[asac][almanews] ALMA Memo 484 Released

[Stacy Oliver]

ALMA Memo # 484

A New Configuration for the ALMA Laser Synthesizer 

Bill Shillue and Larry D'Addario 
National Radio Astronomy Observatory 

2004-06-17

Keywords: laser, synthesizer, photonic, local oscillator 

This memo describes the results of a test of a new configuration of laser
synthesizer for ALMA. The ALMA laser synthesizer is the source of the 1st LO
photonic reference, in which a two-wavelength optical beatnote is
transmitted by optical fiber from the ALMA AOS technical building to each of
the array antennas. It is then used at the antenna as the reference for the
first local oscillator, after a photomixer converts the beatnote into a
radio frequency in the range 27-142 GHz. This report is a condensed version
of a more detailed test report available on ALMAEDM.

In this new configuration, a slave DFB fiber laser (DFB-FL) is phase-locked
to a master laser using an external fiber frequency shifter (FFS). The use
of an FFS as an external tuning element removes the requirement for fast
modulation response from the slave laser that had heretofore been an
obstacle in finding a suitable tunable slave laser. This technique has been
implemented previously for very low phase-noise applications. 

The relatively wide modulation bandwidth allows for a wide bandwidth
phase-lock loop. Coupled with the intrinsically low phase noise of the
DFB-FL, the resulting beatnote is shown to have very low phase noise. 

View a pdf version of ALMA Memo #484. 
http://www.alma.nrao.edu/memos/html-memos/alma484/memo484.pdf


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