Free CFOT Exam Flashcards

9 microns (µm). Cladding is 125 µm on every standard telecom fiber type, so core size — not cladding — is what distinguishes singlemode from multimode.

OM1. OM1 (62.5/125 µm) and OM2 (50/125 µm) are legacy multimode grades largely replaced by laser-optimized OM3/OM4/OM5 in new installs.

Total internal reflection: the core has a higher refractive index than the cladding, so light striking the core-cladding boundary at a shallow enough angle reflects back into the core instead of refracting out.

NA describes the maximum angle at which light can enter the fiber and still be guided by total internal reflection. NA = sqrt(n1² − n2²), where n1 is the core index and n2 is the cladding index.

Step-index fiber has an abrupt core-to-cladding refractive index change; graded-index fiber changes the index gradually across the core. Graded-index reduces modal dispersion, which is why most modern multimode fiber uses a graded-index profile.

850 nm and 1300 nm. Attenuation is roughly 3.5 dB/km at 850 nm versus about 1.5 dB/km at 1300 nm, so 1300 nm multimode links suffer less loss over distance.

1310 nm and 1550 nm. Attenuation is about 0.35 dB/km at 1310 nm and drops to about 0.20 dB/km at 1550 nm, making 1550 nm the lower-loss long-haul window.

Modal dispersion occurs only in multimode fiber, where different light paths (modes) arrive at different times and spread the pulse. Chromatic dispersion occurs in all fiber types because different wavelengths of light travel at slightly different speeds.

Aqua jacket color indicates laser-optimized multimode fiber — OM3 or OM4. Color coding lets a technician identify fiber type at a glance: yellow = singlemode, orange = legacy OM1/OM2, aqua = OM3/OM4, lime = OM5.

Loose tube cable. The fibers sit loosely inside gel-filled tubes that isolate them from cable stress and temperature swings, making it well suited to outside-plant (OSP) environments — but it requires a fan-out kit before connectorizing.

Tight-buffered fiber adds a 900 µm secondary buffer directly over the coated 125 µm glass fiber, letting it be handled and terminated directly — which is why tight-buffer construction is standard for indoor premises cable.

Distribution cable bundles multiple buffered fibers under one common jacket and needs a fan-out/breakout kit before connectorizing. Breakout cable gives each fiber its own individually jacketed subunit, so it can be terminated directly without a fan-out kit.

The ST connector. Its bayonet twist-lock is a legacy design, mostly seen on older multimode installations; SC and LC have largely replaced it in new work.

The LC connector. Its half-size 1.25 mm ferrule (versus the 2.5 mm ferrule of SC/ST) allows twice the port density in patch panels and transceivers, which is why LC dominates high-density data center and enterprise deployments.

UPC (Ultra Physical Contact) — blue, flat/slightly curved polish. A good UPC connector should exceed -50 dB of return loss.

APC (Angled Physical Contact) — green. The 8° angle deflects reflected light out of the core instead of back toward the source, giving APC connectors better than -65 dB return loss — required for PON and RF video-overlay links.

The angled APC ferrule and flat UPC ferrule have incompatible end-face geometries — mating them creates a physical gap and air-gap reflection, causing high loss and potential damage. Always match polish type to polish type.

Epoxy/polish gives the lowest loss but is the slowest field method, requiring adhesive cure time and a multi-grit polishing sequence. Pre-polished splice-on connectors terminate in one to two minutes with a factory-polished ferrule, at the cost of a slightly higher loss.

A coarse-to-fine sequence, roughly 12 µm, then 3 µm, then 1 µm, then a final 0.3 µm film — each step removing the scratches left by the previous, coarser grit.

A cleaver scores the fiber with a blade and then breaks it to leave a flat, perpendicular end-face. An acceptable cleave angle is roughly 1-2 degrees; a splicer or connector will reject a cleave with a larger angle.

Fusion splicing (melting the fiber ends together with an electric arc) typically achieves 0.02-0.05 dB loss. Mechanical splicing (aligning fibers in a V-groove with index-matching gel) typically runs 0.1-0.5 dB — higher, but requires no power source.

It fills the tiny air gap between the two fiber end-faces, matching the refractive index of the glass so light passes through without a Fresnel reflection at the gap — reducing loss.

Mass fusion splicing aligns and fuses up to 12 ribbon fibers simultaneously in a single electric arc, dramatically speeding up high-fiber-count cable splicing versus splicing each fiber individually.

0.3 dB per splice. A splice measuring above this on an OTDR trace or power-meter test is generally flagged for re-splicing.

A splice protector — a heat-shrink sleeve with an internal strength rod — is slid over the splice and heated to seal and mechanically reinforce it before it's stored in a splice tray or closure.

An OLTS (optical loss test set — a light source paired with a power meter). It requires access to both ends of the fiber, one for the source and one for the meter, and gives a single total-loss number.

An OTDR (optical time-domain reflectometer). It only needs access to one end of the fiber — it sends pulses down the fiber and reads reflected/backscattered light to build a distance-based trace of every event.

A connector or mechanical splice appears as a reflective event (a sharp upward spike) because of the small air gap and Fresnel reflection. A fusion splice or macrobend appears as a non-reflective event — just a downward step in the trace with no spike.

A dead zone is the blind distance immediately after a strong reflective event, during which the OTDR detector is saturated and cannot resolve nearby events — which is why a launch fiber is used to push the first connector's dead zone off the link under test.

It's a length of fiber inserted between the OTDR and the link under test so the OTDR's near-end dead zone falls on the launch fiber instead of hiding the loss of the link's first connector.

A VFL injects a visible red laser into the fiber so a technician can visually spot a break, tight bend, or bad connector by eye through the jacket. It does not measure loss or distance precisely — it's a quick fault-finding tool, not a certification instrument.

dB is a relative ratio comparing two power levels (used to express loss or gain). dBm is an absolute power measurement referenced to 1 milliwatt. A loss reading is in dB; a power-meter reading of an actual signal is in dBm.

50% — a 3 dB loss corresponds to a halving of optical power. A 10 dB loss corresponds to a 90% power loss, or one-tenth the original power remaining.

Power budget = transmitter output power (dBm) minus receiver sensitivity (dBm). The measured link loss must stay under this budget, typically with a safety margin of about 3 dB built in for aging and future splices.

Insertion loss measures power lost in the forward direction as light passes through a connector, splice, or component — lower is better. Return loss measures power reflected backward toward the source — a larger (more negative) return loss number is better.

Two wavelengths — 850 nm and 1300 nm — because multimode fiber's loss and bandwidth performance differ significantly between the two windows, so certifying at both gives a complete picture of link performance.

10 times the cable's outer diameter. Bending a cable tighter than this while under tension risks microbending loss or fiber damage.

Approximately 600 lbf. Exceeding this during a cable pull risks stretching or breaking the strength members and damaging the fibers inside.

About 30-40% fill. Staying under this ratio leaves room to pull the cable without excessive friction and leaves capacity for future cable additions.

It reduces friction between the cable jacket and the conduit wall, lowering the pulling tension needed and helping keep the tension under the cable's maximum rated pulling force.

Cable routes, splice locations and counts, and OTDR test traces for each link — the records a future technician needs to troubleshoot or extend the plant without re-testing from scratch.

The OLT (optical line terminal) sits at the central office/head end and feeds the PON. The ONT/ONU (optical network terminal/unit) sits at the subscriber premises and terminates the fiber for the customer's equipment.

Downstream (OLT to subscriber) travels on 1490 nm; upstream (subscriber to OLT) travels on 1310 nm — different wavelengths let both directions share one fiber.

Roughly 15-17 dB. This is one of the largest loss contributors in a PON power budget, since it divides one input signal among 32 output legs.

CWDM (coarse WDM) spaces its channels about 20 nm apart, allowing fewer channels but simpler, cheaper optics. DWDM (dense WDM) spaces channels 0.8 nm or less apart, packing far more wavelengths onto a single fiber at higher cost.

TIA-568. It defines cabling topology, distance limits, connector/polarity conventions, and testing requirements referenced throughout FOA certification content.

OFNP is plenum-rated fiber cable, approved for open air-handling spaces without conduit. OFNR is riser-rated, approved for vertical shafts between floors but not for plenum spaces — plenum-rated cable can substitute for riser cable, but not vice versa.

Verify with a power meter (or confirm the source is off) that the fiber is not carrying live laser light before looking at or near the end-face — never assume a fiber is dark.

Telecom lasers commonly used are Class 1M or 3R, which can be invisible (infrared) yet still damage the retina at close range or with magnifying optics. Always view a possibly-live fiber through a video inspection probe/scope, never a direct-view microscope.

Collect it on dark tape and dispose of it in a sealed, puncture-resistant container — bare glass shards are a skin and eye hazard and should never be left loose on a work surface or floor.