With optical fiber now serving as the primary medium for high-speed communications systems around the globe, more and more fiber technicians are being trained each day to install and maintain these critical networks, which includes testing and troubleshooting fibers on a regular basis. Ensuring the performance and integrity of fiber cables is crucial to maximizing the uptime of any network
While there are numerous types of devices that have been developed for testing fiber cables at both the physical and data levels, important test gear that virtually every technician will carry or use at some point are an OTDR and an OLTS.
While both are important, it’s beneficial to understand the differences between the OTDR and the OLTS.
In this article, we will discuss both the OTDR and the OLTS at a high level in terms of their use applications as well as useful ways of training technicians to operate them.
What is an OTDR?
OTDR stands for Optical Time Domain Reflectometer and it’s a device that is used to test and characterize an optical fiber for continuity and integrity. As part of this, it provides some key performance results and metrics like end-to-end distance, signal loss/attenuation over the span, fiber “events” like in-line components or detrimental issues, and more.
An OTDR works by sending a pulsed laser signal down the length of the fiber, then measures the amount of light reflected or scattered back from various points in the fiber. Analyzing the reflected light, the device is then able to determine the performance results of the fiber and display them to the technician operating the device.
When analyzing the reflected light, OTDR technology analyzes two types of light levels, Rayleigh backscattering and Fresnel reflections. Rayleigh backscattering is a consistent low level of light that occurs naturally as a result of the normal reflection and impurities within the glass fiber. Fresnel reflections, on the other hand, are larger light level reflections that can occur at various points within a fiber and are typically caused by an event of some type.
As a result, Rayleigh backscattering is used by an OTDR to calculate the end-to-end distance and the resulting signal loss/attenuation over the entire span, while Fresnel often results from reflection events in the fiber including connectors or a fiber core mismatch. In both instances, the OTDR device calculates loss values of all of the events and displays this information as part of the results.
A test that is run by an OTDR is often referred to in the industry as a “trace” and the trace often consists of a visual, graphical representation of the fiber performance characteristics along with a table that provides detailed information and/or identifies the types of loss events that arise.
Shown below: OTDR Device in a Test Lab
OTDR Benefits and Applications
OTDR devices are useful for multiple application areas within the fiber network - during the installation of new fiber cables, performing regular maintenance, identifying and troubleshooting issues, and certifying fiber cable performance when determining new equipment for the network.
To share some examples, when installing or deploying a new fiber cable span in a network, an engineering team will need to confirm the integrity and continuity of the span to ensure there are no breaks, significant loss points, and so on. As part of routine maintenance, OTDRs are used to check and validate the fiber span in an attempt to identify any issues (ex: a degrading splice) before they become a major issue. If an issue arises where a fiber is broken or tampered with, an OTDR is the best way of locating that specific issue point in order to resolve it quickly. Lastly, OTDRs are valuable when certifying a network fiber before buying or connecting a new device. If a new device is rated to operate to a maximum of 5km and you don’t check in advance and recognize that the fiber in the network is actually 5.5km, you will have a wasted investment that doesn’t perform as intended.
OTDR Challenges
In general, an OTDR is an essential device and many perform exceptionally well with few limitations and challenges, especially those designed and manufactured by leading entities in the market. There are, however, just a few challenges to overcome or address when using an OTDR.
Dead Zones
Since OTDRs utilize a pulsed optical signal and pulse widths must be adjusted depending on the length of the fiber cable, OTDRs often have what is referred to as a “dead zone” where an event will not be identified. As a pulse is launched from the device into a fiber cable, depending on the pulse width, there can be up to several meters at the very beginning of the fiber within the pulse where an event is not identified. Thus, if there is a loss point or fiber damage immediately after that first connection point, it may not be properly identified in the trace.
To solve this issue, an OTDR launch fiber can be placed in-line ahead of the cable being tested. This extra length of fiber at the beginning simply provides the added distance necessary for the OTDR pulse to launch and settle out while enabling it to then detect issues at the start of the fiber cable.
Training/Teaching OTDR Expertise
OTDRs are fairly sophisticated devices with a lot of options in terms of features and settings. A user is required to understand the available settings, know how to apply the appropriate settings based on the cable being tested, and finally interpret the results correctly. To some, using an OTDR to its fullest capability is considered an art form that requires a solid amount of education and learning.
It’s important that those seeking or needing to use/operate an OTDR are adequately trained. This is not the type of device that someone can pickup as a first-time beginner and use effectively without prior training. Additionally, like most products or devices, every OTDR is different in terms of performance capabilities, features, measurements, and so on. Thus, a technician without proper knowledge and training that doesn’t apply the proper settings and takes an incorrect trace can result in a number of detrimental outcomes.
Fortunately, with more required training within businesses or using external training entities now becoming the norm, along with proper solutions like OTDR Training Platforms that can efficiently replicate a field network in the classroom, OTDR training is readily available around the globe.
What is an OLTS?
An OLTS is an Optical Loss Test Set and it consists of two items, a light source and an optical power meter. gives incredibly accurate insertion loss measurement. While these are often separate devices, some of the newer solutions available in the market incorporate both into the same unit. An OLTS is used to accurately measure the insertion loss of a cable. This is accomplished by injecting a signal using the light source (LED or Laser) at one end and measuring how much light comes out of the other end using the power meter. The light source within an OLTS will typically provide test signals at specific wavelengths depending on the type of fiber being tested. For example, a single mode OLTS may provide the common 1310nm and 1550nm settings, while a multimode OLTS may utilize the common 850nm and 1310nm wavelengths. As OLTS devices have advanced over the years, many now are capable of performing both single mode and multimode testing capabilities in the same shared device, offering more testing flexibility and value to the fiber technician.
Shown Below: An OLTS consisting of an MPO Light Source and and MPO Power Meter
OLTS Benefits and Applications
An OLTS is primarily used to accurately measure the power level and insertion loss of a fiber cable along with the polarity, which is necessary when validating fiber cables and signal continuity. In addition, they can be used as an initial tool for quickly troubleshooting fibers that may be experiencing issues - if the measured power level is not what it should be or there is no light at all, then a technician knows there is a more significant issue with the fiber that will need to be evaluated and addressed.
Simplicity of Use
An OLTS is a fairly straightforward tool for technicians to learn and use. Learning to take end-to-end loss measurements with this test set does not take long to learn and many OLTS solutions have a limited set of features and setup options.
Numerous Options Depending on Needs
There are many types of OLTS device options available in the market, whether utilizing the light source and power meter as individual devices paired together, or an integrated device. There are very basic, low-cost devices and there are more advanced devices with a higher price tag but more features.
Additionally, with higher bandwidth systems like 100G sometimes utilizing a multi-fiber cable approach and MPO connectivity (ex: 12-fiber MPO with 10 fibers running 10G per fiber), devices like MPO power meters are available that enable a technician to test power levels across multiple fibers simultaneously, saving a significant amount of time.
OLTS Challenges
In terms of the device itself, there are no major drawbacks or challenges that arise or must be overcome when using these devices.
Similar to OTDRs and virtually any manufactured products, features and measurements can vary, so a user should do their homework as to which device will provide the most benefits for their specific needs. Does a user require something very basic and inexpensive simply to verify if light is coming out of the end of a cable, or is a more advanced device with a higher degree of accuracy required for the most accurate loss measurements? Lastly, for those needing both, is there a complete set that includes both OTDR and OLTS available from a manufacturer that will enable a technician to meet all of their testing needs?
Differences Between OTDR and OLTS - Summary
After sharing information about both the OTDR and OLTS devices, a few points to remember:
- Both the OTDR and OLTS devices are essential tools for technicians installing, maintaining, and troubleshooting fiber optic networks.
- The OTDR is a more sophisticated device that is used to characterize an optical fiber in a more complete manner, while the OLTS is designed to take accurate power / insertion loss measurements.
- Both the OTDR and OLTS can be used to validate the performance of a fiber cable, but to varying degrees and capabilities. While an OLTS can quickly identify that a power level is not meeting the expectation, an OTDR test would identify where the issue or loss point is in the fiber so that the problem can be resolved.
- The OTDR and OLTS require training, but the OTDR requires a higher degree of knowledge and expertise to operate effectively.
- Efficient network and fiber link simulation solutions for replicating a variety of network spans and events are available to effectively train field technicians to use these important devices.
OTDR, OLTS, and Related Training Solutions
Do you have questions about OTDR and OLTS devices? Are you requiring a more efficient way to train technicians to use OTDR and OLTS devices?
With a focus on providing customized OTDR launch fibers and OTDR training solutions to support a variety of fiber optic testing applications, the team of fiber experts at M2 Optics is available at your convenience to support you in any manner. Contact M2 today for more information.
