It was shown some time ago that the space-time and the space-frequency degrees of
polarization of a stochastic electromagnetic beam are not equivalent to each
other. It is not possible, in general, to obtain a formal relationship between
them. In this Letter, we discuss certain conditions under which they are
directly related. These conditions lead to the concept of polarization-purity.
If an optical beam obeys these conditions, its space-frequency degree of
polarization has the same value at all frequencies present in the spectrum, and
the value is equal to the space-time degree of polarization.
© 2013 Optical Society of America
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