They are the same. I think you can use them interchangeably. Also, you can put "Since then" at the beginning of a sentence also. For example: "Since then, I have been working at the airport." or "Since then, I have not seen him again." If you use "ever since" at the beginning, you need to add more information. For example: "Ever since I went to Italy, I am addicted to pasta." or "Ever since my wife left me, I have had to cook for myself."