function [delay, amp] = modeDelayFFT(lambda, spec_lambda,fibre_length)

% Input wavelength range/nm, spectral interference/fW and fibre length/m
% output complex amplitutde

% redistribution in frequency
freq= 3e5./lambda; % change lambda to frequency/THz
freq = fliplr(freq);
spec_f = fliplr(spec_lambda);

% interpolation
points = length(freq);
freq_modified = linspace(freq(1),freq(points), points);
spec_f_modified = spline(freq,spec_f,freq_modified); %data interpolation

%  FFT
amp = fft(spec_f_modified);

% delay caculation
sampling_f = 1/(freq_modified(2)-freq_modified(1)); % ps
delay = linspace(0, sampling_f, points)/fibre_length; % ps/m