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How to Accurately Measure Laser Line Width

Unlike any other light source, a laser produces a single-color beam of light. Saying a laser emits a single color is the same as saying it emits a single wavelength of light. However, a combination of factors ̵2; some fundamental to the generation of light ̵2; make it impossible for the light to be truly one single wavelength. Instead, each laser emits a range of wavelengths. For ultrastable lasers, that width is very small, and the linewidth requires very accurate measurement.

Things You'll Need

Laser
Beamsplitters and Mirrors
Photodetector
Low-noise electronic amplifier
Electronic spectrum analyzer

Instructions

- 1
  Divide the laser output into two beams using the beamsplitter.
- 2
  Route one of the paths directly to the photodetector.
- 3
  Route the other beam along a different path a few feet longer than the first beam path, crossing it with the original beam close to the photodetector.
- 4
  Recombine the two beams with the beamsplitter and send them both into the photodetector.
- 5
  
  To achieve stability, a laser must be in thermal equilibrium, which takes a little while to achieve.
  
  Allow the laser to stabilize.
- 6
  Connect the output of the photodetector into the amplifier, then plug it into the electronic spectrum analyzer.
- 7
  Measure the spectrum of the photodetector output. The two beams will ̶0;beat̶1; against each other, creating an intensity variation at a frequency twice as fast the linewidth of the laser. That is, a measured frequency of 150 MHz implies a laser linewidth of 75 MHz.

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