An electro-optical communications satellite transponder

The design concept of an electrooptical transponder for communications satellites operating in C-band or Ku-band is presented. A system comprising 12 single-polarization transponders 36-MHz wide is illustrated with a block diagram. The RF carrier uplink signal passes through a low-noise GaAs FET amplifier and is translated to the downlink RF frequency by a mixer and local oscillator; this signal then serves as a subcarrier to intensity modulate an 850-nm GaAlAs semiconductor laser, and the optical signal is optically switched via fiber, demodulated by pin diode or avalanche photodiode, and passed through a GaAs FET preamplifier and amplifier for downlink transmission. Both prefiber and postfiber component groups can be integrated onto monolithic electrooptical chips operating at very low power. Additional features and design options are discussed; the frequency variation involved (12 decades) is illustrated; and the optical switching speed is shown to be noncritical in present TDMA, SS/TDMA, or FDMA configurations.