Laser characterization and stabilization for precision interferometry

Lasers for high-precision optical measurements, in particular for ground-based interferometric gravitational wave detectors, were characterized and stabilized. A compact, automated laser beam diagnostic instrument, based on an optical ring resonator, was developed and used to characterize the output beam of different continuous-wave, single-frequency lasers at wavelengths of 1064nm and 1550nm. The laser beam fluctuations in power, frequency and pointing as well as the spatial beam quality were investigated. The results were used, amongst others, for laser stabilization design. Different laser stabilization methods are reviewed and the laser stabilization concept for the second-generation gravitational wave detector Advanced LIGO is described. Important components of this stabilization were developed, such as the so-called pre-mode-cleaner resonator for filtering various laser beam parameters. Furthermore, several laser power stabilization experiments were performed. A high-sensitivity, quantum-noise-limited detector for power fluctuations consisting of an array of photodiodes was developed and was used to stabilize the output power of a laser in the audio frequency band, achieving an independently measured relative power noise of 2.4×10-9 Hz-1/2 at 10 Hz. In addition, a novel power-fluctuation detection technique, called optical ac coupling, which is based on photodetection in reflection of an optical resonator, was investigated theoretically and experimentally. This technique allows new power stabilization schemes, especially important for next generation gravitational wave detectors, and it can beat the theoretical quantum limit of traditional schemes by up to 6 dB, among other benefits. A sensitivity of 7×10-10 Hz-1/2 for relative power fluctuations was experimentally demonstrated at radio frequencies using an optical ac coupled photodetector.