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+%---------------------------------------------------------------
+% Returns the ABCD matrix for free space propagation.
+%
+% SYNTAX: abcd=free(L);
+%            <...> indicates optional argument
+%
+% L = distance to propagate.
+% n = index of refraction. Default is 1.
+%
+% abcd = |1   L|
+%        |0    1 |
+%
+%---------------------------------------------------------------
+% SYNTAX: abcd=free(L);
+%---------------------------------------------------------------
+
+function abcd=free(L,varargin)
+
+abcd=[
+    1   L
+    0   1
+];