mahmood:
In the past I have used opto couplers to transmit analog signal of low bandwidth. The idea is to translate the analog level into a variable pulse width signal and then send the pulses through the opto coupler as a saturated On/Off signal. At the receiver end the variable pulse signal can be turned back into a DC level with simple RC filter.

I implemented a scheme like this to optically isolate the brightness control of a CRT monitor from a computer where the computer needed to support remote brigntness adjustments. I had an 8 bit DAC produce a DC voltage. This voltage was fed to one side of a comparator that in turn drove the LED in the transmit opto coupler. The other input to the comparator came from a triangle wave signal implemented with its high and low level peaks adjusted slightly higher and lower than the range of the output voltage of the DAC. This ensured that the Opto coupler was always passing a singal of some duty cycle and not going completely off or fully on.

Hope this is useful.
Mike