Why do you think that this is a problem
in the first place?
Why do you think that the problem (if any)
is with the optocoupler and not some other
influence?
From the description (which is inconsistent
between posts) it now sounds like the larger
assembly transitions between open loop
and closed loop ("decreasing, then stops
at V- = Vref"). V- should equal Vref always
if the assembly is closed loop.
"It is like a finger pointing away to the moon"
(student with blank look; slap upside the head)
"Do not concentrate on the finger, or you
will miss all the heavenly glory"
- Bruce Lee, "Enter the Dragon"
That is to say, broaden the scope to include
what is driving the LED, how and why, and
maybe you will see the actual -cause- and
then determine -what- needs compensation
(or perhaps, none as it's a different problem
entirely).