Understanding Two-Way (Bi-Directional) Optical Fiber Measurement With an OTDR

In today's interconnected world, reliable digital communication is essential. To ensure seamless connectivity 24/7, service providers and network operators shoulder the responsibility of maintaining vast fiber optic networks. However, these networks face challenges such as signal losses and fiber breaks, which can disrupt communication. Optical Time Domain Reflectometers (OTDRs) play a crucial role in identifying and resolving these issues swiftly and accurately.

Benefits of Two-Way Fiber Optic OTDR Measurement

Bi-directional OTDR testing provides a comprehensive assessment of fiber optic cables for the following reasons:
 

• Detecting Hidden Issues: Varied characteristics in each direction may hide problems like splice or connector issues. Bi-directional testing ensures accurate assessment.
• Verification of Network Integrity: Comparing measurements from both directions confirms network integrity, revealing asymmetrical issues missed in single-direction testing.
• Improving Fault Localization: Testing in both directions provides more data points, speeding up fault pinpointing and reducing maintenance downtime.
• Assessing Connector and Splice Performance: Bi-directional testing evaluates performance in both directions, ensuring signal quality and network reliability.
• Compliance with Standards: Bi-directional OTDR testing is referenced in industry standards such as TIA-455-231, IEC 61300-3-35, GR-196-CORE, and ITU-T G.650.3. These standards provide guidelines for consistent and accurate testing methodologies to ensure thorough assessment and validation of fiber optic networks.

Example Bi-Directional Test Scenario

When light is injected into an optical fiber, some of it is scattered within the optical fiber to the opposite end of the light‘s path. This is called backscattered light. Each optical fiber has its own backscattered light value, or level. Since the connection loss for fibers with different backscatter levels varies according to the measurement direction, correct loss assessment requires measurement from both ends.

For Example, a fiber whose backscatter level is -60 dB is connected to a fiber of -55 dB at a loss of 3 dB

When an Optical Time Domain Reflectometer (OTDR) measures from End A, the light travels at a backscatter level of -60 dB and then declines by 3 dB at the connection point.However, since the backscatter level beyond the connection point is -55 dB, which is higher than the A side by 5 dB, the backscatter level is displayed as 2 dB due to the difference between the connection loss and the backscatter level of the B side. The loss of event 1 is -2 dB, which looks like the light is amplified. This type of event is also referred to as a “gainer” as shown below at caption 
 

On the other hand, when the OTDR measures from End B, the light travels at a backscatter level of -55 dB and declines by 3 dB at the connection point. However, since the backscatter level beyond the connection point is -60 dB, the light declines by an additional 5 dB. It therefore declines 8 dB in total. The loss of event 1 is displayed, in fact, as 8 dB. Conversely this type of event is also referred to be a “loser” as shown below at caption
 

The correct loss value is the average of the following:

The loss of “gainer” event 1 measured from End A: -2 dB

The loss of “loser” event 1 measured from End B: 8 dB

Therefore, the correct loss = = 3 dB

By averaging the values measured from both ends, the backscatter value of the fibers is deducted and the loss of event 1 is corrected. Analysis of two-way measurement in the AQ7280 and AQ1210 is [Advanced Analysis] → [2-Way Trace].

Two-way or bi-directional OTDR testing is essential for a comprehensive evaluation of fiber optic cables, providing insights into network integrity, fault localization, and overall performance, ultimately ensuring the reliability and efficiency of communication networks.

Find the OTDR That's Right For You

When selecting an OTDR for field use, it's essential to consider factors beyond just basic specifications and cost. Features like multitasking capabilities, user-friendliness, and long-term support can greatly enhance efficiency and ease of use during field operations. Additionally, prioritizing the quality and reliability of the instrument is crucial to ensure accurate and dependable results over the long term.
Yokogawa's reputation for providing robust and reliable OTDRs, backed by decades of experience and innovation in the field, makes them a reputable choice. Their track record for producing instruments capable of withstanding harsh field conditions and delivering accurate measurements is a testament to their commitment to quality and customer satisfaction. Choosing a trusted and established brand like Yokogawa can provide peace of mind and confidence in the reliability and performance of the OTDR investment.