There is a lot of information on the OTDR screen.

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There is a lot of information on the OTDR screen. The slope of the fiber plot shows the fiber attenuation coefficient (loss by length) and is calibrated in dB / km by the OTDR. The fall in the graphical trace of the fiber along the connector allows the loss in dB to be measured. The peak produced by the reflectance of a mechanical connector or joint can also be measured. While some users measure the point-to-point loss of a fiber optic cable network with an OTDR,

Notice the large initial pulse in the OTDR plot shown in the graph above. That is produced by the high power test pulse that is reflected in the OTDR connector and overloads the OTDR receiver. Receiver recovery causes the "dead zone" near the OTDR. In order to avoid problems caused by the dead zone, it is necessary to always use a launch cable of sufficient length when testing the cables.

Connectors and splices are called "events" in OTDR jargon. Both should show a loss, but the connectors and mechanical splices will also show a reflection peak so that you can thus distinguish them from fusion splices. In addition, the height of that peak will indicate the amount of reflection in the event, unless it is so large that it saturates the OTDR receiver. The top of the peak will be flat and will have a tail at the end, which will indicate that the receiver was overloaded. The peak width shows the distance resolution of the OTDR or how close it can detect events.
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OTDRs also detect cable problems caused during installation. If a fiber is broken, the fiber end will appear much shorter than the cable or a splice with high loss in the wrong place. If excessive tension is placed on the cable due to folds or a radius of curvature that is too tight, it will resemble a splice in the wrong place. There is no better help in detecting and solving problems with an OTDR than having good documentation, so that you know what the OTDR should be showing at the points along the fiber.

Visual inspection of connector by microscope

Fiber optic inspection microscopes are used to inspect connectors, in order to confirm that polishing is adequate and to find faults such as scratches, polishing defects and dirt. They can be used both to verify the quality of the finishing procedure and to diagnose problems. A well-made connector has a smooth, polished and scratch-free finish, and the fiber shows no signs of cracks, splinters or areas where the fiber is protruding from the end of the splint or inward.

The increase to visualize the connectors can be of a power of 30 to 400, but it is better to use an average increase. If the increase is very low, fundamental details may not be visible. Performing the inspection with a very large magnification can lead to the person who sees through the microscope being too critical, and rejects good connectors. Multimode connectors should use increases in the 100-200X range and single-mode fiber may use a larger increase, up to 400X. A better solution is to use a medium magnification, but inspect the connector in three ways:

   When viewing directly, the fiber and the hole of the splint can be visualized, and determine if it is of a suitable size, if the fiber is centered in the hole and if the appropriate amount of adhesive has been applied. However, with this form, only the largest scratches are visible. Adding light transmitted through the core will make the cracks visible at the end of the fiber, caused by pressure or heat during the polishing process.

    If you see the end of the connector from a certain angle, while lighting it from approximately the same angle on the opposite side or if you use lighting from a smaller angle and go directly, you will get the best inspection for polishing quality and possible scratches. The shadow effect produced by angular vision or illumination increases the contrast of the scratches against the smooth and mirrored polished surface of the glass.

    However, you must be careful when inspecting the connectors. Sometimes it tends to be too critical, especially if large increases are used. In general, only defects on the fiber core are considered problems. The glass chips around the outer part of the cladding are not unusual and will have no effect on the ability of the connector to couple light in the core of multimode fibers. Also, scratches that are only in the cladding should not cause any loss problem.

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The best microscopes allow you to inspect the connector from different angles, either by tilting the connector or allowing angular illumination to get the best view of what is happening. Verify that the microscope has an easy-to-use adapter to connect the connectors of interest to the microscope.

The video output microscopes that are now available allow you to get an easier view of the connector's end face, and some even have software that analyzes the finish. Although they are much more expensive than normal optical microscopes, they facilitate inspection and greatly increase productivity.

It is important that you remember to verify that the cable has no power before looking at it under the microscope, in order to protect your eyes. The microscope will concentrate all energy that exists in the fiber and focus it on your eye with potentially dangerous results. Some microscopes have filters to stop the infrared radiation of the transmitters in order to minimize this problem.

Indications for field terminations

This method has good and bad aspects. Manufacturing is complex, so these connectors are expensive, almost ten times more than the adhesive / polishing type, since they require careful manufacturing. A part of the extra cost can be compensated with the lower labor costs in your installation. To have less losses, you must make a good cut in the fiber in which the termination is being made, since the cutting of the fiber is an important factor in the losses of a mechanical splice. It is recommended to use a precision cutter such as those used with fiber optic fusers, Even if you do everything correctly, the loss will be slightly higher since you will have the loss of the connection plus the loss by splicing on each connector. The best way to complete the termination is to verify the loss of the splice with a visual fault locator and "twist" as is done with the mechanical splices.


Indications for field terminations

Here are some issues to remember when installing field connectors. If you follow these guidelines, you will save time, money and avoid frustration.


With whatever you do, always follow the manufacturer's instructions about terminations carefully.

Choose the connector carefully and if it is any other type than epoxy or polished, rinse it with the customer. Some customers have strong opinions about the types or brands of connectors used in their work.
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NEVER carry a new type of connector to install in the field until you have installed enough in the office or in the laboratory to be sure that you can install them successfully. This is not the place to conduct experiments or learn.   One of the most important cost factors in the installation of connectors is its performance: how many pass the tests. The most important factor in field termination is the installer's experience.

Reflectance or loss of optical return of the connector

The reflectance or loss of optical return of the connector (also called "return reflection") is the amount of light that is reflected in the fiber towards the light-emitting source as a result of light reflections outside the surface interface polished connector end and air. It is called Fresnel reflection and is caused by the light that is transmitted and undergoes changes in the index of refraction at the interface between the fiber (n = 1.5) and the air (n = 1). Reflectance is the main
problem with connectors, but it can also affect mechanical splices that contain an index equalizer gel
to avoid it.

The reflectance is a component of the loss per connection and represents a loss of 0.3 dB for connectors that have no contact or have space between them, in the case where two fibers do not touch.

Reducing reflectance to the maximum is necessary to obtain maximum performance from high-speed fiber-based single-mode fiber-based systems based on lasers and, in particular, cable television modulated amplitude signals. In multimode fiber systems, reflections are not a problem but may contribute to background noise in the fiber.
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As reflectance is usually a problem in single-mode fiber systems, manufacturers focused on solving the problem of components of this type of fiber; however, multimode fiber connectors also benefit as the reduction in reflectance also implies a reduction in optical loss. Several strategies were used to reduce the reflectance, mainly by means of a convex polishing of the physical contact (PC) at the end of the splint of the connector, which reduces Fresnel's reflection. The technique involves polishing the surface of the end of the fiber to achieve a convex surface or, even better, polishing in the form of a soft angle (angled physical contact or APC) to prevent reflectance.

The reverse current (in the absence of light) must be very small

The reverse current (in the absence of light) must be very small, in order to detect very weak optical signals (high sensitivity).
Fast response (high bandwidth).

The noise level generated by the device itself must be minimal.
There are two types of detectors: PIN photodiodes and APD avalanche.
PIN detectors: Its name comes from the fact that they are composed of a PN junction and between that junction, a new zone of intrinsic material (I) is inserted, which improves the efficiency of the detector. It is mainly used in systems that allow easy discrimination between possible light levels and over short distances.
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APD detectors: Avalanche photodiodes are photodetectors that show, applying a high reverse voltage, an internal effect of current gain (approximately 100), due to impact ionization (avalanche effect). The mechanism of these detectors consists of launching an electron at high speed (with sufficient energy), against an atom so that it is able to tear out another electron.
These detectors can be classified into three types:
Silicon: they have a low noise level and a performance of up to 90% working in the first window. They require a high supply voltage (200-300  V ).
Germanium: suitable for working with wavelengths between 1000 and 1300 nm and with a yield of 70%.
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Two important issues to consider when working with fiber

Safety when working with fiber optics

Some people think that the major concern in fiber optic installations is eye damage when working with the laser. The reality is that lasers that perform perforations on metal or remove warts from the fingers have little relation to the typical fiber optic installation. The optical sources that are used in the optical fiber, generally have much lower power levels (the exception is the high-power telecommunications systems of dense wavelength division multiplexing (DWDM) or cable television). Of course, you should always be careful with your eyes, especially when using a fiber-optic microscope that can concentrate all the light of fiber in your eye.

The real safety problem is always related to the small glass residues that remain when cutting the ends of the fibers that have already been finished or spliced. These wastes or fiber fragments are very dangerous! The cut ends are extremely sharp and can easily penetrate your skin. If they get into your eyes, they are very difficult to remove. Don't even think about what happens if you ingest any. Always wear safety glasses when working with fiber and dispose of fiber waste carefully.

Whenever you work with fiber, follow the rules detailed below.

1. Always wear safety glasses to protect your eyes from fiber debris.

2. Dispose of all-fiber remains properly. Always use a properly labeled container for later disposal and work on a black cloth so that the glass remains are more easily located.

3. Do not throw them on the floor where they can stick to carpets or shoes and move to any other place,

4. Do not eat food or drink near the work area.

The fiber optic splicing and termination processes involve the use of chemical adhesives and cleaners. Follow the instructions for use (detailed in the substance safety data sheet - MSDS) carefully. Remember that even isopropyl alcohol, used as a simple cleaning product, is flammable.

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Zero dirt tolerance

When we work with fiber optics, dirt tolerance is practically zero. The particles present in the air are about the size of the core of single-mode fiber - they absorb a lot of light and can scratch the connectors if they are not removed! Dirt on the connectors is the biggest cause of scratches on the polished connectors, and high loss measurements!

1. Try to work in a clean area. Avoid working near heating system outlets, as these eliminate dust.

2. Always use dust caps on connectors, threaded splice connectors, connection panels or any other material with which you are going to make a connection.

3. Use special fiber optic cleaners or clothes that do not leave lint residue, and isopropyl alcohol to clean the connectors.

4. The splints of the connectors and cables used for the tests will become dirty by discarding the material from the alignment sleeve in the splice bushing, which will create an attenuator. You will see how the front edge of the connector splint turns black! Use metal or ceramic alignment sleeves for testing only.

Development of a cable of 3000 fibers, using tapes of 16 fibers.

Development of a cable of 3000 fibers, using tapes of 16 fibers.

The design and characteristics of cables with 3000 optical fibers are described. In a design tape from 16 fibers that can be used are used. easily divided into 4 types of 4 fibers or 2 of 8. Each. Field tests of cables with diameters of 44 and 46 mm showed good performance. The design of a device for splicing by the method of fusion and separation of fiber tapes is presented. The aim of this proposal is to: increase the number of fibers in the cable and reduce the connection time of the cables.

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Optical LZ with AC delayed.

Optical LZ with altern. delayed. In the 1st embodiment, an optical circulator with 4 ports is used, one of which is used as an input, others as an output, and the optical fibers with Bragg reflector arrays with a linearly varying period are attached to the remaining 2, and the nature change in the period in both cases is the opposite to compensate for variance. In the 2nd case, a communication device with radiation separation according to the polarization state is connected to the port of the optical circulator, and 2 segments of the optical fiber with arrays of Bragg reflectors with ac are connected to the outputs of the communication device. period. In the 3rd case, quarter-wave plates are located at the input of the optical fiber segments. In each case, a delay change is made using linear fiber expansion.


Linear control of the spectral characteristics of components with wavelength selection.

The design of linearly tunable fiber-film components whose characteristics depend on the wavelength of the incoming radiation is considered. The design includes a linearly broadened thin-film optical waveguide with a high refractive index associated with a single-mode fiber coupler. The results of the theory are given. and experiment. studies of linear adjustment of spectral characteristics (branch power, resonance position and polarization of the light emerging from the fiber). Almost linear control has been achieved.