Interface Control Document for the ISC sub-system
Preface
This is a draft of the ICD for the LCGT-ISC sub-system.
Sub-System Definition
The role of the Interferometer Sensing and Control (ISC) sub-system is to keep the interferometer at its optimal operation point and extract the gravitational wave signal with low noise. This sub-system can be functionally broken into two parts: Sensing and Control.
Sensing
In order to control the interferometer, we have to know the state of the interferometer. Sensing part of this sub-system defines the methods to extract information about the state of the interferometer. We sort the degrees of freedom (DOFs) of an interferometer into three categories.
Length Sensing
The word "length" here means the distances between the mirrors. In addition to the geometric lengths, a change in the laser frequency also appears as an apparent change in the lengths for an interferometer. Therefore, the laser frequency is counted as a length here. There are 5 length DOFs to be controlled for a Dual-Recycled Fabry-Perot Michelson Interferometer (DRFPMI).
Name |
Notation |
Description |
Michelson (MICH) |
l- |
Differential change of the Michelson arm lengths |
Power Recycling Cavity Length (PRCL) |
l+ |
Length of the power recycling cavity |
Signal Recycling Cavity Length (SRCL) |
ls |
Length of the signal recycling cavity |
Differential Arm Length (DARM) |
L- |
Differential change of the arm cavity lengths |
Common Arm Length (CARM) |
L+ |
Common change of the arm cavity lengths |
Out of those DOFs, MICH, PRCL and SRCL are called "central part", "short DOFs" or "スモール系". DARM and CARM are called "arm DOFs", "long DOFs" or "ラージ系". CARM is often regarded as being equivalent to the laser frequency variation. DARM contains the gravitational wave signal, so it is the most important DOF.
Alignment Sensing
In order for a laser beam to properly resonate inside the interferometer, the beam and the mirrors of the interferometer must be aligned well. Alignment sensing is a mechanism to monitor errors in the alignment. A mirror has three rotational degrees of freedom, out of which only two (pitch and yaw) are important for an interferometer. For each of pitch and yaw, there are 6 alignment DOFs in an interferometer.
Name |
Description |
Common Stable (CS) |
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Common Unstable (CU) |
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Differential Stable (DS) |
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Differential Unstable (DU) |
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Power Recycling Mirror (PRM) |
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Signal Recycling Mirror (SRM) |
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CS, CU, DS, DU are the DOFs of the arms in the Sidles-Sigg basis [ref].
Auxiliary Signals
There are other signals which may have to be monitored (and possibly corrected for by feedbacks) during the operation of an interferometer.
Name |
Description |
Thermal lensing |
Heat deposited on the mirrors by the laser beam can create a lensing effect. This lens mainly manifests itself as a mismatch of the spatial modes between the recombined beams at the dark port. |
AS_I |
If we use RF readout for the DARM, we may have to monitor the I-phase signal of the AS-port PD, which does not contain any GW signal but comes from junk light at the AS-port. AS-I could saturate the AS-PD in RF and reduce the dynamic range for the GW signal channel (AS-Q). If so, we can feedback the AS-I into the AS-PD current to cancel it. |
SPOB |
SPOB is Sideband Picked Off at Beam-splitter. The light picked off inside the PRC is demodulated at twice the frequency of the sidebands to serve as a measure of the sideband power in the PRC. |
Control
Once the information about the state of the interferometer is obtained, we feedback the information to keep the interferometer at the optimal operation point. Feedback has to be strong enough to keep the error signals in linear regions so that up-conversion and other noises related to the non-linearity are well suppressed.
The control part consists of two successive components: Filtering and Actuation.
Filtering
Type |
Description |
Related Subsystems |
Digital |
Most of the ISC feedback filters will be implemented as digital filters. |
Digital |
Analog |
Analog electronics are used where a wide feedback bandwidth is required.This will be the case mainly for the laser stabilization loops i.e. the frequency and intensity stabilization servos. |
IFO support |
Actuator
Actuators change the state of an interferometer.
Name |
Description |
Related Subsystems |
Suspension actuators |
These actuators move the mirrors. This is the realm of the suspension sub-system.ISC group will set requirements for the suspension actuators |
Suspension |
Laser actuators |
Laser intensity and frequency has to be controllable with a high bandwidth. |
IOO |
Thermal |
If necessary, the thermal deformations of the mirrors have to be compensated by some way, such as illuminating the mirrors with CO2 lasers. |
IFO Support |
Requirements
The requirements for the ISC system ultimately come from the target sensitivity. Again the requirements for the ISC subsystem are divided into Sensing and Control.
Requirements for Sensing
DARM |
The DARM sensing noise should be below the target sensitivity. |
Other DOFs |
The noises in the DOFs other than DARM affect the sensitivity through feedback. The sensing noises should be small enough to allow descent feedback gains for the auxiliary loops. |
Requirements for Control
DARM |
DARM actuator noise directly appears in the DARM noise spectrum. This has to be kept below the target sensitivity. |
Other DOFs |
Actuator noises in the DOFs other than DARM appear in the DARM signal through various couplings. The couplings have to be modeled and estimated to set the requirements for the actuator noises. |
Interface with Other Sub-Systems
The ISC sub-system is realized by connecting hardware components provided by other sub-systems. So establishing good interface with the related sub-systems is imperative. Since the ISC requirements are set directly from the target sensitivity, ISC sets the requirements for other sub-systems in most cases.
Issues |
Description |
Numbers |
Related Subsystems |
Consensus |
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Laser Power |
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IOO |
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Laser Frequency Noise |
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IOO |
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Laser Frequency Control |
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IOO |
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Laser Intensity Noise |
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IOO |
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Laser Intensity Control |
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IOO |
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Modulation |
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IOO| |
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Input Mode Clearner |
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IOO |
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Output Mode Cleaner |
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IOO |
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Seismic RMS |
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Suspension |
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Mirror actuators |
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Suspension |
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Mirror Loss |
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Mirror |
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Mirror Transmittance |
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Mirror |
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Mirror Coating |
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Mirror |
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||Folding RCs|| || ||IOO, Mirror, Suspension, Vacuum|| ||
Vacuum Layout |
Optical layout of the interferometer determines the layout of the vacuum tubes and chambers. |
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Vacuum |
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Vacuum Pressure |
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Vacuum |
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PD |
Requirements for power handling, quantum efficiency, noise, bandwidth etc |
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IOO |
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Analog Electronics Standards |
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IFO support |
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