High-Efficiency Photonic Microwave Downconversion With Full-Frequency-Range Coverage

Microwave frequency downconversion is optically implemented by using two cascaded polarization modulators (PolMs), characterized by full-frequency-range coverage and high conversion efficiency. In this proposed approach, the incident radio-frequency signal (RF) and the local oscillator signal (LO) are applied to the first PolM at the transmitter and the second one at the receiver, respectively. Effective optical carrier suppression can be achieved using the cascaded PolMs, whereas the optical sidebands induced by the RF and the LO signals remain unchanged. Since no additional optical filter is needed, the lower operation frequency limit in other approaches with an optical filter is eliminated, such that a full-frequency-range coverage is ensured in this approach. Meanwhile, the use of optical carrier suppression (OCS) enables high conversion efficiency for microwave downconversion. In the proof-of-concept experiments, the conversion efficiency is improved by over 20 dB within the frequency range from 2 to 15 GHz, as compared to those of cascaded modulators without OCS. Both the upper and the lower limits of such a frequency range can be greatly extended when test instruments with full frequency range are used.