Generation of a Long Linearly Chirped Microwave Waveform Based on a Fourier Domain Mode-Locked Optoelectronic Oscillator Incorporating a Frequency Shifting Loop

Broadband chirped microwave waveform with a large time–bandwidth product (TBWP) is highly needed in a radar system to achieve pulse compression with a large pulse compression ratio. A chirped microwave waveform with a wide bandwidth can be generated optically thanks to the ultrawide bandwidth offered by modern photonics, but the pulsewidth is usually very limited. In this work, we propose and experimentally demonstrate a novel approach based on a Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) to generate a broadband chirped microwave waveform with a long pulsewidth. The key is to increase the effective loop length of the FDML-OEO, which is done by incorporating a recirculation frequency shifting loop (FSL) in the OEO loop. Since the time duration of a microwave waveform is proportional to the OEO loop length, the pulsewidth is increased if a light is recirculating in the FSL for multiple times. The proposed approach is evaluated experimentally. For an FDML-OEO with a length of 1.6 km, a linearly chirped microwave waveform (LCMW) with an increased pulsewidth to
$202.67 \mu \text{s}$
is generated by controlling the light recirculating in an FSL with a length of 3.2 km for 12 times.