Today I have learned that a device called "isolator" exists (eg: SI8712CC-B-IS). Its function is very similar to the optocoupler's. On the good side, it is much much faster. While it is very hard to find an optocoupler with propagation delay less than 1\$\mu\$s, almost all isolators have less than 100ns delay. On the other hand, they are slightly expensive though.
If such a device exists, why do we keep using optocouplers? Is the price the main factor of choice? Can we always use an isolator instead of an optocoupler? How do they differ in general?
Best Answer
What you show and call a "isolator" is a optocoupler with digital output. The term isolator refers to various means to isolate a signal such that there is no common connection. A opto-coupler or opto-isolator (different words for the same thing) is only one method.
Opto-isolators can be broken into a few broad catagories. The most common is a LED shining on a phototransistor. As you found, these usually have a few µs to to 10s of µs propagation delay, particularly when turning off.
Another type uses a photodiode as the detector, with active circuitry to amplify the relatively small signal from the photodiode into a digital signal. These can have less than 100 ns propagation delay. Note however that they require separate power on the receiving side, and generally cost more.
As should be no surprise when you find two different things that address the same problem, each has its advantages and disadvantages. Everything is a tradeoff.