How to Select the Right OTDR
As the use of fiber in premise networks continues to grow, so do the requirements for testing and certifying it. An optical time-domain reflectometer (OTDR) is an optoelectronic instrument in the characterization and certification of fiber-optic links (multi-mode and single-mode LAN/Ethernet). When choosing an OTDR, it is important to select the specific OTDR performance and features according to the required specification to qualify these links accurately. This article describes the key specifications that should be considered when choosing an OTDR.
This specification determines the total optical loss that the OTDR can analyze, that is to say, the overall length of fiber link can be measured by the OTDR. The higher the dynamic range, the longer the distance the OTDR can analyze. The dynamic range specification must be considered carefully for two reasons: 1. OTDR manufacturers specify dynamic range in different ways (playing with specifications like pulse width, signal-to-noise ratio, averaging time, etc). Therefore, it is important to understand them thoroughly and avoid comparing apples and oranges. 2. Having insufficient dynamic range will result in the ability to measure the complete link length and, in many cases, affect the accuracy of the link loss, attenuation and far-end connector losses. A good rule of thumb is to choose an OTDR whose dynamic range is 5 to 8 dB higher than the maximum loss you will encounter.
Dead zones originate from reflective events, such as connectors and mechanical splices, along the link, and they affect the OTDR's ability to accurately measure attenuation on shorter links and differentiate closely spaced events, such as connectors in patch panels, etc. For premises fiber testing and troubleshooting, short dead zones are much more important than dynamic range. The distances are short enough that you do not need a great deal of dynamic range. However, those same short distances require that you have short dead zones in order to detect fiber patch cords and measure the loss at each end of a fiber link. For long-distance fiber testing and communication (longer than 20 km), loss of the fiber itself creates a significant amount of loss, and dynamic range is an important specification for these long fiber links.
Sampling resolution is defined as the minimum distance between two consecutive sampling points acquired by the instrument. This parameter is crucial, as it defines the ultimate distance accuracy and fault-finding capability of the OTDR. The duration of the measurement pulse and the data sampling interval create a resolution limitation for OTDR. The shorter the pulse duration and the shorter the data sampling interval, the better the instrument resolution, but the shorter the measurement range. Thus the choice of the right OTDR also depends on the selected pulse width and distance range.
This is an important feature because a great deal of time can be saved in the analysis of OTDR traces if the user is able to set pass/fail thresholds for parameters of interest (such as splice loss or connector reflection). These thresholds highlight parameters that have exceeded a warning or fail limit set by the user and, when used in conjunction with reporting software, it can rapidly provide re-work sheets for installation/commissioning engineers.
Report generation is another major timesaver, as post-processing time can be reduced by up to 90 percent if the OTDR has specialized post-processing software allowing fast and easy generation of OTDR reports. These reports also include bidirectional analyses of OTDR traces and summary reports for high-fiber-count cables.
Since OTDRs are very expensive and have only specific uses, the selection of OTDRs must be made carefully. In order to select the right patch cable, you're supposed to consider all the above factors comprehensively. Fiberstore's OTDRs are available with a variety of fiber types and wavelengths, including single-mode fiber, multi-mode fiber, 1310nm, 1550 nm, 1625 nm, etc. They also supply OTDRs of famous brands, such as JDSU MTS series, EXFO FTB series, YOKOGAWA AQ series and so on. OEM portable and handheld OTDRs (manufactured by Fiberstore) are available as well.