Opto coupler are designed to provide complete electrical isolation between an input and output circuits. The useful purpose of isolation is to provide protection from high voltage Read more about Opto Coupler
Diode has a property that is ability to conduct current in only one direction. Due to this property diodes are used in rectifier circuits. As rectification is the process to convert AC signal to Pulsating DC.
To remove the AC components from a rectifier circuit a filter is used. A filter circuit is a device to remove the AC components of rectified output. A filter circuit is a combination of inductor and capacitor and is called LC filter.