How to find linear LEDs (light power(mW) vs current)?
Since Diodes and transistor hFE are both inherently non-linear , one way to make it ultralinear is to use a dual version and compensate one side using linear error correction feedback on the other side( sophisticated method).  

Optoisolators are inherently nonlinear may have a  3:1 gain tolerance in output range for any input current. Expecting a linear response on anything more requires expertise in the secondary opto loop using error correction with an Op Amp. 


  One needs to have reasonable specs and tolerances for linearity and bandwidth then solve the technical issues.  Since you have neither no rational solution can be given, other than a book on opto linear design.


LED  dynamic range may be up to 3 decades but very non-linear  and bandwidth constraints are severe.


1st you need a voltage to current feedback converter with OA 
2nd  you need a 2ndary transfer function error correction circuit design with low gain. (&lt;1)


EDIT

Fortunately the experts in opto design Sharp (now licensed to Vishay) have such a solution with dual emitters (LED) for high gain linearity and stability.


0.01 % servo linearity
Wide bandwidth, > 200 kHz
High gain stability, ± 0.005 %/°C typically


  Detectors are very stable and low error and well matched while Emitters are high deviation thus only 1 emitter uses 2 detectors, 1 for voltage to current feedback and the other detector for output to current to voltage sense resistive load.