Fiber Optic Theory and Transmission Principles
Light guidance
Optical fiber works by total internal reflection. The core has a higher refractive index (n1) than the cladding (n2); light striking the core-cladding boundary above the critical angle reflects back into the core and is guided along the fiber. The half-angle of the cone of light the fiber accepts is the acceptance angle; its sine is the numerical aperture (NA = √(n1² − n2²)). A trap question: higher NA is not automatically better - a larger NA accepts more light but supports more modes, which increases modal dispersion and lowers bandwidth.
Fiber types and windows
Multimode (MM) fiber is 50/125 µm or 62.5/125 µm; OM1–OM5 grades differ in bandwidth and, from OM3 onward, are laser-optimized for 850 nm VCSELs. Singlemode (SM) has a core of roughly 8–10 µm (OS1/OS2) and carries one mode at 1310 nm and 1550 nm. Typical attenuation is about 3.5 dB/km at 850 nm and 1.5 dB/km at 1300 nm for MM, and roughly 0.35 dB/km at 1310 nm and 0.25 dB/km at 1550 nm for SM. Dispersion: modal dispersion dominates MM; chromatic dispersion dominates SM. In standard SM, 1310 nm is near the zero-chromatic-dispersion point while 1550 nm is the lowest-loss window - questions commonly swap these.
Decibel math
A dB expresses a ratio: 3 dB = half power, 10 dB = one-tenth. A dBm expresses absolute power referenced to 1 mW. Never add dBm to dBm.
Worked example
A 2 km SM link at 1310 nm: 2 km × 0.35 dB/km = 0.7 dB loss; two connector pairs at 0.5 dB = 1.0 dB; one splice at 0.2 dB. Total = 1.9 dB. Against a 6 dB budget, the margin is 4.1 dB.