4 Ways to Select the Right Fiber Optic Mode and Type
Posted by Scott Mortenson on Feb 17, 2023
We understand the basic composition of a fiber optic cable — core, cladding, buffer — and that there are glass or plastic cores. Both zip light through them, but because this is technology, it gets more complicated as we go along.
The need for fiber optics in data centers for Internet Service Providers (ISPs), long distance telephone systems, networking, medical, military, aerospace, mechanical, automotive, etc., seems to be limitless. If you can think of a specific industry or market, there’s most likely a specific type of fiber optic to make the proper connection.
Single-Mode with Virtually Infinite Bandwidth or Multi-Mode Support on Slower Networks
Single-mode fiber is small (about 9 microns, or 0.009 millimeters) so the laser only goes in one mode and is typically used for telephony and cable television. Because it has a lower loss rate and virtually infinite bandwidth, the laser source is at 1310nm (0.00131 millimeters) and 1550nm.
For reference, a human hair is 0.08 to 0.12 mm thick — or 70-120 microns.
Multi-mode fiber has light shooting through the core (either 50 or 62.5 microns) which supports multiple modes (rays) of light. MMF is generally used with low-cost light sources like LEDs, with wavelengths between 850nm and 1300nm for slower networks running at gigabits per second or more.
Glass or Plastic Optical Fiber?
There’s also Plastic Optical Fiber with a 1mm core used for short, lower speed networks such as those within storage systems and internal data center security. Plastic Clad Silica (or Hard Clad Silica) has a thin plastic cladding on a glass core and is around 200 microns.
Since fiber optics is all about carrying and controlling the light/laser, refraction is something that needs to be considered as it makes the light bend. In multi-mode fiber, depending on the angle, the ray of light can be lost in the cladding, or reflected back into the core. Basically, the light can go bouncing around in the core and it may not do what you want if you’re not aware ahead of time.
Single-mode fiber (SMF) doesn’t share that problem as the core is so small, there’s nowhere for it to bounce. While the choice of core material (glass, plastic) affects chromatic dispersion, there is no problem with modal dispersion.
Bend Insensitive and Step Index Multi-Mode Fibers
Because optical fiber is sensitive to bending, and bending causes stress to the light, which can result in loss, there are also Bend Insensitive Fibers. It adds a “low index layer” of glass to reflect the “lost” light back into the core, making it less sensitive to loss from bending. And it can be used in single-mode and most multi-mode fibers.
Step index multi-mode fiber is made one type of optical material and the cladding is another type offering different optical characteristics.
Since it has dispersion caused by the different path lengths of the various modes traveling in the core, this type of fiber is too slow for situations where speed is needed. Step index fiber is not widely used, mainly for consumer audio and TV links.
One of the factors that limits the bandwidth is dispersion, which is the widening or spreading of pulses of light transmitting data as they travel down an optical fiber.
Because there are different path lengths of the modes, graded index multi-mode fiber offers hundreds of times more bandwidth than step index fiber. The path of light is not continuous, but goes from hundreds of steps to thousands. As the light travels through each step, it is bent slightly to reflect back to the core. Grade index MMF is primarily used for premises networks, LANs, fiber to the desk, CCTV and other security systems.
The Five Grades of Multi-Mode Fiber
Originally, multi-mode fibers came in several sizes for various networks and sources. Eventually, it became standardized as the OM1 standard with 62.5 core fiber and a 125 micron cladding. OM2, with 50/125 fiber, enables gigabit connections over greater spans for LANs.
Newer OM3 or laser-optimized 50/125 fiber is often considered the best choice for multi-mode applications. OM4 fiber offers a higher bandwidth for 10G+ networks. And OM5 is wide-band multi-mode optimized for wavelength division multiplexing (WDM) for Vertical-Cavity Surface-Emitting Lasers (VCSELs) in the 850-950nm range.
To make sure you get the right fiber optic type for your project, you’ll need size (the core and cladding diameters in microns), the attenuation coefficient (dB/km) and bandwidth (MHz-km) for multi-mode. Single-mode fiber needs chromatic polarization-mode dispersion.
The bottom line is there’s a type of fiber optic cable for nearly every need, situation, and project — large and small — to fulfill the client’s requirement with (hopefully) room to grow. Data center facilities may use a mixture of copper cables and glass fiber, and each has its pluses and minuses.