Working Kerr effect; PDM; speedups; removed unused files

[4yp.git] / chromaticDispersion.m

function [xCD, kstart] = chromaticDispersion(x, D, lambda, z, Tsamp)
  %% Simulate chromatic dispersion.
  %% Params:
  %%  - x: input waveform (pulse-shaped)
  %%  - D: dispersion coefficient (ps / (nm km))
  %%  - lambda: wavelength (nm)
  %%  - z: length of fibre (km)
  %%  - Tsamp: sampling time (s)
  %% Output:
  %%  - xCD: x after being dispersed. Energy of xCD is not normalized.
  %%  - kstart: starting index of the discrete signal

  %% Convert everything to SI base units
  c = 299792458; % m/s
  D = D * 1e-6; % s/m^2
  lambda = lambda * 1e-9; % m
  z = z * 1e3; % m

  %% kmax: maximum k at which to sample the inpulse response.
  %%       See [1] Eq. (8), noting that the same omega (Eq. (7))
  %%       can be used for dispersion and dispersion compensation,
  %%       as phi(t) is the same in both Eqs. (5) and (6).
  kmax = floor(abs(D) * lambda^2 * z / (2 * c * Tsamp^2));
  k = -kmax : kmax; % index for discrete function

  t = k * Tsamp;

  % Impulse response
  g = exp(j * pi * c / (D * lambda^2 * z) * t .^ 2);

  lenx = length(x);
  leng = length(g);

  len_fft = max(lenx, leng);

  G = fft(g, len_fft);
  X = fft(x, len_fft);

  xCD = ifft(G.' .* X);
  l = (leng - 1) / 2;
  if l > 0
    xCD = [xCD(l:end); xCD(1:l-1)];
  end

  kstart = 1 - kmax;
end
%% References
%% [1]: S.J. Savory, Digital filters for coherent optical receivers, 2008.