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

[bznm]

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?

 

[Andy Resnick]

The typical linewidth of unstabilized HeNe lasers is about 1GHz, stabilized systems can go down to a few kHz.

 

[sophiecentaur]

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)

 

[dmriser]

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.

 

[Claude Bile]

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.