A Mode Selector to Suppress Fluctuations in Laser Beam Geometry

Our development of a gravitational wave detector requires a Michelson interferometer of extreme sensitivity capable of measuring 10-16 m (i.e. some 10-10 of a wavelength λ of the illuminating laser light). Even after painstaking alignment of the interferometer components, and after considerable improvement of the laser stability, noise contributions much in excess of this goal were observed, due partly to fluctuations of the laser beam geometry. The two most obvious types of geometric beam fluctuations are a lateral beam jitter and a pulsation in beam width; these lead to spurious interferometer signals if the interfering wavefronts are misaligned in their tilts or in their curvatures respectively. The geometry of the laser beam can be considerably stabilized by passing it through an optical resonator. The geometric beam fluctuations, as viewed from this resonator, can be described by a well-centred ground mode TEMoo, contaminated by transverse modes TEM mn , with amplitudes decreasing rapidly with the mode order m + n. In the simplest case, the resonator consists of two identical concave mirrors of high reflectance ρ2. The mirror separation can be chosen such that, while the resonator is tuned for maximum transmittance for the TEMoo mode, the low order transverse modes TEM mn are almost totally suppressed, in amplitude by factors of the order of 1 − ρ2. Considerations leading to a practical implementation are discussed, and experimental results are given.