In free-space optical communications in which the signal is coupled into a single-mode fiber, atmospheric distortion leads to loss of signal and reduced receiver sensitivity. We demonstrate a coherent receiver system in which a Dual Polarization Quadrature Phase Shift-Keying (DP-QPSK) signal is coupled into a photonic lantern, which efficiently separates the light from a large multimode core into single mode fibers. Outputs from the lantern are passed to off-the-shelf integrated coherent receivers and digitized, and the resulting signals are coherently combined with optimal weight coefficients. The reconstructed signal exhibits reduced sensitivity to atmospheric distortion and demonstrates improved performance.
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.
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