PM fiber beat length calculator



About this calculator

This calculator computes the polarization beat length of a polarization maintaining (PM) optical fiber. Beat length is the distance after which the polarization state has made one rotation, i.e. the phase shift between the two polarization modes is 2π2 \pi. Beat length is one way to define a PM fiber's birefringence, the other two being the index difference Δn\Delta n or the propagation constant difference Δβ\Delta \beta between the polarization modes.

Either Δn\Delta n or Δβ\Delta \beta (in m1m^{-1}) can be given as input to the calculator along with the wavelength of the signal. Δβ\Delta \beta is related to the index difference as follows

Δβ=2πλΔn\Delta \beta = \dfrac{2 \pi}{\lambda} \Delta n

so the calculator first calculates Δn\Delta n (if necessary)

Δn=λ2πΔβ\Delta n = \dfrac{\lambda}{2 \pi} \Delta \beta
and then the beat length as
Lb=λΔnL_b = \dfrac{\lambda}{\Delta n}