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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 === * '''Design and purchase item lists''' * 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 * Digital servo / operation * Alignment (initial / fine / automatic) |
. attachment:god_of_green.png |
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| === 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 |
* [[Upgrade 09/Purchase List/Green Locking|Purchase list]] |
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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 |
* 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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| * Ready-to-go panels for AdvLIGO * Automatic alignment of the green beam * . * Green Michelson for precise ETM calibration |
. <<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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| * 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 ? |
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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). |
* 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]] |
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| attachment:Cavity-Common-Diff.png | * Mode matching ([attachment:modematch_END.png mode matching at end table]) * Green generation . [attachment:manual_oven.pdf manual for the covesion oven] * B) [[Upgrade 09/GreenLock/Electronics|Electronics]] |
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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. | . B) [[Upgrade 09/GreenLock/Control Information|jump to Control Information]] . {i} [[Upgrade 09/GreenLock/Noise Requirement|jump to Noise Requirement]] |
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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 | . {i} aLIGO ALS wiki http://ilog.ligo-wa.caltech.edu:7285/advligo/Arm_Length_Stabilisation . {i} Description of the optical / servo configuration [https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=6888 LIGO DCC] . {i} Servo modeling (!SimLink) [https://nodus.ligo.caltech.edu:30889/svn/trunk/docs/upgrade08/Green_Locking/Servo_modeling/ 40mSVN] |
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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. 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). |
Arm Length Stabilization using green laser beams
- attachment:god_of_green.png
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]
- Optical Layout ([attachment:green_optics.png Table]/ In-vac)
- Mode matching ([attachment:modematch_END.png mode matching at end table])
- Green generation
- [attachment:manual_oven.pdf manual for the covesion oven]
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]
