Advanced LIGO subsystems
are the organizational units of the overall project. Follow the links below to view the mission and progress of each subsystem.
|Auxiliary Optics||Core Optics|
|Data Acquisition||Data and|
Advanced LIGO News
LIGO Initiates DRMI Test at Livingston
Having completed the Advanced LIGO buildup of the central portion of the L1 interferometer, a commissioning team will now perform a significant test of the new hardware and associated digital controls. "DRMI" means dual recycled Michelson interferometer; dual because the test will include both signal recycling optics (photo) and power recycling optics. Signal recycling represents a significant upgrade to the Initial LIGO design. LIGO always has utilized power recycling by reflecting light in the "symetric port" of the detector into the full interferometer. The term symmetric port indicates the path between the beam splitter and the laser. Now LIGO also will reflect light from the antisymmetric port -- the output path which is perpendicular to the symmetric port. It's in the antisymmetric port that the detector's interference fringes appear.
In the case of DRMI, a picture is worth more than a thousand words. Click on the thumbnail to view an enlarged diagram of a detector. In simplest terms, the goal of DRMI is to obtain a suitable level of performance from everything that's inside the grey outline. Use the diagram to identify the three key optical paths that make up the DRMI test. Power Recycling Cavity (PRC): Light that travels from the first PRC mirror along the input line, interacts with the beam splitter and reflects from the inner test masses (ITM's -- see photo below). Signal Recycling cavity (SRC): Light that travels between the inner test masses and the final SRC mirror. Michelson (MICH): The pair of lengths between the beam splitter and the beam splitter-facing surfaces of the ITM's. LIGO must exert control of these three regions to make the laser light resonate throughout the entire DRMI. Since PRC, SRC and MICH share the ITM's and beam splitter, any control system action that moves a mirror in one of these cavities will affect the entire path.
Once the mirror alignments are good enough to carry the light through the DRMI, commissioners must instruct the control system to make the light resonate by optimizing the cavity lengths. The control system needs to know the directions in which to move the optics. Sidebands that LIGO addds to the main laser frequency provide this information. At several locations within the DRMI, portions of the beam are sampled by photodiodes that demodulate the light to produce radio frequency signals. These sensor signals yield the inputs to feedback loops that drive the mirrors via electromagnetic actuators. Through the careful choice of sideband frequencies and sampling locations, LIGO can supply the control that's necessary to lock PRC, SRC and MICH together in a resonant condition. Mirror assemblies known as tip-tilts (photo) provide beam steering capability to ensure that control system beams do indeed land on their target photodiodes.
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