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| === Introduction === ''3f locking'' usually refers to a technique to lock an optical cavity of the interferometer. In that, the optical cavity is locked by means of the error signal obtained by demodulating the signal from the reflection photodiode at 3 times the modulation frequency of the sideband. |
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| === Introduction === ''3f locking'' usually refers to a technique to lock an optical cavity of the interferometer. In that the optical cavity is locked by means of the error signal obtained by demodulating the signal from the reflection photodiode at 3 times the modulation frequency of the sideband. |
This technique is different from the classical Pound-Drever-Hall locking technique. In that, by demodulating the signal from the reflection photodiode at the same frequency as that used to generate the sidebands, one obtains a signal proportional to the amplitude of the beat between the first order sidebands and the carrier. |
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| This technique is different from the classical Pound-Drever-Hall locking technique. In that, by demodulating the signal from the reflection photodiode at the same frequency as that used to generate the sidebands, one obtains a signal proportional to the amplitude of the beat between the first order sidebands and the carrier. In the 3f locking, the main component extracted by the triple frequency demodulation is proportional to the beating between the first and the second order sidebands. | [''put here picture illustrating how the PDH signals originate''] In the 3f locking, the main component extracted by the triple frequency demodulation is proportional to the beating between the first and the second order sidebands. [''put here picture illustrating how the 3f signals originate''] |
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| As that happens, the reflected carrier light is affected and it cannot provide anymore suitable error signals to lock the central part of the IFO. Other signals become necessary if the short cavities are to be locked. | In that process, the reflected carrier light is affected and it cannot provide anymore suitable error signals to lock the central part of the IFO. Other signals become necessary if the short cavities are to be locked. |
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Suitable double demodulation signals are available only in the IFO [[IFO detection configuration|narrow-band configuration]], that is with a detuned SRM. For the [[IFO detection configuration|broadband configuration]], as it is going to be used for the 40m Upgrade and in Advanced LIGO, the 3f signals are planned to be the alternative. === 3f locking scheme for the 40m upgrade interferometer === |
3f Locking for the 2009 40m Upgrade
Introduction
3f locking usually refers to a technique to lock an optical cavity of the interferometer. In that, the optical cavity is locked by means of the error signal obtained by demodulating the signal from the reflection photodiode at 3 times the modulation frequency of the sideband.
This technique is different from the classical Pound-Drever-Hall locking technique. In that, by demodulating the signal from the reflection photodiode at the same frequency as that used to generate the sidebands, one obtains a signal proportional to the amplitude of the beat between the first order sidebands and the carrier.
[put here picture illustrating how the PDH signals originate]
In the 3f locking, the main component extracted by the triple frequency demodulation is proportional to the beating between the first and the second order sidebands.
[put here picture illustrating how the 3f signals originate]
3f signals to lock the central part of the interferometer
In the old configuration of the 40m (2001-2009), the lock acquisition of the interferometer starts by locking the Dual Recycling Michelson (DRM) with the arm cavities out of resonance. At this stage the classical PDH error signals are used. Then to complete the locking of the full IFO, the CARM offset is reduced and the arms are slowly brought into resonance.
In that process, the reflected carrier light is affected and it cannot provide anymore suitable error signals to lock the central part of the IFO. Other signals become necessary if the short cavities are to be locked.
Before the CARM offset is completely reduced, the controls signals for the central part are replaced with the double demodulation signals which are virtually not affected by the carrier state in the arms.
Suitable double demodulation signals are available only in the IFO narrow-band configuration, that is with a detuned SRM. For the broadband configuration, as it is going to be used for the 40m Upgrade and in Advanced LIGO, the 3f signals are planned to be the alternative.