Fiber optic cables are characterized by geometric, optical, and transmission parameters that determine their performance, including core/cladding dimensions, numerical aperture, attenuation, and dispersion.

Geometric Parameters

• Core Diameter: The central region of the fiber that carries light. Typical values are 8–10 µm for single-mode fibers (SMF) and 50–62.5 µm for multimode fibers (MMF) .
• Cladding Diameter: Surrounds the core, usually 125 µm for standard fibers, providing total internal reflection .
• Coating/Buffer: Protective layers around the cladding to prevent mechanical damage.
• Fiber V-Number (Normalized Frequency): Determines the number of modes a fiber can support. Calculated as $V=\frac{2\pi a·NA}{\lambda }$, where $a$ is core radius, $NA$ is numerical aperture, and $\lambda$ is wavelength. Single-mode operation occurs when $V<2.405$ .

Optical Parameters

• Numerical Aperture (NA): Indicates the light acceptance angle of the fiber, affecting coupling efficiency and mode propagation .
• Refractive Index Profile: Step-index or graded-index profiles influence modal dispersion and bandwidth.
• Attenuation (Loss Coefficient): Power loss per kilometer, expressed in dB/km. Includes absorption (impurities, intrinsic SiO₂ vibrations), scattering (Rayleigh, Brillouin, Raman), and bending losses (micro/macro bending) . Typical low-loss SMF: ≤ 0.18 dB/km.
• Dispersion: Broadening of optical pulses, limiting bandwidth and distance. Includes modal dispersion (dominant in MMF) and chromatic dispersion (wavelength-dependent, important in SMF) .

Transmission Parameters

• Bandwidth: Maximum data rate supported over a given distance, influenced by modal and chromatic dispersion.
• Cutoff Wavelength: The wavelength above which a fiber operates in single-mode.
• Polarization Mode Dispersion (PMD): Differential delay between polarization modes, critical for high-speed SMF systems .
• Maximum Transmission Distance: Determined by attenuation and system sensitivity; for example, an EDFA-amplified SMF system can theoretically reach hundreds of kilometers depending on fiber loss .

Standards and Testing

Fiber optic cables are governed by international standards to ensure performance and interoperability. Key organizations include ISO, IEC, ITU-T, TIA, and Telcordia, which define specifications for attenuation, dispersion, mechanical strength, environmental resistance, and connector quality . Testing involves measuring optical power, loss, and endface quality using power meters, optical time-domain reflectometers (OTDR), and other instruments .

Summary

The performance of a fiber optic cable depends on a combination of geometric dimensions, optical properties, and transmission characteristics, all of which are standardized and tested to ensure reliable communication. Understanding these parameters is essential for selecting the right fiber type for applications ranging from short-distance data centers to long-haul telecommunication networks.