SharePoint Calculated Column with department from a People Field

Hi all,

I am a beginner in SharePoint online. I created a document library and created a people field called Raised_By. Now i need to create a calculated column to extract the department from Raised_By so it is placed in the calculated field.

When I am creating the calculated column in the list of fields there isn't Raised_By so I cannot create a formula based on this people field.