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How to Measure Laser Line Width

From the forensics lab to the grocery store, lasers are seen throughout the world. One of their inherent characteristics -- one that̵7;s essential to just about every laser application -- is their narrow spread of wavelengths; that is, they put out a single color, or combination of single colors. The technical term describing a laser̵7;s wavelength spread is ̶0;linewidth.̶1; The linewidth determines how accurately a laser can communicate, how precise a hologram it can create and how reliably it can analyze materials, so measuring linewidth is a fundamental need.

Things You'll Need

Laser
Piezoelectric spherical Fabry-Perot cavity
Photodetector
Oscilloscope or other data recorder

Instructions

- 1
  Turn on the laser and let it reach stable operation. Some lasers will be stable almost instantaneously, while some may take a while to reach thermal equilibrium.
- 2
  Send the beam into the Fabry-Perot cavity. The Fabry-Perot has two spherical mirrors facing each other, and will interfere the beams constructively when twice the distance between the opposing mirrors is equal to an integral number of source wavelengths. The beam reflects back and forth within the cavity many times, which leads to a narrow peak when the light is interfering constructively.
- 3
  Put the photodetector at the output of the Fabry-Perot and connect the photodetector to the oscilloscope or other data acquisition instrument.
- 4
  Monitor the output as you automatically vary the Fabry-Perot cavity length. Measure the distance between two neighboring peaks of the Fabry-Perot output. This is the free spectral range (FSR) in your units of measurement. For example, you might find that 6 horizontal divisions on your oscilloscope correspond to the FSR.
- 5
  Calculate the free spectral range for your cavity. The FSR is given by c/(cavity length), where c is the speed of light. For example, a 10 cm-long cavity would have an FSR of
  3 x 10^10 cm/s / (2 x 10 cm) = 1.5 x 10^9/s = 1.5 GHz.
- 6
  Determine the scale of your measurement in units of frequency. For example, if 6 horizontal divisions correspond to 1.5 GHz, then the scale is 250 MHz/division.
- 7
  Measure the width of a single line. If necessary for readability, you can change the scale factor, as long as you keep track. For example, you could change the horizontal scale on the oscilloscope by a factor of 2, which would make each horizontal division now equal to 125 MHz. So if the width of a single line was equal to 2/3 of a 125 MHz division then the laser linewidth would be about 83 MHz.

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