An optical RF memory with an ultra-long storage time of 13 ms

An optical RF memory (ORFM) based on a fiber acousto-optic frequency shifting loop (AO-FSL) for microwave signal storage with a record-long storage time of 13 ms is proposed and experimentally demonstrated. The key to achieving a long storage time is the use of a distributed fiber Raman amplifier which is able to compensate for the loop loss while maintaining an ultra-low noise figure (NF). An acousto-optic frequency shifter is employed in the loop to avoid lasing. To test the ORFM, a microsecond-long microwave signal is amplitude-modulated on an optical carrier and injected into the AO-FSL. Experimental results show that the ORFM enables the storage of microwave signals, including a single-tone and a linearly frequency modulated (LFM) microwave signals, with signal durations up to 10 µs and a record-long storage time of 13 ms. This corresponds to more than 300 circulations in the AO-FSL, representing the highest number ever reported to date. Compared with a digital RF memory, our proposed ORFM eliminates the need for a high-speed analog-to-digital converter (ADC) and digital-to-analog converter (DAC), offering a wide operational bandwidth and simplified system architecture. These advantages make it particularly well-suited for electronic warfare applications where an ORFM having a long storage time and high signal fidelity is needed.