Quick answer: Glass fibre uses a silica core with about 0.2 to 0.35 dB per km loss and reaches kilometres. Plastic optical fiber uses a PMMA core with about 0.15 to 0.2 dB per metre loss and reaches tens of metres. Glass is for telecom and data; plastic is for short consumer, automotive and sensor links.
Glass Fibre Optic Cable vs Plastic Optical Fiber: Core and Reach
Most fibre in a building is glass, but not all of it. The red glowing cable behind a television is plastic optical fiber, and the patch cord in a data centre is glass with a silica core. Both guide light by total internal reflection, and both are sold as fibre optic cable. The difference in core material changes loss by three orders of magnitude, which changes the reach from kilometres to metres and the cost from specialist to disposable.
Core, Cladding and Light Guidance
Both cables use a core with a higher refractive index than the cladding, so light striking the boundary at a shallow angle reflects back into the core. Glass fibre is drawn from a silica preform and coated with a protective layer; the core is typically 8 to 10 micrometres for single mode and 50 or 62.5 micrometres for multimode, with a 125 micrometre cladding. Plastic optical fiber uses a PMMA core of about 1 mm with a fluorinated polymer cladding, which makes it easy to cut and connect but limits the number of reflections the signal can survive.
Silica Core Geometry
A single mode core is about 9 micrometres, small enough that only one mode propagates. Multimode cores are 50 or 62.5 micrometres and carry many modes, which causes modal dispersion and limits distance at high speed. The cladding is 125 micrometres in both cases.
PMMA Core and Large Diameter
A 1 mm PMMA core accepts light from a cheap LED without precise alignment. That is why plastic fibre connectors are simple snap-fit parts. The large core also supports many modes, so bandwidth is low and the signal spreads quickly over distance.
Loss and Bandwidth
Glass fibre loss is measured in decibels per kilometre: about 0.35 dB at 1310 nm and 0.2 dB at 1550 nm for single mode, and about 3 dB per km at 850 nm for multimode. Plastic optical fiber loss is measured in decibels per metre, typically 0.15 to 0.2 dB per metre at 650 nm. That is roughly 150 to 200 dB per kilometre, which limits a usable link to tens of metres. Bandwidth follows the same pattern: glass supports gigahertz over kilometres, plastic supports tens of megahertz over metres.
Loss per Unit Length
A 100 m plastic link loses 15 to 20 dB, which consumes most of a simple LED budget. A 100 m glass multimode link loses about 0.3 dB. The same distance is trivial for glass and marginal for plastic.
Bandwidth and Distance
Plastic fibre is used for 100 Mbps over about 100 m in automotive and industrial systems, and for much lower rates in consumer audio. Glass multimode carries 10G over 300 m and single mode carries 100G over kilometres.
Connectors, Termination and Cost
Glass fibre needs precision cleaving, fusion splicing or polished connectors, and the tools are expensive. Plastic fibre can be cut with a hot knife or a simple cutter and terminated with snap connectors, which is why it appears in consumer products and cars. The cable itself is cheap in both cases; the cost difference is in the termination and the test equipment. A glass link justifies an OTDR and a fusion splicer; a plastic link usually does not.
Termination Methods
Glass uses LC, SC or FC connectors with 125 micrometre ferrules, or a fusion splice. Plastic uses simple push-on connectors or a crimped ferrule. A bad cleave on glass causes reflection and loss; a bad cut on plastic causes a visible light leak.
Where the Cost Sits
The glass cable is inexpensive, but the labour and tools are not. Plastic cable costs more per metre than a basic glass patch cord in some sizes, but the installation is trivial. Choose by the number of terminations and the reach, not by cable price alone.
Choosing Between Them
Use glass for any link that leaves a room, exceeds about 50 m, or needs more than a few hundred megabits. Use plastic for short consumer audio links, automotive networks, industrial sensors and decorative lighting. The two are not interchangeable at the connector level, and a plastic link cannot be extended with a glass patch cord without a converter. If a project needs both, plan a media converter or an active optical cable with the right electronics at each end.
Typical Glass Applications
Telecom access, data centre trunks, campus backbones, CCTV backhauls and any outdoor route. Glass survives distance, temperature and bandwidth demands that plastic cannot meet. A single mode span between buildings can run for decades without a media converter, which is why outside plant standardised on silica rather than polymer.
Typical Plastic Applications
Toslink audio cables, car infotainment rings, robot arms, wind turbine sensors and short machine vision links. Plastic tolerates vibration and tight bends, and the connector cost is low.
Glass fibre and plastic optical fiber compared
| Property | Glass fibre | Plastic fibre | Practical effect |
|---|---|---|---|
| Core material | Silica | PMMA | Different loss |
| Core size | 9 to 62.5 um | About 1000 um | Easy plastic alignment |
| Loss | 0.2 to 3 dB per km | 0.15 to 0.2 dB per m | Kilometres vs metres |
| Bandwidth | GHz over km | Tens of MHz | Data vs control |
| Termination | Cleave and splice | Cut and snap | Tool cost |
| Typical reach | Up to 40 km | Up to 100 m | Different markets |
Real-World Application Scenarios
A data centre uses glass multimode for 10G links up to 300 m and single mode for the campus backbone. A soundbar uses a plastic Toslink cable because the run is 1 m and the connector must cost cents. A car uses plastic fibre for a 5 m infotainment ring because it tolerates vibration and can be assembled on a production line without a fusion splicer.
Related Guides on This Site
For fibre types and construction, read the fiber cable types guide and what fiber optic cable is made of. Distance planning is covered in single mode vs multimode, and connector selection in the fiber connector types overview.
Frequently Asked Questions (FAQ)
Q: Is glass fibre optic cable better than plastic?
A: For distance and bandwidth, yes. Glass loses about 0.2 to 0.35 dB per km, while plastic loses 0.15 to 0.2 dB per metre. Plastic is better for short, cheap, vibration-resistant links where a snap connector is enough.
Q: What is plastic optical fiber used for?
A: Short consumer audio links, automotive networks, industrial sensors and decorative lighting. Its large core and simple connectors suit low-cost, short-reach systems up to about 100 m at low data rates.
Q: Can I join glass and plastic fibre?
A: Not directly. The core sizes and connectors differ, and the loss at a joint would be high. Use a media converter or an active link with the correct optical interface at each end.
Q: Why is glass fibre loss measured per km?
A: Because glass loss is low enough that a kilometre is a useful unit. Plastic loss is much higher, so it is measured per metre. The units reflect the practical reach of each material.
Q: Is glass fibre fragile?
A: The bare fibre is brittle, but a cable with a tight buffer, aramid strength members and a jacket is robust. Respect the bend radius and do not crush the jacket. Plastic fibre is more tolerant of tight bends but has much lower performance.