hi mahmood;
opto-coupler directly can not be used
because of non-linearity.basically there are 3 ways of analog signal isolation
1.transformer coupled
2.optical coupling with nonlinearity corrected
3.capacitivly isolated.
all these techniques use some form of modulation.
check out burr-brown.their ISO parts(isolation amplifiers)like ISO100 and ISO102 may fit your application.
regards
pranav