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laser linewidth
Laser linewidth is the spectral linewidth of a laser beam.
Two of the most distinctive characteristics of laser emission are spatial coherence and spectral coherence. While spatial coherence is related to the beam divergence of the laser, spectral coherence is evaluated by measuring the laser linewidth of the radiation. Although the concept of laser linewidth can have varied theoretical descriptions here, this article provides a simple experimental description. One of the first methods used to measure the coherence of a laser was interferometry.〔O. S. Heavens, ''Optical Masers'' (Wiley, New York, 1963). 〕 An alternative approach is the use of spectrometry.〔F. P. Schäfer'' et al.'', Organic dye solution laser, ''Appl. Phys. Lett.'' 9, 306-309 (1966).〕
==Continuous lasers==
Laser linewidth in a typical single-transverse-mode He-Ne laser (at a wavelength of 632.8 nm), in the absence of intracavity line narrowing optics, can be of the order of 1 GHz. On the other hand, the laser linewidth from stabilized low-power continuous-wave lasers can be very narrow and reach down to less than 1 kHz.〔L. W. Hollberg, CW dye lasers, in ''Dye Laser Principles'', F. J. Duarte and L. W. Hillman (eds.) (Academic, New York, 1990) Chapter 5.〕 Often this type of linewidth is limited by fundamental quantum processes.〔H. Haken, ''Light and Matter'' (Springer, Berlin, 1970).〕 This limit is known as the Schawlow–Townes linewidth〔https://www.rp-photonics.com/schawlow_townes_linewidth.html - Schawlow–Townes Linewidth〕 which can be lower than Hz for some kind of CW lasers. Nevertheless observed linewidths are larger due to a technical noise (from noise in current, vibrations etc.).
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