Differences between revisions 10 and 93 (spanning 83 versions)

Arm Length Stabilization using green laser beams

attachment:god_of_green.png

Conceptual Design

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
Mode matching ([attachment:modematch_END.png mode matching at end table]) Green generation [attachment:manual_oven.pdf manual for the covesion oven]
Electronics
Purchase list jump to Control Information jump to Noise Requirement

links

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]

-  ⇤ ← Revision 10 as of 2009-12-22 23:47:03 → 
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  Editor: KojiArai
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+   ← Revision 93 as of 2010-10-05 20:02:23 → ⇥
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  Editor: KiwamuIzumi
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+Additions are marked like this.
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-<<TableOfContents([2])>>
= Arm length stabilization injecting green laser from the end of the arm =
== Plan ==
 * '''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]
+<<Anchor(TOP)>>
= Arm Length Stabilization using green laser beams =
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- * '''Detailed design'''
  * Optical design / item list

  * Electronics / control design / item list

 * '''Detailed considerations'''
  * Optical Layout (Table / In-vac)
  * Mode matching
  * Main optics specs
  * Green generation
  * Expected performance / noise / control system range
+   . attachment:god_of_green.png
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-  * Digital servo / operation
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-  * Alignment (initial / fine / automatic)
 * '''Development roadmap
'''
  * '''At the end table arrangement'''
+= Conceptual Design =
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-   * NPRO placement / optical assembly at the end
+   <<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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-   * 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
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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
+  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]]

  Mode matching ([attachment:modematch_END.png mode matching at end table])
  Green generation
  [attachment:manual_oven.pdf manual for the covesion oven]
  [[Upgrade 09/GreenLock/Electronics|Electronics]]
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-   * Ready-to-go panels for AdvLIGO
   * Automatic alignment of the green beam
   * .
   * Green Michelson for precise ETM calibration
+    [[Upgrade 09/Purchase List/Green Locking|Purchase list]]
  [[Upgrade 09/GreenLock/Control Information|jump to Control Information]]
  [[Upgrade 09/GreenLock/Noise Requirement|jump to Noise Requirement]]
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-   * Absolute length / mode spacing measurement by green'''
'''
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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 ?
+= links =
|| ''' 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] ||
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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).

attachment:Cavity-Common-Diff.png

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.

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

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.

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).