Fiber Optic Cable Basics: Construction & Certification Measurements

srijeda , 11.05.2016.

Being one of the most popular and cost-effective means for communications, fiber optic cables have been installed in the backbone for years, as fiber makes communications at the speed of light possible. And their greater bandwidth and lower attenuation allow for longer distances and more channels compared with copper wires. The number of articles and papers on fiber optic cables is so large. Some are about their classifications (tight buffer/distribution/breakout/loose tube), some are on their advantage discussion (speed, distance, security), and others are meant to touch upon their related connector color codes (orange, yellow, blue). While in this article, still some basic information is included, like the construction and certification measurements.

Fiber Optic Cable Construction
Fiber optic cable construction includes a core, cladding, coating, strengthening fibers, and a cable jacket.

Fiber Core
Fiber core refers to the light-carrying element at the center of the cable, transmitting optical data signals from an attached light source to a receiving device. The core is a single continuous strand of extruded silica glass or plastic that’s measured in microns (µm) by the size of its outer diameter. The larger the core, the more light the cable can carry. The two most common multi-mode sizes are 50 and 62.5 microns. Single-mode cores are 8.5–9 microns.

The cores of OM1 and OM2 multi-mode fiber (MMF) are made differently than the cores of laser-optimized OM3 and OM4 cable. OM1 and OM2 have a small defect in the core called an index depression. This enables them to be used with LED light sources. In contrast, OM3 and OM4 are manufactured without the center defect, which enables them to be used directly with VCSELS for greater speeds and distance. These OM3 and OM4 MMFs are widely used in Gigabit applications, especially in 10GbE transmission when they are used as the medium for 10GbE transceiver modules (ie. SFP-10G-SR).

Cladding
The optic cladding surrounds the fiber core. It functions as a boundary that contains the light waves and causes the internal refraction, enabling light to travel through the length of the fiber segment.

Coating
There is a protective acrylate coating that surrounds the core and cladding to protect them. This coating keeps the glass from dust and scratches that can affect fiber strength.

Strengthening Fibers
These components are to protect the core against crushing forces and excessive tension during cable handling, especially the installation and termination process. The materials can range from aramid yarn to wire strands to gel-filled sleeves.

Cable jacket
Fiber optic cable jacket is available in PVC and plenum-rated versions. The former type is typically used for patch connections in the data center, wiring closet, and at the desktop, while the latter kind is deployed when you need to route a cable through the buildings air plenum. Plenum cable has a flame-resistant jacket to inhibit the spread of fire.

Performance Measurements
Unlike copper cable, it’s easier to certify fiber optic cable, since it’s immune to electrical interference. When certify it, the following measurements are necessary.

Attenuation/Decibel Loss
Attenuation means the decrease of signal strength as it travels through the fiber optic cable, measured in decibels/kilometer (dB/km). Generally speaking, attenuation problems are more common to MMF.

Return Loss
This loss refers to the amount of light that is reflected from the far end of the cable back to the source. The lower the number, the better. For example, a reading of -60 decibels is better than -20 decibels. Like attenuation, return loss is usually greater with MMF.

Graded Refractive Index
This concept measures how the light is sent down the fiber, commonly mensurated at wavelengths of 850 and 1300nanometers. Compared to other operating frequencies, these two ranges yield the lowest intrinsic power loss (This is valid for MMF only.)

Propagation Delay
This is the time a signal takes to travel from one point to another over a transmission channel.

Optical Time-domain Reflectometry (OTDR)
This enables you to isolate cable faults by transmitting high-frequency pulses onto a cable and examining their reflections along the cable. With OTDR, you can also determine the length of a fiber optic cable because the OTDR value includes the distance the optic signal travels.

Notes on Fiber’s Speed & Modal Bandwidth
After introducing basic information about fiber optic cable, here, I’d like to mention two commonly-seen concepts: the first one, fiber’s speed which is not meaning the speed of the signal in the fiber, but the bandwidth potential of the fiber.

The second one, modal bandwidth which is caused by the fact that light in MMF travels in rays or "modes" that take different times to get to through the fiber, thus dispersion occurs. The longer the fiber, the greater the effect. This is the incentive driving fiber manufacturers to develop better MMFs.

Conclusion
Fiber optic cables are suitable for backbone, horizontal, and desktop applications, providing extremely reliable data transmission. They are less susceptible to temperature fluctuations, and can even be submerged in water or under sea. Fiberstore fiber optic cables come in various types with detailed specifications displayed for your convenient choice. These quality cables are designed with best-in-class performance. Besides, you can also find Push-Pull MPO cable, a kind of fiber patch cable with push-pull MPO connector here. For more information about fiber optic cables or patch cords, you can visit Fiberstore.

Oznake: fiber optic cables, MMF, 10GbE transmission, SFP-10G-SR, patch cords, Push-Pull MPO cable