doc: basic example

Author: miRoox

docs/src/index.md

@@ -16,3 +16,26 @@ To use some light field data types, you need to add [Unitful.jl](https://painter
 ```julia
 Pkg.add("Unitful")
 ```
+
+## Basic Examples
+
+Draw a cross hole and its diffraction pattern:
+
+```@example
+using OpticalPropagation
+using Unitful
+using Plots
+
+cross = MonoLightField2D(
+    [Int((abs(x)<10 && abs(y)<50) || (abs(x)<50 && abs(y)<10)) for y in -200:199, x in -200:199],
+    wavelength=632.8u"nm",
+    size=(1u"mm",1u"mm")
+)
+patt = angularspectrum(cross, 1u"cm")
+
+plot(
+    plot(cross, size=(420,400)),
+    plot(patt, size=(420,400)),
+    size=(840,400)
+)
+```