Fiber Bragg Gratings Reflection Co-efficient

[Aniket1]

I am studying fiber bragg gratings for a project. I read the couple-mode theory and came across a plot of the reflection coefficient v/s wavelength used which had a global maxima at the Bragg wavelength. The plot was drawn for a constant value of the coupling constant κ. I don't understand this assumption since the coupling constant was found to be dependent on the wavelength used in the same manuscript. So a plot with the wavelength as the independent variable, keeping the coupling co-efficient constant doesn't make sense.

 

[BigJDubs]

How does this plot come about?The plot you refer to is typically used to illustrate the effect of a single grating on the reflection coefficient of light at the Bragg wavelength. It is assumed that the coupling constant, κ, is a constant value over the range of wavelengths being considered. The global maximum of the reflection coefficient occurs at the Bragg wavelength, where κ is equal to the ratio of the incident and reflected wave amplitudes. Thus, the plot is representative of the behavior of the grating with a fixed coupling constant across different wavelengths.