OSE Seminar By OSE Alumnus, Dr. Xiaozhen Xu of NASA on RF differential phase measurements of the LISA fiber amplifier

Departmental News

Dr. Jason Jones

Posted: December 5, 2022

Date: Thursday, 12/08/2022

Time:  12:15 PM - 1:15 PM

Location:  PAIS, Room 2540 and via Zoom



One of the key requirements of the LISA laser system is the phase difference between the optical carrier frequency and its sideband.  This phase difference needs to be smaller than the allowed phase noise equivalent to a spacecraft displacement of 1pm/√Hz. This translates to a maximum allowable differential phase noise for the RF beat tones of reference-carrier and reference-sideband. 

We carefully characterized noise sources in the measurement setup and controlled key parameters to reduce noise floor:  

  1. Characterize the phase meter noise at different RF frequency and work within frequency range of minimum noise; 
  2. Characterize amplitude-to-phase conversion of photodiodes at various current levels. Photodiode with low noise at desirable current level is selected; 
  3. Characterize differential phase responses on temperature variation of individual components as well as the entire enclosed setup, which help us to understand the temperature stability requirement needed to meet the phase noise requirement; and 
  4. Enclosed the measurement setup with temperature-controlled Styrofoam enclosure to isolate sensitive components from environmental temperature fluctuations to minimize temperature to phase noise coupling. 
    With these efforts, the differential phase noise from the measurement setup, including the phase modulator, is 5x lower than requirement at 10mHz. The lowered system level noise of our test setup allows proper evaluation of differential phase noise from fiber amplifiers.  We used this setup to evaluate different fiber amplifiers, including the TRL4/5 MOPA unit built at NASA GSFC. The measured differential phase noise measured with our system is now proposed as the new requirement for the LISA laser.  
    In this talk, we will present our RF phase noise measurement setup, and will also summarize our results measured with different fiber amplifiers at different operation and environmental conditions.
    Xiaozhen Xu 1, Kenji Numata  2, Anthony Yu 
    1 Science Systems & Applications, Inc., Greenbelt, Maryland, USA 
    2 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 


Xiaozhen Xu received her PhD from the University of New Mexico in 2014, working with Dr. Jean-Claude Diels. Her thesis work developed a high power UV laser system for laser induced discharge. Since graduation, she has accumulated a broad range of industrial R&D experiences in the development and applications of advanced laser technologies. She is currently a laser system engineer of SSAI, supporting various programs at NASA Goddard Space Flight Center in Maryland, including development of laser system for the LISA program.