So I am trying to figure out the coherence length of my laser source. I am using a mach zehnder interferomter (image attached). I have theoretically calculated $L_c$ to be somewhere between 800-1200 $\mu m$ (equation on wiki page). So anyways.
When I scan the movable mirror through a large range (50 mm), I observe interference fringes everywhere. I thought that outside of the coherence length, there should be no interference. Am I doing something incorrect?
NOte: The top part of the MZI interferomter is an adjustable mirror. NOT TO SCALE.