Arm-length stabilisation for interferometric gravitational-wave detectors using frequency-doubled auxiliary lasers.

Residual motion of the arm cavity mirrors is expected to prove one of the principal impediments to systematic lock acquisition in advanced gravitational-wave interferometers. We present a technique which overcomes this problem by employing auxiliary lasers at twice the fundamental measurement frequency to pre-stabilise the arm cavities' lengths. Applying this approach, we reduce the apparent length noise of a 1.3 m long, independently suspended Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control of the system from the auxiliary laser to the measurement laser.

In situ measurement of absorption in high-power interferometers by using beam diameter measurements.

We present a simple technique to make in situ measurements of the absorption in the optics of high-power laser interferometers. The measurement is particularly useful to those commissioning large-scale high power optical systems.