3.1 Optical Fiber Overview and Light Propagation

Why Optical Fiber on the TECH Exam

Optical fiber is one of the two 25% blueprint domains, and almost every fiber task on the hands-on exam—fusion splicing, OTDR troubleshooting, optical loss testing—starts from the same physical principles. A Technician must be able to reason about how light moves through a fiber, why certain wavelengths are used, and why the core and cladding diameters on a cable jacket matter. This section establishes that vocabulary.

Anatomy of a Fiber Strand

A single optical fiber has three concentric regions:

The core diameter for singlemode is approximately 8–10 µm (commonly called 9 µm); multimode cores are either 50 µm or 62.5 µm. The cladding is almost always 125 µm outside diameter, which is why fiber is described as a ratio like 50/125, 62.5/125, or 9/125. The coating brings the strand to 250 µm (colored tight buffer is 900 µm).

Refractive Index and Total Internal Reflection

Light slows when it enters glass. The refractive index n = c / v tells you the ratio of the speed of light in vacuum to the speed in the material. Silica glass has n ≈ 1.46 at telecom wavelengths. To guide light, the core must have a higher index than the cladding. The boundary between them acts as a mirror for any ray that strikes it above the critical angle θc, defined by sin(θc) = n2 / n1.

When a ray hits the boundary above θc, it reflects back into the core rather than refracting out. This is total internal reflection (TIR), and it is the only reason fiber works. The small index difference is engineered by doping the core with germanium to raise its index.

Numerical Aperture and Acceptance Cone

Only rays that enter the fiber end within a certain cone can be guided. The numerical aperture (NA) quantifies that cone:

NA = sin(θ_max) = √(n1² − n2²)

A larger NA gathers more light but supports fewer guided modes and higher modal dispersion. Multimode fibers have larger NA (~0.20) than singlemode (~0.14). When you couple an LED or VCSEL to a fiber, source NA must not exceed fiber NA or light is lost as cladding modes that leak out within the first meters.

Wavelengths and Transmission Windows

Silica attenuation is wavelength-dependent. The familiar windows are:

• 850 nm — multimode, used by VCSELs in short data-center links
• 1300 nm — multimode single-window and singlemode zero-dispersion region
• 1310 nm — singlemode, used for campus links and OTDR testing
• 1550 nm — singlemode, lowest attenuation window, used for long-haul and 1550 OTDR for bend detection

The OH⁻ water peak around 1383 nm historically separated the 1310 and 1550 windows; modern low-water-peak fibers reduce it. Modern singlemode cable attenuation is typically less than 0.4 dB/km at 1310 nm and 0.3 dB/km at 1550 nm, while multimode is roughly 3.0 dB/km at 850 nm and 1.0 dB/km at 1300 nm.

Attenuation Mechanisms

Attenuation is the loss of optical power with distance. The dominant causes in installed ICT cabling are:

1. Rayleigh scattering — microscopic density fluctuations frozen into the glass during manufacture; this is what an OTDR sees as backscatter.
2. Absorption — residual OH⁻ ions and impurities absorb light.
3. Bending losses — macrobends (visible bends below the minimum bend radius) and microbends (tiny stress from cable crush or poor termination) leak power from the core, more severely at 1550 nm than 1310 nm.
4. Splice and connector losses — point losses added at interfaces; fusion splices are typically below 0.1 dB.

Dispersion: A First Look

As a pulse travels, it spreads out. This spreading—dispersion—limits bandwidth more than attenuation does in most premises links. Two mechanisms matter at this stage:

• Modal dispersion — different modes (rays) travel different path lengths, so they arrive at slightly different times. Only multimode has modal dispersion, and it dominates distance limits at 1 Gb/s and above.
• Chromatic dispersion — different wavelengths travel at slightly different speeds. Singlemode is limited by chromatic (and polarization-mode) dispersion rather than modal.

Cable Jacket and Construction Cues

A fiber cable jacket carries color and print cues that tell you what is inside before you cut it. The 250 µm colored coating inside a loose-tube cable is the fastest identifier: orange typically means OM1 or OM2 multimode, aqua means OM3 or OM4, lime green means OM5, and yellow means singlemode. Jacket print also lists the fiber type, e.g. "50/125" or "9/125 OS2".

Technician-Level Reasoning

When you are handed an unknown fiber at a job site, the correct sequence is: read the jacket print, confirm the core/cladding ratio, identify singlemode vs multimode, and then choose a matching connector, source, and test wavelength. Mismatching a 1310 nm singlemode source into a multimode launch cable, or a UPC connector into an APC adapter, will produce a link that appears to pass on a power meter but fails certification and gives bizarre OTDR traces.