In reference to testing fiber optic cable assemblies, an interferometer is used to measure the endface geometry of the connector after polishing.  An interferometer measures the differences in path length of light reflected off the connector endface.  Interferometer measurements are accurate to within one wavelength of the light used in measurement.


Interferometry is the technique of diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. The instrument used to interfere the waves together is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, quantum mechanics, plasma physics, and remote sensing.

Fiber optic inspection microscopes

Fiber optic inspection microscopes are used to inspect the end face of an optical fiber connector or cleaved fiber for scratches, dirt, or other defects normally associated with poor transmission performance. There are two major types of fiber microscopes available; one is for fiber termination inspection and the other is for inspection of installed connectors on patch panels and hardware devices.

Visual inspection

After termination and polishing, a fiber optic connector undergoes visual inspection to ensure the endface of the fiber does not contain any faults, such as scratches or pitting.  The visual inspection stage ensures that the polished fibers are of consistent quality.  A clean fiber endface, without scratches or pits, provides better optical properties and improves the connector’s re-mateability as well as the overall lifetime of the connector.


Fiber optic connectors are often polished after termination to remove surface defects and to improve optical qualities such as insertion loss and backreflection.  PC and UPC connectors are polished flat (perpendicular to the length of the straight fiber), whereas APC connectors are polished on an 8o angle from the flat.  In all these cases, the ferrule endface forms a dome-shaped geometry that yields good mating properties in the connector.

Individual connectors in the field are usually hand polished on a flat plate with a rubber pad. In factories making preterminated cables, automatic polishing machines are used to polish many connectors at once.


Usually refers to a single filament made of a dielectric material such as glass or plastic, which is used to guide optical signals.  A fiber consists of a core, and cladding with slightly lower index of refraction.  In addition, the fiber is protected by a buffer layer, and often also covered in Kevlar (aramid yarn) and more buffer tubing.  Optical fibers may be used as a channel to guide light for purposes of illumination or for data and communications applications.  Multiple fibers may be grouped together in fiber optic cables. The diameter of the fiber is usually expressed in microns, with the core diameter shown first, followed by the total fiber diameter (core and cladding together).  For instance, a 62.5/125 multimode fiber has a core 62.5μm in diameter, and is 125μm in diameter in total.


A ferrule is a precision ceramic or metal tube within a fiber optic connector that holds and aligns the fiber.  Some fiber optic connectors, such as the MTP™ connector, have a single, monolithic ferrule, which consists of a single solid component that holds several fibers in a row.  Ceramic ferrules offer the best thermal and mechanical performance, and are preferred for most single fiber connectors.


A connector is an intervening device used to fasten or join. In fiber optics, connectors provide impermanent links between two optical cables, or a fiber optic cable and another optical component.  Connectors must also maintain good optical contact between fibers at the connector interfaces.

Fiber connector

A device mounted to the end of a fiber optic cable, light source, or optical receiver, which mates to a similar device to couple light into and out of optical fibers.  Fiber optic connectors provide an impermanent connection between two fiber optic components, and can be removed and reconnected in a new configuration if desired.  Unlike an electrical connector, where contact of conductors is enough to pass the signal, an optical connection must be precision-aligned to permit the light to pass from one optical fiber to another with minimal loss.

Fiber optic connectors are joined to fiber optic cables by a process called termination.  The connector endfaces are then polished to lessen the amount of light lost at the interface between two connectors.  The polished connectors then undergo a series of tests that certify the optical performance of the connector.

Types of fiber optic connector include:
SC, ST, FC, LC, MU, MT-RJ, D4, E2000, Biconic, MT, MTP™, MPO, SMC, SMA

Since fiber optics began, over 80 different styles of connectors have been used commercially. Most have faded from use or never became popular, so only a few connector styles dominate today’s networks.
Multi-mode installations generally use the ST or SC connector, with a growing number of LC connectors for networks over 1 Gb/s, as the transceivers generally use LCs.
Single-mode applications use mostly SC or LCs, but many installations are still using older designs.

Singlemode connectors

In fiber-optic communication, a single-mode optical fiber (SMF) is an optical fiber designed to carry only a single ray of light (mode). This ray of light often contains a variety of different wavelengths. Although the ray travels parallel to the length of the fiber, it is often called the transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber. Single-mode optical fibers are also called monomode optical fibers, single-mode optical waveguides, or unimode fibers.

Multi-mode connectors

Multi-mode optical fiber (multi-mode fiber or MM fiber or fibre) is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus. Typical multi-mode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters—more than sufficient for the majority of premises applications.

SC connector

The SC connector holds a single fiber in a standard-sized (2.5 mm) ceramic ferrule.  The connector body has a square front profile, and is made of moulded plastic.  Clips on either side of the body and the connector key allow for easy push-in connections.  This push-pull latching mechanism makes the SC connector preferred in high-density interconnect applications such as telecommunications closets and premise wiring.  Two SC connectors may be mounted side by side on duplex cable.  SC connectors have been preferred by the TIA/EIA-568-A industry standard for premise cabling because it is felt to be easier to maintain the polarity of duplex cables with this type of connector.

FC connector

The FC connector holds a single fiber in a standard-sized (2.5 mm) ceramic ferrule.  The connector body is made of nickel-plated brass, and features a key-aligned, threaded locking coupling nut for repeatable, reliable coupling.  The threaded coupling nut provides a secure connector even in high-vibration environments, although it takes slightly longer to connect, since it requires turning the connector instead of a simple push and click.  Some FC style connectors exhibit tunable keying, which means the connector key can be tuned to obtain the best insertion loss, or to otherwise align the fiber.

LC connector

LC is a small form factor connector that uses a 1.25 mm ferrule, half the size of the SC. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for single-mode.

MU connector

  The MU connector holds a single fiber in a  1.25 mm ceramic ferrule.  MU connectors are small form factor connectors that emulate the design of the larger SC connector.  The MU exhibits a square front profile and a moulded plastic body that provides simple push-pull latching connections.  The MU connector is well suited for high-density applications. It's more popular in Japan

ST connector

The ST connector holds a single fiber in a standard-sized (2.5 mm) ceramic ferrule.  The connector body is made of a plastic composite, and the connector couples using a twist-lock mechanism. This connector type is often found in data communications applications.  The ST is versatile, and very popular, as well as comparably cheaper than some other connector styles.

MT-RJ connector

MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multi-mode only, field terminated only by prepolished/splice method.

SMA connector

Amphenol developed the SMA from the "Subminiature A" hence SMA, microwave connector.
The SMA connector holds a single fiber. SMA connectors have a threaded coupling nut.  The ferrule is traditionally made of steel, although ceramic versions are available.  SMA connectors come in two varieties: the SMA-905 has a straight ferrule, whereas the SMA-906 has a step-down barrel.  Both types of SMA connector, the 905 and the 906, are standardized by both NATO and the IEC.

Radius of Curvature

Radius of curvature is the measurement of a connector's end face spherical condition.
The radius generated on a connector end face affects connector performance, and so is specified--the radius must be such that when mated with another connector most of the compression that occurs is applied to the material that surrounds the fiber (also referred to as ferrule absorption). In general, the ferrules used are pre-radiused. The radius is maintained during polishing by applying pressure between the connector and a resilient polishing surface, by application of a weight or by setting compression dimensionally (it is increasingly rare, but the forming of a flat ended ferrule into a PC end is still done through the same basic technique of applying pressure against a resilient surface). The harder the resilient polishing surface the larger will be the resultant connector radius (more flat). Conversely, the softer the polishing surface the smaller the connector radius. A proper radius, in conjunction with fiber under-cut, allows for correct fiber-toconnector compression. The industry specification for radius of curvature is 8-25mm. This range allows for maximum connector performance.


The term Apex defines the highest point on the spherical surface at the end face of the connector.

Apex Offset

Apex Offset is the measured distance between the center of the fiber and the actual high point of a polished connector.
Although Apex Offset describes a physical condition of the polished fiber, rather than a performance parameter, it is considered an acceptance criterion in itself. An excessive Apex Offset contributes to high Insertion Loss and high Backreflection readings. Offset can also be defined as angular offset. Angular offset is the angle between the radial line through the fibre centre and the radial line through the apex of the polish.

Fiber Protrusion or Undercut

When a fiber is recessed inside a connector ferrule the term used is “Fiber Undercut.” When a fiber protrudes above the ferrule it is called “Fiber Protrusion.” Measurement of this characteristic is accomplished by using an interferometer. An Interferometer displays the offset of the interference lines that pass over the fiber.
Most polishing sequences begin with aggressive materials, silicon carbide to remove epoxy and diamond lapping films for the beginning and intermediate polishing, that remove both the ferrule and fiber at the same rate. During the last polishing step, however, a less aggressive material, usually silicon dioxide, is used because it attacks only the fiber. If an aggressive film is used for the final polishing step excessive undercut will result.
Excessive Fiber Undercut is usually specified as more than 50nm. Fiber Undercut is a condition that affects both Backreflection and Insertion Loss. When connectors are mated, the ferrule material surrounding the fiber compresses, which optimally allows

Apex Offset Angle

Pass/Fail criteria is not applied for the Apex Offset Angle parameter in the IEC standard for angled connectors because if the apex offset is good when the connector is oriented at the desired angle, then it doesn't matter because point of contact will be at the core for that angle.

Core dip

Core dip is a recessed area of a fiber core located at the top of the fiber. It appears during polishing process because core is usually softer than cladding material and it is easier removed. Core dip is the area where fibers come into contact with each other when connectors are mated. It prevents the proper physical contact between the fiber cores and thus causes insertion loss and return loss.