docs: def directional_coupler_lc(wavelength_nm, n_eff_1, n_eff_2):
      '''
      Calculates the coherence length (100% power transfer) of a
      directional coupler.
  
      Args:
          wavelength_nm (float): The wavelength in [nm] the
              directional coupler should operate at.
          n_eff_1 (float): n_eff of the fundamental (even)
              supermode of the directional coupler.
          n_eff_2 (float): n_eff of the first-order (odd)
              supermode of the directional coupler.
  
      Returns:
          float: The length [um] the directional coupler
          needs to be to achieve 100% power transfer.
  
      '''
      wavelength_m = wavelength_nm * 1.e-9
      dn_eff = (n_eff_1 - n_eff_2).real
      lc_m = wavelength_m / (2. * dn_eff)
      lc_um = lc_m * 1.e6
      return lc_um