When a fiber optic link has a problem, we must quickly locate and resolve it, otherwise it will cause great difficulties and troubles in our work and life. This article will provide you with some comprehensive solutions for quickly locating fiber optic fault points based on different scenarios and tool features.
1. Basic troubleshooting tools for quick positioning
Tool 1: Red light pen (visual fault locator)
VFL mainly used for short-distance continuity detection of ≤5km. The red light pen is connected to one end of the optical fiber, and the other end is used to observe the red light of the optical fiber link with the naked eye:
- If there is no red light output, it is judged that the optical fiber is broken or bent excessively;
- If red light leakage is visible along the line, it is necessary to accurately locate the damaged point such as the bend/rat bite point, etc.
- When using the red light pen, use the flashing mode to enhance the recognition in a strong light environment, and use the optical fiber identifier to distinguish dense optical fiber lines.
Tool 2: Optical power meter
Power meter initial optical attenuation test, measure the receiving end power.
- If the receiving end power is lower than the standard value (such as -25dBm), it means that the overall attenuation is too large, please check the long-distance loss/splitter failure;
- Compare the transmitting end power, if the receiving end power drops significantly, it means the link is interrupted, if it only drops slightly, check for pollution or bending.
- Note that the error of the two measurements must be less than 3dB to ensure accuracy
2. OTDR long-distance accurate diagnosis (>5km)
When using OTDR for testing, the key points of parameter setting and the impact description are roughly as shown in the following table
| Parameters | Setting principles | Impact description |
| Wavelength (λ) | Consistent with the system wavelength (e.g. 1550nm for a 1550nm system) | Avoid misjudgment of bending loss |
| Pulse width | Use short pulse width (high precision) for short distances and long pulse width for long distances | Long pulse width increases blind area |
| Measurement range | 1.5~2 times the fiber length | Prevent misjudgment of terminal events |
| Refractive index | According to the fiber manufacturer’s data, set accurately | A deviation of 0.001 can cause an error of 100 meters |
Curve analysis skills
- Breakpoint: The curve drops suddenly to the noise level (such as a sudden drop at 20km)
- Splice loss: recover after a small drop (＞0.3dB needs to be checked)
- Bending loss: continue to drop slowly, no reflection peak
- Blind area processing: connect 200m transmission fiber after strong reflection to improve resolution
3. Scenario-based troubleshooting process
Home/enterprise user-side troubleshooting methods
1. Restart the optical modem and check whether the interface is plugged in tightly → Solve 90% of loose problems
2. Observe the optical modem indicator light:
- Red light/registration light flashes →Fiber link interruption
- Green light abnormal →Equipment configuration problem
3. Use a red light pen to detect the incoming fiber: leakage points are mostly caused by bending or extrusion damage
Backbone network/computer room level fault handling method
- Splitter node: OTDR test shows abnormal burrs at the fiber jumper node → clean or replace the pigtail
- Optical cable break: OTDR locates the breakpoint distance → on-site verification combined with GIS map (such as manhole cover crushing/construction digging)
- Misconfiguration troubleshooting: check the corresponding relationship between the fiber jumper ports on the patch panel (LC/SC interfaces are easy to connect incorrectly)
Conclusion
When locating the fault point, we recommend using a red light pen for 1 minute to locate short-distance faults, using an optical power meter for abnormal optical attenuation, and using OTDR+curve analysis for complex links. If multiple troubleshooting fails, check the compatibility of the equipment (such as mixed use of single-mode and multi-mode optical fibers) or the transmission data configuration. At the same time, pay attention to the following points:
- For optical fibers that have been opened for service, red light pens are disabled because lasers damage the equipment;
- Before OTDR testing, the interface must be cleaned, as dust can cause false reflection peaks;
- Bend repair standard: single-mode fiber bending radius > 30mm, multi-mode > 15mm
