The principle of bidirectional transistor optocoupler for communication

Communication bidirectional transistor optocoupler (usually also referred to as bidirectional optocoupler) is a special photoelectric conversion device, whose working principle and application are widely involved in multiple fields.
principle
The communication bidirectional transistor optocoupler is mainly composed of light emitting diodes (LEDs) and bidirectional phototransistors (such as phototransistors). Its working principle is based on the photoelectric effect and the switching characteristics of semiconductor devices:
Forward transmission:
When the input terminal (LED) receives an electrical signal, the LED emits light.
The optical signal passes through the isolation layer and illuminates the photosensitive surface of the phototransistor.
After being illuminated, the internal resistance of the photosensitive transistor changes, triggering the switching action of the transistor and achieving the transmission of electrical signals.
In bidirectional optocouplers, due to the bidirectional conductivity of phototransistors, they can process AC signals.
Reverse transmission (may exist in certain types of bidirectional optocouplers):
In some designs, optical signals can also be transmitted in reverse from phototransistors to light-emitting diodes, but this reverse transmission is usually not the main function of bidirectional optocouplers, and their efficiency and characteristics may be limited.
Electrical isolation:
The electrical isolation between the light-emitting diode and the phototransistor is achieved through an optical isolation layer, effectively preventing electrical interference and noise between the input and output terminals