Balanced Homodyne Detection

Planning Documents

Requirements

  1. Optical design
    • Round-trip cavity length?
    • Curvature of curved mirrors? This along with the round-trip length should be chosen to avoid any accidental resonances of higher order modes of the carrier or sideband fields.
    • Cavity finesse (i.e. transmission of the mirrors)? This will set the achieved filtering performance of the OMC, which is in turn motivated by (i) the mode content of the beam incident on the OMC and (ii) the requirement on the amount of junk light that can be tolerated at the OMC readout PDs.
  2. Displacement noise of OMC/LMC mirrors
    • Are we using a single cavity or two?
  3. Scatter loss in mirrors
    • This will affect the backscatter into the IFO
    • This will limit the transmissivity of the cavity for a given finesse.
  4. Cavity design: triangle, quad ring, quad zig-zag (reference the new OMC paper from Koji)
    • What does this mean?
  5. Laser amplitude / frequency noise including DC offsets in various other lock points and motions to simulate bilinear noise
  6. Oscillator noise: AM / FM
    • This will set the requirement on the filtering necessary.
  7. Aux length noise: MICH/PRC/ SRC
  8. Req. on offsets/asymmetries: arm finesse, arm reflectivity, mass imbalance of ETMs
  9. Phase noise of LO
    • This will set the requirement on whatever we use to control the homodyne angle.
  10. Polarization stability requirement for polarization BHD
    • i.e. let's say we have some finite Ip/Is ratio. What does this mean for the readout in signal-referred units?
  11. Backscatter
    • RIN on the LO
    • Phase noise due to motion of the mirrors
  12. Pointing
    • If we opt to not use a mode-cleaner for the LO field, what is the pointing requirement on whatever option we choose?
  13. Suspension
    • What kind of suspension(s) are we planning to use?

    • VOPO and OMCS seem too big to fit on any of our existing tables.

  14. Control scheme
    • How will we control the LO phase?

Some preliminary calculations can be found in this elog

Modeling/Noise Budget

Modeling to determine the BHD noise requirements is being done in the 40m BHD repo. The models use Finesse 2.2 and are called from Python 3 using PyKat.

In order of priority, here is the list of couplings to be modeled. Items denoted with (HOM) require a multi-mode analysis.

  1. Classical noise on LO
    • RIN
    • Phase noise
      • Laser phase noise
      • LO path length noise (homodyne angle)
      • OMC length noise
  2. Asymmetries
    • Reflectance/transmittance imbalance of homodyne BS
    • Finesse imbalance of OMCs
  3. Readout electronics noise
  4. Angular pointing noise (HOM)
    • Mode-overlap variation at homodyne BS
    • Mode coupling into OMCs
  5. Backscatter noise (HOM)

Layout options

Schematic BHD layout options. In the linked document, some options for placement of the BHD optics are schematically sketched (but I don't consider the option of adding a new table). Some more discussion may be found in this elog.

7/8: A more detailed layout sketch, that accounts for LO and AS beam routing, may be found here.

Action items

Polarization BHD

Balanced Homodyne Detection (last edited 2020-04-29 17:41:39 by JonathanrichardsonATligoDOTorg)