Analysis of the estimated optical loss of cable network links

Analysis of the estimated optical loss of cable network links
The estimated optical loss analysis is the calculation and verification of the operational characteristics of an optical fiber system. These include elements such as routing, electronics, wavelength, fiber type, and circuit length. Attenuation and bandwidth are the key parameters for the analysis of the estimated optical loss. The designer must analyze the loss of the links at the beginning of the design stage, before installing a fiber optic system, to ensure that the system will work with the proposed cable network.

In the calculation of the estimated optical loss, both the passive and active components of the circuit can be included. The passive loss is made up of the loss connection by the fiber, the connectors, and the splices. Do not forget the couplers and splices on the link. If system electronics have already been chosen, active components such as wavelength, transmitter power, receiver sensitivity, and dynamic range can be considered. If not yet chosen, the industry-standard or generic loss values ?? can be used for the estimated optical loss. Before starting the system,

The purpose of the estimated optical loss is to ensure that the network equipment will work on the installed fiber optic link. It is logical to be conservative when it comes to specifications. Do not use the best possible specifications for the estimated optical loss of the fibers or the loss per connection in order to leave a margin for the degradation of the components and the installation over time.
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The best way to show how the estimated optical loss is calculated is with an example that illustrates how this calculation is performed for a typical cable network. In this case, it is a two-kilometer multimode / single-mode hybrid link with five connections (two connectors on each end and three connections on the link connection panels) and a splice in the middle. The following figure shows the link design and instantaneous power at any point along with the entire link, drawn to scale to match the drawing of the link above.

As a general rule, the link loss margin should be greater than approximately 3 dB, so that the degradation of the links over time is contemplated. The LEDs on the transmitter can wear out and lose power, the connectors or splices can be degraded or the connectors can get dirty if they are open in case of re-routing or testing. If the cables were accidentally cut, the excess margin will be necessary for the splices to have restoration capacity.

Visits to the installation area

As soon as possible, you should visit the site of the layout, that is, the area in which the network will be installed. It is necessary to walk on foot or in a vehicle each section of an external plant layout, to determine the best options for the location of the cable, the obstacles to be avoided or overcome, and the local bodies that may have ideas about the routing of the cable.

In general, city or other city governments have information about the available ducts or the rules governing the use of telephone or energy poles, which can save design time and effort.

In the installations carried out within already constructed buildings, you must inspect each area to be absolutely certain that you know what the building really is and then make notes on the plans to reflect the real situation, especially all obstacles to the laying of cables, hardware, and walls that require fire protection systems and do not appear on existing plans.
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If you can, take pictures. In buildings under construction, visiting the installation area is also a good idea, just to get an impression of how the final structure will look and meet the work supervisors with whom you will work. They can be the best source of information about who the local authorities are, who will inspect their work and what their expectations are.

Take measurements with the OTDR

Take measurements with the OTDR
All OTDRs will display the plot on a screen and provide two or more markers to locate at points on the screen, in order to measure the loss and distance. This can be used to measure the loss of the length of a fiber, in which case the OTDR will calculate the attenuation coefficient of the fiber or the loss of a connector or splice.

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Fiber attenuation coefficient
To measure the length and attenuation of the fiber, we place the markers at any end of the section of the fiber we want to measure. The OTDR will calculate the distance difference between the two markers and provide the distance. It will also mark the difference between the power levels of the two points where the markers cross the path and calculate the loss or difference in the two power levels in dB. Finally, it will calculate the fiber's attenuation coefficient by dividing the loss by distance and will present the result in dB / km, the normal units for attenuation. If the fiber segment is loud or doesn't look straight,
Loss due to splices or connection
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The OTDR measures the distance to the event and the loss in an event (a connector or splice) between the two markers. In order to measure the loss of the joint, move the two markers near the joint to be measured, and make sure they are approximately the same distance from the center of the joint. The OTDR will calculate the dB loss between the two markers and will give a dB loss reading.

Measurements of connector loss or splices with some reflectance will look very similar, with the exception that you will see a peak in the connector, produced by the reflection of the connector. The OTDR may also use a least squares method to reduce the effects of noise and eliminate the error caused by the loss of fiber between the two markers.

Fiber optic power meters

The power measurement requires a power meter with an adapter that fits the fiber optic connector on the cable being tested, and if you are testing a transmitter, a good fiber optic cable (having a size of adequate fiber, since the coupled power depends on the size of the fiber core) and some help from the network electronics to turn on the transmitter. Remember that when measuring power, the meter must be set at the appropriate wavelength and range (generally dBm, sometimes microwatts, but never "dB" [this unit of measurement is a relative power range that is used only to test the lost]).

In order to measure the power, connect the meter to the cable attached to the source that has the output you want to measure. It can be in the receiver to measure the power of the receiver, or you can use a patch cord reference test lead (which has been tested and known to work) that is connected to the transmitter to measure the output power. Turn on the transmitter/source and allow a few minutes for it to stabilize. Set the power meter for the compatible wavelength and observe the power indicated by the meter. Compare it to the power specified for the system and make sure it is sufficient, but not too much.

Optical loss or insertion loss

Optical loss is the main performance parameter of most fiber optic components. For fiber, it consists of the loss per unit length or attenuation coefficient. For connectors, it involves the loss of connection when it joins another connector. For the cables, it consists in the total loss of the cable components, among which are the connectors, the fibers, the splices and any other component in the cable run being tested. We will use wires to illustrate the insertion loss, and then we will look at other components.

Cable loss is the difference between the power coupled in a cable to the end of the transmitter and what comes out to the end of the receiver. The loss test requires the measurement of the total amount of optical power lost in a cable (including fiber attenuation, connection loss and splice loss) with a light source and power meter (LSPM ) fiber optic or optical loss test equipment (OLTS). The loss test is performed at wavelengths suitable for the fiber and its use. Generally, multimode fiber is tested at 850 nm, and optionally, at 1300 nm with LED sources.

Most tests are performed on preconnected cables, either connection cables (patchcords) or installed cable networks. But fiber manufacturers test each fiber to check for loss, in order to calculate its attenuation coefficient. Connector manufacturers test many connectors to obtain an average value of the loss that the connector will have when it is terminated in the fibers. Manufacturers of other components also test the loss of their components to verify their performance.

The measurement of the insertion loss is made by connecting the cable under test to good reference cables with a calibrated launch power that becomes the loss reference "0 dB". Why do you need reference cables to measure the loss? The test with reference cables at each end stimulates the network of cables with connection cables that are connected to a transmission device. You need a cable to measure the output power of the source for the calibration of the loss reference "0 dB".

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Also, in order to measure the loss of the connectors at the end of a cable, you must attach them to a similar connector and know that it is good. This is an important point that is often not fully understood. When we talk about connector loss, we really mean loss of "connection", that is, the loss of a pair of connected connectors. Therefore, to measure the connectors, these must be attached to reference connectors, which must be high quality connectors so as not to negatively affect the measured loss when attached to an unknown connector.

Must have the right tools for the job

You must have the right tools for the job. Make sure you have the right tools and in good condition before starting work. This includes all the tools for the terminations, for the cables and the testing equipment, do you know if the test leads are in good condition? without them, the well-done terminations will obtain satisfactory measurements in the tests. More and more installers are acquiring their own tools, such as car mechanics, and they say that they ensure that the tools are well maintained. 

Dust and dirt are his enemies, it is very difficult to place terminations or make splices in a place with dust. Try to work in the cleanest place possible; use lint-free cloths (do not use swabs or cloths made with old t-shirts) to clean each connector before connecting or testing it; do not work near the heating grilles, as these will blow dust on you continuously; place covers on connectors and connection panels when not in use; keep them covered and clean.

Do not polish excessively. Contrary to what everyone thinks, polishing too much is as bad as polishing too little. The ceramic splint in most connectors today is much stronger than fiberglass. By polishing it too much, the fiber weakens and thus obtains a surface of the concave fiber, instead of convex as it should be, increasing the losses. A few passes are all that is needed.
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Change granulated sandpaper regularly. When polishing, debris and dirt accumulate on the sandpaper, which can cause problems after polishing many connectors and resulting in poor terminations. Verify the manufacturer's specifications.

The most popular connectors

The ST connector remains one of the most popular multimode connectors because it is economical and easy to install.   The SC connector was designated as standard by the old EIA / TIA 568A standard, but at first, its high cost and difficulty in installation (until recently) diminished its popularity in internal plant installations. Anyway, the new SC connectors are much better, in terms of cost and ease of installation, so it has increased its use. But now the LC connectors are competing with the SC, since the first are the connectors used for transceivers of the systems that operate at gigabit speed, due to their small size and high performance. 

Single-mode networks have used FC or SC connectors in the same proportion as the ST and SC connectors were used in multimode installations. Some D4 connectors are also used. But the LC has become the most popular, as already stated, for their performance and small size.

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Now, the EIA / TIA 568 standard allows any fiber optic connector, as long as it is backed by FOCIS. 
This paved the way for the development of new connectors, which we call "SFF compact connector", which includes the AT&T LC-LC, MT-RJ, Panduit Opti-Jack, 3M Volition, E2000 / LX-5, and the MU connector The

LC connector was the most successful within the United States.

Optical fiber wires.

In a fiber optic transmission system there is a transmitter that is responsible for transforming electromagnetic waves into optical or light energy, which is why it is considered the active component of this process. Once the light signal is transmitted by the tiny fibers, at another end of the circuit there is a third component called the optical detector or receiver, whose mission is to transform the light signal into electromagnetic energy, similar to the original signal.

The basic transmission system is composed in this order, of the input signal, amplifier, light source, optical corrector, fiber optic line (first section), splice, fiber optic line (second section), optical corrector, receiver, amplifier and output signal.

In summary, it can be said that this communication process, the optical fiber works as a means of transporting the light signal, generated by the transmitter of LED'S (light-emitting diodes) and laser.

Light-emitting diodes and laser diodes are suitable sources for fiber optic transmission because their output can be quickly controlled by means of a polarization current. Besides its small size, its luminosity, wavelength and the low voltage needed to handle them are attractive features.

Devices implicit in this process
The main blocks of a fiber optic communications link are the transmitter, receiver, and fiber guide. The transmitter consists of an analog or digital interface, a voltage to current converter, a light source and light to fiber source adapter.

The fiber guide is an ultra-pure glass or a plastic cable. The receiver includes a fiber-to-light sensor connector device, a photodetector, a current-to-voltage converter, a voltage amplifier, and an analog or digital interface. In a fiber optic transmitter, the light source can be modulated by an analog signal Fiber optic jobs near me
Coupling impedances and limiting the amplitude of the signal or in digital pulses. The voltage to current converter serves as an electrical interface between the input circuits and the light source.

The light source can be an LED light-emitting diode or an ILD laser injection diode, the amount of light emitted is proportional to the excitation current, therefore the voltage to current converter converts the input signal voltage into a current that is used to direct the light source. The source to fiber connection is a mechanical interface whose function is to connect the light source to the cable.