BCJR implementation

WDM code added (Pol Cont., Opt MUX/DEMUX, Opt Atten, DP_Fiber) -> the codebase is not optimized to always work with dp signals!

projects/ECOC_2025/theory/coherence_length_plot.m

@@ -0,0 +1,32 @@
+%% Parameters
+df       = linspace(100e3,50e6,10000);   % Laser FWHM linewidth [Hz]
+n_fiber  = 1.467;                      % Fiber group index
+c        = 3e8;                        % Speed of light [m/s]
+
+% Compute coherence length (1/e of mean-fringe decay)
+tau_c = 1./(pi*df);
+L_c   = (c/n_fiber) .* tau_c;          % Coherence length [m]
+
+%% Plot
+figure('Color','w'); 
+loglog(df/1e6, L_c, 'LineWidth',2,'LineStyle','-'); % linewidth in MHz
+xticks([0.1, 1, 10, 50]);
+yticks([1, 10, 100, 1000]);
+yticklabels({'1','10','100','1000'})
+grid on; box on;
+xlabel('Laser linewidth [MHz]','FontSize',12,'Interpreter','none');
+ylabel('Coherence length [m]','FontSize',12,'Interpreter','none');
+title('Coherence Length vs. Laser Linewidth','FontSize',14,'Interpreter','none');
+
+%% Annotate some key points
+hold on;
+freqs = [150e3, 1e6, 10e6, 50e6]; % [Hz]
+for f = freqs
+    x = f/1e6;
+    y = (c/n_fiber) * (1/(pi*f));
+    scatter(x,y,'Marker','x','LineWidth',1,'MarkerEdgeColor','black');
+
+    text(x*1.1,y, sprintf('%.2f MHz', f/1e6), ...
+        'FontSize',10,'HorizontalAlignment','left');
+
+end