Choice of Wavelength for RF over Fiber – 1310nm vs 1550nm
Infra-red wavelengths provide lower loss
RF over fiber uses infra-red lasers because attenuation in the glass fiber is much lower in the infra-red region than at other wavelengths. Longer wavelengths generally means lower loss, however, above the infra-red region thermal background energy increases the noise floor significantly.
Causes of losses in RF over fiber
Losses in fiber are caused by (i) absorption and (ii) scattering. Absorption occurs at a number of specific wavelengths. It is caused by water vapour in the glass absorbing the light energy. These specific wavelengths are consequently referred to as “water bands”. Scattering is caused by light energy bouncing off molecules within the fiber. The amount of scattering which occurs is a function of the wavelength. Shorter wavelengths have higher levels of scattering. Choices of wavelength therefore consider the location of the water bands and the amount of scattering. The wavelengths 1310nm and 1550nm have become global standards. ViaLite RF over fiber links are available at both of these wavelengths.
Why is lower loss important?
Minimizing losses means better signal to noise ratio and dynamic range which means a higher quality signal. It also means a longer potential link distance. ViaLite links are available up to 100km.
Why not use 1550nm all the time?
Since operating at 1550nm provides the best performance, it seems logical to choose 1550nm for every link. However, a major part of the link cost is the laser. Lasers operating at 1550nm are more difficult to manufacture than those at 1310nm and consequently are more expensive. Therefore shorter links (up to 10km) would typically use a 1310nm laser because it provides good performance at a lower cost. Longer link distances (10-100km) where losses become more critical to performance would typically use the higher cost 1550nm laser since these link lengths are not feasible with the fiber losses which occur at 1310nm.
Combining 1310nm with 1550nm for a bi-directional link
Since RF over fiber is inherently mono-directional, using a single fiber for a bi-directional link requires the use of more than one wavelength. In this scenario the use of 1310 nm and 1550nm can be combined. E.g. the transmitter at point A would be 1310nm and the receiver 1550nm, whilst at point B the situation would be reversed.