Commercially available analog fiber optic links provide the wide bandwidth, interference protection, and isolation for transmission of receiver IF bands to the digital processing equipment for the Green Bank Telescopes. An amplitude stability of 10-4 over periods of several minutes is required for continuum observations and baseline stability for broad spectral line observations. Gain variations of 1 percent were observed in a commercially available direct-modulated Fabry-Perot laser fiber optic link, when stress induced birefringence changes occurred in the fiber. Further investigation revealed gain variations were produced by the polarization dependence of responsivity in the photodetectors. Scale models of the cable wraps revealed that rotation of the laser with respect to the photodiode, due to certain cable wrap designs, is the dominant source of gain instabilities, and a clock spring-type cable wrap reduces this effect. However, the potential for gain variations due to vibration of the structure is not solved by careful cable wrap design. Therefore, an optical level control system is developed to ensure amplitude stability requirements are satisfied. In this system, consisting of a distributed feedback laser diode, a Mach Zehnder intensity modulator, and a high-powered photodetector, the microwave power gain is a function of laser power. The gain is stabilized by detecting the change in average photodetector current and modulating the laser diode bias with a correction voltage. With a second-order control loop, the gain changes resulting from the polarization sensitivity of the photodiode are corrected to better than 10-4.