Optical & Fiber
Fiber optics carries information as pulses of light instead of electrical signals — immune to electrical noise, capable of enormous bandwidth, and able to run for kilometers between repeaters. The Associate CET exam covers the core physics (how light stays in the fiber), the parts of a cable, and the optoelectronic devices that convert between light and electricity.
A fiber-optic link sends data as modulated light down a glass or plastic strand. Compared to copper, it offers far greater bandwidth, very low loss over distance, complete immunity to electromagnetic interference (light isn't affected by nearby motors, RF, or power lines), no radiated signal to tap, and electrical isolation between the two ends. The trade-offs are more delicate cable, more expensive terminations, and the need for optoelectronic converters at each end.
The whole trick of a fiber is total internal reflection. The fiber has a core of one glass surrounded by cladding with a slightly lower refractive index. Light traveling down the core hits the core-cladding boundary at a shallow angle and reflects completely back in — bouncing along the fiber with almost no loss instead of leaking out the sides.
Light only stays trapped if it enters the fiber within a certain angle. That range forms the cone of acceptance — light entering inside the cone hits the boundary shallowly enough for total internal reflection; light entering at too steep an angle hits the boundary too directly, refracts into the cladding, and is lost. The width of this cone is described by the fiber's numerical aperture (NA).
| Part | Function |
|---|---|
| Core | The central glass/plastic path that actually carries the light. Higher refractive index. |
| Cladding | Surrounds the core with a lower index so light reflects back in. Not a separate tube — it's a glass layer. |
| Coating / buffer | Plastic layer protecting the glass from moisture and scratches. |
| Strength members | Aramid (Kevlar) yarn that takes pulling stress so the fiber isn't strained. |
| Outer jacket | The cable's protective outer covering. |
Single-mode
Very thin core (~9 µm) — light travels essentially one straight path ("mode"). Lowest dispersion, longest distance, highest bandwidth. Used for long-haul links. Driven by lasers.
Multi-mode
Larger core (~50–62.5 µm) — light takes many paths, which spread the pulse over distance (modal dispersion). Cheaper, easier to couple. Used for shorter runs. Often driven by LEDs.
A fiber link needs devices to convert electricity to light at the transmit end and light to electricity at the receive end:
| Device | Role |
|---|---|
| LED | Source: emits light when forward-biased. Inexpensive, used with multi-mode fiber over shorter distances. |
| Laser diode | Source: narrow, coherent, intense beam. Couples into single-mode fiber for long-haul, high-speed links. |
| Photodiode | Detector: converts received light back into current. Reverse-biased; its leakage current rises with incident light. |
| Phototransistor | Detector: like a photodiode but with built-in gain — more sensitive, slower. |
| Optocoupler | An LED + photodetector in one package — transfers a signal between circuits with full electrical isolation. |
Handling
Don't bend fiber past its minimum bend radius — tight bends cause loss and can crack the glass. Keep connector end-faces spotlessly clean; a speck of dust causes significant loss. Splices and connectors are the main loss points.
Safety
Never look into the end of a live fiber or a laser source — the infrared light is invisible but can damage your eye. Dispose of bare fiber scraps carefully; the glass shards are tiny and hard to see.