Lasers for high optical power interferometers

Many experiments in modern physics set highly demanding requirements on their laser light sources. Precision metrology, laser cooling experiments and the quantum engineering of atoms and molecules are some example areas in which very stable lasers are indispensable. The first generation of interferometric gravitational wave detectors have been among the laser applications with the most challenging requirements, simultaneously requiring low fluctuations in power, frequency and beam pointing as well as high power levels of 10W. Laser sources for second generation gravitational wave detectors need to fulfill even more demanding requirements, which we will discuss in the first section of this chapter. The second section is devoted to the design of lasers for advanced detectors followed by a section in which we discuss their stabilisation. The last section covers some laser concepts for third generation gravitational wave detectors. Requirements on the light source of a gravitational wave detector One of the fundamental noise sources of laser interferometric gravitational wave detectors directly related to the laser light is the shot noise in the interferometer readout. The ratio of a potential gravitational wave signal to the readout shot noise is proportional to the square root of the light power in the interferometer. Hence, gravitational wave detectors need high-power lasers in combination with resonant optical cavities to achieve high circulating power levels in the interferometer. First generation detectors use lasers with approximately 10Wlight power and second generation instruments will require power levels of order 200W. In general, increasing the light power in the interferometer improves the sensitivity until the noise introduced by fluctuating radiation pressure forces on their mirrors reaches the same level as the readout shot noise