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----- == 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)|| || ||Common Unstable (CU)|| || ||Differential Stable (DS)|| || ||Differential Unstable (DU)|| || ||Power Recycling Mirror (PRM)|| || ||Signal Recycling Mirror (SRM)|| || 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 sideband resonating in the PRC to see how much sideband is stored in the PRC.|| ---- === Control === ----- == Requirements == ----- == Interface with Other Sub-Systems == |
[[attachment:ISC_ICD_2010_04_19.doc|ISC ICD 2010/4/19]] |
- Diff for "LCGT/subgroup/ifo/ISC/ICD"