FOA CFOS/A cheat sheet

Total internal reflection occurs only when light strikes the core-cladding boundary above the critical angle; the critical angle is arcsine(n_cladding / n_core).

Numerical aperture NA = sqrt(n_core^2 - n_cladding^2); for step-index fiber the acceptance angle in air is arcsine(NA).

Attenuation in dB = 10 x log10(P_in / P_out); fiber loss per km = (dB loss) / (length in km).

Rayleigh scattering scales approximately as 1/lambda^4, so loss falls steeply from 850 nm to 1310 nm and then flattens toward 1550 nm.

The three standard telecommunications windows are 850 nm (multimode, short reach), 1310 nm (zero dispersion in standard SMF), and 1550 nm (lowest loss, about 0.2 dB/km in SMF).

Multimode fiber carries many modes, so modal dispersion dominates: 62.5/125 um and 50/125 um OM classes differ mainly in bandwidth-length product.

Single-mode fiber core is roughly 8-10 um with a 125 um cladding; the cutoff wavelength must sit below the operating wavelength for true single-mode propagation.

Bend loss appears when the fiber is bent below a critical radius; macrobend loss is worse at long wavelengths, so a 1550 nm test reveals bends a 1310 nm test may miss.

Connector and splice loss are insertion losses measured in dB; reflection is specified as return loss (positive dB, higher is better) or reflectance (negative dB).

OM1 = 62.5/125 um, OM2 = 50/125 um, OM3/OM4 = 50/125 um laser-optimized (OM4 higher bandwidth), OM5 = wideband multimode supporting short-wavelength division multiplexing.

OS1/OS2 are single-mode; OS2 has lower water-peak loss, so it supports the full 1260-1625 nm range including E-band and CWDM wavelengths.

G.652.D is standard single-mode; G.655 is non-zero dispersion-shifted fiber (NZDSF) for DWDM; G.657 is bend-insensitive fiber for tight indoor routing.

Chromatic dispersion is zero near 1310 nm in G.652 and rises to about 17 ps/nm-km at 1550 nm, requiring dispersion compensation or coherent DSP on long links.

Polarization mode dispersion (PMD) arises from core asymmetry and stress; it is statistical, so link budgets use a PMD coefficient in ps/sqrt(km).

Coarse WDM uses 20 nm channel spacing in the 1270-1610 nm range; dense WDM uses 50 or 100 GHz spacing near 1550 nm with tighter laser and filter tolerances.

An optical link budget: receiver sensitivity minus transmitter launch power gives allowed loss; subtract connector, splice, and fiber losses and a margin of typically 3 dB.

Graded-index multimode fiber reduces modal dispersion by continuously refracting rays back toward the core axis, raising bandwidth-length product versus step-index.

TIA-568 defines the structured cabling hierarchy: horizontal cabling, backbone cabling, work area, telecommunications room, and equipment room; maximum horizontal distance is 90 m plus patch cords.

Minimum bend radius: typically 10x cable outer diameter for installation (loaded) and 20x for long-term installed state; follow the manufacturer's data sheet when it is stricter.

Maximum tensile load for installation is usually specified per cable type (often around 220 N for indoor tight-buffered cable); never exceed the manufacturer's rating.

Polarity: duplex LC/SC links use A-to-B or A-to-A plus crossover methods; MPO trunk polarity is Method A (straight), Method B (reversed pairs), or Method C (crossed pairs).

Connector end-face grades per IEC 61753-1: Grade A/B for single-mode and Grade A/B/C/D for multimode, defined by radius of curvature, apex offset, and fiber height.

Color codes: single-mode PC/UPC is typically blue, APC green (8-degree angled), and multimode OM classes use beige (OM1), black (OM2), aqua (OM3/OM4), and lime green (OM5).

Fire ratings: OFNP/OFCP plenum, OFNR/OFCR riser, OFNG/OFNR general purpose; plenum cable is required in air-handling spaces per NEC Article 770.

Fusion splice loss should be under about 0.1 dB and field connector termination under about 0.3 dB; these are common acceptance thresholds in project specifications.

Cable trays, conduits, and innerduct must respect fill ratios; conduit fill is usually limited to about 40 percent of cross-sectional area for multiple cables.

Tier 1 testing uses an OLTS for insertion loss and length; Tier 2 adds an OTDR to characterize splices, connectors, and events along the span.

OLTS testing is bidirectional: measure both directions, report the worse value, and use the correct reference method (one-jumper, two-jumper, or three-jumper) per TIA-526-14 for multimode or TIA-526-7 for single-mode.

Encircled flux launch conditions are mandatory for accurate multimode insertion loss measurement; non-compliant launch overstates or understates loss.

OTDR dead zones: event dead zone (short) and attenuation dead zone (longer) are set by pulse width; shorter pulses improve resolution but reduce dynamic range.

OTDR trace analysis: a reflective event is a spike (connector), a non-reflective step is a splice or bend, and a downward step is loss; a backscatter slope change indicates a fiber mismatch.

Link loss budget = sum of connector losses + splice losses + fiber attenuation x length + margin; design margin is commonly 3 dB for indoor and larger for outdoor plant.

Troubleshooting order: verify reference and test setup first, then inspect and clean end faces, then measure insertion loss, then run OTDR to localize the fault.

Contamination is the leading cause of connector failure; inspect with a scope and clean with lint-free wipes and approved solvent before every mating.

Ethernet distance design: 10GBASE-SR about 300-400 m on OM3/OM4, 10GBASE-LR 10 km on SMF, 40GBASE-SR4 and 100GBASE-SR4 use MPO parallel optics on OM3/OM4.