In this tutorial, I explain the concept, probabilistic sensor fusion model and the sensor model used in Octomap library.
related publication: OctoMap: An Efficient Probabilistic 3D Mapping Framework Based on Octrees
1)Octamap Volumetric Model
2)Probabilistic Sensor Fusion Model
octree storing free (shaded white) and occupied (black) cells. Image is taken from Ref 
3)Sensor Model for Laser Range Data
Image is taken from Ref .