![cut off wavelength of single mode fiber cut off wavelength of single mode fiber](https://www.rp-photonics.com/img/step_index_fiber_mode2.png)
Therefore we must choose an arbitrary point as the boundary.įigure 37 shows the distribution of optical power across the diameter of a typical single-mode fibre. Even though the signal decays exponentially there is no exact cutoff point. One important point is that there is no abrupt boundary that defines the extent of the mode field. The mode field can be considered the effective core of the fibre although the real core size is typically somewhat smaller. We introduce the concept of the mode field. The mode field is defined as the distance between the points where the strength of the electric field is decayed to 0.37 (1/e) of the peak. In single-mode fibre the core diameter is not sufficient.įigure 37. In many situations, not the least being when we join fibre, we need a number that will give us a measure of the extent of the region that carries the optical signal. 2.4.1 Single-Mode CharacteristicsĢ.4.1.1 Mode Field Diameter (MFD) and Spot SizeĪs we have seen optical power (in single-mode fibre) travels in both the core and the cladding. The electromagnetic wave extends from the core into the cladding and therefore a percentage of each electromagnetic wave travels in the cladding. This does not mean that a percentage of the rays or a percentage of the photons travel in the cladding. The electric and magnetic fields decrease exponentially as we move away from the axis of the fibre but nevertheless a significant amount of the optical power travels in the cladding. The important point here is that the light must be thought of as an electromagnetic wave. This is not the time to think about rays of light or even of photons.įigure 36 shows a schematic of a quantum of light (photon) travelling down a single-mode fibre. The best way to conceptualise transmission here is to think of light as an electromagnetic wave in a waveguide. The simple way to think about single-mode propagation is to say that the core of the fibre is so small compared with the wavelength of the light that the light is confined to go in one path (or mode) only - straight down the middle. Understanding Optical Communications:Optical Fibre Previous