# 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

The laser may be lasing on multiple modes (multiple longitudinal modes, multiple transverse modes, or both). Each of the individual modes may have a very long coherence length, while the many modes taken together would have a much shorter coherence length.

Suppose that one of the modes has, say, 40% of the power, and that four other modes have 15% of the power each. With a long path-length-difference, you will still easily see the fringes from the dominant mode interfering with itself.

This is a case where the temporal coherence properties of the laser are too complicated to properly describe using just one number called the "coherence length". If you plot fringe visibility vs path-length difference, it would NOT be a simple decaying exponential function.

• OP: what do you observe in your measurement? – garyp Jul 8 '14 at 17:39
• What I measured was the intensity as I moved the adjustable mirror. I realized that I was accidentally changing the path length at different locations, causing there to be fringes at the new location. As for modes, there is a second with 5 percent of the power. So my measurement was just a rookie mistake with adjusting the angle of the mirror in the bottom right. – yankeefan11 Jul 8 '14 at 18:07