A fiber optic cable is a cable that transmits data as pulses of light through strands of ultra-pure glass, rather than as electrical signals through copper wire. Each glass strand is thinner than a human hair, yet a single fiber can carry up to 32 terabytes of data per second. Fiber cables form the backbone of the modern internet, connecting continents through undersea lines and delivering high-speed broadband to homes and businesses.
What’s Inside a Fiber Cable
A fiber optic cable has five basic layers, each serving a distinct purpose. At the center is the core, a strand of exceptionally pure silicon dioxide (glass) so transparent that looking through five miles of it would be like peering through a household window. Some lower-cost cables use plastic cores instead, but glass dominates in telecommunications.
Surrounding the core is the cladding, a layer of slightly different glass with a lower refractive index. This difference in refractive index is what keeps light trapped inside the core. Both the core and cladding are manufactured together in a permanently fused state, forming a single optical unit.
Outside the cladding sits a protective coating, typically made of acrylate applied in two layers. This coating acts as a shock absorber, shielding the delicate glass from nicks, scratches, moisture, and physical impacts. Beyond that, strengthening fibers (often made of aramid yarn, the same material in bulletproof vests) give the cable tensile strength. Finally, an outer cable jacket wraps everything together and protects against the environment.
How Light Carries Data
Fiber cables work through a principle called total internal reflection. When light enters the glass core at a shallow enough angle, it bounces off the boundary between the core and cladding rather than passing through it. The core’s refractive index (about 1.46) is just slightly higher than the cladding’s (about 1.45), and this tiny difference is enough to trap the light completely, bouncing it down the length of the fiber until it reaches the other end.
A transmitter at one end converts electrical data into rapid pulses of light. These pulses travel through the core, reflecting thousands of times per meter, and a receiver at the far end converts the light back into electrical signals. Because light moves incredibly fast and the glass is so pure, fiber can carry enormous amounts of data over long distances with very little signal loss.
Single-Mode vs. Multi-Mode Fiber
Not all fiber cables are the same. The two main types differ in core size, light source, and how far they can send data.
Single-mode fiber has a tiny core, just 8 to 9 micrometers across. It uses a laser to send a single beam of light straight down the center. This focused approach allows data to travel much farther without degrading. Single-mode fiber routinely supports distances of 10 kilometers (about 6 miles) for standard 10-gigabit connections, and specialized configurations for fiber-to-the-home networks can reach 30 kilometers. It’s the standard choice for long-distance telecommunications, internet backbones, and connections between buildings.
Multi-mode fiber has a wider core of 50 or 62.5 micrometers, which allows multiple beams of light to travel simultaneously at slightly different angles. It typically uses LEDs or a type of small laser called a VCSEL as its light source. The tradeoff for that wider core is shorter range, generally up to 2 kilometers for older standards, though modern high-speed versions (rated OM3 through OM5) are designed specifically for data centers where distances are short but speed demands are extreme, supporting 200 and 400 gigabit connections.
How Fiber Compares to Copper
The practical differences between fiber and copper are significant. In direct testing, fiber optic cables delivered latency of 3 to 4 milliseconds compared to 13 to 14 milliseconds for copper wire cables, making fiber roughly three to four times faster in response time. That gap widened slightly as temperatures increased, with copper performance degrading more than fiber under heat.
Because fiber transmits light instead of electricity, it’s immune to electromagnetic interference. Copper cables pick up noise from nearby power lines, motors, and radio signals, which can corrupt data and require error correction. Fiber works reliably in electrically noisy environments like factories, hospitals, and areas near heavy equipment. It also can’t be tapped as easily as copper, since intercepting a light signal without breaking the connection is extremely difficult.
Fiber does have physical limitations. All fiber cables have a minimum bend radius that must be respected during installation. The general rule is that the cable shouldn’t be bent tighter than 20 times its diameter while being pulled, and no tighter than 10 times its diameter once installed. A standard cable about 13 millimeters wide, for example, needs bends no tighter than 260 millimeters (about 10 inches) during installation. Bending past these limits can crack fibers or increase signal loss, and the damage isn’t always visible from outside.
Common Connector Types
Fiber cables plug into equipment using standardized connectors, and you’ll encounter a few common types depending on the setting. LC connectors are compact and widely used in data centers, telecom networks, and office LANs. SC connectors are slightly larger with a square, push-pull design common in telecom and data center environments. ST connectors use a bayonet-style twist lock and appear in older networks, LANs, and medical imaging equipment.
For high-density environments where dozens or hundreds of fibers need to connect in tight spaces, MPO/MTP connectors bundle multiple fibers into a single plug. These are standard in modern data centers handling large volumes of traffic.
Where Fiber Cables Are Used
Fiber’s most visible role is in telecommunications. Undersea fiber cables replaced copper long ago and now carry virtually all intercontinental internet traffic. On land, fiber runs from city to city and increasingly all the way to homes through fiber-to-the-home (FTTH) networks.
In medicine, fiber optics enable endoscopic surgery by delivering both light and a visual feed through thin, flexible cables inserted into the body. Dentists use fiber optic tools to shine focused light into teeth when searching for cracks and cavities. The military uses fiber for everything from underwater sensor systems to command communications, partly because the cables are resistant to electronic jamming. In cars, fiber optic lines carry signals between sensors and onboard computers and handle interior and exterior lighting. Aerospace applications range from in-flight communication cables to sensors that monitor structural stress.
Hollow-Core Fiber: The Next Generation
Researchers at the University of Southampton have developed a new type of fiber that replaces the solid glass core with air. Because light travels faster through air than glass, these hollow-core fibers transmit data up to 45% faster and reduce signal loss by 35% compared to standard fibers. The signal loss in their latest design is just 0.091 decibels per kilometer, which means fewer signal amplifiers would be needed in long cable runs, cutting both cost and energy use.
A single hollow-core fiber strand can also support a bandwidth of up to 54 terahertz, compared to 10 terahertz for conventional fiber. That’s roughly five times the capacity for carrying separate data channels simultaneously. The drop in latency could matter for real-time applications like remote surgery, online gaming, and training large AI models where even small delays add up. Microsoft has already begun testing these fibers in live network segments, confirming they work with existing telecom equipment, which opens the door to gradual adoption without replacing entire infrastructure at once.