Alan,
This has been discussed couple of years ago here on forum..
It's not possible to use optical mouse for this purpose, since optical mouse uses a camera and various algorithms to derive the movement from changing image. The result of this is non-repeatability.. which is fine for mouse, but not OK for encoder.
You will find a lot on this subject here:
http://www.iceinspace.com.au/forum/s...=optical+mouse
and here
http://www.iceinspace.com.au/forum/s...=optical+mouse
Of course, opto-mechanical mouse is something else.. and it is used (but they are not easy to find these days).
Other option is to use encoders (wheels and opto-interrupret modules) from old printers, scanners..
http://www.iceinspace.com.au/forum/s...hlight=encoder
http://www.iceinspace.com.au/forum/s...hlight=encoder
http://www.iceinspace.com.au/forum/s...hlight=encoder