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Encyclopedia of Laser Physics and Technology

Mode Radius

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Definition: a measure of the transverse extent of a laser mode or laser beam

The transverse extent of the optical intensity distribution of a mode (e.g. of an optical cavity or a waveguide) is usually specified as a mode radius, which is defined like a Gaussian beam radius: the mode radius is the radius where the intensity has dropped to 1/e2 of the intensity on the beam axis. For non-Gaussian mode shapes, it is also common to use a Gaussian fit and take its radius as the result.

The mode diameter (sometimes also called mode field diameter, MFD) is simply twice the mode radius.

It is not recommended to use the common terms mode size or spot size in a quantitative sense, because it is then not clear whether the radius or the diameter is meant.

Mode Radius of a Single-mode Fiber

For single-mode fibers, the mode radius may be estimated from the core radius a and the V number, using Marcuse's equation [1]:

mode radius of single-mode fiber

This shows that the mode radius becomes smaller for higher frequencies, which have higher V values. The equation is fairly accurate for V values between 0.8 and 2.5. In the multimode range (V > 2.405), it applies to the fundamental mode.

The mode radius then determines the effective mode area of the fiber according to

formula for mode area from mode radius

Bibliography

[1]D. Marcuse, “Loss analysis of single-mode fiber splices”, Bell Syst. Tech. J. 56, 703 (1977)

See also: modes, Gaussian beams, beam radius, effective mode area, fibers, single-mode fibers

Category: fibers and other waveguides


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Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.

You may order the print version via Wiley-VCH.

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