Fiber Optic Technology Fundamentals
Domain 1 tests the physics that explains every later measurement: how light is guided, why some fibers support many modes, and how power is expressed. Expect definitions, simple calculations, and interpretation of source/detector behavior.
Guidance and modes
- A step-index fiber guides by total internal reflection at the core–cladding boundary. The critical angle is measured from the normal: θc = arcsin(n2/n1). TIR occurs only when the ray inside the core strikes the boundary at an angle greater than θc.
- Numerical aperture in air is NA = √(n1² − n2²). It is dimensionless. The acceptance half-angle in air is θa = arcsin(NA).
- Multimode fiber (50 or 62.5 µm core) carries many modes and suffers modal dispersion; single-mode fiber (about 8–10 µm core) carries one mode and has a much smaller NA, typically around 0.1.
- Common windows are 850 and 1300 nm for multimode, and 1310 and 1550 nm for single-mode. Attenuation is quoted in dB/km; connector and splice loss in dB; absolute power in dBm.
Worked example
A multimode fiber has n1 = 1.48 and n2 = 1.46. NA = √(1.48² − 1.46²) = √(2.1904 − 2.1316) = √0.0588 = 0.2425. The acceptance half-angle in air is arcsin(0.2425) ≈ 14.0°. Reasoning: light entering within a 14° cone of the fiber axis is guided; a ray launched at 20° exceeds the acceptance cone and will not be guided efficiently. Note that the critical angle at the boundary is arcsin(1.46/1.48) ≈ 80.6° from the normal, which corresponds to about 9.4° from the axis - the two angles are related but not equal.
Traps that cost marks
- Treating NA as an angle in degrees, or setting NA equal to the acceptance angle.
- Measuring the critical angle from the fiber axis instead of the normal.
- Assuming single-mode fiber has a large core or high NA.
- Mixing dB and dBm: a 3 dB loss halves power; it does not mean −3 dBm.
- Claiming 1300 nm is a zero-attenuation window; it is a low-attenuation window and, for standard single-mode, near zero chromatic dispersion.