19-06-2012, 01:31 PM
Optical time-domain reflectometer
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An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. (This is equivalent to the way that an electronic time-domain reflectometer measures reflections caused by changes in the impedance of the cable under test.) The strength of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.
An OTDR may be used for estimating the fiber's 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. To measure the attenuation of multiple fibers, it is advisable to test from each end and then average the results, however this considerable extra work is contrary to the common claim that testing can be performed from only one end of the fiber.
In addition to required specialized optics and electronics, OTDRs have significant computing ability and a graphical display, so they may provide significant test automation. However, proper instrument operation and interpretation of an OTDR trace still requires special technical training and experience.
OTDRs are commonly used to characterize the loss and length of fibers as they go from initial manufacture, through to cabling, warehousing while wound on a drum, installation and then splicing. The last application of installation testing is more challenging, since this can be over extremely long distances, or multiple splices spaced at short distances, or fibers with different optical characteristics joined together. OTDR test results are often carefully stored in case of later fiber failure or warranty claims. Fiber failures can be very expensive, both in terms of the direct cost of repair, and consequential loss of service.
OTDRs are also commonly used for fault finding on installed systems. In this case, reference to the installation OTDR trace is very useful, to determine where changes have occurred. Use of an OTDR for fault finding may require an experienced operator who is able to correctly judge the appropriate instrument settings to locate a problem accurately. This is particularly so in cases involving long distance, closely spaced splices or connectors, or PONs.
OTDRs are available with a variety of fiber types and wavelengths, to match common applications. In general, OTDR testing at longer wavelengths, such as 1550 nm or 1625 nm, can be used to identify fiber attenuation caused by fiber problems, as opposed to the more common splice or connector losses.
The optical dynamic range of an OTDR is limited by a combination of optical pulse output power, optical pulse width, input sensitivity, and signal integration time. Higher optical pulse output power, and better input sensitivity, combine directly to improve measuring range, and are usually fixed features of a particular instrument. However optical pulse width and signal integration time are user adjustable, and require trade-offs which make them application specific.
Reliability and quality of OTDR equipment
The reliability and quality of an OTDR should be determined on the basis of its accuracy, measurement range, ability to resolve and measure closely spaced events, the speed at which it makes measurements, and its ability to perform satisfactorily under various environmental extremes and after various types of physical abuses. In addition to its cost, the instrument should also be rated on the features provided, its size, its weight, and how simple it is to operate.
Accuracy is defined as the correctness of the measurement (i.e., the difference between the measured value and the true value of the event being measured).
The measurement range of the OTDR is defined as the maximum attenuation that can be placed between the instrument and the event being measured, for which the instrument will still be able to measure the event within acceptable accuracy limits.
Instrument resolution is a measure of how close two events can be spaced and still be recognized as two separate events. The duration of the measurement pulse and the data sampling interval create a resolution limitation for OTDRs: the shorter the pulse duration and the shorter the data sampling interval, the better the instrument resolution, but the shorter the measurement range. Resolution is also often limited when powerful reflections return to the OTDR and temporarily overload the detector circuitry. When this occurs, some time is required before the instrument can resolve a second fiber event. Some OTDR manufacturers use a “masking” procedure to improve resolution. The procedure shields or “masks” the detector from high-power fiber reflections, preventing detector overload and eliminating the need for detector recovery.
Industry requirements for the reliability and quality of OTDRs are in GR-196, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment.
Types of OTDR-like test equipment
The common types of OTDR-like test equipment are:
Full-feature OTDR
Hand-held OTDR
Fiber Break Locator
RTU in RFTSs
The equipment is summarized below, and detailed in GR-196, Generic Requirements for Optical Time Domain Reflectometer (OTDR) Type Equipment.
Full-feature OTDR
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. 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. Most full-feature OTDRs are powered from an AC source and or battery source.
Hand-held OTDR and Fiber break locator
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. 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. 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 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.
In general, hand-held OTDRs and fiber break locators are lighter and smaller, simpler to operate, and more likely to operate using battery power than full-feature OTDRs. The intent is for hand-held OTDRs and fiber break locators to be inexpensive enough for optical technicians to be equipped with one as part of their standard tool kit.