Single-Mode vs Multi-Mode

RF over fiber uses single-mode rather than multi-mode fiber because the latter does not support the bandwidth and link distances required for typical applications such as remoting satellite communication, GPS and wireless camera antennas.

The structure of a fiber optic cable

A fiber optic cable has four parts:

• Core
• Cladding
• Coating (buffer)
• Jacket.

Light of an appropriate wavelength, usually between 850nm and 1610nm, travels down the core of a fiber made from high purity silica glass. The cladding around the core is made from glass with a lower index of refraction than the core to ensure total internal reflection. The difference in refractive index is engineered by doping either the core or the cladding – usually with germanium, boron or fluorine. The glass is then covered in a protective plastic “buffer” coating and an outer “jacket” to guard against moisture and damage.

Single-mode

A single-mode fiber usually has a narrow core of 8-9um in diameter which allows only a single-mode of light to propagate – hence the description “single-mode”. The smaller diameter leads to a lower number of internal reflections, resulting in faster signal propagation and lower loss which translates to longer potential link distance. The narrowness of the core also minimizes optical dispersion. Low optical dispersion allows wide signal bandwidths.

The narrow core is more difficult to manufacture and consequently more costly than multi-mode fiber. However, this is relatively insignificant when compared to the cost of the light-generating lasers. Single-mode lasers are required to generate a narrow spectral width. These are significantly more expensive than their lower spec multi-mode counterparts.

Multi-mode

Multi-mode fiber is used primarily for short distance, narrowband applications such as Ethernet and cable TV. Multi-mode fiber has a larger diameter core, typically 50 or 62.5um. This larger core allows multiple modes of light to propagate and hence a higher capacity for data throughput. However, the larger core diameter means the light experiences more internal reflections and therefore the signal propagation is slower and can travel only a short distance when compared to single-mode. Moreover, the higher optical dispersion limits the bandwidth of the signal.

A longer link distance means more dispersion which in turn equates to a narrower bandwidth. In a data center application, where narrowband signals require transmission of only a few hundred meters, multi-mode is up to the job. However, the higher losses and reduced propagation speed means distances of several km and upwards – e.g. transmission over a network backbone or via dark (leased) fiber – are not feasible using multi-mode.

A comparison of single-mode and multi-mode

Single-mode versus multi-mode

In short, single-mode offers higher performance and longer distance but is more expensive to buy and use. The most appropriate choice is dependent on the application. Applications requiring relatively short link lengths and narrow bandwidths can be supported using multi-mode fiber which is obtainable at a lower cost. However, higher bandwidths and/or link distances over a few hundred meters require the performance delivered by single-mode fiber. E.g. a 36MHz-wide L-band signal will need to use single-mode.

RF over fiber and single-mode

RF over fiber applications almost always involve antenna remoting. The antenna could be a large satellite dish, a roof-mounted GPS antenna or the receive antenna of a wireless camera. Typically, multi-mode does not support the bandwidths and link distances involved in such applications and therefore single-mode fiber is used.