There are many fiber tools available for testing at different stages of the network, to meet various test requirements. These tests are used to reveal the total loss, optical return loss (ORL) and fiber length, can be in a single fiber or a complete network. In addition, the test may require further examination of the different elements of the measured link. Whether identify the characteristics of each component in the link, locate potential problems for a fiber, or find fault in the network, all will inevitably have to use optical time domain reflectometer (OTDR) – from commissioning to the optical network troubleshooting and maintenance, OTDR is the ideal choice. This article will describe the basic principles of an otdr test, for you better understanding the instrument specifications.
What Is An OTDR?

OTDR shows the link condition by reading the light level sent back from the optical pulse. Note that there are two types of reflected light: a continuous low-level light produced by fiber is called Rayleigh backcattering light, high reflection peak at the connection point is called the Fresnel reflection. Rayleigh backscattering is used as a distance function to calculate the attenuation level in fiber optic (unit of dB/km), shown as the linear slope of OTDR trajectory. This phenomenon comes from reflection and absorption of the fiber impurities inside the inherent. When light hits on some impurities, impurity particles will redirect light in different directions, while generating a signal attenuation and backscattering. The longer wavelength, the less the attenuation. Therefore, transmit the same distance in the standard fiber requires smaller power. The below picture illustrates the Rayleigh backscattering.

The second reflection (Fresnel reflection) OTDR uses can detect the physical events along the link. When the light reaches the refractive index mutated position (such as from glass to air), a large part of the light is reflected back, resulting Fresnel reflection, it may be thousands of times stronger than the Rayleigh backscattering. Fresnel reflection can be identified through the OTDR track peak. Examples of such reflection are fast connectors, mechanical splices, optical fiber, fiber breaks or open connectors.
What Is The Blind Zone?
Fresnel reflection leads to an important specification of OTDR, namely blind spot. There are two types of blind spots: events and attenuation. Both are generated by the Fresnel reflection, with the changing distance (meters) depends on the different changes in the reflected power to represent. Blind spot is defined as the duration time, in the meantime detector by high intensity reflected light effects of temporary “blindness”, until it returns to normal can re-read the light signal, imagine, when you night driving with the oncoming cars encounter, your eyes will be short-term blindness. In the OTDR field, time is converted to distance, therefore, the more reflective the detector longer recovery time, leading to longer blind. Most manufacturers are available in the shortest possible pulse width, and single mode fiber -45 dB, -35 dB multimode fiber reflection to specify blind. Therefore, reading the table footnote of specifications is very important because manufacturers use different test conditions for measuring blind area, with particular attention to the pulse width and reflectance values. For example, single-mode fiber -55 dB reflection provides shorter blind specifications than that of -45 dB, just because -55 dB is a lower reflection, the detector recovery faster. In addition, the use of different methods to calculate the distance will be a shorter blind zone than that of the actual value.
Event Blind Zone
Event blind zone is the minimum distance of another event OTDR can detect after Fresnel reflection. In other words, is the minimum fiber length needed between the two reflection events. Still the diving mentioned before as an example, for example, when your eyes can not open because the glare stimulation from the opposite car, after a few seconds, you will find that there is an object on the road, but you can not identify it correctly. Turned to OTDR, can be detected by continuous event, but can not measure the loss. OTDR merging successive events, and returns to a global reflection and loss on all the combined events. To establish specifications, the most common industrial way is to measure the distance between each side of the -1.5 dB of peaks.You can also use another method, measure the distance from the event starts until the reflection level from the peak down to -1.5 dB. This method returns to a longer blind, manufacturers use less.
OTDR event blind zone as short as possible is very important, so that it can detect the closely spaced events in the link. For example, testing requirements of OTDR event blind zone is very short in the building network, because the fiber jumper connecting various data centers is very short. If the blind area is too long, some connectors may be missed, the technicians can’t identify them, which makes the job of positioning potential problems become more difficult.
Attenuation Blind Zone
Attenuation blind zone is after the Fresnel reflection, OTDR can accurately measure the minimum distance of successive events loss. Also use the example above, after a longer period of time, your eyes are fully restored, the ability to identify and analyze possible attributes of objects on the way. Shown below, the detector has enough time to recover, so that it can detect and measure the loss of successive events. The minimum distance required from the beginning of reflection events, until the reflection is reduced to the fiber backscattering level of 0.5 dB.
The Importance Of Blind Zone
The short attenuation zone makes the OTDR can not only detect the continuous events, but also can return to the close event loss. For example, you can now learn the loss of short fiber jumpers in network, which can help technicans to understand the situation within the link.
Blind zone also affected by other factors: pulse width. Specifications using the shortest pulse width in order to provide the shortest blind zone. However, the blind zone is not always the same length, as the pulse wider, blind spots will be stretched. Use the longest possible pulse broadband will lead to blind particularly long, but it has a different purpose.
Conclusion
On the market, there are many types of OTDR – from based fault locator to advanced equipment, can meet the different test and measurement requirements. To buy the right OTDR, you must consider the basic parameters. Because if the selected model is not suitable for the application, only based on the overall performance and price to choose equipment will appear problems. OTDR with complex specifications, the vast majority are a result of compromise. A deep understanding of these parameters and know how to verify these parameters can help buyers to make the right choice meeting their demand, maximizing productivity and cost effectiveness.