Fiber Optic Connectors Excessively Polished

Indicators of over-polish on fiber optic connectors have poor repeatability in return loss and insertion loss measurements. For most "spring ferrule connectors" eg; LC, SC, MU, FC connectors the over polish is difficult to measure.

In most cases, connector manufacturers specify the critical length from the end of the ceramic ferrule to the bottom of the metal fiber optic flange. This critical length will then translate into proper exposure relative to the connector housing and ensure proper mating engagement when in a fiber optic adapter. Since there are no length gauges available to do this properly, and the length of ferrules in metal fiber optic flanges is often not available, an alternative method of monitoring connector polish life is to "calculate" the internal maximum time to repolish.

The maximum number varies from vendor to vendor and can only be truly determined through long-term reliability testing involving vibration, environmental, and other mechanical testing.

Spring-loaded fiber optic connectors are largely polished and rely heavily on the projected and calculated spring force of each fiber optic connector to ensure a secure fit inside the adapter's split sleeve. For example, if the connector has been ground (more than 3 times from the start), it will be difficult to control insertion loss and return loss for repeatable reading. The most notable is return loss, which will show more variation in the reading due to changes in spring force.

The number of times a connector can be repolished is not documented, but generally I instruct all new manufacturers to limit repolishing to 2. This is to prevent excessive removal of ceramic ferrule material and subsequent shortening of the ferrule from the tip to the bottom of the metal flange.

Another "visual" method of monitoring fiber optic connector polish is to use a reticle scale to measure the initial ceramic bevel length on the ferrule face, the bevel length varies with each connector/ferrule as the connector is polished Depending on the manufacturer, the bevel length can be shortened, especially in over-polished connectors.

For other types of actively used connectors, such as SMA, this connector is designed to have a specific pre-polished length. This is the critical length that both connectors must maintain in the connector. Mated fiber optic adapters contact each other and provide repeatable insertion loss readings. If the finish of one of the connectors exceeds even .0001, no contact will occur and cause a high loss mating. Polish lengths thereafter also apply to older ATT Biconic style connectors that require final polish to be maintained at .0003 inches.