Can my employer require a Doctor's note to return back to work ?

Yes, your employer can require this.  Absent some form of discrimination, an employment/union contract to the contrary, or a stated company policy that runs counter to this, it is legal.  Most employment relationships are what is known as "at will".  This means that basically the employer can hire or fire someone for any reason or no reason whatsoever, as well has increase/decrease salary/hours, promote/demote, and generally impose requirements as they see fit.  In turn, you can work for an employer, or not, your choice.