A technique is introduced that determines power distribution in fibers from the measured near-field pattern, assuming that: (1) the optical power distributes uniformly among degenerated modes with the same propagation constant, (2) enough modes are excited to ensure the validity of calculation by geometrical optics, and (3) the phase of each propagation mode has no correlation. Experiments verifed that the fibers have the function of flattening power distribution among modes with the same propagation constant. This fact shows that assumption (1) does not severely limit the applicability of the technique. Wave optical calculation is done to determine the numbers of modes that must be excited to satisfy assumption (2). As an example of application of the technique, differential mode attenuation of graded-index fibers is determined from longitudinal variation of the measured near-field pattern.
Most of the clutter received by an L‐band air route surveillance radar obeys Weibull distribution with various values of the shape parameters. The L‐band radar has a frequency of 1.3 GHz and pulsewidth of 3 μs. Hence, the spatial resolution is as large as 450 m. The X‐band radar, on the other hand, ns. frequency of 9.4 GHz and pulsewidth of 80 ns. The spatial resolution is as small as 12 m; we have found that the clutter amplitude distribution obeys a composite Weibull distribution when reflectors are ordinary terrain and such structures as buildings. Clutter suppression techniques for Weibull and non‐Weibull echos are proposed.
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