We may examine the linear properties of an ET1104 opto-isolator as follows.
Note that there was no reason in particular for choosing an ET1104 to examine. There just happened to be one on hand. Also, the 1.33K to the LED was used only because it happened to come to hand at the moment. It could just as well have been a 1K.
Happily too, there is absolutely no sign of parasitic oscillation in either configuration. Even putting 1.0 µF across the emitter follower's 150Ω load only resulted in some phase shifting with no observed propensity to oscillation at all.
The LED forward voltage drop (the reverse is the drop of the 1N4148) is seen as follows:
The forward voltage drop of the LED is very nearly 1V across the entire linear operating range (see below). Therefore, the LED current at 10 mA above is (7V - 1V) / 1.33K = 4.5113 --> 4.5 mA.
The current transfer ratio here is taken as 10 / 4.5 = 2.22:1.
However, changing the x-axis from 2V / Div. to 5V / Div., we extend the view the emitter follower output and examine a break point, a non-linearity which may have consequences for us:
When we examine the slopes on both sides of the break point, we find as follows: