A century after the existence of gravitational waves was predicted by Albert Einstein in his general theory of relativity, the first direct detection was made on September 14 in 2015. Continuous upgrades and improvements of the Advanced LIGO (aLIGO) gravitational wave observatories have increased their sensitivity and made this detection possible. In the final sensitivity configuration, the detectors are limited by quantum noise. Prototypes are required to carry out further developments to reduce the noise level below quantum noise. In order to reach and subsequently surpass the Standard Quantum Limit (SQL) of interferometry, the AEI 10m Prototype facility was developed at the Albert Einstein Institute (AEI) in Hanover. The achievement of extreme stability in frequency of the laser source for the sub-SQL interferometer is required. For this purpose, a 21 m long, fully suspended triangular ring cavity was established to provide a stable length reference on which the laser frequency is stabilized. To reduce the frequency noise of the free-running laser, the scope of this thesis was the assembly and commissioning of the frequency reference cavity. The initial design has been improved to suppress the noise sources affecting the cavity length. To ensure continuous stabilization of the system, an automatic alignment of the input beam to the cavity was developed and implemented. The cavity optics are suspended as triple pendulums for isolation from seismic noise. Around the pendulum resonance frequencies from 0.5 Hz to 10 Hz, the cavity optics are actively controlled by means of a feedback control loop. This has been improved by the use of new digital filters for a degree of freedom dependent damping and reduced the mirror motion at the fundamental longitudinal resonance frequency to 2x10^-2 μm/sqrt(Hz). A further suppression of cavity mirror motion was achieved by implementing a new concept for active cavity length stabilization. This technique uses the mode cleaner feedback signal to control the length of the frequency reference cavity below 2 Hz by acting on the longitudinal motion of curved cavity mirrors. This damping improvement reduced the mirror motion to 2x10^-3 μm/sqrt(Hz). The triple suspensions are commissioned and the stabilization of the mechanical resonances is successful. In addition, a new concept of low-frequency cavity length control has been implemented. The foundations for achieving the performance of laser frequency stabilization have been established to enable the operation of the sub-SQL interferometer.