Tuesday, 24 of October of 2017

Laser Interferometer

A Sunday’s afternoon experiment: how difficult is it to build a basic laser interferometer? Actually surprisingly easy to my opinion. You take the following setup:

And build that in real life, as shown below. The laser is on the top of the image, the beamsplitter just below that.

Routing bank code

below and to the left of the beamsplitter are two mirrors. For the lase a green diode laser is used (cheap one from china). Red and blue lasers worked as well. I used a green one at low level, at full power the fringes were much less pronounced, probably due to a mode change in the laser crystals (the laser is a frequency doubling type, I noticed mode shape changes when modulating the laser current). A beam splitter harvested from an old optical drive is used, but you can probably use just a piece of glass as well. For the mirrors aluminum coated glass is used, a normal piece of mirror glass would do the trick as well I guess. The mirrors have to be exactly aligned and are therefore mounted on an old prism. The prisms serve just as a holder here.

The outgoing interference beam exits to the right, where it finds two objects on its way. These remove some stray reflections and are not contributing to the effect. as a detector a piece of paper is used, but of course when a simple light detector is used it is possible to visualize (or listen to) the signal. Worth the try for next time.

After a lot of fiddling (and indeed, the positioning of the parts is a meticulous job) the fringe pattern shows! in the example one-and-a-half fringe is shown, better alignment was too much effort. Below you will see how the fringe looks when the mirrors are less well aligned (just to show that I am not displaying some other interference pattern or just two dots).

Tiny vibrations could very well be detected (a resolution of a hundred nanometers must be easily possible with this simple setup), when someone walks by the fringe pattern moves a lot and even talking is clearly visible in the pattern. Also a nice experiment: when a lighter’s gas is spilled in the measurement path the fringes are shifting, similar to putting a droplet ispropanol (or just alcohol) just below the measuring path so that the vapor gets in the light path.

Now where to use this for? well, it is usable as a long distance laser microphone: point the measurement beam on someone’s window and you are able to listen to sound near that window because the window will vibrate along with the sound pressure. You will have to point the beam exactly perpendicular. You could use the device as a very accurate measurement tool. Well, a lot more, keep you posted when finding a good application.