| Radiation from lasers is different from conventional optical light because, like
microwave radiation, it is approximately monochromatic. Although each laser has
its own fine spectral distribution and noise properties, the electric and magnetic
fields from lasers are considered to have precise phase and amplitude variations
in the first-order approximation. Like microwaves, electromagnetic radiation with
a precise phase and amplitude is described most accurately by Maxwell’s wave
equations. For analysis of optical fields in structures such as optical waveguides and
single-mode fibers, Maxwell’s vector wave equations with appropriate boundary
conditions are used. Such analyses are important and necessary for applications in
which we need to know the detailed characteristics of the vector fields known as
the modes of these structures. They will be discussed in Chapters 3 and 4.
For devices with structures that have dimensions very much larger than thewavelength,
e.g. in a multimode fiber or in an optical system consisting of lenses, prisms
or mirrors, the rigorous analysis of Maxwell’s vector wave equations becomes very
complex and tedious: there are too many modes in such a large space. It is difficult to
solve Maxwell’s vector wave equations for such cases, even with large computers.
Even if we find the solution, it would contain fine features (such as the fringe fields
near the lens) which are often of little or no significance to practical applications. In
these cases we look for a simple analysis which can give us just the main features
(i.e. the amplitude and phase) of the dominant component of the electromagnetic
field in directions close to the direction of propagation and at distances reasonably
far away from the aperture. |
|
|
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