%% ============================================================
%  Wavelength–Frequency Conversion Utilities
% ============================================================

% Example usage:
%   f = lambda2freq(1310e-9);              % 1310 nm -> Hz
%   lambda = freq2lambda(224e12);          % 224 THz -> m
%   delta_lambda_nm = df2dlambda(224e12, 400e9); % 400 GHz @ 224 THz -> nm
%   delta_freq_GHz  = dlambda2df(1310e-9, 3.45); % 3.45 nm @ 1310 nm -> GHz

%% ---- Core conversion functions ----
function f = lambda2freq(lambda)
% lambda2freq Convert wavelength [m] → frequency [Hz]
    c = physconst('lightspeed');
    f = c ./ lambda;
end

function lambda = freq2lambda(f)
% freq2lambda Convert frequency [Hz] → wavelength [m]
    c = physconst('lightspeed');
    lambda = c ./ f;
end

%% ---- Differential conversions ----
function d_lambda = df2dlambda(f_center, d_f)
% df2dlambda Convert frequency spacing Δf [Hz] → wavelength spacing Δλ [m]
% around a given center frequency f_center [Hz].
% Uses first-order differential: Δλ ≈ (c / f^2) * Δf

    c = physconst('lightspeed');
    d_lambda = (c ./ (f_center.^2)) .* d_f;
end

function d_f = dlambda2df(lambda_center, d_lambda)
% dlambda2df Convert wavelength spacing Δλ [m] → frequency spacing Δf [Hz]
% around a given center wavelength λ_center [m].
% Uses first-order differential: Δf ≈ (c / λ^2) * Δλ

    c = physconst('lightspeed');
    d_f = (c ./ (lambda_center.^2)) .* d_lambda;
end

%% ============================================================
%  Example section (can be commented out)
% ============================================================

if ~isdeployed
    fprintf('--- Example conversions ---\n');

    lambda_nm = 1310;                 % nm
    lambda = lambda_nm * 1e-9;        % m
    f = lambda2freq(lambda);          % Hz
    fprintf('λ = %.1f nm  →  f = %.3f THz\n', lambda_nm, f/1e12);

    d_f = 2000e9;                      % 400 GHz spacing
    d_lambda = df2dlambda(f, d_f);    % [m]
    fprintf('Δf = %.0f GHz @ %.1f nm  →  Δλ = %.3f nm\n', d_f/1e9, lambda_nm, d_lambda*1e9);

    % Verify reverse direction
    d_f_back = dlambda2df(lambda, d_lambda);
    fprintf('Δλ = %.3f nm @ %.1f nm  →  Δf = %.0f GHz\n', d_lambda*1e9, lambda_nm, d_f_back/1e9);
end