Failure Analysis and Quality Improvement Case for 100G LR4 Optical Module (Transmitter Channel Issue)

Failure Analysis and Quality Improvement Case for 100G LR4 Optical Module (Transmitter Channel Issue)

1. Case Background
On July, 2025, one unit of the 100G LR4 optical module was returned by the customer. No specific failure description was provided by the customer. Our quality team immediately conducted a comprehensive failure analysis on the returned module.

2. Preliminary Test Findings
Under room temperature testing, the module exhibited the following issues during self-loopback testing on a bit error rate tester (BERT):

• Channel 2 of the receiver failed to establish a link.

• DDM (Digital Diagnostic Monitoring) values for Channel 2 were significantly lower than those of the other channels.（Picture as blow）

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• Further inspection using an oscilloscope revealed low optical output power and poor eye diagram quality on the second transmitter channel.

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• The normal optical power and eye diagram should be as follows (the actual optical power needs to be increased by 7.5 dBm):
  The displayed optical power is: -5.25 dBm, and the actual optical power is: 2.2 dBm.

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3. Root Cause Analysis
Upon further investigation:

• The abnormal channel showed an optical power of only -5.25 dBm (as displayed), while the actual expected output power (after accounting for a 7.5 dBm offset) should have been approximately 2.2 dBm.

• The eye diagram was severely degraded, indicating poor signal integrity.

• The issue was traced to component misalignment during assembly, resulting in low optical coupling efficiency.

• This type of defect is not recoverable through tuning or firmware adjustment and requires replacement of the transmitter optical sub-assembly (TOSA).

4. Preventive Measures
In addition to immediate corrections, the following process improvements have been introduced:

• Increased thermal cycling intensity and duration for incoming component screening.

• Extended burn-in time from 48 hours to 72 hours for finished modules.

• 100% final inspection

5. Recommendations for Customer Use
To minimize ESD and contamination risks, customers are advised to:

• Always keep dust caps on when the module is not in use.

• Clean fiber end faces with alcohol-soaked swabs before connection.

• Avoid bending fiber cables less than 5 cm in radius.

• Ensure proper grounding during handling and installation.

6. Conclusion
This case highlights the importance of process control in optical module assembly. Through root cause analysis and targeted corrective actions, we have strengthened our quality assurance system and reduced the risk of similar failures in the future.