Fiber optic cables are made primarily of ultra-pure glass, specifically silicon dioxide (silica), the same compound found in quartz and ordinary sand. Each fiber is thinner than a human hair, yet it carries data as pulses of light across enormous distances. The glass itself is just the starting point. A finished optical fiber is a layered structure, with each layer made from different materials that serve a specific purpose.
The Glass Core and Cladding
Every optical fiber has two concentric glass layers: a central core where light travels, and an outer cladding that keeps the light trapped inside. Both are made of silica, but their compositions are slightly different. The core is doped with tiny amounts of germanium oxide, which raises the glass’s refractive index just enough so that light bouncing against the boundary between core and cladding reflects back inward rather than escaping. This principle, called total internal reflection, is what makes the whole system work.
The cladding sometimes contains fluorine, which has the opposite effect of germanium: it lowers the refractive index. By raising the index in the core and lowering it in the cladding, manufacturers create a sharp optical boundary that guides light efficiently. The actual concentration of these additives is small, typically measured in single-digit percentages. The vast majority of both layers is still pure silica glass.
How the Glass Is Made
You can’t just melt sand and pull a fiber from it. Optical fiber requires glass of extraordinary purity, far beyond anything used in windows or bottles. Impurities measured in parts per billion can absorb light and weaken the signal, so manufacturers use chemical vapor deposition to build the glass from scratch.
The process starts with a hollow silica tube mounted on a lathe. Chemical vapors, including silicon and germanium compounds, flow through the tube while an external torch heats it to extreme temperatures. The chemicals react and deposit as thin layers of ultra-pure glass on the inside of the tube. Layer by layer, the core and cladding build up with precisely controlled compositions. Once deposition is complete, the tube is collapsed under heat into a solid glass rod called a preform. This preform is essentially a scaled-up version of the final fiber, with the same core-and-cladding structure but roughly the size of a baseball bat.
The preform is then fed into a drawing tower, where one end is heated to around 2,000°C. Gravity pulls a thin strand of molten glass downward, and it’s drawn out into a continuous fiber at high speed. The diameter is monitored in real time and kept to about 125 micrometers, roughly the width of a thick piece of hair.
Protective Coatings
Bare glass fiber is fragile. It’s strong under tension but vulnerable to scratches, moisture, and bending. So immediately after being drawn from the preform, the fiber passes through a coating applicator that adds one or two layers of polymer.
The most common coating material is acrylate, a UV-cured polymer applied in two layers. The inner (primary) coating is soft and cushions the glass against tiny imperfections that could become crack points. The outer (secondary) coating is harder and provides abrasion resistance. Together, these coatings roughly double the fiber’s diameter to about 250 micrometers. For everyday telecom and internet applications, acrylate coatings work well at normal temperatures.
Harsher environments call for tougher materials. Polyimide coatings handle temperatures from negative 150°C up to 300°C, making them common in oil well sensors, jet engines, and industrial monitoring. Adding a thin carbon layer underneath the polyimide blocks hydrogen gas from penetrating the glass, which would otherwise degrade signal quality over time. With this carbon buffer layer and special processing, fibers can withstand temperatures approaching 400°C for decades. Silicone coatings offer another option, with good flexibility and chemical resistance across a wide temperature range.
The Cable Structure Around the Fiber
What most people see and handle isn’t the fiber itself but the cable built around it. A single cable may contain anywhere from one to several hundred individual fibers, each with its own coating, bundled together inside additional protective layers.
A typical cable includes a buffer tube (usually plastic) surrounding each coated fiber, strength members made of aramid yarn (the same material in bulletproof vests) or fiberglass rods, and an outer jacket of polyethylene or similar durable plastic. Cables designed for burial underground often add a layer of steel or aluminum armor. Submarine cables, which carry internet traffic across ocean floors, include copper conductors for powering signal repeaters along with heavy steel wire armoring to resist water pressure and anchors.
Plastic Optical Fiber
Not all fiber optics are glass. Plastic optical fiber uses a core made of polymethyl methacrylate (acrylic) or similar transparent polymers, with a cladding of fluorinated polymer. The core is much larger than glass fiber, often around 1 millimeter in diameter, which makes it easier to connect and handle but limits its performance. Light loss in plastic fiber is far higher than in glass, so it’s only practical for short distances: think home audio connections, car networks, and industrial links within a single building. For anything beyond a few hundred meters, glass fiber is the standard.
Specialty Glass Fibers
Standard silica glass transmits light well in the near-infrared range used by telecom lasers, but it becomes opaque at longer wavelengths. For applications like thermal imaging, chemical sensing, and mid-infrared lasers, other glass compositions take over.
Fluoride glass fibers replace silica with combinations of metal fluorides. The most well-known formulation, called ZBLAN, contains zirconium fluoride (53%), barium fluoride (20%), lanthanum fluoride (4%), aluminum fluoride (3%), and sodium fluoride (20%). These fibers transmit light at wavelengths where silica is useless, extending deep into the mid-infrared. Variations swap in hafnium or indium fluoride to shift the transmission window or improve durability.
Chalcogenide glass fibers push even further into the infrared. They’re built from sulfur, selenium, or tellurium combined with elements like germanium, arsenic, or antimony. A common example is arsenic trisulfide glass. These materials transmit wavelengths out to 10 micrometers or beyond, which is valuable for detecting specific chemical signatures in gas sensing and spectroscopy. They’re softer and more fragile than silica, so they remain specialty products rather than mainstream infrastructure.
In all cases, the fundamental design is the same: a higher-index core surrounded by a lower-index cladding, wrapped in protective coatings. What changes across applications is the specific chemistry chosen to match the wavelength of light being carried and the environment the fiber needs to survive.