Important OTDR Parameters

Posted by Kevin Miller on Wed, Nov 16, 2022 @ 10:11 AM

With global connectivity and data consumption rising at an exponential rate every year, the need for robust, faster, and more reliable data transmission is at its peak. Supporting this demand, fiber optic technology is fundamental and critical to delivering high-speed communication services and supporting the modern economy. 

A Deloitte report highlighted connectivity expectations in the Netherlands, by 2030, fiber deployments are expected to be the dominant means of access locally, along with most other regions of the world.

When operating a fiber optic network, maintaining performance and the physical fiber infrastructure is critical, so technical teams rely on a set of sophisticated devices that help to validate and troubleshoot optical fibers. The Optical Time-Domain Reflectometer (OTDR) is one key device that helps assess the integrity of network fibers. In this article, we will briefly discuss OTDRs along with a few of the key parameters that one must gain experience with in order to operate these devices effectively.

What is an OTDR?

An OTDR is a fiber optic instrument that evaluates an optical fiber for continuity and performance, providing important measurements, including the length and signal loss. Since an OTDR is the primary tool for characterizing an optical fiber, it is highly useful for the initial validation of newly constructed fiber spans, periodic testing to ensure optimal performance, and troubleshooting to identify the location of physical fiber issues when they occur.

Example of OTDR device in a test lab
Shown: example of OTDR device in a test lab

How Does an OTDR Work?

At a high level the OTDR includes a laser source and a detector. During a test, the OTDR sends a pulse of light down the fiber, then analyzes the reflections and scattering of the light that occur both naturally as it is transmitted through the fiber and from events that occur in fiber such as splices, breaks, and terminations. 

Specifically, the device analyzes Rayleigh scattering, along with Fresnel reflections. Both concepts are essential to determining the measurements that are generated by the OTDR.

Rayleigh scattering occurs when light scatters due to fluctuations in the fiber core, some of this light is reflected back toward the OTDR which is considered backscattering. As this is a naturally occurring phenomenon during light transmission, this is a known factor that can be measured accurately and is used to measure the attenuation or light loss across the length of the fiber.

Fresnel reflections are associated with the reflection of light in the optical fiber that occurs when there are instances of material transitions causing a change in the Index of Refraction or IOR (also called the refractive index). Examples of this include splices, terminations, breaks, and fiber core alignment mismatches that will register as loss-related events during a test.

Analyzing both Rayleigh backscattering and Fresnel reflections, an OTDR can provide a complete characterization of an optical fiber that provides the operator with key loss measurements (Insertion Loss and Return Loss) for the end-to-end span, along with identifying all other loss-inducing events like splices, in-line components (connectors, splitters, etc), degradations, or complete breaks. The OTDR is then able to accurately identify not only the total length of the fiber, but the specific locations of the various events, and all of the information is displayed visually on the device called a “trace”. The operator can then use this information as necessary, whether it is simply to save/record measurements for periodic validation testing or to identify the location of a specific detrimental issue that needs to be addressed.

Key OTDR Parameters

Because there are several important factors related to light transmission, to obtain accurate measurements, it is essential the user understands these critical elements and not only selects the appropriate device but sets the parameters accordingly based on the fiber and application. While OTDR manufacturers have gone to great lengths to make their devices as user-friendly as possible, the idea of a user “just pushing the test button” in most cases without a deeper understanding of the device and setting proper parameters will rarely yield the most accurate results.

Once an individual is adequately trained, these devices offer an array of features that enable the user to manually program and adjust the key parameters, giving them significant control and allowing them to adjust and adapt settings appropriately to the particular fiber being tested.

Pulse Width

The pulse width refers to the amount of time the signal pulse is on and being emitted into the fiber. Since time and distance are related, a longer pulse width coincides with a longer signal distance, while a shorter pulse width coincides with a shorter signal distance. Because the OTDRs optical light signal is used for creating and analyzing instances of backscattering and reflection and it will experience both inherent and event-induced loss over the fiber length, a proper pulse width must be selected to ensure it will reach the end of the fiber while also identifying all of the events that might occur. Using too short of a pulse width for a longer fiber or one with a high number of loss events will not have enough signal power to reach the end while using too long of a pulse on a shorter fiber will overload the OTDR and reduce resolution and the ability to identify events. Finding the happy medium between reaching the end and identifying all events is the key to this parameter.

Dead Zones

Dead zones are another key parameter to understand which results from Fresnel reflections. This phenomenon occurs when a higher amount of reflected light temporarily blinds the detector and it takes some time along the signal path until it can read/analyze again. Two types of dead zones exist - attenuation and event. An attenuation dead zone is the distance after a reflective event before an OTDR can accurately measure the fiber attenuation, while an event dead zone is the distance the OTDR needs until it can detect another event.

Since the goal is to minimize the dead zones during an OTDR trace, selecting the appropriate pulse width is important. The longer the pulse width, the greater the dead zone, which is another reason why pulse width selection is critical. Using too long a pulse width on a shorter fiber will create larger dead zones and since the OTDR cannot identify events in a dead zone, a dead zone can cause events and fiber issues to be missed completely.

Since a dead zone occurs at the very beginning of the fiber under test immediately after the connection point, it is a best practice to place an OTDR launch fiber ahead of the fiber being tested. This provides an extra length of fiber that allows more time for the signal to settle out and helps to significantly minimize the initial dead zone, allowing the operator to identify events at the beginning of the fiber they might otherwise miss.

OTDR Sidekick Launch Fiber Module
Shown: OTDR Sidekick Launch Fiber Module

Dynamic and Distance Range

The dynamic range is another key parameter of an OTDR that determines the maximum length of the fiber that is observed during a test trace. In more technical terms, it is the distance between the point of the initial backscatter and the noise floor at the end of the fiber under test. The dynamic range value is measured and expressed in decibels (dB) and is essentially an analysis of power levels. OTDRs offering a larger dynamic range value can test longer lengths of fiber compared to those offering a smaller dynamic range value. Therefore, equating a dynamic range value with a fiber distance value is important when evaluating or choosing an appropriate OTDR for the application.

The distance range, not to be confused with dynamic range, is the maximum distance the OTDR will display when taking a trace. If testing a 20km link, the distance range should be set beyond this to ensure the device is representing the complete length of fiber while capturing the necessary details in the resulting trace.

Sampling Time & Resolution

Sampling time and resolution play an important role in the fault-finding capacity of the OTDR. These metrics help to define the instrument's ultimate distance accuracy - the more sampling points and time, the better the resolution and distance accuracy. Therefore, it is beneficial to optimize this key parameter as well. 

Learn More - OTDR Tools and Resources

The OTDR is one of the most critical tools available to fiber optic network operators and technicians for optimizing network performance and troubleshooting critical issues. 

M2 Optics, the leading provider of packaged optical fibers for test lab and field applications, not only uses OTDRs each day during manufacturing procedures, but provides several OTDR-related solutions and resources like Launch Fibers, Fiber Monitoring Systems, and a Dynamic Range Calculator. Click a link to learn more:

Topics: fiber optic testing, otdr