Advanced LIGO subsystems
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Advanced LIGO News
Livingston Locks Advanced LIGO Detector
Contributed by David Shoemaker
Advanced LIGO made a great step forward on 27 May 2014 with the first locking of a complete detector, Livingston's L1 interferometer. This achievement indicates that the critical subsystems of the interferometer are functioning successfully, are working well together and are ready for the addition of final parts. Let's back up to understand how we got here.
By 2013 the Project had assembled most of the interferometer equipment and was installing that equipment at the two Observatories. As soon as possible, we started to make bring the installed parts into operation to test how they worked together, and to tune the performance of the components to match the seismic conditions - LIGO calls this 'integrated testing'. The first integrated experiments involved the Pre-Stabilized Laser and the Input Mode Cleaner, carried out at the Livingston Lab. The Mode Cleaner is a resonant optical cavity which transmits only the laser light that matches 1) the optical axis of the cavity, 2) the fundamental spatial mode of the light (a simple Gaussian intensity pattern), and 3) the resonant frequency determined by the cavity length. (An integral number of wavelengths are required for one round trip, so that multiple passes will cause the waveforms to overlap and build up.) The mirrors of the cavity are very stable in position, allowing this cavity to act as a stabilizer for the beam position, form, and frequency. Using this starting system, we could see that the laser and input mode cleaner were working, along with the seismic isolators, mirror suspensions, and a myriad of servo control systems that maintain the cavity lengths, mirror angles and mirror positions.
A second integrated test at the Hanford Lab involved making an optical cavity out of two Advanced LIGO test mass mirrors over the full arm length of 4km. This test provided another opportunity to evaluate the ability of new components to work together, this time with the additional challenge of a 2.5-mile cavity length. The tighter constraints on angles led to improvements in the seismic isolation system, made possible by the fact that the system can be tuned 'on the fly' by changing the parameters in the digital servo controls that take seismic sensor signals and apply them to the magnetic motors in the isolators. We also demonstrated at this time that we could use a secondary system of light (green, at 532nm wavelength) to more crudely first measure the position of the mirrors and in a controlled way bring the length of the cavity into resonance for the main sensing light (infrared, at 1064nm wavelength).
A third integrated test used all of the Advanced LIGO components that are in the main vertex building (a test that excluded the long arms and the end mirrors). This Michelson interferometer, with additional mirrors to form resonance for the incoming laser light and for the outgoing signal light, requires all hardware and software to work properly, but keeps us from being overwhelmed by complexity; the simplicity of working over relatively short distances (15 meters or 50 feet) and without the two 4km arm cavities allows faster progress on the tuning of the components and software. In parallel with this effort, the mirrors at the distant ends of the 4km arms at Livingston were installed and precision-aligned to point back to the vertex.
In March 2014, at LIGO Livingston Observatory, the installation phase was completed, enabling the starting of interferometry for the complete detector. Within days individual arms were brought into resonance and held there - 'locked' in our terminology - and the green and infrared light systems were debugged and adjustments made. Several weeks of long nights followed, when the integrated testing team chose to avoid daytime seismic noise and distractions. One by one the systems were tuned, by improving alignment, servo control systems, electronics, and automatic sequences of locking the various cavities to their correct lengths; the results of all the previous experiments at Hanford and Livingston helped inform the team how to proceed. The team made a handful of attempts to creep up on just the right lengths to bring all the cavities into resonance, but each time the servo systems would lose control at the last moment.
Then, just before midnight on the night of 26 May US Central Time, the last new understanding of how to combine signals and approach the ideal operating point was achieved, and the system swung into a perfect lock - the 4km arm cavities, the Michelson interferometer, and the input power recycling and output signal recycling cavities all sitting in the ideal positions to within a tiny fraction of the one-micron wavelength of the light. A small earthquake caused the system to lose lock after an hour; future tuning will make the system much more robust.
You can view the lock sequence that's graphed above as it appears to LIGO commissioners in the control room by dowmloading this seven-minute video clip from mid-June 2014 [350Mb, M4V format). The clip plays at double speed and represents 14 minutes of real time.
Much work remains. The signal chain of light used to sense gravitational waves was not yet in place, and some automatic alignment systems were not yet operating. Also, a long and arduous path of taking the system from 'just locked' to and astrophysically interesting sensitivity - the commissioning - will be needed. But once the system locks, it is far easier to characterize the instrument and make this needed progress. It is a quantum step forward toward LIGO's goal. The plot below, taken from the Livingston Advanced LIGO electronic log, shows encouraging commissioning progress that has occurred since first lock. The continuing downward shift of LIGO's noise spectrum toward the Advanced LIGO goal now becomes Livingston's singular focus.
LIGO Hanford is finishing installation as part of a planned staggered completion of aLIGO construction. We expect to start locking Hanford's H1 interferometer by August 2014. LIGO's goal is that both instruments will reach a level of tuning for actual detections by the end of 2014. Then, observing runs can follow in 2015 and beyond, and the project will be on its way to opening the new field of gravitational wave astronomy.
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