Please advise required data rate and isolation voltage and any applicable isolation requirements.
Capacitive coupling will work well if your isolation requirements allow it.
Inductive coupling with eg an oscillator allows low power - effectively an "RF" link.
Formal opto couplers tend to require mAs of current.
Some, such as rhe eg IXYS CPC1001N are specified for operation down to0.1 mA and may operate at lower currents. Other opto couplers may also operate at lower than usual current but care is needed in design to not get a "works in some cases" situation. Available data rate may be affected by low currents.
You could make your own opto coupler with eg an LED and a PIN diode and get good results at very low currents.
Extra information: Sampling rate can be as low as 10 Hz.
If you transfer say 20 bits of data per sample and operate at 10 Hz.
Bits/second = 200 Bits.
At 2 Kbps duty cycle = 10%
At 20 kbps duty cycle = 1%
At 200 kbps duty cycle = 0.1%
So if you have a say 5 mA optocoupler operating at 20 kbps the mean current is 5 mA x 1% = 50 uA
= 5% of power budget.
Lower mA opto couplers and faster data rates reduce average current proportionally.