FOA CFOS/A key terms

The effect that keeps light guided inside a fiber core: when light strikes the core-cladding boundary at a shallow enough angle, it reflects back into the core instead of refracting out. It is the basis of all step-index and graded-index multimode and single-mode transmission.

The minimum angle, measured from the boundary normal, at which light striking a lower-index medium is completely reflected rather than partly refracted out. Rays arriving steeper than this angle escape the core, so it defines which launch angles the fiber can guide.

A number describing how much light a fiber can accept and still guide, equal to the sine of the widest half-angle of an accepted cone. Higher NA means easier coupling from sources but more modal dispersion in multimode fiber.

The half-angle of the cone of light that a fiber will accept and propagate, measured in air at the launch face. It is derived from the numerical aperture and determines how forgiving a source-to-fiber alignment must be.

The plot of refractive index versus radius across a fiber. A step profile has a uniform core index; a graded profile falls smoothly toward the cladding, which reduces modal dispersion. Profile shape strongly affects bandwidth and bend performance.

Multimode fiber whose core has one uniform refractive index. Rays travel different path lengths, causing large modal dispersion, so it is limited to short distances and low bit rates such as legacy 100 Mb/s and 1 Gb/s links.

Multimode fiber whose core index gradually decreases toward the cladding. Outer rays travel faster in lower-index glass, partly equalizing arrival times, which raises bandwidth enough for gigabit and 10G links over hundreds of meters.

Pulse spreading caused by different modes traveling different paths at different speeds through a multimode fiber. It limits bandwidth-distance product and appears as inter-symbol interference at the receiver. It does not occur in single-mode fiber.

Pulse spreading because different wavelengths travel at slightly different speeds through glass. Even a single-mode signal has finite spectral width, so this effect grows with distance and bit rate and must be managed in long-haul and high-rate links.

Differential delay between the two polarization states of a signal in single-mode fiber, caused by small asymmetries and varying with time and temperature. It becomes significant at very high bit rates over long installed spans.

The difference in propagation delay among mode groups in a multimode fiber, measured as a function of launch offset. It explains why launch conditions matter for high-speed multimode testing and why encircled flux methods were adopted.

A figure of merit, in MHz·km, giving the modal bandwidth available over one kilometer. The usable bandwidth falls as distance rises, so a link's reach at a given bit rate can be estimated from this value and the source's spectral width.

The absorption band near 1383 nm caused by hydroxyl ions in ordinary silica fiber. Legacy fiber cannot use that window, while low-water-peak fiber allows the full 1260-1625 nm range, enabling coarse and dense wavelength division multiplexing.

Loss per unit length of fiber, usually stated in dB/km at a named wavelength. It combines absorption and scattering losses and is the main input for computing a link's optical power budget.

Loss caused by microscopic density fluctuations in glass that scatter light out of the guided direction. It falls as wavelength rises, which is why longer wavelengths generally have lower fiber loss.

Loss caused by bending fiber around a radius large enough to be visible, which lets tightly guided light leak from the core. It grows as bend radius shrinks and as wavelength lengthens, and is controlled by bend-insensitive fiber designs.

Loss from many tiny random bends along a fiber, often caused by poor cable buffering, tight ties, or rough surfaces. It is usually invisible to the eye but can add significant loss and is assessed by the mandrel wrap test.

The diameter of the light-carrying region in a single-mode fiber, slightly larger than the physical core. MFD, not core diameter, governs splice and connector loss, so mismatched MFDs between fibers cause additional loss.

The wavelength below which a nominally single-mode fiber will support more than one mode. Above cutoff the fiber carries one mode; the value depends on fiber length and bend condition and matters for short patch cords.

The allowable loss between transmitter output and receiver sensitivity, in dB. It is calculated from source power, receiver sensitivity, and margins, then compared with the sum of fiber, splice, and connector losses to confirm a link will work.

The minimum optical power a receiver needs to achieve a specified bit error rate. It is one endpoint of the power budget calculation and is stated in dBm, so it must be compared with expected received power in the same units.

An absolute optical power unit referenced to one milliwatt, where 0 dBm equals 1 mW. Transmitter output and receiver sensitivity are stated in dBm, while losses and gains are stated in plain dB.

The predicted total loss of a link, found by adding fiber loss, splice loss, and connector loss, then adding a design margin. It must not exceed the optical power budget for the link to pass design review.

The optical power lost through a component such as a connector pair, splice, or passive device, measured as the drop in dB between input and output. It is the standard acceptance metric for installed links.

The ratio, in dB, of reflected power to incident power at a component or link, with a larger positive value meaning less reflection. Poor return loss disturbs transmitters and can degrade analog and high-rate systems.

The fraction of incident light reflected at an interface, often stated in dB as a negative value. Air gaps and rough end faces raise reflectance, so polishing quality and contact are closely linked to it.

A connector end-face geometry where the fiber is slightly domed so mating ferrules touch directly, eliminating the air gap. It lowers reflectance compared with flat polish; angled variants go further for high-reflection-sensitive systems.

A connector polish with the end face ground at an angle, typically eight degrees, so reflections are directed into the cladding rather than back to the source. APC and PC ferrules must never be mated together.

A launch-condition specification for testing multimode fiber that limits how much power travels in outer modes. Standardized launch reduces measurement variation between testers, making loss results comparable between installers and across projects.

A method of conditioning launch for multimode loss testing by wrapping several turns of fiber around a mandrel to strip outer modes. It approximates a standard launch and is an accepted alternative where encircled flux sources are unavailable.

An instrument that sends short pulses and plots backscattered light versus distance, revealing splice and connector locations, losses, and breaks. It locates faults but its loss readings are directional and need care to interpret.

The distance after a reflective event during which an OTDR cannot resolve further events, caused by receiver recovery. Attenuation and event dead zones differ, and short patch cords can hide near-end connectors if the zone is too long.

A false OTDR trace event caused by light reflecting back and forth between two strong reflectors, appearing as a spike beyond the real end of fiber. Recognizing ghosts prevents misreading a link as longer or more lossy than it is.

The minimum curve radius a cable or fiber may be bent without added loss or damage, usually stated as a multiple of cable diameter. Violating it during installation is a common cause of latent loss and fiber breakage.

The maximum longitudinal force allowed on a cable during installation, specified by the manufacturer and often expressed in newtons or pounds. Exceeding it stretches fibers, raising attenuation or causing later breaks.

A smaller conduit placed inside a larger duct to subdivide space and protect cables. It allows multiple cables in one duct, aids future pulls, and is often color-coded to identify pathways and ownership.

Joining two fibers permanently by aligning and melting their ends with an electric arc. A well-tuned splice has very low loss and reflectance and is the preferred method for outside plant and long spans.

Optical power lost at a joint between two fibers, caused by core offset, angle mismatch, end-face quality, or mode field mismatch. Acceptance values are typically tight, and bidirectional OTDR testing gives the true value.

Carrying several independent signals at different wavelengths on one fiber. Coarse and dense variants differ in channel count and spacing, and dense systems demand tight wavelength control and low-reflectance components.

The consistent arrangement of fiber positions through a link so that transmit at one end reaches receive at the other. Duplex and multi-fiber systems use different methods, and incorrect polarity is a common cause of a link that tests good but does not work.