Why An optoisolator is just a light source and a light detector sealed in one opaque package. The input signal drives an LED, its light crosses a small transparent gap, and a phototransistor on the other side conducts in response. Because the only coupling is light, the input and output circuits share no electrical connection, so the device can pass a control signal while blocking hundreds or thousands of volts of potential difference and breaking ground loops.
Watch out The helium-neon laser choices describe lab-grade optical sources, far too bulky, fragile and expensive for a part costing a few cents; photomultipliers are high-gain detectors for extremely faint light, not for signal isolation.
Optoisolator = LED in, phototransistor out: light crosses the gap, voltage does not.