AM-PM conversion in the residual phase noise of two-point electro-optical frequency division

Electro-optic comb-based two-point optical frequency division (eOFD) provides a robust architecture for generating low-phase-noise microwave signals by transferring the relative frequency stability of a dual-wavelength laser. However, this transfer process introduces residual phase noise that limits the achievable performance. Here, we show that electro-optic comb generation redistributes optical power while preserving the noise floor, leading to increased relative intensity noise (RIN) on individual comb lines. This RIN is converted into amplitude noise of the intermediate-frequency (IF) signal during heterodyne photodetection and is subsequently converted into microwave phase noise via amplitude-to-phase (AM-PM) conversion in the mixer. To suppress this effect, we employ a saturated RF amplifier after photodetection to compress amplitude fluctuations. A 12 dB reduction of the microwave phase noise is achieved, reaching -140 dBc/Hz at a 10 kHz offset frequency. This work identifies the dominant role of RIN-induced AM-PM conversion in limiting the phase noise of eOFD systems and provides a targeted strategy to suppress this effect.