fitting my periodic function

Hi firends,

I have a set of input and output data, and I also have the function with three free variables. How can I fit this function without getting into local min and max. My function is

a ./ abs(1-exp(-b*300e-6 + 2j*(2*pi*c./(lam*1e-9))*300e-6)).^2

with a,b and c being free variables.

Another fitting function for this problem is in the form of

(a^2*(1-b^2)^2+(4*a^2*b^2*sin(2*pi*c*300e-6/lam*1e-9)^2))/((1-b^2)^2+4*b^2*(sin(2*pi*c*300e-6/lam*1e-9))^2)

again with a,b and c being free variables.

[Walter Roberson]

Question: are the free variables constrained to be real, or could they be complex? Is the output constrained to be real? (That would be a major constraint due to the complex component of the expression.)