Working Principle & Characteristics of OTDRs
What is an OTDR?
An Optical Time Domain Reflectometer (OTDR) is a fiber optic instrument used to characterize, troubleshoot and maintain optical telecommunication networks. OTDR testing is performed by transmitting and analyzing pulsed laser light traveling through an optical fiber. The measurement is said to be unidirectional as the light is insert at extremity of a fiber optic cable link.
Using information obtained from the resultant light signature reflected or scattered back to the point of origin, the OTDR acts as an optical radar system, providing the user with detailed information on the location and overall condition of splices, connections, defects and other features of interest.
OTDR Working Principles
The accuracy and utility of OTDR testing would not be possible without the science that preceded it. Understanding the physics behind the instrument provides invaluable insight into the working principles of OTDR.
When Albert Einstein theorized that electrons could be stimulated to emit a particular waveform, the seed of possibility that would eventually lead to the first operational laser in 1960 was born. While the applications envisioned at that time probably did not include worldwide telecommunications using fiber optics, this technology has now become synonymous with twenty-first century connectivity.
Over the years, many breakthrough discoveries have been leveraged in the development of OTDR testers.
An OTDR contains a laser diode source, a photodiode detector and a highly accurate timing circuit (or time base). The laser emits a pulse of light at a specific wavelength, this pulse of light travels along the fiber being tested, as the pulse moves down the fiber portions of the transmitted light are reflected/refracted or scattered back down the fiber to the photo detector in the OTDR. The intensity of this returning light and the time taken for it to arrive back at the detector tells us the loss value (insertion and reflection), type and location of an event in the fiber link.
Light is returned to the photo detector through a number of mechanisms:
The inherent value of OTDR testing comes from diagnosing the condition of a fiber optic cable that would otherwise be impossible to see. This is essential when the link contains multiple splices and connections that can be subject to failure.
The optical return loss (ORL) and reflectance can be used to diagnose conditions where more loss than expected is occurring at a specific location in the fiber run. The total fiber attenuation can also be assessed, since the amount of backscatter provides an indication of this value.
These same principles are used to calculate distance measurements that are invaluable when repair, troubleshooting or maintenance needs arise. The end of the fiber link or a fiber break will be detectable through Fresnel reflection, since a break or unterminated fiber end is also a change in material media (glass to air). In addition to the overall length of the fiber, the distance to faults, splices and connections can be determined with a graphical presentation of the findings accompanying the analysis.
As the functional utility of OTDR testing increases along with the demand for enhanced testing speed, accuracy, report generation and storage capabilities, the variation in product offerings continues to diversify. The two predominant categories are bench-top and hand-held. A bench-top OTDR is essentially a feature-rich instrument with a direct AC power source, whereas a hand-held or compact OTDR is typically a lightweight, battery-powered device intended for use in the field.
Beyond this basic division, the features and options available for an OTDR should be carefully considered based on the intended use. One important consideration is the type of fiber you will be testing - multimode, single-mode, or both. Another variable is the length of fiber you will be testing. Products designed for long haul applications typically have higher dynamic range capabilities that would not be required for testing shorter fiber optic links, such as FTTA.
Usability features also vary by product, which is yet another reason the intended application for the OTDR should be the most important factor in product selection (Import factors for choosing an OTDR). For example, a light weight product might not be necessary for a stationary test, but if the testing is going to be performed by technicians climbing cell towers or working in an otherwise active setting, weight, as well as features like battery life and ruggedization of the product enclosure become more important.
With the wide variety of applications for OTDR testing, setting parameters accurately for the task at hand will ensure accurate measurements. Using an auto-test function may be sufficient for some tests, but manual setting of parameters is still advisable given the variation in length, type, and complexity of optical fiber runs. Once the correct parameters for testing a given fiber run have been established, these OTDR testing configurations can be recalled from an instruments memory the next time the same or similar run is evaluated.
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