Self Built Interferometer Measures Nanometer Displacement

[jrcgarry] hacked together this awesome interferometer which is able to measure displacements in the nanometer range. Commercial interferometers are used in research labs to measure tiny displacements on the nanometer scale, and can cost tens of thousands of dollars. [jrcgarry] used beam splitters from BluRay drives, mirrors from ebay and a 5mw laser diode.

We’ve covered the use of interferometers before. But never an instrument built from scratch like this. Interferometers exploit the wave-like nature of a beam of light. The beam is split and sent down two separate paths, where the beams bounce off mirrors to return to the beam splitter to be recombined. Because of its wave light nature the beams will interfere with each other. And as the beams have traveled different distances they may be in or out of phase. Resulting in either constructive (brighter) or destructive (darker) interference.

Because the wavelength of light is on the order of 100s of nanometers, by observing the interference patterns you can monitor the displacement of the mirrors with respect to each other at nanometer resolution. [jrcgarry] doesn’t use the interferometer for any particular application in this tutorial but it’s a great demonstration of the technique!

THP Entry: A Repurposed Luminiferous Aether Detector

laserIn the late 1800s, no one knew what light was. Everyone knew it behaved like a wave some of the time, but all waves need to travel through some propagation medium. This propagation medium was called the luminiferous aether and an attempt to detect and quantify this aether led to one of the coolest experimental setups of all time: the Michelson-Morely experiment. It was a huge interferometer mounted on a gigantic slab of marble floating in a pool of mercury. By rotating the interferometer, Michelson and Morely expected to see a small phase shift in the interferometer, both confirming the existence of a luminiferous aether and giving them how fast the Earth moved through this medium.

Of course, there was no phase shift, throwing physics into chaos for a few years. When [Beaglebreath] first learned about the Michelson-Morely interferometer he was amazed by the experimental setup. He’s built a few interferometers over the years, but for The Hackaday Prize, he’s making something useful out of one of these luminiferous aether detectors: a functional laser rangefinder capable of measuring distances of up to 60 inches with an error of 0.000005 inches.

The core of the system is an HP 5528A laser interferometer system. [Beaglebreath] has been collecting the individual components of this system off of eBay for several years now, and amazingly, he has all the parts. That’s dedication, right there. This laser interferometer system will be mounted to a simple camera slider, and with the interferometer measurements, humidity and temperature measurements, and some interesting code (running on one of these for hacker cred), [Beaglebreath] stands a good shot at measuring things very, very accurately.

The devil is in the details, and when you’re measuring things this precisely there are a lot of details. The original Michelson-Morely interferometer was affected by passing horse-drawn carriages and even distant lightning storms. While [Beaglebreath] isn’t using as long of a beam path as the OG interferometer, he’ll still have a lot of bugs to squash to bring this project to its full potential.


SpaceWrencherThe project featured in this post is an entry in The Hackaday Prize. Build something awesome and win a trip to space or hundreds of other prizes.

Hyperspectral Imaging With A DSLR

It’s a relatively simple task to find evidence of helium by just looking at the sun; all you need is a prism, diffraction grating, and a web cam. DIY spectrometers have been around for ages, but most of them only produce a spectrum, not a full image complete with spectral data. Now it’s possible to take an image of an object, complete with that objects spectra using a DSLR, some lenses, a PVC pipe, and the same diffraction grating from your DIY interferometer.

The idea behind a hyperspectral imager is to gather the spectral data of each pixel of an image. The spectral data is then assembled into a 3D data cube, with two dimensions dedicated to the image, and the third dimension used to represent wavelength. There are a surprising number of applications for this technique, ranging from agriculture and medicine to some extremely creepy surveillance systems.

The authors of this paper (freakin’ huge PDF) used a piece of PVC pipe, three camera lenses, a diffraction grating, and a small paper aperture to construct their hyperspectral imager. Images are captured using a standard, multi exposure HDR method, assembling the raw data from the camera into a hyperspectral image with MATLAB.

There’s a ton of awesome info in the PDF, covering how the authors calibrated their system for different lighting conditions, interpreted the RGGB Bayer sensor in the camera, and a few examples of what kind of image can be constructed with this kind of data. That’s a recommended read, right there.

Thanks [Yannick] for the tip.