Measure Laser Wavelength With A CD And A Tape Measure

Obviously the wavelength of a laser can’t be measured with a scale as large as that of a carpenter’s tape measure. At least not directly and that’s where a Compact Disc comes in. [Styropyro] uses a CD as a diffraction grating which results in an optical pattern large enough to measure.

A diffraction grating splits a beam of light up into multiple beams whose position is determined by both the wavelength of the light and the properties of the grating. Since we don’t know the properties of the grating (the CD) to start, [Styropyro] uses a green laser as reference. This works for a couple of reasons; the green laser’s properties don’t change with heat and it’s wavelength is already known.

It’s all about the triangles. Well, really it’s all about the math and the math is all about the triangles. For those that don’t rock out on special characters [Styropyro] does a great job of not only explaining what each symbol stands for, but applying it (on camera in video below) to the control experiment. Measure the sides of the triangle, then use simple trigonometry to determine the slit distance of the CD. This was the one missing datum that he turns around and uses to measure and determine his unknown laser wavelength.

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Creating A Scanning Monochromator

If you need a specific wavelength of light for research purposes, the naïve way of obtaining that is a white source light, a prism, and a small slit that will move across your own personal Dark Side of the Moon album cover. This is actually a terrible idea; not only won’t you have a reference of exactly what wavelength of light you’re letting through the optical slit, the prism itself will absorb more of one wavelength of light than others.

The solution is a monochromator, a device that performs the same feat of research without all the drawbacks. [Shahriar] got his hands on an old manual monochromator and decided to turn it into a device that performs automatic scans.

The key of a monochromator is a diffraction grating, a mirrored surface with many fine parallel grooves arranged in a step pattern. Because of the surface of the diffraction grating, it’s possible to separate light according to its spectrum much like a prism. Unlike a prism, it’s effectively a first surface mirror meaning all wavelengths of light are reflected more or less equally.

By adding a stepper motor to the dial of his monochromator, [Shahriar] was able to automatically scan across the entire range of the device. Inside the monochromator is a photomultiplier tube that samples the incoming light and turns it into a voltage. By sampling this voltage and plotting it with MATLAB, [Shahriar] was able to plot the intensity of every wavelength of light within the range of the device. It’s all expertly explained in the video below.

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