What does OTDR stand for and How to Choose?

This article aims to provide important information about OTDR for those who are new to the telecommunications field. And guide them on how to choose the right OTDR for their needs.
1.What is an OTDR?
OTDR means Optical Time-Domain Reflectometer (OTDR), is an optoelectronic instrument. It can be used to measure optical fiber attenuation, joint loss, optical fiber fault location and understand the loss distribution along the length of the optical fiber. It is an essential tool in the construction, maintenance and monitoring of optical cable.

optical fiber OTDR

2. Where is the application?
(1) Test the integrity of the optical fiber cable
(2) Build, certify, maintain, and troubleshoot fiber optic systems
(3) Determine the condition and performance of optical fiber cables
(4) Test components along the cable path, such as connection points, bends, or splices
3. What are the parameters of OTDR?
3.1 Wavelength
In general, the same wavelength should be used when testing the fiber as when transmitting it.
(1)850 nm and/or 1300 nm wavelengths for multimode fiber links
(2)1310 nm and/or 1550 nm and/or 1625 nm wavelengths for single-mode fiber links
(3)Filtered 1625 nm or 1650 nm for in-service troubleshooting of single-mode fiber links
(4)1490 nm wavelength for FTTH systems (optional — test can be performed at 1490 nm, but a common recommendation is to test at 1550 nm to minimize additional investments)
3.2 Dynamic Range
This parameter reveals that the OTDR can analyze the maximum optical loss at different noise levels and also determine the furthest distance that the OTDR can measure, usually expressed in decibels (dB).
Usually, the greater the dynamic range (in dB), the greater the distance that can be reached. But if there are connectors, splicing points, splitters, etc. in a section of optical fiber, the maximum testing distance of an OTDR will be reduced.
3.3 Pulse Width
Due to the propagation loss in the fiber link, the larger the pulse width, the more energy it carries and the longer the distance it can reach. With the increase of pulse width, dynamic range will increase, then OTDR can test longer distance of optical fiber.
3.4 Dead Zones
The dead zone determines the ability to detect and measure two closely spaced events on the fiber link, which are associated with the shortest pulse. With the increase of pulse width, dynamic range will increase, and so will the dead zone.
There are two types of dead zones:
(1)Event dead zone (EDZ) is the minimum distance between two consecutive reflection events that an OTDR can recognize.
(2)Attenuation dead zone (ADZ) is the minimum distance that can be measured from a reflected event (such as a pair of connectors) after a non-reflected event (such as a splice).
As shown in the figure below, there are two 2-meters wire jumpers of in the optical fiber link. If the EDZ is 10 m, you cannot test the start and end of the jumper at the same time. But If the EDZ is 1 m, you can detect both start and end.

In the actual fiber measurement process, dead zone is inevitable. We can choose a device with a smaller dead zone and reduce the error by measuring it multiple times.
3.5 The relationship among dynamic range, wavelength and distance

relationship among dynamic range, wavelength and distance of OTDR

(1) The greater the dynamic range, the longer the testing distance of OTDR
(2)The greater the wavelength, the longer the testing distance of OTDR
4. How to choose an OTDR?
Different OTDR can meet different test and measurement requirements, so you need to know its specifications and parameters before buying. Here are some questions to help you choose before you buy an OTDR:
(1)What fiber type will you be testing? Multi mode or single mode?
Single mode fiber should choose Single mode OTDR, and multi mode fiber should multi mode OTDR.
(2)What is the maximum distance you might have to test? 700 m, 25 km, 120 km?

mini otdr

If you have to test 120km, the OTDR BD4000 is good enough for testing. And there are different wavelengths and dynamic ranges that can be customized.

fiber optic otdr
(3)What kind of measurements will you perform? Installation , trouble shooting?

The installation can choose wavelength 1310 and 1550, for troubleshooting, wavelength 1625 or 1650 is recommended.

(4)Where is your work place?

If you are going to climb a cell tower or a tall building, you need to pay attention to the size and weight of the OTDR and choose a lighter one. Like mini ODTR BD300.

An OTDR is a key field tester for maintaining and troubleshooting fiber optic infrastructures. Before selecting an OTDR, consider the applications that the instrument will be used for and check the OTDR’s specifications to ensure that they are suited to your applications.

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