nm, 1250 nm and 950 nm. These absorption peaks define three distinct windows of operation.

Optical Fiber Dispersion

Dispersion is an inherited property of fibers that can be attributed to the propagation of an optical pulse in the time zone due to the difference in velocity of different optical spectral components that are associated with that optical pulse.

We have to keep in mind that each optical pulse has different spectral components or multiple frequencies. Every optical spectral component has its own velocity and can travel through a different path.

Due to this, each component reaches the out end of a communication channel (fiber) at different intervals of time. This difference in time experienced by different spectral components leads to the longitudinal propagation of the pulse of a cylindrical waveform.

We can say in simple word loss in the bandwidth of the signal results in dispersion, as well as loss per distance traveled (in terms of attenuation). The bandwidth of optical fiber is a measurement of transmission capacity, limited by the total dispersion of the fiber or the widening of the transmitted pulse. This limits the ability to transmit information as the pulses are distorted and widened with transmissions, overlapping each other making the receiver indistinguishable.

Likewise, dispersion restricts the transmission distance and as a bandwidth of the same, it is a function of optical fiber length, as the effect is greater as the fiber length is longer. This type of bandwidth spread can be divided into three categories:

Modal dispersion

Modal dispersion is caused by different modes or path that follow a beam of light into the fiber, resulting in different rays traveling different distances of light and reaching the other end of the fiber at different times.

Polarization mode dispersion (PMD)

Polarization mode dispersion (PMD), produced because the fiber is not exactly a cylindrical waveguide, results in a phenomenon known as birthing which causes a diffusion pulse that loosens the balance between the polarization components.
Pulse broadening

Chromatic dispersion

Chromatic dispersion, which arises as a result of dispersion material and waveguide dispersion (the fiber's own content and geometry) and is due to the variation of refractive index of an optical medium with wavelength.

Nonlinearities

Assuming that the optical communication system behaves linearly is an appropriate approximation when the medium power level (magnitude of the order of mW) and transmission rate does not exceed 2.5 Gbps.

But at high speeds (around 10 Gbps) or higher powers, the effects of certainty appear to be significant, and in the case of WDM systems these effects are also very significant with powers and moderate transmission rates, which determine the number of channels in the system and separation between them. 

The nonlinearities of the optical fibers can be divided into two categories:

➤This results in the dependence of the index of refraction with applied field intensity to those known as the Kerr effect, which in turn is proportional to the square of the amplitude: Self-phase modulation (SPM), cross-phase modulation (CPM) and four-wave mixing (FWM).

➤Due to the interaction of light waves with photons (molecular vibrations) in the silicon of the core, produced by the diffraction effect in the fiber: Stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).

Last Word

My dear friends, very good information has been given about optical fiber parameters, if all of you find any deficiency then please comment and tell me. There is no limit to knowledge, yet I have tried to give you complete information about optical fiber parameters. Tell me how you liked this article " Parameters Of Optical Fiber ".

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