function deltaBeta = calcPhaseMatching(f_i, f_j, f_k, f_0, Ds)
% Approximate phase mismatch for degenerate FWM close to the ZDW.
% Inputs are frequencies in Hz.
%   f_i, f_j : pump frequencies (equal in the degenerate case)
%   f_k      : signal frequency
%   f_0      : zero-dispersion frequency
%   Ds       : dispersion slope in ps / (nm^2 km)

c = physconst('LightSpeed');

pump_frequency = 0.5 .* (f_i + f_j);
lambda_zdw_m = c ./ f_0;
dispersion_slope_si = Ds .* 1e3;

deltaBeta = -(2 .* pi .* lambda_zdw_m.^4 ./ c.^2) .* ...
    dispersion_slope_si .* ...
    (pump_frequency - f_0) .* ...
    (pump_frequency - f_k).^2;

end