Thank you very much! Your answer solve me many problems. ^_^ 

For a discussion of important factors, PMID: 25402854 might be a decent start.

See the following article concerning light diffusion in different brain brain regions of mouse;

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0067626

See recent work by Ed Boyden's group as well addressing this issue;

http://syntheticneurobiology.org/

Bottom line: if you're using a fiber, and place the tip ~200 um from the stimulation target (a common distance), your target will see only ~10% of the light intensity measured at the tip of the fiber.

Another useful source, that synthesizes from a number of publications:

http://www.openoptogenetics.org/index.php?title=Transmission_Through_Brain_Tissue

This includes measurements by Svoboda's group that look at the lateral spread for an LED on the cortical surface.

http://www.openoptogenetics.org/index.php?title=Transmission_Through_Brain_Tissue