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

In summary, a He-Ne laser emits light with a single mode of laser cavity. If the light does not have a monochromatic wavelength, it will beat with itself inside the cavity.
  • #1
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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  • #2
The typical linewidth of unstabilized HeNe lasers is about 1GHz, stabilized systems can go down to a few kHz.
 
  • #3
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)
 
  • #4
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.
 
  • #5
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.
 

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

1. What does it mean for a He-Ne laser light to be monochromatic?

Monochromatic light refers to light that has a single wavelength or color. This means that all the photons emitted by the laser have the same energy level and frequency, resulting in a pure and consistent color of light.

2. How can I test for monochromaticity in a He-Ne laser light?

One way to test for monochromaticity is to use a spectrophotometer. This device measures the wavelengths of light and can determine if there are multiple wavelengths present. Alternatively, you can also use a diffraction grating or a prism to separate the different wavelengths of light and observe if there is a single, dominant wavelength.

3. What factors can affect the monochromaticity of a He-Ne laser light?

Several factors can affect the monochromaticity of a He-Ne laser light, including the quality and alignment of the laser cavity, the purity of the gas mixture, and the accuracy of the mirrors used to reflect the light. Temperature changes and external vibrations can also impact the monochromaticity of the laser light.

4. Is it possible for a He-Ne laser light to be partially monochromatic?

Yes, it is possible for a He-Ne laser light to have a dominant wavelength but also have some minor fluctuations or variations in wavelength. This is known as partial monochromaticity and can be caused by imperfections in the laser setup or environmental factors.

5. How can I improve the monochromaticity of a He-Ne laser light?

To improve the monochromaticity of a He-Ne laser light, it is important to ensure that the laser cavity is high quality and well-aligned. Using high-quality mirrors and controlling environmental factors such as temperature and vibrations can also help improve monochromaticity. Additionally, using a narrow-band filter can help remove any residual wavelengths, resulting in a purer monochromatic light.

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