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0PASSIVE ELEMENTS IN OPTICAL LINE TRACT AND DETERMINE THE VALUE
OF ATTENUATION IN THE LINE TRACT Komilov Rasul
Teacher of the Department of Telecommunication Engineering of TATU named after
Muhammad al-Khorazmi https://doi.org/10.5281/zenodo.10723576
Abstract. This article discusses the passive elements of the optical line tract and formulas for calculating losses in the line tract.
Keywords: Optical fiber, Attenuator, Coupler, Circulator, Filter, WDM, Mach-Zehnder filter, Bragg grating, Fabry-Perot filter, Attenuation.
Introduction
Optical fiber communication has become one of the preferred methods of communication both in industrial and domestic settings. The use of optical fibers is preferred because of their ability to provide a high bandwidth, immunity from electromagnetic interference and quality transmission over long distances.
The optical line tract is composed of various components, including passive elements. Passive elements are components that do not generate energy and are used to manipulate the optical signal as it moves along the optical fiber. Some of the passive elements used in the optical line tract include attenuators, couplers, circulators, and filters.
And further, optical fibers are prone to attenuation, which is the loss of optical power over a particular distance. Optical power loss, commonly referred to as signal degradation or transmission impairment, affects the quality of communication in a network, and the degradation limit stands as the threshold within which the communication is still acceptable. This paper discusses the shapes of passive elements and the determination of attenuation value in the optical line tract.
The passive elements of the optical line tract
Attenuators are passive devices that reduce the power level of an optical signal in optical fiber communication. Attenuators are necessary in controlling the strength of the signal and ensure that the receiver receives the signal at the optimum level. Attenuators work by absorbing or scattering the power level of the signal. Attenuators are manufactured in various shapes, including the stepped shape and tapered shape. The stepped attenuator is made up of many pieces of fiber with different lengths that attenuate the signal some more as the signal moves from one length to another. The tapered attenuator, on the other hand, has a reduced core diameter, which reduces the signal transmission as it progresses along the optical fiber.
Couplers are used in optical fiber communication to connect two or more optical fibers to enable the exchange of signals between them. Couplers are manufactured in various shapes, including Y, T, and X shapes. The Y-shape coupler has two channels, with the input being split into two channels with an equal amount of power, and the output signals are combined into a single channel. T and X shaped couplers are designed to combine or split signals into multiple channels.
Circulators are passive components that ensure that optical signals flow in a particular direction along the optical fiber. Circulators are made up of three or more ports, where one port is the input, and the remaining ports are the output. The signal flows in a clockwise direction, from one port to another. Circulators are manufactured in various shapes, including the Y-junction and ring shapes.
Filters are passive devices that allow or reject specific wavelengths of light in a signal. Filters are essential components in wavelength division multiplexing (WDM), where different wavelengths are combined on the same fiber. Filters are manufactured in various shapes, including the Fabry-Perot filter, Bragg grating, and Mach-Zehnder filter. The Fabry-Perot filter uses two reflective surfaces to form a cavity where the light within the designated wavelength is reflected, and the other wavelengths are transmitted. The Bragg grating filter changes the refraction index of the fiber and allows only a particular wavelength to be reflected. The Mach-Zehnder filter uses two arms to redirect the signal into two paths, and the path difference results in destructive interference at specific wavelengths.
Figure 1. Passive elements
Types of attenuation
Optical losses in optical fibers occur due to several reasons. The primary reason is scattering, which is the reduction in signal power due to the reflection of the signal light by the optical fibers' constituent particles. The second reason is absorption, which is the loss of the signal light's energy by the fiber material. Lastly, micro-bending is the third reason, and it entails the signal light's dissipation caused by the fiber's curvature. The attenuation resultant from microbending is the most severe. The attenuation can be expressed in two ways; the loss can be in terms of dB/Km or dB/connector. The dB/Km represents the loss of signal power per kilometer, while the dB/connector expresses the power loss per connection.
To determine the value of attenuation by calculation
Attenuation value is the measure of attenuation resulting from propagation in optical fibers. The attenuation value of optical fibers is critical in determining the quality of communication, and it is parameterized by several factors, including wavelength, fiber diameter, and any additional losses arising from splicing, connectors or bends. The primary objective of determining the attenuation value is to ensure the optical signal transmitted across the optical fiber is within the acceptable range. A fiber's acceptable range of attenuation depends on the device used, i.e., light emitting diodes versus a laser, and it varies from 0.25 dB/Km to 1dB/Km.
If the extinction occurs due to swallowing, then it is calculated by the following formula:
ac
= 8,68^tgS ■ 103, dB/km
(1)
1 swallowing ~>— ^
If the extinction occurs due to scattering, then it is calculated by the following formula:
K
«scatter = K4 , dB/km (2)
The attenuation value is determined by measuring the optical power at the receiver when the transmitter is off. The receiver is installed prior to the attenuation point, where there is no significant power loss. Then, an external attenuator, variable in attenuation levels, is installed between the receiver and the attenuation point. The attenuator is adjusted until the received power is below the minimum power, which the receiver can detect. The difference between the initial power and the power when the receiver can no longer detect signals is then recorded as the attenuation value.
Conclusion
The optical line tract incorporates passive components that are used to manipulate the signal as it moves along the optical fiber. Passive components such as attenuators, couplers, circulators, and filters are fundamental in ensuring the quality of a signal is maintained. These components are manufactured in different shapes, including stepped and tapered attenuators, Y, T, and X-shaped couplers, Y-junction, and ring-shaped circulators, and Fabry-Perot, Bragg grating, and Mach-Zehnder filters. The shapes are designed to achieve particular outcomes that are essential in optical fiber communication.
The determination of attenuation value is crucial in deciding the fiber's quality and the corresponding fiber optic communication system. Attenuation affects the quality of communication in such systems, and it is caused by absorption, scattering, and bending. The attenuation value is measured by installing a receiver before the attenuation point and then adjusting an external attenuator until there is no power to detect. The difference between the initial power and the power with no detection ability is recorded as the attenuation value. This process is essential in maintaining the quality of communication in optical fiber communication systems, particularly for long-distance transmission.
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