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Interferometer Characterization at the 40m prototype
Goals
To help the DRMI commissioning that will be performed at the sites.
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.
Contents
- Interferometer Characterization at the 40m prototype
-
Plan for DRMI Characterization at the 40m
- Schnupp asymmetry measurement and its adjustment
- PRC length measurement and its adjustment
- SRC length measurement and its adjustment
- Recycling gain measurements
- Beam centering and measurement of spot position on BS
- Reflectivity check
- Sensing matrix check
- Calibration of Actuator responses
- Diagonalization of Length Sensing and Controls
- f2a filter adjustment
- Length Noise budgeting
- Angular Motion Noise budgeting
- 3f locking test
- Sub-tasks
- References
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 by tuning the asymmetry.
- According to a simulation the asymmetry should be within the precision of XXX mm to achieve more than 95 % of the maximum 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) [#ref4 [4]]
- Since we've already been able to lock both FP arms the second method was performed to measure the asymmetry at the 40m.
Results
- Here is the result from the first measurement [#ref4 [4]]
- {{{Lsa = 0.0364 [m] ± 3.2 [mm]
}}}
Discussion and Decision
- Based on the results we should ...
PRC length measurement and its adjustment
Requirements
![/!\](/moin_static1911/modern/img/alert.png "/!\")
The required precision should be reviewed also from the point of view of the sensing matrix - {{{ 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 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 both sidebands.
- 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]] [#ref5 [5] ] [#ref6 [6]][#ref7 [7]]
How to measure
- There are two major methods to measure the PRC length:
(1) A technique uses another auxiliary laser to scan the frequency.
(2) To sweep the sidebands' frequency.
Results
SRC length measurement and its adjustment
Requirements
- The required precision should be reviewed also from the point of view of the sensing matrix
- {{{ 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
- [#PRC Essentially the same as the PRC length measurement]
Results
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
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
}}}
Reflectivity check
Design
- {{{ REFL = XXX (when PRMI, carrier locked)
}}}
- This test is not really necessary but it's still useful for finding a bug.
How to
- Measure the light power coming into a PD at the REFL port.
Results
- {{{ REFL = XXX (when PRMI, carrier locked)
- REFL = XXX (when PRMI, sidebands locked) REFL = XXX (when DRMI)
Sensing matrix check
-
Design
How to
- Shake each DOF and measure the response at a port of interest.
Results
Calibration of Actuator responses
-
- This task is much related to the noise budgeting. In the process of the noise budgeting one should characterize the open-loop gains such that the suppressed and non-suppressed noise can be correctly estimated. For this purpose the calibration of actuators on BS, ITMs, PRM and SRM are necessary to characterize the control loops. Unlike the optical gains, the actuator responses shouldn't vary, therefore the use of the actuator responses as references in estimation of the noise budget is always fairly reliable.
How to
The usual way of the calibration starts from the Michelson sensor because the error signal is just a sinusoidal wave and easy to estimate the optical gain. Once the estimation of the optical gain finishes the actuator on BS and ITMs can be calibrated by shaking them and taking transfer functions from them to the Michelson error signal [#ref8 [8]] [#ref9 [9]].
- After the calibration of the BS and ITMs, the calibration of PRM can be done by locking PRMI (Power-Recycled MIchelson). Locking the PRMI makes the sensor of PRC linear, and consequently one can calibrate the PRC sensor by shaking the well-calibrated ITMs. In a similar way one can calibrate PRM and SRM.
- In summary here is the steps to calibrate the responses
- {{{ 1. Calibration of the Michelson sensor
- Calibration of the BS and ITMs actuators
- Calibration of the PRC sensor and the PRM actuator response
- Calibration of the SRC sensor and the SRM actuator response
- In summary here is the steps to calibrate the responses
}}}
Results
- See [#ref9 [9]] for the details
- {{{ BS = 1.643e-9 / f^2 [m/counts]
- ITMX = 3.568e-9 / f^2 [m/counts] ITMY = 3.542e-9 / f^2 [m/counts]
- {{{ BS = 1.643e-9 / f^2 [m/counts]
}}}
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 [#ref8 [8]].
- To avoid the coupling the MICH control should also actuate on PRM and SRM to minimize the coupling.
How to
f2a filter adjustment
Requirement
Precision < XX %
- Pushing a suspension to the length direction cause a misalignment because of the inherent force to torque coupling on a suspended mirror [#ref10 [10]].
How to
- There is a script that measures the f2a ratio and apply filters to cancel them.
Results
Length Noise budgeting
Requirements
How to
- Suspension noise
- Laser amplitude noise
- Electronics noise (including ADC and DAC)
- Coupling from Angular motions
Results
Angular Motion Noise budgeting
Requirements
How to
- Optimize control loops (local damping, OPLEVs and WFSs)
- Suspension noise
- Electronics noise (including ADC and DAC)
Results
3f locking test
Requirements
How to
Results
Sub-tasks
Tuning of Locking protocol
How to
- Threshold values for the triggered locking
- Schmit triggers
- Auto locking scripts
IFO modeling
How to
There are several tools that can model and simulate interferometers in numerical ways. Here we introduce two major tools; finesse [#ref11 [11] ]and Optickle .
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] J.Rollins, "Schnupp asymmetry measurement" [http://nodus.ligo.caltech.edu:8080/40m/4821 40m elog #4821 (2011)]
[5] 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)]
[6] 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)]
[7] 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)]
[8] 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)]
[9] K.Izumi, " Calibration of actuators : BS, ITMX and ITMY " [http://nodus.ligo.caltech.edu:8080/40m/4721 40m elog #4721]
[10] P.Fritschel, " Digital Suspension Filter Design "[http://www.ligo.caltech.edu/docs/T/T010140-01.pdf, LIGO-T010140-01(2001)]
[11] A.Freise and K.Strain, " Interferometer Techniques for Gravitational-Wave Detection "[http://relativity.livingreviews.org/Articles/lrr-2010-1/ Living Review (2010)]