Detect fast movement using photodiode

[Pic2008]

I have used LDR and a LED to detect small objects moving across it ~ 1 cm.
The object is 5mm x 5 mm.
This design is useful to detect slow object.

But I would like to detect very fast movement across LED and photodiode.

However the output of a photodiode is analog. How do i go about interfacing this circuit to PIC16F877? To detect the photodiode input, I plan to enable PortB INTB interrupt so it will send a serial message to PC immediately. Is this circuit sufficient to detect fast movement or do I need IR or laser? The distance between tx and rx is ~1cm.

Any suggestion is welcome.