function g2 = dcs_g2_Db(Db, tau, sdsep, mu_a, mu_sp, alpha, beta)
%
% this function generates analytical field auto-correlation decay for a semi-
% infinite medium, then uses the Siegert relationship to derive the
% intensity temporal auto-correlation decay assuming ergodicity
%
% sdsep, mu_a and mu_sp units must be mm

zo = (mu_sp + mu_a)^-1;
zb = 1.76 / mu_sp; % this assumes n_tissue=1.35
r1 = (sdsep^2 + zo^2)^(1 / 2);
r2 = (sdsep^2 + (zo + 2 * zb)^2)^(1 / 2);
k0 = 2 * pi * 1.35 / (785 * 1e-6); % wavelength=786e-6 mm!
kd = (3 * mu_sp * mu_a + mu_sp^2 * k0^2 * alpha .* (6 * Db .* tau)).^(1 / 2);
kd0 = (3 * mu_sp * mu_a).^(1 / 2);
fit_g1 = exp(-kd .* r1) / r1 - exp(-kd .* r2) / r2;
fit_g1_norm = exp(-kd0 .* r1) / r1 - exp(-kd0 .* r2) / r2;

g1_norm = fit_g1 / fit_g1_norm;
g2 = 1 + beta * (g1_norm).^2;
