  Optical homodyne detection is examined in view of joint probability
distribution. It is usually discussed that the relative phase between
independent laser fields are localized by photon-number measurements in
interference experiments such as homodyne detection. This provides reasoning to
use operationally coherent states for laser fields in the description of
homodyne detection and optical quantum-state tomography. Here, we elucidate
these situations by considering the joint probability distribution and the
invariance of homodyne detection under the phase transformation of optical
fields.
