Linearly Chirped Fiber Bragg Grating group delay

[baby_1]

Homework Statement

Here is my problem : I want to know where does group delay equation come form?

Homework Equations

As I checked reference 21 it only indicates

The Attempt at a Solution

I'm following how can I start and solve this problem

 

[Fred Wright]

For a clear explanation of group delay in LCFBG see http://arxiv.org/pdf/physics/0604091.pdf