| 摘要 |
<p>A demodulation system (12) used in connection with an analog optical link (14) that provides a wide dynamic range and SNR potential for large RF signal bandwidths. An optical carrier signal modulated with an RF signal is applied to an optical splitter (30) in the demodulation system (12) that splits the signal into first and second carrier signals. One of the carrier signals is applied to a coarse demodulator (34) that provides either PM or FM demodulation to generate a demodulated signal representative of the signal that includes the RF signal and the additive inverse of an error signal. The error signal includes the theoretical minimum noise floor, as well as excess noise and distortion introduced by the coarse demodulator when the frequency or phase deviation is large. The output from the coarse demodulator (34) is inverted, and integrated in the FM case, and then applied to a phase modulator (56) along with the second optical carrier signal from the optical splitter (30). The phase modulator (56) modulates the optical carrier signal with the additive inverse of the demodulated signal from the coarse demodulator (34) so that the portion of the RF signal occurring in both the optical carrier signal and the demodulated signal are cancelled, and the optical carrier signal is modulated with the error signal. The modulated carrier signal, which now has a small frequency or phase deviation, is applied to a fine demodulator (60) that demodulates the signal to generate the error signal. The error signal and the demodulated signal from the coarse demodulator (34) are then combined, and what remains is a substantial copy of the RF signal with minimal excess noise and distortion. Unbalanced Mach-Zehnder interferometers (32, 62) and photodetectors (36, 38, 64, 66) can be provided in the coarse demodulator (34) and the fine demodulator (60) to provide the demodulation.</p> |
