Gravitational wave detectors are highly sensitive instruments to measure gravitational waves. To increase their observation time and decrease maintenance and upgrade periods, newly developed improvements need to be tested thoroughly. The AEI 10 m prototype is a testing facility for potential upgrades to gravitational wave detectors, providing an extremely low noise environment.The sub-SQL interferometer at the AEI 10 m prototype is a Fabry-Perot Michelsoninterferometer. Once set up, it will measure differential length changes between its interferometer arms limited by the standard quantum limit (SQL). This will allow techniques to surpass the SQL in gravitational wave detectors to be tested.This thesis presents the design and implementation of various subsystems for thesub-SQL interferometer at the AEI 10 m prototype. Aluminum gallium arsenide (AlGaAs) test mass mirror coatings promise reduced coating Brownian noise. Nonetheless, it remained to be shown that the thickness uniformity and damage threshold of AlGaAs coatings fulfill the requirements of gravitational wave detectors. Here, the thickness uniformity RMS was measured to be 0.41 ±0.05 nm, and the lower limit for the laser-induced damage threshold of the mirrors was measured to be 64 ±5 MW/cm2.A scatter measurement device is set up to characterize optics for the AEI 10 mprototype. It provides bidirectional reflection distribution function measurements ofhigh-quality optics. It can measure optics with a total integrated scattering of below0.01 ppm. The beam splitter of the sub-SQL interferometer and its triple-suspension were designed. The optical requirements of the beam splitter are defined and discussed. Its shape and a split coating are designed to reduce the power in arising ghost beams. The beam splitter’s triple suspension was built, and its resulting performance was evaluated, fulfilling the set requirements.An output mode cleaner for the sub-SQL interferometer was designed and built, in-cluding its double suspension. The design focuses on reducing optical losses while fulfilling the mode filter requirements. Its losses were measured to be 0.1 ±0.4 % while fulfilling the required mode filter performance based on an output beam profile of aLIGO.