Developments on Fiber Connector End Face Inspection
The performance of an installed optical fiber link hinges on the condition of the connector end faces. Test suppliers have been pushing end face inspection for good reason – most service calls are due to problems with connectors. Standards bodies are busy making improvements to their documents on visual inspection. This work is being done in subcommittee 25 working group 3, the working group responsible for developing standards for customer premises cabling. Recent developments on end face inspection is the topic of this blog.
The end face inspection requirements are described in the 1st committee draft of ISO/IEC 14763-3/Amendment 1 “Testing of optical fibre cabling.” This document contains information for inspecting different kinds of end faces as found in installed fiber optic cabling and test cords. Amendment 1 makes changes to Annex B in ISO/IEC 14763-3 and relies on information found in IEC 61300-3-35 “Visual inspection of fiber optic connectors and fiber-stub transceivers.”
At one time Annex B was normative, e.g., “shall”, “required”, but was changed to informative, e.g., “should”, “recommended”, mostly because the automated analysis of end faces was inconsistent amongst suppliers of inspection equipment. For example, rotation of a connector might produce a different measurement result. Keep in mind that end face inspection is used to identify excessive scratches especially on the fiber core and identify other defects. Other defects include contamination such as dirt, fiber connector’s enemy.
The advantage of automated inspection for installers and other customers is that testing is faster and removes judgment calls. It also makes documentation easier, which can reduce finger-pointing after the job is done. Should the standards change, updating existing equipment can be done in the field.
A calibration standard for visual inspection equipment is currently being developed by a task group dedicated to developing calibration artifacts that all inspection equipment suppliers can use. These artifacts will be distributed to participating companies for testing on their respective equipment as part of a round robin. The round robin has two objectives:
- Compare the scratch detection analysis of inspection equipment.
- Develop a calibration artifact that could be used with inspection equipment.
This study will undoubtedly provide guidance for future revisions of visual inspection standards.
The previously mentioned Annex B in the amendment to ISO/IEC 14763-3 includes these specifications:
- The types of connectors that can be measured such as single fiber LC/PC, multiple fiber MPO, and APC end faces.
- Inspection equipment with considerations to field of view, magnification, and eye safety.
- Return loss requirements for cabling interfaces such as 20 dB for multimode connectors, 35 dB for single-mode connectors, and 60 dB for single-mode APC connectors.
Annex B also contains inspection requirement tables for cabling interfaces with various return losses. In each of the tables, limits for scratches and defects found within the core and cladding are provided. For example, there are two requirements for a scratch on the core of multimode fiber having a 20 dB return loss:
- In the range from 0 micron to 65 micron, any number is allowed for scratches less than or equal to 3 micron in width.
- In the range from 0 micron to 65 micron, no scratches are allowed that are wider than 3 micron.
During a recent review of Amendment 1 it was noted that IEC 61300-3-35 does not provide requirements for inspection of single-mode connectors with 35 dB return loss. Since 35 dB is the performance grade for connectors that ISO 11801-1 (generic cabling standard) specifies, it was agreed that IEC 61300-3-35 should be updated. The update has already started.
In summary, inspecting connector end faces is a must and undoubtedly all vendors are capable of identifying problematic defects. For standards, a proposal has been submitted to update the optical fiber inspection tables found in IEC 61300-3-35. To avoid confusion, it is desired to use this standard as the de facto document for inspection. Upon completion of the round robin, establishment of a calibration artifact, and further improvements in the inspection equipment, Annex B will most likely become normative (shall be required).