Common Faults and Solutions in Optical Fiber Transmission

Although optical fiber transmission is efficient and reliable, various faults may also be encountered in practical applications. Fiber optic fault detection is a systematic project. Problems may occur from the physical layer to the protocol layer, and the symptoms are similar (such as packet loss, interruption), which requires systematic elimination. The user might have just encountered a network outage and is under pressure to carry out emergency repairs, in need of a quick and actionable solution.

Typical classifications of optical fiber faults: physical damage (accounting for the majority), connection issues, equipment failures, environmental interference, and configuration errors. The physical layer is the most common, so the issues of cleaning and bending should be emphasized particularly. If the user is a novice, they may overlook the basic point that optical fibers cannot be bent at right angles.

Several key scenarios have come to mind: the new link is not working after construction (most likely due to fusion or joint issues), sudden interruption during operation (caused by external force damage), and intermittent faults (bending or dirt). The solutions should be sorted by priority, for example, visual inspection first and then loading the instrument.

It is particularly important to note that users may not have professional equipment like OTDR, so they have to use methods that can be judged by the naked eye, such as checking the on-off status with a flashlight. This technique is very practical.

Although protocol layer failures are few, users are prone to misjudgment. For instance, if the optical power is normal but not working, it might be due to compatibility issues with the device or rate Settings. Users have to be reminded not to just focus on the physical layer.

ZORA Indoor OS2 Fiber Optic Cable

ZORA outdoor non-armored OM3 loose tube fiber optic cable

What are the common optical fiber transmission faults and their solutions?

The following is a review in the order from the physical layer to the logical layer:

I. Physical Layer Faults (Most Common)

1. Fiber optic breakage or damage:

Reasons: Improper construction (excessive pulling, flattening, right-angle bending), external force damage (excavation, rodent gnawing), natural disasters, aging and deterioration.

Phenomenon: Complete communication interruption, no light or extremely low light at the receiving end of the optical power meter, OTDR test shows reflection peaks or large loss points.

Solution:

•  Locate the breakpoint: Use OTDR to precisely locate the fault point (distance and location).

•  Fix:

Splicing: Locate the break point, cut off the damaged part, and re-splice the optical fiber (a splicing machine is required).

Replacement: If the damage is severe or it cannot be spliced (such as deep in the pipeline), the entire optical cable should be replaced or a spare core should be used.

•  Prevention: Standardize construction, reserve sufficient bending radius (static >10 times the fiber diameter, dynamic >20 times the fiber diameter), use protective sleeves to avoid external force compression.

Reasons: The radius of the coil fiber is too small (less than the minimum bending radius), the optical cable is squeezed and twisted inside the cabinet/distribution frame, and there are sharp bends in the cable routing path.

Phenomenon: Macro bend: The optical power drops significantly (or even is interrupted). Micro-bending: It causes additional losses and may lead to a high bit error rate or instability.

Solution:

• Check the wiring: Carefully inspect the entire link, especially around the patch panels, terminal boxes, corners, and near the equipment interfaces.

• Release bending: Remove overly small bends or compressions to ensure that all bending radii meet the requirements (generally, macro bends >30mm, and micro-bends should be avoided).

• Re-winding: Use a larger winding ring inside the terminal box or patch panel.

• Use a wire manager: Standardize the wiring to avoid tangling and squeezing.

• Prevention: When designing and installing, reserve sufficient space and strictly adhere to the minimum bending radius specification.

4. Poor quality of optical fiber fusion points or connectors:

Reasons: Improper parameter Settings of the fusion splicer, electrode aging, poor treatment of the optical fiber end face, poor cutting Angle, and poor fusion splicing environment (dust, wind); Inferior connectors or flanges.

Phenomenon: Excessive loss at this point (OTDR test shows a high-loss event point, and the optical power meter shows that the total link loss exceeds the standard).

Solution:

• OTDR test positioning: Determine the location and nature of high-loss points (fusion points or connection points).

• Rewelding/remanufacturing: Remanufacturing substandard welding points or connectors. Ensure the use of qualified consumables and standardized operations.

• Replace components: Replace inferior connectors or flanges.

• Prevention: Use qualified welding machines and consumables, calibrate welding machines regularly, train qualified welding personnel, and select brand-name connection devices.

5. Optical fiber end face mismatch:

Reason: Attempting to connect different types of optical fibers (such as SMF to MMF), or using incompatible end face grinding types (such as APC to UPC). Although it can be physically plugged in, the loss is huge.

Phenomenon: Extremely high insertion loss, unable to establish normal communication.

Solution:

• Check the type and end face of the optical fiber: Confirm that the same type (SMF/MMF) and the same end face grinding (UPC/UPC or APC/APC) optical fibers and connectors are used at both ends of the link.

• Replace matching components: Use jumper or adapter of the correct type and end face.

Ii. Equipment/Signal layer failure

1. Optical module failure:

Reasons: Aging, overheating, electrical faults, and quality defects of optical modules (such as SFP/SFP+/QSFP, etc.).

Phenomenon: Abnormal indicator light at the device port (not on, constantly on but not flashing, color error), the optical power meter shows no light at the transmitting end or abnormally low/high optical power (exceeding the module specification), no light at the receiving end or normal optical power but the device cannot recognize the module or cannot be UP.

Solution:

• Check the indicator lights: Observe the status lights of the device ports and optical modules.

• Measure optical power: Use an optical power meter to measure the optical power at the transmitting end and the receiving end, and compare it with the module specification.

• Check the device logs: Inspect the switch/router logs for any errors reported by optical modules (such as' RX loss of signal ', 'Module temperature high', 'Unsupported transceiver').

• Interchange test: Swap the suspicious optical module with a normally functioning module of the same model (on the same type of port and optical fiber) for testing. If the fault shifts along with the module, it is a module problem.

• Replace the optical module: Replace it after confirming the module fault.

• Prevention: Ensure good heat dissipation of the equipment and select compatible and reliable optical modules.

2. Equipment port failure:

Reasons: Hardware damage to the device port, configuration errors (such as closing the port, rate/duplex mode mismatch), software bugs.

Phenomenon: The physical link (optical fiber) test is normal, but the port cannot be UP, frequently UP/DOWN, negotiation fails, and the rate is incorrect.

Solution:

• Check the configuration: Confirm that the port has not been 'shutdown', and the rate (' speed ') and duplex mode (' Duplex ') are configured correctly (usually set to 'auto').

• Check the log: Inspect the device log for error messages regarding the port.

• Change the port: Switch the jumper to another port of the same type on the device for testing.

• Restart port/device: Try restarting the port or the entire device.

• Upgrade firmware: If you suspect a software Bug, try upgrading the device's firmware.

• Replace the equipment board card/report for repair: If it is confirmed that the port hardware failure occurs, the board card needs to be replaced or the equipment needs to be reported for repair.

3. Insufficient or overloaded optical power:

Reason:

Shortcomings: Excessive link loss (such as long distance, too many connection points, bending, etc.), low transmission optical power, and decreased receiving sensitivity.

Overload: The transmitted optical power is too high (especially when using long-range modules over short distances), and the link loss is too small (no attenuation over short distances).

Phenomenon: Insufficient optical power leads to high bit error rate, packet loss, intermittent interruption or complete disconnection. Optical power overload may lead to saturation, damage or abnormality at the receiving end.

Solution:

• Measure optical power: Use an optical power meter to measure at the receiving end, ensuring that it is between the receiving sensitivity of the optical module and the overload point.

• Check the transmission power: If possible, measure whether the optical power at the transmitting end is within the specification range.

• Calculate the link budget: Compare the link budget at the time of design (transmission power - receiving sensitivity > total link loss) to confirm whether the actual loss is within the budget.

• Adjustment plan:

      -Shortcomings: Reducing connection points, repairing high-loss points, replacing with higher-power transmission modules (with caution), using optical amplifiers (relays), shortening the distance.

      -Overload: Connect a fiber optic attenuator in series at the transmitting end or the receiving end to reduce the optical power reaching the receiving end.

• Prevention: Accurately calculate the link budget during the design stage, select appropriate modules (avoid using long-distance modules for short-distance ones), and standardize construction to reduce losses.

Iii. Environmental/External Factors

External interference:

Reasons: Strong electromagnetic field interference (near high-power motors, transformers, radio transmission sources), radiation (near nuclear facilities).

Phenomenon: Noise may be introduced, leading to an increase in bit error rate, and in severe cases, interruption may occur. Note: The optical fiber itself is resistant to electromagnetic interference, but the interface circuits of the equipment may be affected.

Solution:

• Stay away from interference sources: Re-plan the optical cable route to avoid areas with strong interference.

• Shielding: Use metal armored optical cables or provide good grounding and shielding for the equipment.

• Check the grounding of the equipment: Ensure that the optical terminal equipment is well grounded.

   

  
  

• 

• 

General steps for troubleshooting

1. Clarify the phenomenon: Is it a complete interruption? Intermittent? Slow speed? High bit error? Scope of influence?

2. Preliminary Inspection:

Visual inspection: Check the status of the device port indicator light (Link/Active), and whether there is any obvious physical damage, excessive bending or loosening of the optical fiber patch cord.

Device status: Log in to the device to check the port status (UP/DOWN), error count, and log information.

Simple test: Shine a flashlight (visible light) on one end of the optical fiber and observe at the other end if there is red light (only applicable to visible wavelength optical fibers and multimode optical fibers over short distances, strictly prohibited for laser communication optical fibers!) Danger! .

3. Optical power test:

Measure the optical power at the receiving end using an optical power meter. This is the fastest way to determine whether there are optical signals in the physical layer and the strength of the signals. Compare the module specifications and link budgets.

4. Segmented elimination:

Divide the long link into several sections (e.g., Device A -> patch cord A -> patch Panel A -> backbone fiber -> Patch panel B -> Patch cord B -> Device B).

Starting from the equipment at both ends, gradually insert patch cords, test to the patch panel, etc., to locate at which section or connection point the fault occurs.

5. OTDR Test (Precise Positioning) :

When the optical power test is abnormal or it is necessary to precisely locate the breakpoints or high-loss points, use OTDR. It can display the loss distribution of the entire optical fiber, the positions of event points (connectors, fusion points, bends, breakpoints), and the loss values. Professional personnel and equipment are needed.

6. Clean the connectors: In most suspected physical layer failures, cleaning the connectors is the primary and most cost-effective step!

7. Substitution method:

Replace the suspicious components with the known normal patch cords and optical modules.

Swap the patch cords for testing (for example, switch the TX patch cord of device A to the RX port for testing).

Test by changing the device port.

8. Check the configuration: Confirm that the device port configuration (on/off, rate/duplex mode, VLAN, etc.) is correct.

9. Check logs: Carefully analyze the system logs and port logs of devices such as switches, routers, and optical transceivers.

Important Safety Tips:  

Never look directly at the optical fiber port! The laser energy transmitted in optical fibers is extremely high. Even invisible light (such as 1310nm, 1550nm) can cause permanent and irreversible damage to the eyes! Before performing any optical fiber operations (plugging and unplugging, testing), make sure that the optical module is not working or the light source has been disconnected.

Wear goggles when operating optical fibers.

Keep the working environment clean and avoid dust contaminating the end face.

By systematically applying the above methods and tools, most optical fiber transmission faults can be effectively located and resolved. Cleaning connectors, measuring optical power, OTDR positioning, and the segmented elimination and replacement method are the core troubleshooting approaches.