Enhanced FROSTI Design for Integration into LIGO

Future upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) such as A+ hinge on achieving higher operating power in the main interferometer and better quantum noise performance to further increase its astrophysical reach. We developed a novel approach to laser wavefront control called the Front Surface Type Irradiator (FROSTI) to help achieve these sensitivity targets, by providing complementary corrective capabilities to LIGO’s existing thermal compensation system. Initial in-vacuum tests of a prototype demonstrated that FROSTI can provide an effective correction capability that meets the design requirements. Other challenges were revealed in the course of the prototype testing, however, including an unexpected non-deterministic behavior in the Resistance Temperature Detector (RTD) that fluctuates without changes in power input, the need to reduce the weight of the FROSTI for eventual installation on the suspension frame of the test masses, more accurate RTD calibration for temperature monitoring and power balancing, and others. We report the results of further in-vacuum testing conducted to investigate and resolve these issues, and present an enhanced FROSTI design suitable for integration into LIGO.