Return to Memo list

ALMA Memo # 507

ALMA First LO Reference : Elimination of Large Phase Fluctuations Due to Lightwave Polarization Effects

B. Shillue and S. AlBanna
2004-12-15

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.