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What is a laser spectrum? How many wavelengths can we see in it, and why?

For a regular laser, the red ones you can buy at the store, is there just one red wavelength in the laser? Or are there multiple? Could someone please explain why?

I was thinking that there’s just one wavelength but I’m not sure which wavelength since there’s so many different types of red.

Much appreciated if someone could give their input or sources.

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2 Answers 2

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You are probably talking about the helium-neon laser:

The best-known and most widely used He-Ne laser operates at a wavelength of $632.8$ nm (in air), in the red part of the visible spectrum.

This particular wavelength is due to transitions between two energy levels of the neon atoms. And because of the LASER (Light Amplification by Stimulated Emission of Radiation) mechanism it is a spectrum with an extremely narrow bandwidth.

helium neon laser spectrum
(image by Deglr6328 at the English-language Wikipedia)

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  • $\begingroup$ Thanks! Do we only see 7 wavelengths in the laser spectrum since we can only see the visible specturm which has 7 colours? I'm having trouble understanding as each colour covers a range of wavelengths. $\endgroup$
    – qris
    Apr 6 at 0:26
  • $\begingroup$ @qris There is only this single one red wavelength in the laser spectrum, and no other red colour. $\endgroup$ Apr 6 at 0:30
  • $\begingroup$ There are infinitely more than 7 colors. This is a profound misunderstanding of what color means. Every unique wavelength or even every mixture of different wavelengths will cause different color perceptions. We just group them with names because it makes life a lot easier in some (most) contexts. $\endgroup$ Apr 6 at 2:38
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What is a laser spectrum?

The spectrum of the laser is a function telling you how much power (or power spectral density) the laser outputs at each frequency or wavelength.  

How many wavelengths can we see in it, and why?

It depends on the laser. Some are single-mode, producing a spectrum consisting of only a single, narrow, band of wavelengths. Others are multi-modal, producing (typically) several such "peaks" in their spectrum.

Note also that a laser peak is "narrow" but it is not infinitely so. A laser diode output peak might be as wide as 1 nm, or as narrow as a few femtometers (when the peak is very narrow, it's easier to measure in frequency space than in wavelengths, and can range down to a few 100's of kHz).

For a regular laser, the red ones you can buy at the store, is there just one red wavelength in the laser? Or are there multiple?

For a typical red laser pointer, built around a diode laser, there will be only one main peak in the spectrum. However, given that extreme spectral purity is not required for this application, it's likely that there would be some "side-lobes" on this peak. These are peripheral peaks that might be anywhere from 3 to 30 dB weaker than the main peak.

I was thinking that there’s just one wavelength but I’m not sure which wavelength since there’s so many different types of red.

As Thomas Frisch says in another answer, the red produced by a helium-neon gas laser is at 632.8 nm.

Low-cost laser pointers (again, using diode lasers) are likely to be closer to 650 nm, as that happens to be a wavelength that it's relatively easy to produce in diode laser technology. 635 nm and 670 nm diode lasers are also common.

In a comment on Thomas's answer you asked,

Do we only see 7 wavelengths in the laser spectrum since we can only see the visible specturm which has 7 colours?

The visible spectrum is not just 7 discrete colors. It is a continuous band of wavelengths from about 380 to about 750 nm (there is some variation between individuals in the maximum and minimum visible wavelengths). Any wavelength between these extremes is visible, including 395 nm, 450.23 nm, and 720.12345679 nm.

That said, laser output spectra are not usually narrow enough to distinguish 720.12345679 from 720.12345678 nm, and also often vary due to the temperature of the laser itself.

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