Multimode & Singlemode Fiber Basics
Of all the differences between multimode & singlemode fiber, the most fundamental differences are the size of the fiber’s core and the associated attenuation or loss and bandwidth of the fiber. The fiber itself consists of 3 basic portions – the core, cladding and buffer or coating. The core is the most central portion of the fiber where the light travels. There are 3 basic fiber core diameter sizes for fiber systems that transport traditional video, voice, and data signals. Singlemode fiber has a core diameter of nominally 9μm while multimode fiber has either a 50μm or 62.5μm core diameter. For the most common types of fibers, the cladding is always 125μm while the protective coating has a diameter of 250μm. Other buffers and jacketing materials help build the fiber up to more practical and rugged cable structures.
The basic rule of thumb is that the smaller the core diameter, the higher the fiber’s bandwidth and the lower the attenuation (loss in dB per kilometer). The fiber’s attenuation and bandwidth are also dependent on wavelength. The table below illustrates the approximate attenuation of both multimode & singlemode fibers.
| Fiber Type | Multimode | Multimode | Singlemode | |
| Core diameter | 50μm | 62.5μm | 8 – 10μm | |
| Attenuation (dB/km) | 850nm | 2.5 | 3.5 | N/A |
| 1300/1310nm | 0.8 | 1.4 | 0.3 | |
| 1550nm | N/A | N/A | 0.2 |
Depending on the data rate and distance of the fiber transport systems they will be either loss or bandwidth limited. The fiber will either attenuate the optical signal to such a point where the receiver cannot reliably recover the information or the bandwidth of the fiber will distort the signal to where it cannot be recovered even though there is plenty of optical signal at the receiver.
The bandwidth limitations of MM fiber can severely limit the transmission distance of high bandwidth video signals. Fiber attenuation is one of the basic characteristics of fiber that needs to be understood and taken into consideration when designing any fiber transmission system.
