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Optical Contamination Studies in LIGO

Optical Contamination Studies in LIGO

- Contributed by Daqun Li

Contamination Studies Apparatus If you have never visited the sub-basement labs at Caltech, you would probably never imagine that LIGO's contamination studies are being vigorously conducted in a rambunctious lab deep down in the West Bridge physics building. Step inside, and hear the vacuum pump singing and the air vent huffing, feel the four walls shaking and the floor beneath quaking--all as a result of a central air compressor located nearby. Is this a difficult place to detect gravitational waves? Definitely! But not a bad spot to carry out LIGO's contamination studies project because our apparatus, shown in the photo at right, is quite invulnerable to this noisy environment--immune even to a 4.0 earthquake! And most importantly, we are achieving good results.

Small Contaminants, Big Problems

Optical contamination of the Core Optics by material outgassing--particularly hydrocarbons--is a serious issue for LIGO. Because of the large optical circulating power we use, even small contaminations can cause dramatically large optical absorption and scatter, decreasing the sensitivity of the LIGO interferometer by a reduction in circulating power. Further, optical absorption deforms the curvature of the Core Optics and even induces a thermal lensing effect, resulting in an overall performance degradation of the LIGO interferometer. To avoid this, LIGO has set a strict loss requirement of 10 ppm/yr in scatter loss and one ppm/yr in absorption loss. The purpose of contamination studies is to make sure that any non-optical materials LIGO plans to use do not induce losses exceeding the LIGO loss requirement.

We have three high-finesse (~30000) contamination test cavities qualified previously in vacuum under a continuous light illumination at 1064 nm with a power level comparable to that in LIGO. Ringdown (measuring total cavity loss) and mode spacing (measuring cavity absorption loss) techniques are employed to measure cavity losses at sensitivities of ~1.5 ppm/cavity for total cavity loss and ~0.3 ppm/cavity for cavity absorption loss. We put pre-baked materials with an initially acceptable Residual Gas Analyzer scan into our cavity chambers for final acceptance test. This is performed by independently monitoring the total cavity loss and cavity absorption loss over a period from which annual cavity losses (scatter and absorption) are extrapolated to compare with the LIGO's requirement. So far we have successfully qualified LEDs, photodiodes, kapton cables, teflon wires and vac-seal for use in LIGO. Viton seals, air baked steel and a Faraday isolator are being tested, so let's hope to see them in LIGO vacuum chambers soon!