diff --git a/meanas/fdfd/waveguide_cyl.py b/meanas/fdfd/waveguide_cyl.py
index 0d3be0b..d476caa 100644
--- a/meanas/fdfd/waveguide_cyl.py
+++ b/meanas/fdfd/waveguide_cyl.py
@@ -25,6 +25,9 @@ def cylindrical_operator(
     """
     Cylindrical coordinate waveguide operator of the form
 
+    (NOTE: See 10.1364/OL.33.001848)
+    TODO: consider 10.1364/OE.20.021583
+
     TODO
 
     for use with a field vector of the form `[E_r, E_y]`.