Lab 17 Wrap Up

Tonight Team "Wednesday" was tasked with creating a Mach-Zehnder interferometer (MZI) from scratch on the table. While some online notes suggest that an MZI is easier to align than a Michelson, I think everyone in the class would disagree. The MZI has an extra degree of freedom at each of the two 45-degree angle mirrors that is coupled to its rotation around the vertical axis. Of these two degrees of freedom, the Michelson only has the rotational degree of freedom - the return mirrors can be placed anywhere along the direction of the arms and still obtain laser fringes. Here's a diagram showing the MZI coupled degrees of freedom:

Aligning optics on a table is a little like riding a bicycle. After a while your body just "gets it" as you learn the effect of exercising each of the many degrees of freedom in an optic's mount. Of course you need to have an overall alignment strategy as well, such as working optic by optic down-beam. In this alignment, the "aha" moment is when you translate and rotate the mirror at the same time, keeping the location "A" constant. Having a second location "B" helps a lot.

We learned several other alignment tricks. "Chopping" a card in and out of beam rapdily by hand as the card is moved downstream gives a strong and fairly persistent visual impression of the beam's location in 3D akin to as if there were smoke or mist to mark the beam's location. This trick is especially helpful if there are features (bolt-hole row, tape w/ line down middle) on the table showing the direction the beam should go. I'll post up the other trick later.

Here are some in-progress alignment photos:

This one showing mirror translation along a bolt-hole row, and below, showing by-eye alignment looking down a bolt-hole row from one end of the table to make sure everything is in the right place. It is amazing how important it is to keep moving around the table and eye-balling optic locations! It is just too darned easy to plop things down off-by-one because of the repetitive pattern on the table top.