A preliminary version of the ALMA line length correction system, which will stabilize the electrical length of the optical fibers used to distributed the LO reference signals from the center to each antenna, has been constructed in the form of a semi-portable test instrument. The length stabilization technique is dependent on use of a frequency-stable master laser. Such a laser is not yet available, but the University of Electro Communications (UEC) in Japan has a laser with very good long term stability. Therefore the test instrument was brought to Japan and a series of measurements was made using the UEC laser with the goal of verifying the accuracy of the length stabilization technique. The tests allowed us to set an upper limit on the delay variation in the stabilized path of about 70 fsec over 1 hour. It is possible that the system meets the ALMA goal of about 6 fsec delay variation (1.26 µm fiber length variation) over at least 100 sec, but establishing this was beyond the limits of these tests.
View a pdf version of ALMA Memo #445.
Last modified: 2003-02-04
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