In the previous article, we have seen the physical structure and basic principles of data transmission in fiber optics. Mainly there are two categories of fiber optic cables available in the market,

• Single mode fiber
• Multimode fiber

When planning to build a fiber optic network, the first decision you often encounter is single-mode fiber or multimode fiber. Hopefully, I can make that decision easier for you by explaining the differences between the two, and why you should choose one over the other.

Single Mode fiber

Generally, single-mode fibers have a small core size (less than 10 µm) that permits only one mode or ray of light to be transmitted. Because of this, there is little light reflection created and there’s no interference or overlap between the different wavelengths of light. This will lower fiber attenuation and create the ability for the signal to travel on a longer distance. Thus single mode fibers usually used in long distance and higher bandwidth applications.

Single-mode cable (OS1 & OS2) has a small (8–10-micron) glass core that is much smaller than multimode and only one pathway of light or mode of propagation(OS stands for optical single-mode). With only a single wavelength of light passing through its core, single-mode fiber realigns the light toward the core center instead of simply bouncing it off the edge of the core as multimode does. OS1 is applied to the inside-plant tight-buffered cable. OS2 is applied to loose-tube cables.

Single-mode cable is almost always YELLOW, so it is easy to identify.

Multi-Mode Fiber

Multimode cable has a large-diameter core that lets multiple modes of light pass through it. This means more data can pass through the multimode fiber core at a given time. This will create more light reflections and higher dispersion and attenuation rate, reducing the quality of the signal over long distances.

Multimode cores come in two core sizes and five varieties: 62.5-micron OM1, 50-micron OM2, OM3, OM4 and OM5(OM stands for “optical multi-mode”). All have the same cladding diameter of 125 microns, but 50-micron fiber cable has a smaller core (the light-carrying portion of the fiber). Although all can be used in the same way, 50-micron cable, particularly OM5 and laser-optimized OM3 and OM4, provide longer link lengths and/or higher speeds and are recommended for premise applications (backbone, horizontal and intra-building links). Generally, multimode fibers are used in short distance, data and audio/video applications in LANs.

Multimode cables come in different colors so they can be easily recognized. OM1 and OM2 come with an ORANGE; OM3/OM4 is typically AQUA or OM4 is sometimes ERICA VIOLET to help distinguish it from OM3; the latest generation of multimode fiber, OM5, is LIME GREEN.

In many scenarios, you may hear that 9/125,50/125,62.5/125. In this case, the first portion is indicating the diameter of the core portion and the second portion indicates the total diameter size. Different multimode cables have emerged due to the requirement of adding more data transfer speed and longer distance. So all the differences between OM cables will be based on that.

OM3 & OM4 are superior to OM1 & OM2

Both OM1 and OM2 work with LED based equipment that can send hundreds of modes of light down the cable, while OM3 and OM4 are optimized for laser (eg. VCSEL) based equipment that uses fewer modes of light. LEDs cannot be turned on/off fast enough to support higher bandwidth applications, while VCSELs are capable of modulation over 10 Gbit/s and are used in many high speed networks. For this reason, OM3 and OM4 are the only multimode fibers included in the 40G and 100G Ethernet standard. Now OM1 and OM2 are usually used for 1G which are not suitable for today’s higher-speed networks. OM3 and OM4 are used for 10G mostly at present. But OM3 and OM4 can support the 40G and 100G speed, which may make them the tendency.

Light Propagation Difference

The light propagation between single mode fiber and multimode fiber is totally different. Multimode fiber has two types of light propagation—step index and graded index, while single mode fiber has only one step-index. And the light propagation reduces less in the single mode fibers transmission than that of multimode fibers.

In the previous article, we have seen what is refractive index and it was also discussed that the refractive index of the core portion is having a large dependency for total internal reflection. The multimode step index and multimode graded index can be described only based on that.

When the core portion of multimode fiber is having a fixed refractive index and light passes through this core will result in the sudden step changes at the time of total internal reflection. This sudden step change(reflection to another side) will result in a multimode step index.

When the core portion of multimode fiber is having a non-uniform refractive index. i.e. Index of refraction is increasing as one goes near the center while decreasing near outer core edges. The index of refraction is maximum at the center of the core. This will result in the generation of a multimode graded index.

Multimode step index fiber cable is widely used due to the benefits of large size. It is easier to make and hence less expensive. It is used for shorter distances at very low pulse frequencies.

Which One to Choose?

When selecting single mode fiber or multimode fiber, the most important thing to consider is the distance requirement. Within a data center, it’s typical to use multimode fibers which can get you 300-400 meters. If you have very long runs or are connecting over a longer distance, single mode fiber can get you 10km, 40km, 80km, and even farther. You just need to use the appropriate optics for the distance required, and again, the prices go up accordingly. It is estimated that the transmission costs of multimode fiber, including both transmitter and receiver sides, will be in the range of $ 500 to $ 800. And the transmission systems designed for use with single mode fiber will typically cost more than $ 1000. So single mode fiber or multimode fiber, what’s your favorite?

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