Fiber Optics: Basic Technology, Components, and Applications
Core physics
Light travels in a fiber core because the core has a slightly higher refractive index than the cladding; total internal reflection traps light when the angle of incidence at the core-cladding boundary exceeds the critical angle. Two consequences matter for exams: a larger index difference (higher numerical aperture, NA) accepts light over a wider cone of angles but supports more modes and higher intermodal dispersion. Single-mode fiber has a tiny core (about 8-10 µm) and carries one mode at 1310/1550 nm; multimode (50 or 62.5 µm) is used with LEDs or VCSELs over shorter distances and suffers modal dispersion that shrinks bandwidth-length product.
Loss and windows
Silica loss is dominated by Rayleigh scattering at short wavelengths and infrared absorption at long ones, leaving windows near 850, 1310, and 1550 nm; 1550 nm is lowest loss, while 1310 nm has zero material dispersion in standard single-mode. Know attenuation units: dB/km, with connector and splice losses in dB and link budget computed by simple addition.
Components and applications
Candidates should recognize connectors (SC, LC, ST, MPO), polish styles (PC, UPC, APC; APC is angled and must not be mated to flat polish), splices (fusion and mechanical), couplers/splitters, WDM devices, transceivers, and patch panels. Applications include telecom long haul, data centers, FTTH, CATV, sensing, and industrial links.
Traps and a rule of thumb
Traps: confusing NA with attenuation; assuming APC and UPC are interchangeable; forgetting that bend-insensitive fiber still has a minimum bend radius; mixing multimode types (OM3/OM4/OM5) with single-mode transceivers. Rule of thumb: a 3 dB loss equals half the power, so a 10 dB budget leaves about one-tenth of launched power; sum every connector pair (~0.3-0.75 dB), splice (~0.1-0.3 dB), and fiber loss before comparing to the transceiver budget.