Interferometer Characterization at the 40m prototype

Goals

• To help the DRMI commissioning that will be performed at LLO.

Motivations

• In the aLIGO schedule a DRMI test will start on May 2012 at LLO. Prior to the DRMI test we, the 40m lab, should help their commissioning to make the things smooth and hence allow to finish the commissioning in the shortest time. Moreover any commissioning tests that will be performed at LLO should be well predicted and tested at the 40m so that the people at the site can easily pass through all the commissioning tests and possibly can spend time for a real trouble shooting (commissioning) to fix unexpected issues.

• So for this purpose some recipes must be prepared by the 40m lab. Each recipe includes descriptions about how to make a commissioning test, how to estimate important parameters and the results at the 40m. Additionally some useful scripts must be developed at the 40m.

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Plan for DRMI Characterization at the 40m

Schnupp asymmetry measurement and its adjustment

Requirement

•  Schnupp asymmetry = 0.0342 [m] +/- XXX [mm]

• The Schnupp asymmetry determines the reflectivity r and transmissivity t of the Michelson for the f1 and f2 sidebands when the carrier is kept in the dark condition. In the design the f2 sideband should be critical coupling in the dual recycling cavity [#ref1 [1]] [#ref2 [2]]. To achieve the critical coupling we should adjust r and t properly.

• According to a simulation the asymmetry should be within the precision of XXX mm to achieve more than 95 % of the power build up.

How to measure

• Here two example methods are introduced:

  • (1) based on a measurement of the MICH transmissivity at frequency of AM sidebands [#ref3 [3]]

  • (2) based on a measurement of the difference in the optical phase of each arm at frequency of PM sidebands (needs each FP cavity locked)

• Since we've already been able to lock both FP arms the second method was performed to measure the asymmetry at the 40m.

Results

• The result at the 40m was ... and the precision was +/- XXX mm. This implies us to fix the length of the asymmetry.

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PRC length measurement and its adjustment

Requirements

• {{{ lprc = 6.7538 [m] +/- 3 [mm]

}}}

• In order to successfully lock a power recycled interferometer with Fabry-Perot arms a technique broadly used is to choose the PRC length such that the sidebands are resonate in PRC. In our design the PRC length has been chosen to let both f1 and f2 sidebands resonate in PRC [#ref1 [1]] [#ref2 [2]]. The precision required to get those sidebands resonate is simply depending on the linewidth of PRC for both sidebands.
• As shown in the plot below the length must be adjusted within 3 mm precision to get more than 90 % of max build up for the f2 sideband.
  • attachment:PRClength_scan.png

  • Fig.1 An example plot of macroscopic PRC length scan and the intracavity power.

  • This plot is too naive because doesn't include the effects from FP arms and SRC. Will be updated soon

• There are several techniques to measure the absolute length of a cavity. Here are the references [#ref2 [2]] [#ref4 [4] ] [#ref5 [5]][#ref6 [6]]

How to measure

• There are two options to measure the PRC length. One is to use another auxiliary laser and the other is to sweep the modulation frequency.

Results

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SRC length measurement and its adjustment

Requirements

• {{{ lsrc = 5.39915 [m] +/- 1.5 cm }}}
• Similar to the PRC length, the f2 sidebands should resonate in SRC to get cleaner signal [#ref1 [1]]. According to the linewidth of SRC for the f2 sidebands the length should be something.
• As shown in the plot below the length must be adjusted within 1.5 cm precision to get more than 90 % of max build up for the f2 sideband.
  • attachment:SRClength_scan.png

  • Fig.2 An example plot of macroscopic SRC length scan and the intracavity power.

  • This plot is too naive because doesn't include the effects from FP arms and PRC. Will be updated soon

How to measure

Results

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Recycling gain measurements

Design

•  Carrirer    = XX
   f1 sideband = YY
   f2 sideband = ZZ

• Since the recycling gain is related to loss in the recycling cavity, measuring the recycling gain tells us the amount of loss in the recycling cavity. The usual amount of loss is expected to be XX ppm per round trip. If we find unacceptably big loss, it might be due to a clipping or some obvious loss.
• In addition to the gain of the carrier light, the recycling gain of f1 and f2 tells us the reflectivity of the Michelson which is determined by the Schnupp asymmetry.

How to

Results

•  Carrirer    = XX
   f1 sideband = YY
   f2 sideband = ZZ

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Beam centering and measurement of spot position on BS

Requirements

{{{ off-centering on each optics < XXX mm }}}

How to

results

• {{{ YAW = XXX mm PIT = XXX mm

}}}

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Reflectivity check

How to

Results

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Sensing matrix check

How to

Results

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Calibration of Actuator responses

How to

Results

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Diagonalization of Length Sensing and Controls

Requirements

• {{{ Precision < 1 % (?)

}}}

• When the DRMI is locked the MICH control signal is fed back to the BS actuator, but the actuation on BS intrinsically changes the PRC and SRC length as well as the MICH length ( lx-ly). This means the noise from MICH can be transferred to the PRC and SRC, resulting in the degradation of noise performance in the PRC and SRC [#ref7 [7]].

• To avoid the coupling the MICH control should also actuate on PRM and SRM to minimize the coupling.

How to

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f2a filter adjustment

Requirement

•  Precision < XX %

• Pushing a suspension to the length direction cause a misalignment.

How to

• There is a script that measures the f2a ratio and apply filters to cancel them.

Results

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Length Noise budgeting

Requirements

How to

• Suspension noise
• Laser amplitude noise
• Electronics noise (including ADC and DAC)
• Coupling from Angular motions

Results

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Angular Motion Noise budgeting

Requirements

How to

• Optimize control loops (local damping, OPLEVs and WFSs)
• Suspension noise
• Electronics noise (including ADC and DAC)

Results

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3f locking test

Requirements

How to

Results

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Sub-tasks

Tuning of Locking protocol

How to

• Threshold values for the triggered locking
• Schmit triggers
• Auto locking scripts

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IFO modeling

How to

• There are several tools that can model and simulate interferometers in numerical ways. Here we introduce two major tools; finesse [#ref8 [9] ]and Optickle .

• See this page for details

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References

[1] R.Abbott et al, " Advanced LIGO Length Sensing and Control Final Design "[https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=12213 LIGO-T1000298-v2 (2010)]

[2] A.Stochino, " Design and Characterization of Optical Cavities and Length Sensing and Control System of an Advanced Gravitational Wave Interferometer " [https://nodus.ligo.caltech.edu:30889/svn/trunk/alberto/thesis/main/main.pdf 40m svn (2010)]

[3] O.Miyakawa, " Michelson asymmetry length " old 40m Elog (2004) attachment:osamu_elog.png

[4] M.Rakhmanov et al, " Characterization of the LIGO 4km Fabry-Perot cavities via their high-frequency dynamic response to length and frequency variations " [http://iopscience.iop.org/0264-9381/21/5/015/ CQG (2004)]

[5] A.Araya et al, " Absolute-length determination of a long-baseline Fabry-Perot cavity by means of resonating modulation sidebands " [http://www.opticsinfobase.org/abstract.cfm?URI=ao-38-13-2848 Applied optics (1999)]

[6] M.Rakhmanov et al, " An optical vernier technique for in situ measurement of the length of long Fabry-Perot cavities " [http://iopscience.iop.org/0957-0233/10/3/013/ Meas.Sci.Technol (1999)]

[7] R.ward, " Length Sensing and Control of an Advanced Prototype Interferometric Gravitational Wave Detector "[https://dcc.ligo.org/cgi-bin/private/DocDB/ShowDocument?docid=9237 PhD Thesis LIGO-P1000018-v1 (2010)]

[8] P.Fritschel, " Digital Suspension Filter Design "[http://www.ligo.caltech.edu/docs/T/T010140-01.pdf, LIGO-T010140-01(2001)]

[9] A.Freise and K.Strain, " Interferometer Techniques for Gravitational-Wave Detection "[http://relativity.livingreviews.org/Articles/lrr-2010-1/ Living Review (2010)]

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