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. An alternative approach is the use of spectrometry.

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. Often this type of linewidth is limited by fundamental quantum processes. This limit is known as the 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.).

Laser linewidth from high-power, high-gain pulsed-lasers, in the absence of intracavity line narrowing optics, can be quite broad and in the case of powerful broadband dye lasers it can range from a few nm wide to as broad as 10 nm.

Laser linewidth from high-power high-gain pulsed laser oscillators, comprising line narrowing optics, is a function of the geometrical and dispersive features of the laser cavity. To a first approximation the laser linewidth, in an optimized cavity, is directly proportional to the beam divergence of the emission multiplied by the inverse of the overall intracavity dispersion. That is,

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