Quasi-phase-matching. Generalize the discussion of the text leading from Eq. (2.4.1) to Eq. (2.4.6) by allowing the lengths of the inverted an non inverted sections of nonlinear optical material to be different. Let 
be the period of the structure and l be the length of the inverted region. Show how each of the equations in this range is modified by this different assumption, and comment explicitly on the resulting modification to the value of dQ and to the condition for the establishment of quasi- -phase matching phase matching.
