  The radio emission of pulsar B1451-68 contains two polarization modes of
similar strength, which produce two clear orthogonal polarization angle tracks.
When viewed on a Poincare sphere, the emission is composed of two flux patches
that rotate meridionally as function of pulse longitude and pass through the
Stokes V poles, which results in transitions between orthogonal polarization
modes (OPMs). Moreover, the ratio of power in the patches is inversed once
within the profile window. It is shown that the meridional circularization is
caused by a coherent OPM transition (COMT) produced by a varying mode ratio at
a fixed quarter-wave phase lag. The COMTs may be ubiquitous and difficult to
detect in radio pulsar data, because they can leave no trace in polarized
fractions and they are described by equation similar to the rotating vector
model. The circularization, which coincides with flux minima at lower
frequency, requires that profile components are formed by radiation with an
oscillation phase which increases with longitude in steps of 90 degrees per
component. The properties can be understood as an interference pattern
involving two pairs of linear orthogonal modes (or two nonorthogonal elliptic
waves). The frequency-dependent coherent superposition of coplanar oscillations
can produce the minima in the pulse profile, and thereby the illusion of
components as separate entities. The orthogonally polarized signal which is
left after such negative interference explains the enhancement of polarization
degree which is commonly observed in the minima between profile components.
