general enquiry

if the response time of the photo diode is 1ns , what speeds it can sense the signal ??

Here's some useful theory.

Look at Fig. 3.

Note also that the pulse width, light intensity of the activating light source and the "fall time" value (and some reasonable "rest time") have to be considered when determining the max frequency that the photo diode can handle.

Obviously, max frequency will be considerably less than [LATEX]\frac{1}{rise time}[/LATEX]

Here's a thread that might also be useful.