Post-sale Analysis Report of 25G BIDI Module

Returned Product Failure Analysis Report of 25G BIDI Module

1.Basic Information

2.Failure Description

During the incoming re-test of the returned 25G BIDI 40km optical transceiver, the following failures were observed: the TX optical power was low, measured at -7.76 dBm, which is below the specification limits. Meanwhile, the eye diagram performance was poor, with insufficient eye opening, crossing point deviation, and inadequate margin, failing to meet IEEE 802.3 and industry standard eye mask requirements.

3.Analysis Procedure

Step 1: Re-test to Confirm Failure

To eliminate test system errors, the returned module was re-tested using a standard test board, a calibrated optical oscilloscope, and an optical power meter. The re-test results were consistent with the customer's feedback: TX optical power was -7.76 dBm (abnormal), and the eye diagram failed, showing insufficient eye opening and noticeable noise.

Step 2: Visual & End-face Inspection

A detailed inspection of the optical interface (the ceramic ferrule end-face inside the LC adapter) was performed under a magnifying microscope. Significant contamination was observed on the end-face, including but not limited to dust, fiber residues, or flux/oil-based contaminants. This contamination reduced the optical coupling efficiency and was the direct physical cause of the low TX optical power and degraded eye diagram.

Step 3: Re-test After End-face Cleaning

After performing a standardized cleaning of the end-face using professional fiber optic end-face cleaning tools (lint-free swabs + specialized cleaning solution), the module was re-tested. Post-cleaning, the TX optical power recovered to within specification limits, and the eye diagram returned to normal, with good eye opening, stable crossing point, sufficient margin, and full compliance with the mask requirements.

4.Root Cause Analysis

Based on the above analysis, the root cause of this failure is confirmed as: End-face Contamination at the optical interface.

The contaminants attached to the fiber end-face caused the following effects:

The contaminants attached to the fiber end-face caused the following effects:

（1）Increased insertion loss, resulting in the output optical power dropping to -7.76 dBm;

（2）Introduced reflection and scattering in the optical path, degrading signal integrity and resulting in poor eye diagram performance;

（3）After cleaning the end-face, the optical performance fully recovered, proving that there are no abnormalities in the internal optical components or circuitry of the module.

The possible sources of contamination include:

（1）Insufficient environmental cleanliness control during production and assembly;

（2）End-face contamination during transportation or usage;

（3）Cross-contamination from dirty test adapter end-faces.

（4）Improper handling by the customer during usage, insertion/extraction, or storage, resulting in the optical port end-face coming into contact with contaminants (e.g., finger touching, failure to replace the dust cap in a timely manner, or exposure in a non-clean environment).

5.Conclusion

The root cause of the low TX optical power and poor eye diagram of this module is end-face contamination at the optical port. This is a recoverable cleanliness-related issue, not a functional failure of the components or circuitry. After proper cleaning, the module's optical performance has fully recovered to the qualified status.