The difference between the event dead zone and attenuation dead zone of optical time domain reflectometer and its impact on testing
In the field of optical fiber network testing, OTDR (optical time domain reflectometer) is an indispensable core tool. It can accurately locate fault points and measure fiber length and loss. Understanding the difference between its key parameters - "event dead zone" and "attenuation dead zone" - is crucial to interpreting test results and improving test efficiency. This article will deeply analyze the difference between the two, the calculation formula and its impact on testing, and take the TFN RM7 series OTDR as an example to show how its excellent performance optimizes test results.
1. Core concepts: Event dead zone VS Attenuation dead zone
1.1. Event dead zone (EDZ):
Definition: It refers to the minimum fiber distance required for OTDR to detect and distinguish the next adjacent reflection event again after detecting a strong reflection event (such as active connectors, mechanical joints).
Essence: It reflects the "spatial resolution" of OTDR, that is, the ability to distinguish two closely adjacent reflection events.
Impact: An excessively large event dead zone will cause small events (such as the second connector at the end of a short fiber segment) near a reflection point (such as a patch panel jumper) to be "drowned" and unable to be detected, resulting in missed judgments.
Key factors: Mainly affected by the transmit pulse width. The narrower the pulse, the smaller the event dead zone.
1.2. Attenuation dead zone (ADZ):
Definition: It refers to the minimum fiber distance required for the OTDR's receiver to recover and accurately measure the loss value of the non-reflective event (such as a splice point, bending loss) immediately after the event is detected after a strong reflection event.
Essence: It reflects the "loss measurement accuracy recovery capability" of the OTDR, that is, the ability to accurately measure small loss changes immediately after a strong reflection point.
Impact: An excessively large attenuation dead zone will cause the splice loss or small bending loss that occurs immediately after a strong reflection point (such as a connector) to be unable to be accurately measured, resulting in misreading of loss values or misjudgment of event types.
Key factors: It is also affected by the transmit pulse width, but is also closely related to the design and recovery speed of the OTDR receiving circuit. Usually the attenuation dead zone is larger than the event dead zone.
2. Analysis of calculation formula
Understanding the calculation of dead zone helps to select appropriate parameters (mainly pulse width) in actual testing:
2.1. Approximate calculation formula of event dead zone (EDZ):
EDZ ≈ (c*PW) / (2*n)
c: speed of light in vacuum (≈ 3 × 10⁸ m/s)
PW: pulse width of emitted light (unit: seconds, s)
n: group refractive index of fiber core (typical value SMF≈1.468)
Explanation: The formula shows that pulse width (PW) is the core factor determining the size of dead zone. The narrower the pulse (the smaller the PW), the smaller the calculated event dead zone (EDZ). The 2 in the denominator is because light propagates back and forth in the optical fiber.
2. Approximate estimation of attenuation dead zone (ADZ):
The attenuation dead zone does not have a unified formula as accurate as the event dead zone, but it is usually related to the event dead zone and is significantly larger than the event dead zone. A common empirical relationship is:
ADZ ≈ 5*EDZ to ADZ ≈ 10*EDZ or even larger.
-
Explanation: This emphasizes that even if the next event can be detected (small event dead zone), to accurately measure the loss immediately after the strong reflection point (attenuation dead zone), a longer distance is still required for the receiving circuit to recover from the saturation state.
3. The impact of dead zone on OTDR testing
The size of the dead zone directly determines the testing capability of OTDR in complex links, especially in short-distance, high-density connection scenarios:
3.1. The impact of too large event dead zone:
Event missed detection: It is impossible to distinguish densely deployed connection points (such as connections between short jumpers in data center cabinets), resulting in missed fault point location.
Misjudged event type: Two adjacent events may be misjudged as a composite event.
Impact on short link testing: It is almost unusable in access network, FTTH or short jumper testing between devices.
3.2. Impact of too large attenuation dead zone:
Inaccurate loss measurement: The loss value of the first splice or small bend immediately after a strong reflection event (such as a connector) will be underestimated, overestimated, or even impossible to measure.
Misjudgment of event nature: It is difficult to distinguish between the non-reflection event (splice loss) immediately adjacent to the reflection point and the trailing edge of the reflection event itself.
Impact on link quality assessment: It may lead to inaccurate performance assessment of key connection points (such as splices close to OLT/ONU).
4. TFN RM7: Small dead zone, great performance, new benchmark for test accuracy
TFN RM7 series OTDR has a deep understanding of the key impact of dead zone on test accuracy in design. Through technological innovation, it has achieved industry-leading ultra-small dead zone performance, significantly improving the accuracy and efficiency of testing. Refer to its parameters:
Excellent event dead zone (EDZ): Depending on different models and application scenarios, the event dead zone of the RM7 series is as low as 0.6 meters (such as RM7-S3, RM7-S4, RM7-S5, RM7-C1, RM7-SM1), and some models are 1 meter (RM7-M1). This means that it can clearly distinguish the next event only 0.6 meters away from the strong reflection point, which is very suitable for accurate testing in high-density, short-link scenarios such as in data center cabinets and after FTTH splitters, effectively avoiding event omissions.
Excellent attenuation dead zone (ADZ): The attenuation dead zone of the RM7 series is also excellent, reaching 2.5 meters (such as RM7-S3, RM7-S4, RM7-S5, RM7-C1, RM7-SM1) or 4 meters (RM7-M1). Combined with its event dead zone as low as 0.6 meters, its ADZ/EDZ ratio is very well controlled, ensuring that accurate loss measurement capabilities can be restored within a very short distance after the event is detected. This allows the first fusion loss or tiny bending loss adjacent to the connector to be accurately evaluated, greatly improving the reliability of loss measurement.
Flexible pulse width selection: RM7 provides extremely rich pulse width options (from a minimum of 0.04ns to cover a variety of test ranges), and users can flexibly choose according to test distance and accuracy requirements. Selecting a narrower pulse width is the most direct and effective means to actively reduce the dead zone and improve short-distance or high-precision test capabilities. The minimum pulse width of RM7 is only 0.04ns, which lays the physical foundation for achieving ultra-small dead zones.
High dynamic range and fine resolution: With a dynamic range of up to 50dB (such as RM7-S5) and a loss resolution of 0.001dB, RM7 ensures event detection and loss measurement position accuracy in small blind areas, while also ensuring long-distance testing capabilities and sensitivity to small loss changes, providing comprehensive testing performance.
Conclusion
Event dead zone (EDZ) and attenuation dead zone (ADZ) are core indicators for measuring OTDR performance. They jointly determine the instrument's test accuracy and reliability in complex optical fiber links, especially in short-distance and multi-connection point scenarios. The event dead zone is about "visibility" (whether the next reflection event can be detected), and the attenuation dead zone is about "accurate measurement" (whether the loss value after the adjacent reflection point can be accurately measured). From the formula EDZ ≈ (c*PW) / (2*n), it can be seen that choosing a narrow pulse is the key to reducing the dead zone.
TFN RM7 series OTDR, with its industry-leading ultra-small event dead zone (minimum 0.6 meters) and excellent attenuation dead zone (as low as 2.5 meters), combined with rich narrow pulse options and powerful hardware performance, effectively solves the bottleneck of traditional OTDR in dead zones. It significantly reduces the risk of missed events and mismeasured loss, enabling engineers to obtain clearer, more accurate, and more reliable test results in high-demand scenarios such as dense wiring in data centers, complex FTTH splitting, and key nodes in metropolitan area networks, providing a powerful tool guarantee for efficient construction, precise operation and maintenance, and rapid troubleshooting of optical fiber networks. Choosing the RM7 OTDR with a small dead zone means choosing more excellent test accuracy and efficiency.