Method for compensation of thermally induced modal distortions in the input optical components of gravitational wave interferometers

The next generation of interferometric gravitational wave detectors will employ laser powers approaching 200 W to increase shot-noise limited sensitivity. Optical components that transmit the laser light will exhibit increased thermal lensing induced by bulk absorption and concomitant changes in the material refractive index, resulting in significant changes in the modal characteristics of the beam. Key interferometer components such as electro-optic modulators and Faraday isolators are particularly at risk, since they possess relatively large absorption coefficients. We present a method for passive correction of thermally induced optical path length (ΔΛ) changes induced by absorption in transmissive optical components. Our method relies on introducing material in the optical path that possesses a negative index temperature derivative, thereby inducing a compensating opposite ΔΛ. We experimentally demonstrate a factor of 10 reduction in higher order spatial mode generation for terbium gallium garnet, a Faraday isolator material