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.

13 thoughts on “Measure Laser Wavelength with a CD and a Tape Measure

  1. Once you have two laser sources of known wavelength, you could use this technique to build your self a cheap spectrometer using a webcam. If the grating is the proper distance from the sensor, and is fixed relative to the sensor, and you know the pixel size and number of pixels, then you can determine the wavelength by which pixel the light is centered on. You can use a lens to get better resolution, and a cylindrical lens to get a line that fills more pixels in a column. I have a picture of one I built, but I am not sure how to add a photo here.

      1. Typically you use a low pressure mercury-argon lamp to calibrate. These lamps have about 7 lines in the visible spectrum and a lot more in the IR and UV range that make it real easy to do a thorough calibration. 2 points is not really enough if you have a high resolution spectrometer to compensate for linearity issues. I used to use a minerallight UV lamp without the filter to calibrate mine, you cant let them run long as the mercury evaporates and the pressue build up increasing the strength of the Hg lines which overpower the weaker Ar lines. Also lots of short wave UV.

    1. The warm-up time changes the power, but the wavelength is stable. The diode-pumped Nd:YVO4 chip will lase at 1064nm in this application. It is capable of lasing at 946nm (this is the basis of blue laser pointers), but this is suppressed by the coatings used on the crystal. The low power green pointers then use intra-cavity frequency doubling (half the wavelength, also called the second harmonic) with a non-linear crystal of some type (typically LBO or BBO), while the high power pointers (500mW or more) typically use a PPLN (periodically poled lithium niobate) crystal. The PPLN technique is very temperature sensitive (the temperature determines the phase-matched wavelength) so there can be significant changes in power as the laser warms up. Lab lasers that use a PPLN have temperature controllers to regulate the PPLN crystal temperature.

  2. Sure the laser wavelength can be measured directly. All you need is an interferometer with a movable mirror and a photodiode on the other end of the beam splitter. Count the pulses on the photodiode and measure the mirror movement. 2x mirror movement / number of photo diode pulses gives you the wavelength directly.

    1. The standard Redbook track pitch is for 74min (650MB) discs. It’s very common for discs vary the track pitch to allow for higher capacities. For example, an 80min (700MB) disc would have an expected track pitch around 1.6um*(74min/80min) = 1.48um.

  3. This is pretty neat. And it also works in reverse, and with single slit gratings. This means that once you have a laser source of known wavelength, you can determine the width of thin wires (e.g. your hair) with a very high degree of accuracy

  4. Indeed. Also you can measure the diameter of fine powders such as talcum powder by using a microscope slide covered with the talc to diffract the laser beam and observing the resultant intensity patterns. I remember doing this at A-Level in college. Very cool stuff since lasers were rather alien technology to a teenager in Britain in 1994.

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