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LIGO Caltech News
A Whole Lot of Shakin' Going On!
Forty-Meter Achieves Recycled Configuration
We have a great earthquake detector here in the LIGO metrology lab. Too bad that isn't our prime objective, because we have been doing it very well! We have watched as our "fringes" swing like crazy from the magnitude 4+ quake in Big Bear City and seen even smaller movements which we suspect are from quakes with magnitude as low as 1.8 in the nearby San Bernardino mountains. Confirmation of these events comes from the U.S. Geological Survey's List of Recent Earthquakes for California-Nevada. Many thanks to them for solving some of our mysteries!
Our real objective is to measure the surface of the LIGO Core Optic
Components (COCs), some of the most perfect optics created to date. This
is the first time a measurement of such accuracy has been attempted using a
wavelength of 1,064 nanometers (nm). There are 40 optics in all to be
measured, with radii of curvature varying from infinite (flat) to 7400
meters. All the optics are coated, and the reflectivities of different
optics vary from 50% to 99.999998% at 1,064nm. In measuring these surfaces
we use a WYKO 6000 Fizeau phase shifting interferometer, modified for use
at 1,064nm. Our test setup is pictured at left.
The LIGO pathfinder optic (seen at center in the photo) is being used as
the test piece, and it has a surface roughness of <0.8 nm root mean square (rms). The
vertical lines on the TV monitor (at right in the photo) are interference
fringes representing the optical distance between the reference flat
(attached to the face of the interferometer at left) and the test piece.
These fringes are the ruler which we use to measure the flatness of our
COCs. Each fringe, dark to dark, represents 532 nm (1/2 wave) of
difference between the surfaces. In this particular picture, the two
surfaces are aligned to within five micrometers, with the difference mostly
in the horizontal direction. The entire setup is installed on a vibration
isolation system, a 1,900 pound floating table made by Newport Corp.
The measurement consists of a series of snapshots taken of the null cavity
(reference and test surfaces aligned to within 500 nm) as the
interferometer phase shifts through about 2000 nm. The phase shifting is
accomplished by piezoelectric transducers which push uniformly on the
reference flat mount, such that the cavity length between the reference and
test piece changes by about two waves. The custom software supplied with
the WYKO combines the data from these snapshots and produces a map of the
cavity, shown here at right.
The variations seen in this plot are the combination of the test piece, the reference flat, and the air in between. We are currently working to separate out the reference flat so that it may be subtracted from subsequent measurements of LIGO optics.
The Core Optics Components have been polished to better than 0.8 nanometers rms deviation from a perfect surface. In larger terms that compares to slicing the earth at it's equator and grading the cut so that the standard deviation is only one inch from a perfectly flat plane. These optics are very smooth! They were measured at the polisher, CSIRO (Commonwealth Scientific and Industrial Research Organisation) of Australia, and some were measured at the National Institute of Standards and Technology. We owe many thanks to both of these institutions for there excellent work and also for providing guidance in the art of metrology as we attempt something never done before: to measure an optic coated at 1,064nm to within one nanometer rms!
After the 40 meter interferometer was reconfigured for recycling in the Summer and Fall of 1997, first resonant servo locking was then achieved that December. (See our February 1998 newsletter for an earlier, related article.) The successful operation closely followed the design and hardware modifications to the 40 meter facility implemented by Jennifer Logan, as a post-doctoral project in LIGO. During 1998, under Jenny's leadership, and with substantial participation by a visiting team from the University of Michigan (Dick Gustafson, Keith Riles, and Jamie Rollins) the detailed behavior of this new and most complex-to-date system has been studied.
Pictured below, Bill Kells and Jennifer Logan (left), and Dick Gustafson (right) take a moment to savor the sweet success of recycling.
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"Recycling," for LIGO, is needed because most of the input laser light is reflected from the interferometer as a whole and not put to use. Recycling thus amplifies the available laser power as seen by the gravitational wave sensitive portion of the interferometer. However a new level of optical and servo topology complexity is necessary. The interferometer becomes a system of three tightly coupled resonant cavities (the two long arm Fabry-Perot cavities, and the pivotal recycling cavity). To "acquire" such a ordered resonant lock state--when starting from an initially disordered, random set of mirror positions and orientations--is a fascinating example of a phase transition. Though such transitions are notoriously difficult to predict, we have found that acquisition is both easy to "tune-up" for and frequent. Since the 40 meter set-up is closely analogous to the LIGO interferometer design, the success of this work greatly bolsters our confidence in anticipated LIGO lock acquisition.
Basic parameters of the recycled interferometer, such as the recycling "gain," or net laser power amplification, have been measured. More than a few surprises were encountered along the way. For instance the interferometer as a whole was discovered to be undercoupled (analogous to an endothermic phase transition), whereas it had been predicted to be slightly overcoupled (exothermic transition). Still, any discrepancies like these have been so far attributable to ignorance of detailed component parameters, not to fundamental oversights.