Anyway, over the course of the semester I developed a method for aligning the various parts and I will admit I didn’t use them this past weekend. Nevertheless, it was a snap to get the pattern on the 8in and 16in arms. It’s just bizarre since I had gotten so good at getting the patterns that I twice made a portable version so I could roll it into my classes and to a science poster session on campus (the latter required bouncing over some rough sidewalks).

Well, like I said, I plan to order some fully-gimbaled mirrors this summer which should help a bit, though all the stuff we’re using is pretty high-end (mostly ThorLabs stuff).

And, yeah, I am consistently amazed at the fact that this was done with white light. Like I said, I now have a god-like reverence for experimentalists, particularly those who lived in more “primitive” times.

Your math is a little off there, I think. The radial frequency at 532nm is 3.5e15 rad/s, so a 1nm linewidth must be closer to 6e12 rad/s. I used two equations, including yours, but the easier (I find) is to work out the derivative of omega wrt lambda and then approximate that as a discrete change to get s, which leads to . Which will give an answer 2 less than my first attempt.

I’m surprised at the difficulty you are having as regards the length of the arms. It shouldn’t be a coherence problem for as long as the arm lengths are the same, and you don’t pass the laser through very turbulent air or some medium, then the phases should still all match up. As was mentioned in a comment on your earlier post, it is possible to make an interferometer with purely white light, but the arms need to be exactly equal in length. But the fact that you can take the interferometer apart and change the arm lengths show that you clearly know what you are doing as regards the set-up.

Interferometers really are fascinating to play with! The thing that always gets me is that virtually every interferometer we use was developed way before the laser, so alignment must have been truly difficult. I was flicking through Newton’s Optics some months back and noticed that in his “Newton’s rings” experiment he could measure the wavelength of yellow light to 1nm scales (see The Second Book Of Opticks: Part I. on http://www.gutenberg.org/files/33504/33504-h/33504-h.htm where he can distinguish between 1/88952th and 1/89063th art of an inch) Which is all the more impressive as he considered it to be a particle …

Although, one oddity that I found was that if the optical components (i.e. beam splittlers and mirrors) were too far from the laser source I couldn’t get an interference pattern. I assumed that was because the light was no longer coherent, but it was way less than 27.5 m (though I am using the 40 mW version I think).

At any rate, as I’m about to put up in another post, over the weekend I was messing around with it and figured out that I was most definitely seeing the cavity modes. It was odd, though, since it was so different from what I was seeing before.

I have come to the determination that books on optics, notably MZIs, are full of crap.

I’d suggest that the fact that the fringe spacing changes with angle is a strong hint that your MZI is not perfectly aligned – the k-vectors from each arm must be exactly aligned and the beams be exactly centred. In other words, the two arms are not in the same mode after the 2nd beam splitter and so you will have imperfect alignment. As you change the angle you are effectively changing the relative phase of one arm that is projected along the beam from the other arm.

Also, as a sanity check. Your coherence length – relating the temporal coherence to a distance using c – is almost certainly longer than 22um. At 532nm a laser linewidth of 1nm corresponds to a coherence length of 300um. The width of those lasers is typically less than 0.1nm. You could check this by building a Michelson and varying the path length difference, just as Daniel suggested.

It is not at all critical that the interferometer is square. Ideally both arms are the same length, which works with any parallelogram. The hard part of this is in mode-matching the individual arms into the output. Prefect mode-matching of interferometers is actually very difficult!

After this goes on for awhile, the average effect is to wash out the ensemble average interference in BS2. The output will be 50/50 from Channels 1 and 2. The decoherence advocates say, this is like the situation being classical and sort of “explains” the particle-like behavior. Putting aside other criticisms, my proposal showed how to recover the evidence that amplitudes continue to come out of both Channels, and requires an asymmetrical BS1 (ie, unequal split.) BTW the best link about that proposal is at http://fqxi.org/community/forum/topic/949.