ALMA Memo # 484 

Title A New Configuration for the ALMA Laser Synthesizer

Author (institution), Bill Shillue, National Radio Astronomy Observatory

Author (institution)	Larry D’Addario, National Radio Astronomy Observatory 
 


Date 17/06/2004 

Keywords: laser, synthesizer, photonic, local oscillator 
<br><br>
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[1].
      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 [2,3]. 
      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.
      
References

[1] Report available on ALMAEDM: BEND-50.03.09.00-001-A-REP, “Laser Synthesizer - Test Report on Configuration with Fiber Laser (Slave) Phase-Locked with External Frequency Shifter”, 11/17/2003
[2 ]J. Ye, J. L. Hall, “Optical Phase Locking in the Microradian Domain: Potential Aplication to NASA Spaceborne Optical Measurements,” Optics Letters, Vol. 24, No. 24, Dec 15, 1999, pp. 1838-1840
[3] L. Ma, P. Jungner, J. Ye, J.L. Hall, , “Delivering the same Optical Frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Optics Letters, Vol. 19, No. 21, Nov. 1, 1994, pp. 1777-1779