Using optical/photonic techniques to generate, process, transport and measure RF, microwave and mmWave signals. The premise: above ~40 GHz, electronics struggle (frequency-multiplier chains multiply noise with the carrier, need different hardware per band, and grow in size/weight/power), whereas photonics can synthesise high, tunable, low-noise carriers from optical sources.
The field is decades old but historically lab- and defence-bound; the open question is commercial scale, not feasibility (see ref).