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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]:
This shows that the mode radius becomes smaller for higher frequencies, which have higher V values. The equation is fairly accurate for V values above 1. 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
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, mode size converters, fibers, single-mode fibers
