Which Is Best to Test Your Fiber Optic Systems: OLTS or OTDR?

Once all your fiber connections are made, how do you know if your newly installed fiber optic system will function properly? The only way to tell is through testing.

After a fiber optic system is installed, its performance must be verified. Testing gives owners and installers peace of mind and verifies that the system meets all design requirements. It also creates a baseline for future system testing and troubleshooting. (If problems arise later, for example, then current test readings can be compared to original results to identify the problem.)

According to ANSI/TIA-568.3-D, there are two tiers of fiber optic system testing:

• Tier 1 (basic) tests are listed in the standards as required. They’re simple tests that check link attenuation, continuity/length and polarity to certify fiber system performance.
• Tier 2 (extended) tests are listed in the standards as optional additions to Tier 1 tests. They can be used for troubleshooting or to check splice and connector loss, fault location, optical return loss, etc. to certify fiber system performance.

Using the right equipment is key to carrying out proper test procedures. There are two types of devices that can be used in the field to conduct fiber system tests and evaluate fiber link quality:

• OLTS (optical loss test set) devices
• OTDR (optical time domain reflectometer) devices

The differences between OLTS and OTDR

Let’s take a closer look at what makes OLTS and OTDR testing different.

An OLTS device is used for Tier 1 testing. It involves the connection of a light source to one end of the cable and an optical power meter to the other. After injecting a certain level of light from the light source into one end of the cable, the power meter measures exactly how much light comes out of the other end. This reveals the total amount of light lost, which represents insertion loss within the fiber channel. The OLTS device provides a simple “pass” or “fail” status based on test results. If the test fails, an OLTS device isn’t able to pinpoint the cause or location of the problem.

An OTDR device is used for Tier 2 testing. While OLTS testing utilizes both ends of a fiber cable (a light source at one end and an optical power meter at the other), OTDR testing requires access to only one end of a cable. Instead of sending light down the entire length of the cable, OTDR works based on reflection and backscattering. It measures the amount of light that reflects or scatters back after the technician uses the device to send a pulsed laser signal down the fiber. The amount of reflected and backscattered light is used to evaluate fiber performance.

When to use each testing approach

Based on what we’ve shared so far, it may seem clear that OLTS is the foundation for proper fiber testing—and that’s true. But there’s also industry discussion and confusion about when OTDR should be used, and whether it can be substituted for OLTS testing.

OLTS is the most accurate method to determine total insertion loss across an entire fiber link. OLTS devices are also easy to use. Choose your application (100GBASE-SR4, for example), and you’re ready to test. Instead of generating a complex report, the tester makes it simple to determine whether the link passes or fails.

Compared to OLTS, OTDR takes a more complex approach to testing. Historically, the industry has used OTDR for troubleshooting because of the amount of information it provides about link quality. There are specific techniques that must be used to properly conduct an OTDR test.

OTDR devices are useful tools, especially for engineering purposes. They help engineers evaluate fiber optic system design by reporting on conditions like end-to-end distance, signal loss/attenuation over the span and events that cause specific performance issues, such as dirty endfaces, poor fiber core alignment or other problems. They can also assess the performance of specific connection points (a splice or connection) and that connection point’s location within the link.

When it comes to measuring insertion loss, however, some OTDRs use an inferred calculation (meaning it’s not certain). As a result, the tests are not as accurate or repeatable as OLTS tests when it comes to measuring insertion loss.

For these reasons, OTDR isn’t a replacement for OLTS—it’s an addition to OLTS.

Belden’s testing specifications and recommendations

Although it isn’t today, could OTDR be used for Tier 1 testing in the future? Perhaps. In fact, this is currently being discussed within TIA standards committees. Currently, however, OTDR isn’t accepted as commonplace or as a standard.

To gain access to our 25-Year Product Warranty and Lifetime Application Assurance, Belden specifies OLTS over OTDR for three reasons:

• OLTS accurately measures total fiber insertion loss
• OLTS is required by industry standards
• OLTS ensures application compliance

We accept OTDR testing only for PON systems, and the test must be completed in both directions (on both ends of the cable). It’s important to note that the complexity of conducting these tests and reading their results can slow a project down.

If you want to conduct fiber system testing quickly and effectively, with confidence that the results will be reliable and easy to understand, then OLTS is the way to do it. Using OLTS to conduct Tier 1 testing means that installers receive the warranty they need, can hand over the project to the owner sooner and get paid for their work faster so they can move on to the next project.

Belden works closely with standards organizations like TIA, IEEE, BICSI and ISO. We make it a priority to provide insight to shape cabling standards. In fact, Belden has been part of TIA standards since the first commercial building standards were developed. As updates in the OLTS vs. OTDR discussion take place, we’ll be sure to keep you updated.

Related resources:

• Singlemode or Multimode Fiber: Demystifying the Differences
• How Do Fiber Optic Cables Work? Your Question Answered
• TIA Star Award Winners: Why This Honor Means So Much to Belden