Modifications were made to an existing cavity ringdown to efficiently spatially filter the laser beam, precisely align the cavity, and add control of optical feedback. The optical components comprising the cavity were characterized with two different methods of beam injection into the half symmetric cavity. The first method injected the waist of the beam through the back of the flat mirror. This method produced strong ringdown signals. From the time constants of the decay, a mirror transmissivity of 0.00122 ± 0.00005, corresponding to a reflectivity of 0.9988,was determined. This value is lower than desired and is an indication that the mirrors should be carefully and thoroughly cleaned so that they can reclaim the manufacturer's reflection level of 0.9998. The second method injected the beam by a weak reflection off a pellicle beam splitter. This technique harnessed the optical feedback from the resonant cavity causing stronger and more regular ringdown events. Time constants recorded produced multiple transmission fractions through the pellicle beam splitter depending on the angle of the pellicle beam splitter. The values for the three recorded data sets were 0.998 ± 0.025, 0.9994 ± 0.0003, and 0.9991 ± 0.0001. Using the second method of injection, the reflectivity of a distributed Bragg reflector was measured to be 0.938 ± 0.005. Data should be recollected because there were questionable artifacts in the decay curves.