Abstract:
The ALMA 1st LO reference is sent from the central Array Operations Site
Technical Building to each of 64 antennas over optical fiber. Two
high-coherence and phase-locked lightwaves are transmitted that are
separated by a variable frequency ranging from 27-142 GHz. The round-trip
stabilized fiber optic distribution system has been previously described
in this memo series [1-3,13]. In August of 2003, a first generation
version of the line length correction system was tested on a prototype
ALMA antenna. These were the first systematic measurements of the system
on the moving structure. During these measurements we noticed an
undesirable and unexpected phase fluctuation which was correlated with the
antenna azimuth and elevation position. Those tests are described in an
internal test report [4]. Further tests and meetings took place in an
effort to resolve this issue [5-8]. In addition, an ALMA memo was written
describing a theoretical treatment of the measured phase fluctuation [9],
supporting the measurement results which showed that the phase fluctuation
was due to the absolute polarization change (caused by the fiber movement)
of the two lightwaves, and that the phase fluctuation magnitude was
inversely proportional to the degree of polarization alignment of the two
lightwaves. To put it another way, if the state-of-polarizations (SOPs) of
the two lightwaves were different at the receiver end, then any movement
of the fiber would cause a phase change. The purpose of this memo is to
summarize the main points from the references listed above, and
additionally to describe more recent measurements that utilize
improvements to the 1st LO reference baseline design. This is mainly an
experimental report, there is a related theoretical study as well [14].
View a pdf version of ALMA Memo #507.
Last modified: 2005-02-20
alma-memos@nrao.edu