I want to ask that for very high speed switching
will ordinary diode be used or some special diode?
Is optical switching faster than any other type of switching?
If no then which type of switching is fastest?
Optical switching is faster.
You should consult diodes datasheet and its manufacturer's sites. Fast diode is e.g.FRED (fast recovery epitaxial diode). www.intersil.com , www.infineon.com