Experimentally measuring coherence length of laser

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.

• I don't understand your diagram. It does not look like a Mach-Zehnder. (A Michaelson would be better suited, I think.) And a laser often has a coherence length much longer than 1 mm. Why do you think that the coherence length is so small? What kind of laser is it? What are the rectangular things? The object in the lower right looks like a mirror. I suppose the object in the upper left is a beamsplitter. Are the remaining two mirrors? Which components move, and in which direction. – garyp Jul 8 '14 at 16:45