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Fiber Optics OPTICAL TIME DOMAIN REFLECTOMETER (OTDR)
Fiber Optics • An Optical time domain reflectometer (OTDR) is used to test a fiber optic cable by injecting a series of high powered optical pulses into the fiber under test. • It measures the light that is scattered or reflected back from points along the fiber.
Fiber Optics • The strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length. • The OTDR uses what is referred to as the backscatter method to test a single mode fiber link and find faults.
Fiber Optics • The OTDR provides a graphic display of the results of the fiber under test.
Fiber Optics • An OTDR may be used for estimating the fiber length and overall attenuation, including splice and mated-connector losses. • It may also be used to locate faults, such as breaks, and to measure optical return loss.
Fiber Optics • It only requires equipment connection at one end of the fiber link. • By adding a cable at the far end it allows measuring the loss of the entire cable, but negates the big advantage of the OTDR, that it makes measurements from only one end of the cable.
Fiber Optics • The pulse is attenuated on the outbound leg and the backscattered light is attenuated on the return leg, the return signal is a function of twice the fiber loss and the backscatter coefficient of the fiber. • The OTDR can identify fiber breaks, splices and connectors in the link as well as measure loss.
Fiber Optics • OTDRs are generally used to test all outside plant installations of single mode fiber. • The high powered signal pulse sent from an OTDR is to powerful to effectively test inside plant fiber, which is usually the installed multimode fiber. • OTDRs are also used to optimize splices in a fiber cable plant.
Fiber Optics • Using a long pulse length, it may be possible to measure attenuation over a distance of more than 100 km. • A short pulse length will improve distance resolution of optical events, but will also reduce measuring range and attenuation measurement resolution.
Fiber Optics • All attenuated pulses along a fiber are referred to as events, these events are plotted on a graph and represent each connector, splice or break in a fiber.
Fiber Optics • Backscatter is produced mainly by impurities in the fiber core. • The trace slopes down to the right due to the pulse of light being attenuated as it travels away from the OTDR.
Fiber Optics • OTDRs have a “dead zone” that may extend a hundred meters from the unit in which accurate readings are unavailable. • You can overcome this limitation if you use a launch cable DEAD ZONES APPEAR AT THE BEGINNING OF THE TRACE
Fiber Optics • A reflection combined with a loss is usually either a mechanical splice or a connector, but could also be a fracture in the fiber.
Fiber Optics • As the locations of connectors and mechanical splices is normally known the identification of the type of event should be easy to determined
Fiber Optics • A point loss which has no reflection is usually either a fusion splice or a bend. THE OTDR TRACE SHOWED THIS EVENT AS A NON REFLECTIVE LOSS, SIMILAR TO AN EVENT CAUSED BY A SPLICE OR A BEND.
Fiber Optics • Note that if a good splice is testing really bad it can mean that their is a bend nearby and the OTDR is not able to split the two close together events.
Fiber Optics • In this particular example, the APC connector at the end of the fiber run produced very little reflection. In some cases the OTDR might show this as a Warning and state that the Receive Event Not Found. FIBER CONNECTOR AT END OF FIBER RUN
Fiber Optics • The end of the fiber shows a strong reflection as it is terminated in a polished connector. • If the end was shattered or immersed in water (as can happen in a broken cable situation) then there may be a smaller reflection or no reflection at all.
Fiber Optics THIS IS THE END OF THE FIBER.
Fiber Optics • The end of a fiber almost always shows a high level of noise as depicted in the graph below.
Fiber Optics • Historically OTDRs ranged in the $50,000 dollar price range, today a good OTDR will cost around $10,000 dollars and some of the basic OTDRs are around $5,000 dollars. OLDER OTDRs FROM THE LATE 90s.
Fiber Optics • Lets briefly look at the different types of OTDRs; Full-feature OTDR Hand-held OTDR Fiber Break Locator RTU (Return Test Unit)
Fiber Optics • Full-feature OTDRs are traditional, optical time domain reflectometers. • They are feature-rich and usually larger, heavier, and less portable than either the hand-held OTDR or the fiber break locator. Despite being characterized as large, their size and weight is only a fraction of that of early generation OTDRs.
Fiber Optics • Often a full-feature OTDR has a main frame that can be fitted with multi-functioned plug-in units to perform many different fiber measurement tasks. • Larger, color displays are common. The full-feature OTDR often has a greater measurement range than the other types of OTDR-like equipment. • Often it is used in laboratories and in the field for difficult fiber measurements.
Fiber Optics • Hand-held (formerly mini) OTDRs and fiber break locators are designed to troubleshoot fiber networks in a field-type environment often using battery power. • The two types of instruments cover the spectrum of approaches to fiber optic plant taken by the communications providers
Fiber Optics • Hand-held, inexpensive (compared to full-feature) OTDRs are intended to be easy-to-use, light-weight, sophisticated OTDRs to collect field data and perform rudimentary data analysis upon. • They may be less feature rich than full-feature OTDRs. Often they can be used in conjunction with PC-based software to perform easy data collection with the hand-held OTDR and sophisticated data analysis with the PC-based software
Fiber Optics • The hand-held OTDRs are commonly used to measure fiber links and locate fiber breaks, points of high loss, points of high reflectance, link end-to-end loss, and Optical Return Loss (ORL) for the link.
Fiber Optics • Fiber break locators are intended to be low-cost instruments specifically designed to locate the position of a catastrophic fiber event, e.g., fiber break, point of high reflectance, or high loss. • The fiber break locator is an opto-electronic tape measure that is designed to measure only distance to catastrophic fiber events.
Fiber Optics • The RTU is the testing module of the RFTS described in GR-1295, Generic Requirements for Remote Fiber Testing Systems (RFTSS). • An RFTS enables fiber physical plant to be automatically tested from a central location. A central computer is used to control the operation of OTDR-like test components located at key points in the fiber network
Fiber Optics • These test components will scan the fiber to locate problems. • If a problem is found, its location is noted and the appropriate Operations Systems (OSs) are notified to begin the repair process.