In short
- Light stays in the core by total internal reflection. The core has a slightly higher refractive index than the cladding around it.
- Numerical aperture (NA) sets the acceptance cone. NA = √(ncore² − nclad²). A higher NA accepts light from wider angles and couples sources more easily.
- Single-mode or multimode follows from the V-number. Below V ≈ 2.405 the fiber carries one mode: small core, long reach. Larger cores carry many modes and more power over short links.
- OH content decides the spectral window. High-OH silica for UV and visible light, low-OH silica for the near infrared.
- A fiber assembly is more than the fiber. Ferrule, core diameter, length, the connector at each end (SMA 905, FC, ST, SC, LC or bare) and the jacket decide whether it fits your set-up. IZAK designs and supplies fibers made to order.
- See it tested: two videos show the IZAK Fiber Ferrule Inspection System checking 8 ferrules per side, 4 Tx and 4 Rx, for optical communication.
Modern communication depends on speed and reliability, and optical fiber, a thin and flexible strand of glass or plastic, sits at the heart of it. This guide explains how fiber works, the physics behind it, the main fiber types and where fiber is used.
What is fiber optics?
Fiber optics sends information, or energy, as light through thin strands of glass or plastic. Copper wires carry electrical current; an optical fiber carries photons. That difference gives fiber far more bandwidth, much lower loss over distance and complete immunity to electromagnetic interference.
Today optical fiber is the backbone of the internet and telephone networks. It also delivers laser energy in surgery, carries light to and from spectrometers, and works as a sensor in places where electronics cannot survive.
How an optical fiber guides light
A standard fiber has three layers: a core that carries the light, a cladding with a slightly lower refractive index, and a protective coating or buffer. The index step between core and cladding is small, typically well under one percent for glass telecom fiber, yet it is enough to trap the light.
When light inside the core meets the core–cladding boundary at a grazing angle, beyond the critical angle θc = arcsin(nclad/ncore), it is reflected completely back into the core. This is total internal reflection. The light zig-zags along the fiber, or in single-mode fiber propagates as one guided wave, with almost no energy escaping through the walls.
The science behind optical fibers
Four ideas explain how a fiber performs: total internal reflection, numerical aperture, bend radius and the transmission properties of the glass.
Total internal reflection
Light that meets the core–cladding boundary at a shallow enough angle is reflected back into the core. This needs a core index higher than the cladding index. Beyond the critical angle the reflection is complete, which is why a fiber can carry light over long distances with very low attenuation.
Numerical aperture
The numerical aperture (NA) is a dimensionless number that describes how much light a fiber can collect. It equals the sine of the largest entry angle, measured in air, at which light is still guided by total internal reflection: NA = √(ncore² − nclad²). A higher NA accepts light over a wider range of angles and makes coupling from LEDs and lasers easier.
Bend radius
The minimum bend radius is the smallest radius to which a fiber can be bent without a significant loss of performance. In a tighter bend, part of the light meets the boundary below the critical angle, leaks from the core into the cladding and is lost. Bend radius matters whenever cable is installed, routed through trays and enclosures or coiled inside an instrument.
Attenuation, dispersion and bandwidth
Attenuation is the loss of signal along the fiber, in dB/km, caused by scattering, absorption and bending. Dispersion is the spreading of light pulses in time: different modes, and different wavelengths, travel at different speeds. Bandwidth, the information a fiber can carry, is set by both, and keeps improving with new fiber designs. The typical values below are a quick reference.
NA = √(ncore² − nclad²) = sin θa in air. Typical values: about 0.12–0.14 for single-mode fiber, 0.20–0.275 for graded multimode, 0.22–0.50 for large-core fibers.
Loss from scattering, absorption and bending. Standard single-mode fiber reaches about 0.35 dB/km at 1310 nm and about 0.2 dB/km at 1550 nm; multimode at 850 nm is a few dB/km.
Pulses spread as they travel. Modal dispersion comes from different paths in multimode fiber; chromatic dispersion from different wavelengths travelling at different speeds.
A tight bend lets light leak out of the core. Standard fiber is usually routed with bends of about 30 mm or more; bend-insensitive grades tolerate 7.5–10 mm.
How much information a link carries over a given length. Attenuation limits the distance; dispersion limits the data rate.
About 8–10 µm for single-mode fiber, 50 or 62.5 µm for telecom multimode, and 100 µm to over 1 mm for power delivery and illumination.
Fiber NA, acceptance angle and mode calculator
Enter the core and cladding indices, the core diameter and the wavelength. The tool returns the numerical aperture, the acceptance cone in air, the critical angle, the V-number and whether the fiber is single-mode at that wavelength.
Step-index model in air (n₀ = 1). Number of modes ≈ V²/2 for a step-index multimode fiber; a graded-index fiber carries about half as many.
Low-OH and high-OH silica fibers
Silica fibers are also classified by their hydroxyl (OH) content. OH groups in the glass absorb strongly in the infrared, with absorption peaks near 0.95, 1.24 and 1.38 µm and a fundamental band at about 2.7 µm. The same OH content improves the glass in the ultraviolet.
Low-OH fibervisible to near-IR · about 0.4–2.4 µm
Low OH content keeps the infrared absorption peaks small. Choose low-OH fiber for telecom wavelengths, near-IR spectroscopy, fiber sensing and delivery of near-IR lasers.
High-OH fiberUV to near-IR · about 0.19–1.2 µm
High OH content improves UV transmission and resistance to UV-induced darkening (solarization). It suits UV–visible spectroscopy, fluorescence, medical illumination and UV lasers, at the price of higher infrared loss.
In short, OH content mainly affects the infrared: fewer OH absorption peaks mean lower loss on long near-IR links, while high-OH fiber serves medical devices, spectroscopy, sensors and UV lasers. The rule is simple: choose the fiber by the wavelengths you need to transmit, then check the environment (temperature, radiation, bending) in which it will work.
Single-mode vs multimode fiber
The number of modes a step-index fiber carries follows from its V-number, V = π·d·NA/λ, where d is the core diameter and λ the wavelength. When V is below 2.405 only the fundamental mode propagates.
Core about 8–10 µm, one mode, no modal dispersion. The choice for long-haul telecom, cable TV backbones, interferometric sensors and any set-up that needs a clean Gaussian-like output beam.
Core from 50 µm to 1 mm or more, many modes. Easy to couple, carries more power and supports high data rates over short links: buildings, data centers, laser energy delivery in surgery and therapy, illumination.
Distance and data rate point to single-mode; coupling tolerance, optical power and cost of the source point to multimode. A fiber is single-mode only above its cutoff wavelength.
Key differences
- Core size. Single-mode fiber has a much smaller core, which allows only one light path; multimode fiber has a larger core and many paths.
- Data transmission. Single-mode fiber carries data over longer distances with higher bandwidth; multimode fiber suits shorter links at high data rates.
- Applications. Single-mode for long-distance telecom and high-speed links; multimode for local area networks, data centers, and laser energy delivery for surgery, imaging and treatment.
The right choice depends on distance, data rate and budget.
Hollow-core fibers
In a hollow-core fiber the light travels in air or vacuum inside a micro-structured glass cladding. Because the core index is lower than the glass, the light is not guided by total internal reflection. It is confined by a photonic bandgap or by anti-resonant reflection from thin glass membranes around the core.
Very little light travels in glass, so hollow-core fiber offers low material absorption, low nonlinearity, low latency and a high damage threshold. It is used to deliver high-power and ultrafast laser pulses, to carry wavelengths that solid silica absorbs in the mid-IR and deep UV, and for gas-filled sensing cells. The transmission windows can be tuned by the cladding design, from the UV to the far infrared, which also makes hollow fiber attractive for precise light delivery in medical systems.
Components of a fiber optic system
- TransmitterA laser or LED converts the electrical signal into modulated light.
- Optical fiber and connectorsThe transmission medium; connectors and splices join the sections.
- AmplifiersBoost the optical signal on long routes without converting it back to electricity.
- ReceiverA photodetector converts the light back into an electrical signal.
Every connector is a potential loss point. A dirty, scratched or off-center fiber end-face in a ferrule can add tenths of a dB or more, and in high-power systems it can burn. That is why end-face inspection is part of every serious fiber production and assembly line.
Fiber assemblies: ferrule, length, diameter and connectors
When you order a fiber you order an assembly: the fiber itself plus what is fitted to each end and around it. Five choices decide whether it carries your light and fits your set-up: the fiber type, the core diameter, the length, the termination at each end and the jacket.
The precision sleeve at the end of the fiber, usually ceramic (zirconia) or stainless steel. The fiber is bonded inside it and the end face is polished, so the core sits centred and flat against the mating connector or optics. Ferrule concentricity and end-face quality set how repeatable every reconnection is.
Multimode fibers for sensing and spectroscopy typically have cores of 50, 105, 200, 400, 600 or 1000 µm, with an NA of 0.12 to 0.50. A larger core collects more light and eases alignment; a smaller core gives a smaller spot and a sharper image on a slit or detector.
From short patch cords to tens of meters. Over short lengths the connectors and coupling dominate the loss; over long runs the fiber attenuation at your wavelength, and the minimum bend radius along the route, matter more.
High-OH silica for UV and visible light, about 190–1200 nm; low-OH silica for visible to near-IR, about 400–2400 nm. See the OH section above.
Each end can be different, for example SMA 905 on the light-source side and a bare polished ferrule on a sensor head. The connector must match the instrument it plugs into.
PVC for flexible lab cords, stainless-steel armor or Kevlar reinforcement for rough handling, and high-temperature jackets near furnaces, engines or process lines.
Common end terminations and where they are used:
- SMA 905: threaded and robust; the usual choice for large-core multimode fibers, spectrometers, power meters and light sources.
- FC/PC and FC/APC: threaded and keyed, 2.5 mm ferrule. APC is polished at 8° to cut back-reflection.
- ST and SC: bayonet (ST) or push-pull (SC) connectors with a 2.5 mm ferrule.
- LC: compact push-pull connector with a 1.25 mm ferrule, common in dense communication equipment.
- Bare or custom ferrule: a cleaved or polished end for a sensor head, probe or custom holder.
For a deeper look at choosing NA, core size and coupling into a spectrometer slit, see our fiber optic design page.
Custom optical fibers made to order
IZAK designs and supplies optical fibers and fiber assemblies to your requirement: the fiber type, the core diameter, the length, the termination at each end and the jacket, tested before delivery. Choose the options below, add the specification to your quote request and send it. We check the choice against your wavelength and set-up and reply with a quotation.
Then complete the form below: your fiber specification is added to the message, ready to send.
Benefits of fiber optics
- High bandwidth. Far more capacity than copper cable.
- Long-distance transmission. Signals travel much farther with little degradation.
- Immunity to electromagnetic interference. Light in glass is not affected by nearby electrical equipment or lightning.
- Safety and security. No sparks, and a fiber is hard to tap without being detected.
Applications of fiber optics
The backbone of the internet, telephone and cable TV networks: data centers, mobile fronthaul, and high-speed fiber connections to homes and offices.
Laser surgery and therapy, endoscopic imaging and illumination, diagnostic spectroscopy.
Temperature, strain, pressure and chemical sensing in harsh environments, including fiber Bragg gratings and distributed sensing.
Light, EMI-immune data links and fiber sensors in aircraft and spacecraft.
Industrial cutting and welding heads, laboratory beam delivery and pumping of fiber lasers.
No sparks and no radiated signal; a fiber is hard to tap without being detected.
The future of fiber optics
Fiber keeps evolving toward thinner, more flexible fibers with ever greater capacity. Photonic crystal fibers (PCFs) open new options for data transmission and sensing, and multicore and few-mode fibers multiply the capacity of a single cable.
Conclusion
Fiber optics has changed how we communicate, work and entertain ourselves. Fast, long-distance and secure transmission of light makes it a foundation of modern technology, and new applications keep appearing in medicine, sensing and industry.
Fiber testing solutions by IZAK Scientific
Testing is essential to the quality and efficiency of every fiber network and fiber-based product. IZAK Scientific develops precision fiber test equipment for a range of applications and evaluates fiber performance against industry and application-specific standards. Each solution is customized to the client, from telecom component production to industrial sensing, with precision, adaptability and quality as the priorities.
If your fiber delivers a laser beam, the output still has to be measured. See our guide to choosing a laser beam profiler, read how we built our fiber ferrule inspection system, or learn more about optical sensors in radiometry.
Watch: the IZAK Fiber Ferrule Inspection System
The IZAK Fiber Ferrule Inspection System is our product for testing optical fibers in production. In these videos it inspects a set of 8 ferrules on each side of the unit under test: 4 transmit (Tx) and 4 receive (Rx) channels, as used in parallel optical-communication links. Every ferrule end face is imaged and analysed, so a single bad channel is found before the part is assembled into a transceiver or a network.
What is measured on each ferrule:
- Adhesive symmetry around the fiber.
- Defects and cleanliness of the end face: scratches, chips and contamination.
- Fiber center offset relative to the ferrule.
- Transmission losses of each channel.
The second video shows the image-processing side: how the software finds the fiber and the ferrule in each image, measures them and gives a pass/fail result per channel, with tester software or an API for your production line.
Engineering tool
Gaussian Beam CalculatorCoupling a laser into a single-mode fiber? Find the waist, Rayleigh range and divergence of your beam.IZAK products for this application
Instruments from our catalog that match the measurements and set-ups in this article.
Fiber inspectionFiber Ferrule Inspection SystemEnd-face imaging for concentricity, contamination and defects.View product →
Optical powerMulti-Channel Power MeterParallel optical power on several fiber-coupled channels, with logging software.View product →Need a fiber or fiber assembly made to order? Request a custom fiber quote →
Need something that is not in the catalog? We design custom electro-optical systems →
References
- Agrawal, G. P. (2012). Fiber-Optic Communication Systems. Wiley.
- Hecht, J. Understanding Fiber Optics, 6th ed. (2018).
- Senior, J. M., & Jamro, M. Y. (2009). Optical Fiber Communications: Principles and Practice, 3rd ed. Prentice Hall.
- Snyder, A. W., & Love, J. (1983). Optical Waveguide Theory. Chapman and Hall.
- Keiser, G. (2011). Optical Fiber Communications, 4th ed. McGraw-Hill.
- Marcuse, D. (1974). Theory of Dielectric Optical Waveguides. Academic Press.
- Palais, J. C. (2005). Fiber Optic Communications, 5th ed. Prentice Hall.
- Ghatak, A., & Thyagarajan, K. (1998). Introduction to Fiber Optics. Cambridge University Press.
- Rajan, G. (2017). Fiber Optic Sensors: Fundamentals and Applications. SPIE Press.
- Kersey, A. D., Davis, M. A., Patrick, H. J., LeBlanc, M., Koo, K. P., Askins, C. G., Putnam, M. A., & Friebele, E. J. (1997). Fiber grating sensors. Journal of Lightwave Technology, 15(8), 1442–1463.
Frequently asked questions
QWhat is the numerical aperture of an optical fiber?
QHow do I know if a fiber is single-mode at my wavelength?
QShould I use low-OH or high-OH fiber?
QCan IZAK supply a custom optical fiber?
Tzachi Sabati
CEO, IZAK Scientific
Physicist specializing in photonics and quantum technologies, with deep expertise in quantum sensors and advanced optical systems. Leads the Advanced Quantum Lab course at the Technion, bridging academic excellence with industry innovation. At IZAK Scientific, provides cutting-edge photonics-based solutions, developing customized inspection and sensing systems for R&D and production. Passionate about advancing quantum sensing applications and integrating novel technologies to meet industry needs.
