In the circulator, you can set the values for insertion loss, isolation and return loss, whereas in the ideal circulator, there is no return loss and isolation and you can set the value for insertion loss. An optical null simply creates a zero optical signal where is it used in the design.
Circulators are used to achieve Bi-directional transmission over a single fiber. It is 3 or 4 port device. It separates the signals travelling in opposite direction such that Signal entering port 1 will leave from the port 2. In case, a emitted light is reflected it will exit from another port i.e. port 3.
However, the difference berween circulator, ideal circulator and optical null is very well explained by Alistu above.
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