Visual Inspection Of Connectors With A Microscope
Visual inspection of the end surface of connector ferrules with a microscope is used for finding dirt or scratches on fiber optic connectors and inspecting polish-type connectors during the termination process to find possible defects. This requires a microscope which has a fixture to hold the connector in the field of view and a light source to illuminate it properly. Fiber optic inspection microscopes vary in magnification from 30 to 800 power, with 100-200 power being the most widely used range. Some microscopes also can inspect cleaved fibers which are usually viewed from the side, to see breakover and lip.
Fiber Optic Inspection Microscopes
Fiber optic microscopes come in many varieties starting with simple inexpensive portable microscopes generally made by modifying simple optical microscopes to hold the connector being inspected. Special designs for fiber optics will have more sophisticated optics, several illumination options, accessories to hold several types of connectors and probably IR filters to protect the eye from invisible light(see Eye Safety below.) Video microscopes are also popular, offering easier viewing and even saving files of inspections, but at a higher price.
Inspecting Connectors with a Microscope
Visual inspection of the end surface of a connector is one of the best ways to determine the quality of the termination and diagnose problems like dirt on the connector or scratches. A well made connector will have a smooth, polished, scratch free finish, and the fiber will not show any signs of cracks or pistoning (where the fiber is either protruding from the end of the ferrule or pulling back into it).
The proper magnification for viewing connectors is generally accepted to be in the range of 30-400 power, with lower power, usually 100X use for multimode and 200-400 used for more critical singlemode connectors. Lower magnification, typical with a jeweler’s loupe or pocket magnifier, will not provide adequate resolution for judging the finish on the connector. Too high a magnification tends to make small, ignorable faults look worse than they really are. A better solution is to use medium magnification, but inspect the connector three ways: viewing directly at the end of the polished surface with coaxial or oblique lighting, viewing directly with light transmitted through the core, and viewing at an angle with lighting from the opposite angle or with quite oblique lighting.
Viewing directly allows seeing the fiber and the ferrule hole, determining if the ferrule hole is of the proper size, the fiber is centered in the hole and a proper amount of adhesive has been applied. Only the largest scratches may be visible this way, however. Adding light transmitted through the core will make cracks in the end of the fiber, caused by pressure or heat during the polish process, visible.
Viewing the end of the connector at an angle, while lighting it from the opposite side at approximately the same angle or using low-angle lighting and viewing directly will allow the best inspection for the quality of polish and possible scratches. The shadowing effect of angular viewing or lighting enhances the contrast of scratches against the mirror smooth polished surface of the glass.
One needs to be careful in inspecting connectors, however. The tendency is to sometimes be overly critical, especially at high magnification. Only defects over the fiber core are generally considered a problem. Chipping of the glass around the outside of the cladding is not unusual and will have no effect on the ability of the connector to couple light in the core on multimode fibers. Likewise, scratches only on the cladding should not cause any loss problems.
More Examples
dirty connector
Dirt on a connector. The bright center is light transmitted through the core of the fiber. Note how the cladding is dark, not transmitting any significant light.
scratched connector
A scratched singlemode connector. See the small size of the illuminated core?
contaminated connector
Ferrule contaminated by liquid.
Eye Safety
Microscopes focus the light into the eye, so if optical power is present in the fiber, it will be focused into the eye. Since light in most fiber systems is in the infrared (IR) and invisible to humans, it will not be detected. Most fiber optic systems have power levels too low to be harmful but some might – especially telecom and CATV systems. Always check power levels with a power meter before inspecting connectors with a microscope. If possible, only use microscopes with IR filters to prevent IR light from entering the eye.
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