FOA guide 


Connector and Splice Loss Testing

Note: In fiber optics, a single connector has no loss. The "loss of a connector" is defined as a "connection loss" caused by a mated pair of connectors. The lab method used to establish the average loss value of a connector design is shown below. The loss of connectors on a patchcord or short cable is given by FOTP-171 and the loss of an installed cable plant is measured by OFSTP-14 (MM) or OFSTP-7 (SM.)

In order to establish a typical loss for connectors, it is necessary to test all connectors in a standardized fashion. Measurements of connector or splice losses are performed by measuring the transmitted power of a short length of cable and then inserting a connector pair or splice into the fiber and measuring the change of loss as a result of adding a connection. This test ( designated FOTP-34 by the TIA) can be used for both multimode and singlemode fiber, but the results for multimode fiber are very dependent on mode power distribution.


FOTP-34 has three options in modal distribution: 1)EMD (equilibrium modal distribution or steady state) , 2) fully filled, and 3) any other conditions as long as they are specified. Besides mode power distribution factors, the uncertainty of the measured loss is a combination of , inherent fiber geometry variations, installed connector or splice characteristics, and the effects of the splice bushing used to align the two connectors.

This test is repeated hundreds or thousands of times by each connector or splice manufacturer, to produce data that shows the repeatability of their connector design, a critical factor in figuring margins for installations using many connectors. Thus loss is not the only criteria for a good connector, it must be repeatable, so its average loss can be used for these margin calculations with some degree of confidence.

Connector and Splice Durability

Another factor important to a connector is the durability of the design, shown by its ability to withstand many matings without degradation in loss. Testing connector durability is simply a matter of repeated mating and demating of a connector pair while measuring loss. Since the loss is a function of both connectors and alignment sleeve, it is helpful to determine which are the contributors to degradation. Plastic alignment sleeves, when used with ceramic connectors, for example,will usually wear out much faster, shaving plastic off onto the connector ferrules and causing increased loss and return loss. When testing durability, periodic inspection of the connector end faces and ferrules with a microscope to determine wear or contamination is very important.

Splice durability is one of withstanding many cycles of environmental stress, since splices are often used in splice enclosures in pedestals or mounted on poles where they are exposed to the extremes of climatic changes. Manufacturers usually test a number of splices through many environmental cycles and accelerated aging to determine their durability. Such tests may take years.

Connector Reflectance

If you have ever looked at a fiber optic connector on an OTDR, you are familiar with the characteristic spike that shows where the connector is. That spike is a measure of the reflectance (sometimes also called  optical return loss) of the connector, the names used for the amount of light that is reflected back up the fiber by light reflections off the interface of the polished end surface of the connector and air. It is also called fresnel reflection and is caused by the light going through the change in index of refraction at the interface between the fiber (n=1.5) and air (n=1).

That return spike is one component of the connector's loss, representing about 0.3 dB loss for a non-contact or air-gap connector (two air/glass interfaces at 4% reflection each), the minimum loss for non-contacting connectors without an index-matching fluid. But in high-bit rate singlemode systems, that reflection can be a major source of bit-error rate problems. In some singlemode systems, the reflected light interferes with the laser diode transmitter, causes mode-hopping and can be a source of noise. Minimizing the light reflected back into the laser is necessary to get maximum performance out of high bit rate laser systems, especially the AM modulated CATV systems. In multimode systems, reflections can add to background noise in the fiber. 

Since this is more a problem with singlemode systems, manufacturers have concentrated on solving the problem for their singlemode components but multimode connectors benefit also. Several schemes have been used to reduce reflectance, mainly reducing the gap between connectors to a few wavelengths of light using a physical contact (PC) polish on the end of the connector ferrule, which reduces the fresnel reflection. The usual technique involves polishing the end surface of the fiber to a convex surface or at a slight angle to prevent direct back reflections.  Another, even more effective solution, is to polish the end of the singlemode connector ferrule at a small angle (about 8 degrees) to cause any reflected light to be absorbed in the fiber cladding.  These are called angle-polish connectors (APC) and are widely used for CATV and high big rate digital systems. 

More on different methods of reflectance testing.


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