We studied and demonstrated a wavelength discriminant structure that consists of one circulator, one or more Fiber Bragg Gratings and two photodiodes. The discriminants are built in NASA’s LCRD (Laser Communication Relay Demonstration) flight modems to measure the transmitter and pilot laser wavelengths on orbit. The performance of the discriminants is evaluated in ambient and thermal vacuum chamber environment. The paper reports on results of a few discriminants working at different wavelengths and power levels. The trending of the discriminant performance under ambient and TVAC cycles is discussed. The discriminant can achieve sub-picometer wavelength accuracies if calibrated properly.