I am learning about the lasers and I find one thing confusing about the lasers.

I understand that there is a range of wavelength laser, it is never exactly monochromatic. So, I understand the concept of FWHM to define a linewidth.

What I have difficulty understanding is this: Why is it often stated in terms of frequency (i.e., kHz, MHz, etc) and not wavelength (in nm)? Is it not more intuitive to work in the wavelength range?

Also, I am confused about some terminology itself. Sources often write "narrow linewidth", but when they also for the opposite, say "high bandwidth". Would it not be better to call it "broad linewidth"?

  • $\begingroup$ Recall frequency times wavelength is $c$. $\endgroup$
    – Jon Custer
    Mar 7 at 2:10
  • 1
    $\begingroup$ When a beam travels from one medium to another, the wavelength changes but the frequency stays the same. $\endgroup$
    – mmesser314
    Mar 7 at 4:19

1 Answer 1


Thanks to the constant speed of light in a vacuum, we have $c = \lambda f$, so there is a direct relation between frequency and wavelength. This allows us to use those values interchangeably. Moreover, the energy of a given photon, which is related to the frequency $f$ of the field, not only does not depend on the medium in which light propagates but also depends on it linearly ($E=h f$), making that value sometimes more intuitive to use. However, very often, the choice of a specific unit of energy depends on what is common standard among a given community. For example, in the field of spectroscopy, you often see that the energy of photons is presented in units of $\text{cm}^{-1}$, i.e., $1/ \lambda$.

Regarding your second question, I haven't encountered anyone referring to broad lasers as high-bandwidth sources, as that term typically applies to systems processing various signals. Nonetheless, the meaning still should suggest that the source is broad.


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