Differences between revisions 14 and 85 (spanning 71 versions)

Arm Length Stabilization using green laser beams

attachment:green_man.jpg

We are developing a technique of Arm Length Stabilization (ALS) for advanced LIGO. The technique employs two auxiliary green lasers at each end of the arm. Basically these lasers are able to sense the displacement of the each arm independently on the main laser, eventually make full lock acquisition of advanced interferometer more robust and reliable.

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Designs and Plans

time line

The time line describes where we are now and where we go. time line

Conceptual design

Description of the optical / servo configuration [https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=6888 LIGO DCC]
Servo modeling (SimLink) [https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/Green_Locking/Servo_modeling/ 40mSVN]

Green Preliminary Design

Installation status

Design and Purchasing Lists
- Optical design / item list
- Electronics / control design / item list
- Purchase list
Detailed considerations
- Optical Layout ([attachment:green_optics.png Table]/ In-vac)
  - ETM / end table
  - ITM wedge effect
  - Periscopes for beam steering from PR3/SR3 transmissions to the PSL viewport
  - PSL viewport modification (viewport cover / tube / PSL box / periscope on the PSL table)
  - PSL table
  - Laser & SHG specs --- power, polarization, Xtals
    - - references
    - [1] G.D.Boyd and D.A. Kleinman, [http://jap.aip.org/japiau/v39/i8/p3597_s1 "Parametric interaction of Focused Gaussian Light Beams", J.A. Phys. 39, 3597 (1968)]
    - [2] C. Zhang, Y.Qin and Y.Zhu, [http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-33-7-720 "Perfect quasi-phase matching for the third-harmonic generation using focused Gaussian beams",Opt. Lett. 33, 720 (2008)]
- Main optics specs (review)
- Mode matching ([attachment:modematch_END.png mode matching at end table])
- Green generation
  - [attachment:manual_oven.pdf manual for the covesion oven]
- Expected performance / noise / control system range
- Electronics
- Digital servo / operation
- Alignment (initial / fine / automatic)
jump to Control Information

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+= Arm Length Stabilization using green laser beams =
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  .   attachment:green_man.jpg
[[BR]]

   . We are developing a technique of Arm Length Stabilization (ALS) for advanced LIGO. The technique employs two auxiliary green lasers at each end of the arm. Basically these lasers are able to sense the displacement of the each arm independently on the main laser, eventually make full lock acquisition of advanced interferometer more robust and reliable. 

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-= Arm length stabilization injecting green laser from the end of the arm =
== Plan ==
+[[BR]]


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== Designs and Plans ==
=== time line ===
 * The time line describes where we are now and where we go. B) [[Upgrade 09/GreenLock/timeline|time line]]
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-  * Description of the optical / servo configuration [https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=6888 LIGO DCC]
  * Servo modeling ("SimLink") [https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/Green_Locking/Servo_modeling/ 40mSVN]
+ * Description of the optical / servo configuration [https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=6888 LIGO DCC]
 * Servo modeling (!SimLink) [https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/Green_Locking/Servo_modeling/ 40mSVN]
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-=== Detailed design ===
  * Optical design / item list
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+Line 29:
-  * Electronics / control design / item list
+==== Green Preliminary Design ====
   .     <<ImageLink(green_vertex_preliminary_design.png,Upgrade 09/GreenLock/Preliminary design for the vertex,height=200)>>
   <<ImageLink(green_ETM_preliminary_design.png,Upgrade 09/GreenLock/Preliminary design for the end,height=200)>>
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-  * '''Detailed considerations'''
   * Optical Layout (Table / In-vac)
   * Mode matching
   * Main optics specs
   * Green generation
   * Expected performance / noise / control system range
   * Digital servo / operation
   * Alignment (initial / fine / automatic)
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+Line 34:
-=== Development roadmap ===
  * '''End table arrangement'''
   * NPRO placement / optical assembly at the end
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-   * SHG at the end table
   * Control system placement
   * Coarse alignment to the cavity
   * Fine alignment
   * Lock of the green beam to the cavity
  * '''At the PSL table'''
   * SHG at the PSL table
  * '''Digital control'''
   * Virtual Green lock by RCG
   * GPIB interfaces
  * '''Vertex tank optical arrangement'''
   * In-vac steering (PO transmission / Periscope / Mirrors)
   * Vertex phase noise measurement
    * Electronics placement
    * Control system implementation
  * '''Performance evaluation'''
   * f_noise of green beam / PDH performance
+[[BR]]
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-   * Relative f_noise between the PSL green and the Arm transmitted green
   * ALS (Arm Length Stabilization) performance
  * '''Sophistication of the scheme'''
   * Automatic handing off scripts
+-------
== Installation status ==
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-   * Ready-to-go panels for AdvLIGO
   * Automatic alignment of the green beam
   * .
   * Green Michelson for precise ETM calibration
+ * '''Design and Purchasing Lists'''
  * Optical design / [[Upgrade 09/Purchase List/Green Locking|item list]]
  * Electronics / control design / [[Upgrade 09/Purchase List/Green Locking|item list]]
  * [[Upgrade 09/Purchase List/Green Locking|Purchase list]]
 * '''Detailed considerations'''
  * Optical Layout ([attachment:green_optics.png Table]/ In-vac)
   * [[Upgrade 09/GreenLock/end table|ETM / end table]] <<ImageLink(endtable.jpg,Upgrade 09/GreenLock/end_table)>>
   * [[Upgrade 09/Optical Layout#wedge|ITM wedge effect]]
   * [[Upgrade 09/Optical Layout|Periscopes for beam steering from PR3/SR3 transmissions to the PSL viewport]]
   * PSL viewport modification (viewport cover / tube / PSL box / periscope on the PSL table)
   * PSL table
   * Laser & SHG specs --- power, polarization, Xtals
    . '' - references''
    . [1] G.D.Boyd and D.A. Kleinman, [http://jap.aip.org/japiau/v39/i8/p3597_s1 "Parametric interaction of Focused Gaussian Light Beams", J.A. Phys. 39, 3597 (1968)]
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-   * Absolute length / mode spacing measurement by green''' '''
+    . [2] C. Zhang, Y.Qin and Y.Zhu,  [http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-33-7-720 "Perfect quasi-phase matching for the third-harmonic generation using focused Gaussian beams",Opt. Lett. 33, 720 (2008)]
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-----
== Noise Requirement ==
I'd say the arm cavity length should not change more than 1/100 of the resonance width in the time scale of 10sec.[[BR]] For this to be fulfilled, the RMS fluctuation of the cavity length must be suppressed below 1e-11m level.[[BR]] Does this make sense ?
+  * Main optics specs (review)
  * Mode matching ([attachment:modematch_END.png mode matching at end table])
  * Green generation
   . [attachment:manual_oven.pdf manual for the covesion oven]
  * Expected performance / noise / control system range
  *  B) [[Upgrade 09/GreenLock/Electronics|Electronics]]
 
  * Digital servo / operation
  * Alignment (initial / fine / automatic)
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-=== Noise Sources ===
==== PLL phase noise ====
==== Phase noise from the cavity common mode motion ====
When the cavity is locked to the green laser, the differential motion of the two mirrors will be suppressed by the servo.[[BR]] However, the common motion of the cavity mirrors will not be suppressed. This common motion will show up as phase noises [[BR]] of the lasers. The time derivative of phase noise is equivalent to frequency noise. The equivalent displacement noise seen [[BR]] by the cavity is dL=(w*x*L)/c, where w is the angular frequency, x is the displacement noise spectrum of the common motion, [[BR]] L is the length of the cavity and c is the speed of light (see [attachment:PhaseNoise.pdf attachment:PhaseNoise.pdf] for derivation).
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-attachment:Cavity-Common-Diff.png
+ .  B) [[Upgrade 09/GreenLock/Control Information|jump to Control Information]]
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-The two lasers (PSL and green) see the same motion of the cavity but from the opposite sides. Hence, the effect of this phase [[BR]] noise to the two error signals of the green and the PSL lasers will be 180 deg. out of phase. The feedback from the green laser to [[BR]] the cavity length will, therefore, create a noise for the PSL laser.
+[[BR]]
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-In order to estimate the contribution of this noise to the green lock, I plotted the estimated phase noise in the following figure.[[BR]]  attachment:PhaseNoise.png
+-------
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-I first took a spectrum of ETMX OSEM pos signal to see the motion of the mirror with damping.[[BR]] The blue curve in the figure shows the calibrated OSEM spectrum using the well known 2V/mm OSEM [[BR]] calibration and the whitening filter shape (3Hz zero, 30Hz and 100Hz pole).[[BR]] However, OSEM signal is not a good measure of the seismic noise below the pendulum resonant frequency[[BR]] because the suspension cage and the mirror move together at low frequencies.[[BR]] As a tentative solution, I put a filter to make the spectrum look like 1/f^2 below 0.8Hz.[[BR]] This is diffinitely a hacky solution, and should be replaced with a correctly measured [[BR]] seismic spectrum.
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-The estimated seismic motion was converted to the phase noise using the above formula.[[BR]] The RMS displacement noise above 0.1Hz is about 3e-12 m, which satisfies the requirement for [[BR]] the green lock stability (1e-11m).
+== Links ==
 .  B) [[Upgrade 09/GreenLock/Noise Requirement|jump to Noise Requirement]]

 . {i} aLIGO ALS wiki    http://ilog.ligo-wa.caltech.edu:7285/advligo/Arm_Length_Stabilisation
 . Description of the optical / servo configuration [https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=6888 LIGO DCC]
 . Servo modeling (!SimLink) [https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/Green_Locking/Servo_modeling/ 40mSVN]

[[GreenLock|back to the top]]
attachment:god_of_green.png