How to be sure that a He-Ne laser light is monochromatic

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Discussion Overview

The discussion revolves around determining whether the light emitted by a Helium-Neon (He-Ne) laser is monochromatic. Participants explore various methods and concepts related to laser modes, linewidths, and detection techniques, focusing on both theoretical and practical aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant inquires about using a diffraction grating to identify multiple wavelengths in the laser light, noting the importance of the grating's resolving power.
  • Another participant mentions that the typical linewidth of unstabilized He-Ne lasers is about 1 GHz, while stabilized systems can achieve linewidths down to a few kHz.
  • A suggestion is made to use a reference high stability source and an optical mixer to examine the beat frequency between the two sources as a method to assess monochromaticity.
  • It is proposed that a laser can beat with itself if it is not single mode, but detecting this requires specialized equipment due to the fast response times needed.
  • A participant emphasizes that a grating may not have sufficient resolution for certain measurements and suggests using a Fabry-Perot interferometer instead.
  • Another point raised is the distinction between "single mode" and "monochromatic," clarifying that single mode operation does not necessarily imply monochromatic light due to the presence of linewidths associated with modes.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between single mode operation and monochromatic light, indicating that there is no consensus on the definitions and implications of these terms. Multiple methods for assessing monochromaticity are discussed, but no single method is universally accepted as definitive.

Contextual Notes

Participants highlight the importance of understanding the relationship between mode spacing and the free-spectral range of the cavity, as well as the limitations of various detection methods. The discussion reflects a range of assumptions about the capabilities of equipment and the definitions of key terms.

bznm
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How can I be sure that the emission of a He-Ne laser contains only one single mode of laser cavity?

The only thing that I know is that if I use a diffraction grating and the light isn't monochromatic, I'll see maximums of the same order at different angles, but I also know that if wavelengths are very close I may not see them. I have to mind the resolutive power of the grating (R=mN). If N1=1000 lines/mm and N2=500 lines/mm and the grating paces are D1=10^-6 m and D2=2*10^-6 m, will I see different maximums if the light isn't monochromatic?

Do you know other ways to know if the light of a He-Ne is monochromatic?
 
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The typical linewidth of unstabilized HeNe lasers is about 1GHz, stabilized systems can go down to a few kHz.
 
Andy Resnick said:
The typical linewidth of unstabilized HeNe lasers is about 1GHz, stabilized systems can go down to a few kHz.

So, presumably, you could take a know reference high stability source and use an optical mixer to examine the beat between the two? (To answer his question)
 
You actually do not need a local oscillator (second source). If the laser is not single mode, it will beat with itself inside the cavity. The difficulty is detection. If the free spectral range of the cavity (mode spacing) is 5 GHz, you need a detector with response times on the order of 1/5GHz. (200ps). The technique is a lot easier in the RF regime, but it can be done with good equipment.

A grating will not have the resolution for this, typically for measurements on this scale (think hyper-fine structure measurements) one uses a Fabry-Perot interferometer.

Also, single mode lasing does not mean monochromatic light. Generally, you just get a linewidth that is governed by the cavity finesse.
 
One way is to calculate the longitudinal mode spacing and compare it to the free-spectral range of the cavity.

If the mode spacing >> the cavity FSR, it should operate on a single longitudinal mode.

P.S. Don't conflate "single mode" with "monochromatic". Not the same thing (because modes have their own linewidths).

Claude.
 

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